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Roots and the associated soil directly affected by root activity, termed the rhizosphere, have both been extensively studied and recognized for their crucial role in soil functioning. The formation of the rhizosphere is primarily driven by the effect of roots on shaping the physical structure of the soil, which in turn has direct feedbacks on the interactions between physical, biological and chemical processes. As a result, the rhizosphere is a hot spot for microbial activity, cycling of nutrients and turnover of organic matter. Despite the pivotal role of soil structure in controlling rhizosphere processes, we still lack a quantitative description and understanding of the interrelationships of root-systems and soil in the creation and stabilization of soil structure. We provide a comprehensive review of current knowledge and novel insights into processes that drive the formation and stabilization of soil structure in the rhizosphere. These processes are regulated by multiple indirect and direct pathways, involving root growth, the production of rhizodeposits and root hairs, as well as the activity of soil microorganisms and fauna. Further, we highlight that rhizosphere processes may persist and evolve after root death to an extent currently largely unknown. Finally, we identify five pertinent challenges that should be addressed to fully apprehend rhizosphere processes and thus harness the potential resilience of plant-soil interactions. These challenges include refining structural assessment and sampling of rhizosheaths, examining the rhizosphere in-situ and bridging the gap between solid phase and pore scale research. In our view, overcoming these obstacles can be accomplished by combining the power of imaging and isotopic approaches, especially at the field scale, encompassing diverse soils and plant species. The ultimate objective of future research should be to upscale rhizosphere processes by conducting more field experiments in concert with modeling efforts, under the umbrella of collaborative interdisciplinary research. • Plant roots are inherently bound to soil structure dynamics through rhizosphere processes. • Rhizodeposits and root hairs trigger rhizosheath formation through multiple routes. • Roots foster formation of unique habitat by affecting soil porosity and aggregation. • Soil fauna and legacy of dead roots are overlooked factors for rhizosphere structure formation and stability. • Bridging solid phase and pore scale research needed to advance rhizosphere research. [ABSTRACT FROM AUTHOR]

Correlations between organic carbon (OC) and fine mineral particles corroborate the important role of the abundance of soil minerals with reactive surfaces to bind and increase the persistence of organic matter (OM). The storage of OM broadly consists of particulate and mineral-associated forms. Correlative studies on the impact of fine mineral soil particles on OM storage mostly combined data from differing sites potentially confounded by other environmental factors. Here, we analyzed OM storage in a soil clay content gradient of 5–37% with similar farm management and mineral composition. Throughout the clay gradient, soils contained 14 mg OC g−1 on average in the bulk soil without showing any systematic increase. Density fractionation revealed that a greater proportion of OC was stored as occluded particulate OM in the high clay soils (18–37% clay). In low clay soils (5–18% clay), the fine mineral-associated fractions had up to two times higher OC contents than high clay soils. Specific surface area measurements revealed that more mineral-associated OM was related to higher OC loading. This suggests that there is a potentially thicker accrual of more OM at the same mineral surface area within fine fractions of the low clay soils. With increasing clay content, OM storage forms contained more particulate OC and mineral-associated OC with a lower surface loading. This implies that fine mineral-associated OC storage in the studied agricultural soils was driven by thicker accrual of OM and decoupled from clay content limitations. [ABSTRACT FROM AUTHOR]

The biogeochemical functioning of soils (e.g., soil carbon stabilization and nutrient cycling) is determined at the interfaces of specific soil structures (e.g., aggregates, particulate organic matter (POM) and organo‐mineral associations). With the growing accessibility of spectromicroscopic techniques, there is an increase in nano‐ to microscale analyses of biogeochemical interfaces at the process scale, reaching from the distribution of elements and isotopes to the localization of microorganisms. A widely used approach to study intact soil structures is the fixation and embedding of intact soil samples in resin and the subsequent analyses of soil cross‐sections using spectromicroscopic techniques. However, it is still challenging to link such microscale approaches to larger scales at which normally bulk soil analyses are conducted. Here we report on the use of laboratory imaging Vis–NIR spectroscopy on resin embedded soil sections and a procedure for supervised image classification to determine the microscale soil structure arrangement, including the quantification of soil organic matter fractions. This approach will help to upscale from microscale spectromicroscopic techniques to the centimetre and possibly pedon scale. Thus, we demonstrate a new approach to integrate microscale soil analyses into pedon‐scale conceptual and experimental approaches. Highlights: Quantification of soil constituents using Vis‐NIR spectroscopy.New approach to use resin embedded soil core sections with intact structure.Reproducible quantification of soil constituents important for soil carbon storage.Vis‐NIR as promising tool for upscaling from microscale to pdeon scale. [ABSTRACT FROM AUTHOR]

The high aggregate stability of Andosols and the direct effects of sample drying led to several inconsistencies during physical soil organic matter fractionation. We have determined that NaCl addition displayed little influence on clay dispersion. At the microscale, we observed the re‐aggregation of the clay fraction caused by freeze‐drying. This issue was avoided by analyzing aliquots of soil suspension. Thus, we recommend reserving a small soil liquid aliquot to be subjected to microscopy analysis. [ABSTRACT FROM AUTHOR]

Phosphorus (P) constitutes one of five macronutrients essential for plant growth and development due to the central function of phosphate in energy metabolism, inheritance and metabolic control. In many ecosystems, plant available soil-P gets limited by soil aging. Hence, plants have developed adaptation strategies to cope with such limitation by an efficient plant and ecosystem internal P-cycling during annual growth. The natural floodplain habitat of fast-growing Populus x canescens is characterized by high soil-P availability. It was thus expected that the P-nutrition of P. x canescens had adapted to this conditions. Therefore, different P-fractions in different twig tissues were investigated during two annual growth cycles. The P-nutrition of P. x canescens markedly differs from that of European beech grown at low soil-P availability (Netzer F, Schmid C, Herschbach C, Rennenberg H (2017) Phosphorus-nutrition of European beech (Fagus sylvatica L.) during annual growth depends on tree age and P-availability in the soil. Environ Exp Bot 137:194-207). This was mainly due to a lack of tree internal P-cycling during annual growth indicated by the absence of P-storage and remobilization in twig bark and wood. Hence, strategies to economize P-nutrition and to prevent P-losses had not developed. This fits with the fast-growth strategy of P. x canescens at unrestricted P-availability. Hence, the Pnutrition strategy of P. x canescens can be seen as an evolutionary adaptation to its natural growth habitat. [ABSTRACT FROM AUTHOR]

Differentiation of biochar and native soil organic matter (SOM) is required to assess the effect of biochar amendment on in-situ changes of SOM. Therefore, we used C 4 biochar produced at high temperature (1200 °C) by gasification (BC GS ) and measured the 13 C abundance of density and particle size fractions. We quantified the BC GS effects on distinct native C 3 -SOM pools of a grassland topsoil one year after BC GS amendment. The chemical composition was analyzed with solid-state 13 C CPMAS NMR, whereas information on the nanostructure of BC GS were obtained by Raman microspectroscopy measurements. Our aim was to assess BC GS induced chemical changes of SOM and physical fractions and to validate the accuracy of BC GS detection by 13 C NMR spectroscopy. Quantification by isotopic measurements and 13 C NMR spectroscopy for aromatic C yielded similar estimates of BC in soils. Of the total BC GS , 52% were recovered as free particulate organic matter (POM) and 33% were located in aggregated soil structures isolated as occluded POM particles. Around 4% of the total BC GS was detected in the clay fraction. After one year of field exposure, the surface of the BC GS particles decreased in unordered graphitic-like structures. The higher ordered BC residue is supposed to be more recalcitrant. The native SOC stock increase ( p = 0.06, n = 4) in the clay fraction indicated increased sequestration of organic matter as mineral-bound SOM due to BC GS amendment. With respect to soil functionality, the BC GS amendment induced a tremendous shift from a soil system dominated by organo-mineral associations to POM−dominated OC storage, resulting in increased soil air capacity. [ABSTRACT FROM AUTHOR]

Organic carbon (OC) stored in permafrost affected soils of the higher northern latitudes is known to be highly vulnerable to ongoing climatic change. Although the ways to quantify soil OC and to study connected C dynamics from ecosystem to global scale in the Arctic has improved substantially over the last years, the basic mechanisms of OC sequestration are still not well understood. Here we demonstrate a first approach to directly study micro scale soil structures mainly responsible for soil OC (SOC) stabilization using nano scale secondary ion mass spectrometry (NanoSIMS). A cross section from a permafrost layer of a Cryosol from Northern Alaska was analysed using a cascade of imaging techniques from reflectance light microscopy (RLM) to scanning electron microscopy (SEM) to NanoSIMS. This allowed for the direct evaluation of micro scale soil structures known to be hot spots for microbial activity and SOC stabilization in temperate soils. The imaging techniques were supported by classical soil analyses. Using this unique set of techniques we are able to evidence the formation of micro-aggregate structures in the vicinity of plant residues in permafrost soils. This clearly indicates biogeochemical interfaces at plant surfaces as important spheres for the formation of more complex soil structures in permafrost soils. Organo-mineral associations from these hot spots of microbial activity were recovered from plant residues (free particulate organic matter, fPOM) as fine grained mineral fraction with a typically low C/N ratio. This nicely illustrates the link between classical bulk analysis and state of the art spectromicroscopic techniques. [ABSTRACT FROM AUTHOR]

Earthworms are important drivers for the formation of soil structure and play a key role in soil organic matter (SOM) dynamics. Our previous long-term (126 weeks) laboratory experiment showed that carbon (C) loss declined through time in soil when litter was mixed and consumed by earthworms ( Lumbricus rubellus ). Eventually, the C loss was lower than in treatments where litter was mechanically mixed into soil with exclusion of earthworms. However, it is not clear if the solely physical manipulation of soil or biological activity of earthworms lead to different SOM quality, which would result in a distinction in C loss and consequently C sequestration. Thus, we differentiated between physical (mechanical mixing) and earthworm effects on SOM composition. Two types of soil were used in the experiment: clay and sand, and these were incubated with alder ( Alnus glutinosa ) and willow ( Salix caprea ) litter, respectively. The combination of soils and litter types corresponds to the natural combinations at the sampling sites. To explain underlying mechanisms of a lower C loss in the earthworm vs. mechanically mixed treatment, we separated SOM fractions in order to gain pools defined in the Rothamsted model. Chemical differences between initial litter and the active and slow pool of SOM obtained by fractionation were studied. No significant differences between the earthworm and mechanically mixed treatment were found in C, nitrogen (N), and phenol contents, composition of major chemical groups of litter studied by solid-state 13 C NMR spectroscopy, and composition of aromatic components of SOM studied by analytical pyrolysis (Py GC/MS). This lack of differences in chemical composition suggests that greater SOM sequestration in the earthworm treatment is likely to be connected with physical protection of SOM inside cast aggregates rather than with chemical changes in SOM mediated by earthworms. [ABSTRACT FROM AUTHOR]

Organic matter (OM) amendments, originating from waste materials, could be used to enhance soil organic carbon (SOC) storage and fertility of cropland soils. However, there is a limited understanding about the long-term effect of different urban waste compost amendments on soil organic matter (SOM) formation processes and their impact on SOC storage. Accordingly, the long-term effects of different OM amendments on the amount and composition of particulate and mineral associated SOM were investigated. Surface soils were sampled from a Luvisol under cropping rotation, which received biannually 0.4 kg organic carbon m −2 in form of three different urban composts or cattle manure for a period of 15 years. Despite similar C input, different urban waste compost amendments resulted in contrasting C storage. While there were no C stock changes for municipal solid waste compost amended soils, composts from organic waste and green waste and sewage sludge increased SOC stocks in a similar range as conventional farmyard manure. In bulk soils, SOC stocks were increased by approximately 30%, in occluded particulate OM <20 μm by 155% (organic waste compost) and 71% (green waste and sewage sludge compost). Carbon storage in clay fractions showed approximately 20% higher values in all treatments. Organic matter amendments result in C–N coupling as for N proportional stock increases were recorded. The high variability of the composition of different amendment types was reflected in the particulate OM fractions but not in the highly uniform composition of the mineral-associated OM. Bulk soil wettability was not affected by the amendments, as only POM fractions showed increased hydrophobicity, but not clay fractions. Clay fractions showed high alkyl/O-alkyl and low C/N ratios, characteristic for microbial material. The OM composition in the clay fraction is determined by the microbial residues that are independent in their composition from the input material. The increased C storage in the clay fractions of the soil amended with organic wastes and green waste and sewage sludge compost and farmyard manure might be promoted by a better microbial use efficiency leading to C sequestration as microbial compounds. [ABSTRACT FROM AUTHOR]

Permafrost-affected soils of the northern circumpolar region represent 50% of the terrestrial soil organic carbon ( SOC) reservoir and are most strongly affected by climatic change. There is growing concern that this vast SOC pool could transition from a net C sink to a source. But so far little is known on how the organic matter ( OM) in permafrost soils will respond in a warming future, which is governed by OM composition and possible stabilization mechanisms. To investigate if and how SOC in the active layer and adjacent permafrost is protected against degradation, we employed density fractionation to separate differently stabilized SOM fractions. We studied the quantity and quality of OM in different compartments using elemental analysis, 13 C solid-phase nuclear magnetic resonance (13 C- NMR) spectroscopy, and 14 C analyses. The soil samples were derived from 16 cores from drained thaw lake basins, ranging from 0 to 5500 years of age, representing a unique series of developing Arctic soils over time. The normalized SOC stocks ranged between 35.5 and 86.2 kg SOC m−3, with the major amount of SOC located in the active layers. The SOC stock is dominated by large amounts of particulate organic matter ( POM), whereas mineral-associated OM especially in older soils is of minor importance on a mass basis. We show that tremendous amounts of over 25 kg OC per square meter are stored as presumably easily degradable OM rich in carbohydrates. Only about 10 kg OC per square meter is present as presumably more stable, mineral-associated OC. Significant amounts of the easily degradable, carbohydrate-rich OM are preserved in the yet permanently frozen soil below the permafrost table. Forced by global warming, this vast labile OM pool could soon become available for microbial degradation due to the continuous deepening of the annually thawing active layer. [ABSTRACT FROM AUTHOR]

To better understand how carbon and nitrogen mineralization are linked in soils, we conducted a long-term incubation experiment and compared carbon and nitrogen dynamics in the bulk soil and in soil fractions. Topsoil of a Rendzic Leptosol from a beech forest site near Tuttlingen, Germany, was separated into three particle size classes: sand (2000-20 μm), silt (20-2 μm), and clay (<2 μm). Bulk soil and particle size fractions were incubated in replicate, allowing periodic destructive sampling of triplicates at day 0, 14, 42, 84, 140, 210, and 280. We monitored CO2-C respiration, NH3-N emissions, nitrogen mineralization, pool sizes of total and salt extractable (0.01 M CaCl2) organic carbon and nitrogen, and microbial biomass carbon and nitrogen. The chemical composition of selected samples was further characterized by 13C-NMR spectroscopy. Fractionation did not influence carbon mineralization (∑ fractions ≈ bulk soil), which decreased in the order sand > clay > silt. The fractions respired between 10.4% (sand fraction), 8.8% (clay fraction) and 4.4% (silt fraction) of total soil organic carbon. However, nitrogen mineralization was affected by the fractionation procedure (∑ fractions < bulk soil) and followed the order clay > silt > sand. Fractionation increased the surface area and hence provided accessory mineral surfaces, which allowed new binding of especially nitrogen-rich compounds, in addition to ammonium fixation via cation exchange. As indicated by lower metabolic quotients, microbial carbon mineralization was more efficient in the bulk soil compared to the calculated sum of fractions. In the clay fraction, carbon mineralization rates, salt extractable organic carbon contents, and microbial biomass carbon and nitrogen contents declined strongly towards the end of the incubation. This indicates that in the clay fraction, organic carbon was not available for microbial degradation and that microorganisms were strongly carbon-limited causing a subsequent inhibition of nitrogen immobilization. Density fractionation revealed that organic matter in the sand fraction consisted mainly of particulate organic matter present as light material containing partly decomposed plant remnants. The organic matter in the clay fraction was mostly adsorbed on mineral surfaces. Organic matter in the sand and in the clay fraction was dominated by O/N-alkyl C indicating low recalcitrance, but the C/N ratio of organic matter narrowed with decreasing particle size. Our results suggest that carbon and nitrogen mineralization are decoupled in the mineral-associated fractions of the soil. The specific interactions of both carbon and nitrogen containing components with the mineral matrix strongly modulate the mineralization dynamics. Therefore, isolated considerations of C/N or alkyl C to O/N-alkyl C ratios of organic matter are insufficient as indicators for decomposition in plant residues. The combined consideration of C/N and alkyl C to O/N-alkyl C ratios provides a first impression about the degree of decomposition in plant residues. However, bioavailability in fractions where organic matter is mainly stabilized by spatial inaccessibility and by organo-mineral interactions cannot be explained by these ratios, but can be examined by an incubation approach. [ABSTRACT FROM AUTHOR]

The stabilization of soil organic matter (SOM) is triggered by three main mechanisms: (i) low bioavailability due to aggregation, (ii) recalcitrance due to the chemical structure, and (iii) association of the SOM with mineral surfaces. In the present study we used particle size SOM fractions (sand, silt and clay), derived from the Ah soil horizon from a Norway spruce forest in Southern Germany, to study the effects of different stabilization mechanisms on the bioavailability of soil organic carbon (SOC) in a one year incubation experiment. The respired CO2 was hourly recorded, additionally 13CO2 was analysed 20 times and 14CO2 three times during the incubation experiment. To better differentiate between particulate OM (POM) and mineral associated OM (MIN), the incubated fractions and bulk soil were separated according to density (1.8 g cm−3) after the incubation experiment. 13C-CPMAS NMR spectroscopy was used to study the chemical composition of the incubated samples. We demonstrate a clear increase in SOM bioavailability due to aggregate disruption, as the calculated theoretical CO2 evolution of the SOM fractions recombined by calculation was 43.8% higher in relation to the intact bulk soil. The incubated sand fraction, dominated by POM rich in O/N-alkyl C, showed a prolonged bioavailability of SOC moieties with mean residence times (MRT) of 78 years. Interestingly, the silt fraction, dominated by highly aliphatic, more recalcitrant POM, showed low mineralization rates and slow MRT's (192 years) close to values for the clay fraction (171 years), which contained a large amount of mineral-associated SOM. The recorded 13/12CO2 signatures showed a high depletion in 13C during the initial stage of the incubation, but an enrichment of the respired 13CO2 of up to 3.4‰ relative to the incubated SOM was observed over longer time periods (after 3 and 4 days for bulk soil and sand, respectively, and after 14 days for silt and clay). Therefore, we found no evidence for a 13C enrichment of SOM as driven by metabolic isotopic fractionation during microbial SOM mineralization, but an indication of a change in the isotopic composition of the C-source over time. [ABSTRACT FROM AUTHOR]

Abstract: Since a NanoSIMS high-resolution secondary ion mass spectrometry (SIMS) instrument was first used for cosmochemistry investigations over a decade ago, both interest in NanoSIMS and the number of instruments available have significantly increased. However, SIMS comes with a set of challenges that are of both technical and conceptual nature, particularly for complex samples such as soils. Here, we synthesize existing research and provide conceptual and technical guidance to those who wish to investigate soil processes at the submicron scale using SIMS, specifically with NanoSIMS. Our review not only offers advice resulting from our own operational experience but also intends to promote synergistic research on yet unresolved methodological issues. We identify and describe the basic setup of a NanoSIMS instrument, and important issues that may arise as a soil sample specimen are prepared for NanoSIMS analysis. This is complemented by discussions of experimental design, data analysis, and data representation. Next to experimental design, sample preparation is the most crucial prerequisite for successful NanoSIMS analyses. We discuss the requirements and limitations for sample preparation over the size range from individual soil particles to intact soil structures such as macroaggregates or intact soil cores. For robust interpretation of data obtained by NanoSIMS, parallel spatial, textural (scanning electron microscopy, atomic force microscopy), or compositional analyses (scanning transmission X-ray microscopy) are often necessary to provide necessary context. We suggest that NanoSIMS analysis is most valuable when applied in concert with other analytical procedures and can provide powerful inference about small-scale processes that can be traced via isotopic labeling or elemental mapping. [Copyright &y& Elsevier]

Ultrasonication is widely used in soil organic matter (SOM) fractionation studies to break up soil aggregates to disperse the soil into its primary particles. An increasing number of studies also aim to incubate SOM fractions obtained by physical soil fractionation to study the bioavailability of different soil organic C (SOC) pools. To evaluate possible influences of ultrasonic soil disruption on short-term SOM bioavailability as well as the content and composition of water-extractable organic matter (WEOM) and salt-extractable (10 mM K2S04) organic matter (SEOM), we conducted a laboratory incubation experiment with aggregated soil material from an Ap horizon of a cropland soil. Bulk soil and subsamples in which aggregates had been disrupted by ultrasonication were incubated in triplicate for 18 d at 20°C. During the incubation, C02 production was measured continuously. At the beginning and at the end of the experiment, the chemical composition of SEOM was analyzed for the different experimental variants by ultraviolet (UV)-absorbance and solid state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C-CPMAS NMR) spectroscopy. Additionally, we studied the effects of WEOM removal on heterotrophic soil respiration to assess artifacts by unavoidable WEOM removal during soil wet sieving, which is an important step in soil fractionation. After repeated leaching of unsonicated and sonicated soil samples, 0.5 to 1.0% of the total organic C (OC) pool was associated with the WEOM fraction. Ultrasonic disruption of soil aggregates resulted in increased amounts and changed composition of SEOM as well as in a 27% increase in C02 production compared with intact soils. Incubation of ultrasonicated samples resulted in decreased O/N alkyl C and increased aromatic C contents in SEOM, indicating mineralization of potentially mobile organic matter (OM). Ultrasonic treatment during soil fractionation leads to enhanced dissolved OM (DOM) leaching and increased SOM bioavailability, which may bias results from incubation experiments on separated SOM fractions. [ABSTRACT FROM AUTHOR]

Abstract: The specific features of the nano-scale secondary ion mass spectrometry (NanoSIMS) technology with the simultaneous analysis of up to seven ion species with high mass and lateral resolution enables us to perform multi-element and stable isotope measurements at the submicron scale. To elucidate the power of this technique, we performed an incubation experiment with soil particles of the fine silt and clay fractions (from an Albic Luvisol), with occluded particulate organic material and intact soil aggregates (from a Haplic Chernozem), using a 13C and 15N labelled amino acid mixture as tracer. Before and during 6-day incubation after the addition of the label, samples were consecutively prepared for NanoSIMS analysis. For this purpose, two different sample preparation techniques were developed: (i) wet deposition and (ii) the sectioning of epoxy resin embedded samples. Single soil particles (fine silt/clay fraction) showed an enrichment of 13C and 15N after label addition that decreased over time. On aggregates of particulate organic matter, re-aggregated during the 6-day incubation experiment, we could show a spatially heterogeneous enrichment of 13C and 15N on the particle surface. The enrichment in 15N demonstrated the diffusion of dissolved organic matter into intact soil aggregate interiors. The prospects of NanoSIMS for three dimensional studies of stable C and N isotopes in organo-mineral associations is demonstrated by the recorded depth profiles of the organic matter distribution on mineral particles. [Copyright &y& Elsevier]

In a lysimeter experiment with juvenile beech trees ( Fagus sylvatica L.) we studied the development of depth gradients of soil organic matter (SOM) composition and distribution after soil disturbance. The sampling scheme applied to the given soil layers (0–2 cm, 2–5 cm, 5–10 cm and 10–20 cm) was crucial to study the subtle reformation of SOM properties with depth in the artificially filled lysimeters. Due to the combination of physical SOM fractionation with the application of 15N-labelled beech litter and 13C-CPMAS NMR spectroscopy we were able to obtain a detailed view on vertical differentiation of SOM properties. Four years after soil disturbance a significant decrease of the mass of particulate OM (POM) with depth could be found. A clear depth distribution was also shown for carbon (C) and nitrogen (N) within the SOM fractions related to bulk soil. The mineral fractions <63 µm clearly dominated C storage (between 47 to 60% of bulk soil C) and N storage (between 68 to 86% of bulk soil N). A drastic increase in aliphatic C structures concomitant to decreasing O/N-alkyl C was detected with depth, increasing from free POM to occluded POM. Only a slight depth gradient was observed for 13C but a clear vertical incorporation of 15N from the applied labelled beech litter was demonstrated probably resulting from faunal and fungal incorporation. We clearly demonstrated a significant reformation of a SOM depth profile within a very short time of soil evolution. One important finding of this study is that especially in soils with reforming SOM depth gradients after land-use changes selective sampling of whole soil horizons can bias predictions of C and N dynamics as it overlooks a potential development of gradients of SOM properties on smaller scales. [ABSTRACT FROM AUTHOR]

A holistic understanding of plant strategies to acquire soil resources is pivotal in achieving sustainable food security. However, we lack knowledge about variety‐specific root and rhizosphere traits for resource acquisition, their plasticity and adaptation to drought. We conducted a greenhouse experiment to phenotype root and rhizosphere traits (mean root diameter [Root D], specific root length [SRL], root tissue density, root nitrogen content, specific rhizosheath mass [SRM], arbuscular mycorrhizal fungi [AMF] colonization) of 16 landraces and 22 modern cultivars of temperate maize (Zea mays L.). Our results demonstrate that landraces and modern cultivars diverge in their root and rhizosphere traits. Although landraces follow a 'do‐it‐yourself' strategy with high SRLs, modern cultivars exhibit an 'outsourcing' strategy with increased mean Root Ds and a tendency towards increased root colonization by AMF. We further identified that SRM indicates an 'outsourcing' strategy. Additionally, landraces were more drought‐responsive compared to modern cultivars based on multitrait response indices. We suggest that breeding leads to distinct resource acquisition strategies between temperate maize varieties. Future breeding efforts should increasingly target root and rhizosphere economics, with SRM serving as a valuable proxy for identifying varieties employing an outsourcing resource acquisition strategy. Summary statement: This study provides novel insights into the diversity of soil resource acquisition strategies within a single plant species. By comparing landraces and modern cultivars of maize and considering plant‐soil‐microbe interactions, we reveal functional differences in root and rhizosphere economics and trait plasticity that shed light on their ecological role and breeding relevance. [ABSTRACT FROM AUTHOR]

Avoiding chemical and physical artifacts during sampling is crucial for realistic analyses of mineral and other colloids in soil. We developed a sampler, which allows for the in situ collection of Fe oxides that precipitate in their natural environment in a Bg horizon of a Calcaric Gleysol. Simultaneous measurements of redox-sensitive parameters confirmed temporal changes from Fe-reducing to Fe-oxidizing conditions on site. [ABSTRACT FROM AUTHOR]

Earthworms co-determine whether soil, as the largest terrestrial carbon reservoir, acts as source or sink for photosynthetically fixed CO2. However, conclusive evidence for their role in stabilising or destabilising soil carbon has not been fully established. Here, we demonstrate that earthworms function like biochemical reactors by converting labile plant compounds into microbial necromass in stabilised carbon pools without altering bulk measures, such as the total carbon content. We show that much of this microbial carbon is not associated with mineral surfaces and emphasise the functional importance of particulate organic matter for long-term carbon sequestration. Our findings suggest that while earthworms do not necessarily affect soil organic carbon stocks, they do increase the resilience of soil carbon to natural and anthropogenic disturbances. Our results have implications for climate change mitigation and challenge the assumption that mineral-associated organic matter is the only relevant pool for soil carbon sequestration. Gerrit Angst et al. report the function of earthworms as biochemical reactors in soil by converting labile plant-derived carbon into stabilized microbial-derived carbon. They show that earthworms increase the resilience of carbon in soil to natural and anthropogenic disturbances. [ABSTRACT FROM AUTHOR]

Summary: Biophysicochemical rhizosheath properties play a vital role in plant drought adaptation. However, their integration into the framework of plant drought response is hampered by incomplete mechanistic understanding of their drought responsiveness and unknown linkage to intraspecific plant–soil drought reactions.Thirty‐eight Zea mays varieties were grown under well‐watered and drought conditions to assess the drought responsiveness of rhizosheath properties, such as soil aggregation, rhizosheath mass, net‐rhizodeposition, and soil organic carbon distribution. Additionally, explanatory traits, including functional plant trait adaptations and changes in soil enzyme activities, were measured.Drought restricted soil structure formation in the rhizosheath and shifted plant–carbon from litter‐derived organic matter in macroaggregates to microbially processed compounds in microaggregates. Variety‐specific functional trait modifications determined variations in rhizosheath drought responsiveness. Drought responses of the plant–soil system ranged among varieties from maintaining plant–microbial interactions in the rhizosheath through accumulation of rhizodeposits, to preserving rhizosheath soil structure while increasing soil exploration through enhanced root elongation.Drought‐induced alterations at the root–soil interface may hold crucial implications for ecosystem resilience in a changing climate. Our findings highlight that rhizosheath soil properties are an intrinsic component of plant drought response, emphasizing the need for a holistic concept of plant–soil systems in future research on plant drought adaptation. [ABSTRACT FROM AUTHOR]

Emerging evidence points out that the responses of soil organic carbon (SOC) to nitrogen (N) addition differ along the soil profile, highlighting the importance of synthesizing results from different soil layers. Here, using a global meta‐analysis, we found that N addition significantly enhanced topsoil (0–30 cm) SOC by 3.7% (±1.4%) in forests and grasslands. In contrast, SOC in the subsoil (30–100 cm) initially increased with N addition but decreased over time. The model selection analysis revealed that experimental duration and vegetation type are among the most important predictors across a wide range of climatic, environmental, and edaphic variables. The contrasting responses of SOC to N addition indicate the importance of considering deep soil layers, particularly for long‐term continuous N deposition. Finally, the lack of depth‐dependent SOC responses to N addition in experimental and modeling frameworks has likely resulted in the overestimation of changes in SOC storage under enhanced N deposition. [ABSTRACT FROM AUTHOR]

The complexity of soils extends from the ecosystem-scale to individual microenvironments, where micro-scale interactions between microbiota, organic matter (OM) and mineral particles are thought to control the long-term fate of soil carbon and nitrogen. It is known that such biogeochemical processes show disproportionally high reaction rates within nano- to micro-meter sized isolated zones ('hot spots', e.g. rhizosphere) in comparison to surrounding areas. However, the majority of soil research is conducted on large bulk (> 1 g) samples, which are often significantly altered prior to analysis and destructively analyzed. Thus it has previously been impossible to study elemental flows (e.g. C and N) between plants, microbes and soil in complex environments at the necessary spatial resolution within an intact soil system. Especially the high sensitivity and lateral resolution of nano-scale secondary ion mass spectrometry (NanoSIMS) offers one smart approach to study the composition and fate of organic matter at biogeochemical interfaces of biological, organic and mineral soil microscale structures. Especially the possible imaging of stable isotopes (e.g. 13C, 15N) at high resolution enables the direct tracking of microscale processes via experiments using stable isotope enrichment. However, NanoSIMS can only provide meaningful results if applied in concert with other imaging (e.g. focused ion beam (FIB-SEM), micro-CT) and classical analytical techniques (isotopic and elemental analysis) to both integrate different spatial scales and multiple layers of information. Most notably classical bulk analyses are the backbone to connect the imaging of microscale soil features and phenomena active at Pedon and ecosystem scale. Exemplary a range of studies on properties and processes at soil microenvironments with increasing complexity will be presented, reaching from microaggregates to intact rhizosphere and soil cores. [ABSTRACT FROM AUTHOR]

We examined the potential of stable-isotope Raman microspectroscopy (SIRM) for the evaluation of differently enriched C-labeled humic acids as model substances for soil organic matter (SOM). The SOM itself can be linked to the soil water holding capacity. Therefore, artificial humic acids (HA) with known isotopic compositions were synthesized and analyzed by means of SIRM. By performing a pregraphitization, a suitable analysis method was developed to cope with the high fluorescence background. Results were verified against isotope ratio mass spectrometry (IRMS). The limit of quantification was 2.1 × 10C/ C for the total region and 3.2 × 10C/ C for a linear correlation up to 0.25 C/ C . Complementary nanoscale secondary ion mass spectrometry (NanoSIMS) analysis indicated small-scale heterogeneity within the dry sample material, even though-owing to sample topography and occurring matrix effects-obtained values deviated in magnitude from those of IRMS and SIRM. Our study shows that SIRM is well-suited for the analysis of stable isotope-labeled HA. This method requires no specific sample preparation and can provide information with a spatial resolution in the micrometer range. [ABSTRACT FROM AUTHOR]

The physical, chemical, and biological processes forming the backbone of important soil functions (e.g., carbon sequestration, nutrient and contaminant storage, and water transport) take place at reactive interfaces of soil particles and pores. The accessibility of these interfaces is determined by the spatial arrangement of the solid mineral and organic soil components, and the resulting pore system. Despite the development and application of novel imaging techniques operating at the micrometer and even nanometer scale, the microstructure of soils is still considered as a random arrangement of mineral and organic components. Using nanoscale secondary ion mass spectroscopy (NanoSIMS) and a novel digital image processing routine adapted from remote sensing (consisting of image preprocessing, endmember extraction, and a supervised classification), we extensively analyzed the spatial distribution of secondary ions that are characteristic of mineral and organic soil components on the submicrometer scale in an intact soil aggregate (40 measurements, each covering an area of 30 pm x 30 pm with a lateral resolution of 100 nm X 100 nm). We were surprised that the 40 spatially independent measurements clustered in just two complementary types of micrometer-sized domains. Each domain is characterized by a microarchitecture built of a definite mineral assemblage with various organic matter forms and a specific pore system, each fulfilling different functions in soil. Our results demonstrate that these microarchitectures form due to self-organization of the manifold mineral and organic soil components to distinct mineral assemblages, which are in turn stabilized by biophysical feedback mechanisms acting through pore characteristics and microbial accessibility. These microdomains are the smallest units in soil that fulfill specific functionalities. [ABSTRACT FROM AUTHOR]

Redox-driven biogeochemical cycling of iron plays an integral role in the complex process network of ecosystems, such as carbon cycling, the fate of nutrients and greenhouse gas emissions. We investigate Fe-(hydr)oxide (trans)formation pathways from rhyolitic tephra in acidic topsoils of South Patagonian Andosols to evaluate the ecological relevance of terrestrial iron cycling for this sensitive fjord ecosystem. Using bulk geochemical analyses combined with micrometer-scale-measurements on individual soil aggregates and tephra pumice, we document biotic and abiotic pathways of Fe released from the glassy tephra matrix and titanomagnetite phenocrysts. During successive redox cycles that are controlled by frequent hydrological perturbations under hyper-humid climate, (trans)formations of ferrihydrite-organic matter coprecipitates, maghemite and hematite are closely linked to tephra weathering and organic matter turnover. These Fe-(hydr)oxides nucleate after glass dissolution and complexation with organic ligands, through maghemitization or dissolution-(re)crystallization processes from metastable precursors. Ultimately, hematite represents the most thermodynamically stable Fe-(hydr)oxide formed under these conditions and physically accumulates at redox interfaces, whereas the ferrihydrite coprecipitates represent a so far underappreciated terrestrial source of bio-available iron for fjord bioproductivity. The insights into Fe-(hydr)oxide (trans)formation in Andosols have implications for a better understanding of biogeochemical cycling of iron in this unique Patagonian fjord ecosystem. [ABSTRACT FROM AUTHOR]

Background and Aims Stomatal regulation allows plants to promptly respond to water stress. However, our understanding of the impact of above and belowground hydraulic traits on stomatal regulation remains incomplete. The objective of this study was to investigate how key plant hydraulic traits impact transpiration of maize during soil drying. We hypothesize that the stomatal response to soil drying is related to a loss in soil hydraulic conductivity at the root–soil interface, which in turn depends on plant hydraulic traits. Methods We investigate the response of 48 contrasting maize (Zea mays) genotypes to soil drying, utilizing a novel phenotyping facility. In this context, we measure the relationship between leaf water potential, soil water potential, soil water content and transpiration, as well as root, rhizosphere and aboveground plant traits. Key Results Genotypes differed in their responsiveness to soil drying. The critical soil water potential at which plants started decreasing transpiration was related to a combination of above and belowground traits: genotypes with a higher maximum transpiration and plant hydraulic conductance as well as a smaller root and rhizosphere system closed stomata at less negative soil water potentials. Conclusions Our results demonstrate the importance of belowground hydraulics for stomatal regulation and hence drought responsiveness during soil drying. Furthermore, this finding supports the hypothesis that stomata start to close when soil hydraulic conductivity drops at the root–soil interface. [ABSTRACT FROM AUTHOR]

Soil organic carbon management is a nature-based carbon dioxide removal technology at the same time contributing to soil health and agricultural productivity. The soil science communities are refuting the traditional assumptions of the nature of soil organic matter (SOM) as based on 'humic substances' that are operationally-defined and have not been observed by contemporary, in situ spectromicroscopic techniques. Instead, new theories suggest that the interactions between molecular diversity of organic compounds, their spatial heterogeneity and temporal variability controls the formation and persistence of SOM. A mechanistic understanding of these processes occurring within organo-mineral and organo-organic assemblages requires non-invasive techniques that minimize any disturbance to the physical and chemical integrity of the sample. Here, we present a theory-driven review where a combination of in situ methods serve as potential solutions to better understand the persistence and dynamics of SOM and its effects on nutrient distribution at a micro- and nano-scale. We explore underlying theories in light of advances in available methodologies, their historical development and future opportunities. Examples of interdisciplinary approaches that have been utilized in other areas of science but not in soils offer both deductive and inductive analytical opportunities. We show how different conceptual methods across scales inform each other, and how important and indispensable high-resolution investigations are to resolving next-generation questions. [ABSTRACT FROM AUTHOR]

Increased human‐derived nitrogen (N) loading in terrestrial ecosystems has caused widespread ecosystem‐level phosphorus (P) limitation. In response, plants and soil micro‐organisms adopt a series of P‐acquisition strategies to offset N loading‐induced P limitation. Many of these strategies impose costs on carbon (C) allocation by plants and soil micro‐organisms; however, it remains unclear how P‐acquisition strategies affect soil C cycling. Herein, we review the literature on the effects of N loading on P limitation and outline a conceptual overview of how plant and microbial P‐acquisition strategies may affect soil organic carbon (SOC) stabilization and decomposition in terrestrial ecosystems.Excessive input of N significantly enhances plant biomass production, soil acidification, and produces plant litterfall with high N/P ratios, which can aggravate ecosystem‐level P limitation.Long‐term N loading can cause plants and soil micro‐organisms to alter their functional traits to increase P acquisition. Plants can release carboxylate exudates and phosphatases, modify root morphological traits, facilitate the formation of symbiotic associations with mycorrhizal fungi and stimulate the abundance of P‐mineralizing and P‐solubilizing micro‐organisms. Releasing carboxylate exudates and phosphatases could accelerate SOC decomposition, whereas changing symbiotic associations and root morphological traits (e.g. an increase in fine root length) may contribute to higher SOC stabilization. Increased relative abundances of P‐mineralizing and P‐solubilizing bacteria can accelerate P mining and SOC decay, which may decrease microbial C use efficiency and subsequently lower SOC sequestration.The trade‐offs between different plant P‐acquisition strategies under N loading should be among future research priorities due to their cascading impacts on soil C storage. Quantifying ecosystem thresholds for P adaption to increased N loading is important because P‐acquisition strategies are effective when N loading is below the N threshold. Moreover, understanding the response of P‐acquisition strategies at different levels of native soil N availability could provide insight to divergent P‐acquisition strategies across sites and ecosystems. Altogether, P‐acquisition strategies should be explicitly considered in Earth System Models to generate more realistic predictions of the effects of N loading on soil C cycling. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

For the prediction of permafrost nitrogen (N) climate feedbacks, a better process-based understanding of the N cycle in permafrost ecosystems is urgently needed. Therefore, we characterized and quantified soil organic matter, gross soil microbial ammonification and nitrification and soil-atmosphere exchange of nitrous oxide (N 2 O) of boreal permafrost ecosystems on the southern edge of the Eurasian permafrost area in situ. Soil organic carbon (SOC) and total nitrogen (TN) stocks (top 0.5 m) of tree-free lowland peatland (LP) soils exceeded those of gravel-rich upland forest (UF) soils by an order of magnitude. Nuclear magnetic resonance spectroscopy revealed more recalcitrant organic matter at greater depth and more bioavailable organic matter substrates in upper peat horizons. In line with this result, gross ammonification and nitrification generally decreased with increasing sampling depth. Gross rates of mineral N turnover in active layers were comparable to those of temperate ecosystems. Despite substantial gross ammonification, the low nitrification:ammonification ratios and negligible soil N 2 O emissions depicted however a closed N cycle at UF and LP characterized by N limitation. In strong contrast, the lowland peat soils underneath alder trees (LA), being associated with diazotrophic bacteria in root nodules, showed an accelerated N turnover with very high gross rates of ammonification (3.1 g N m−2 d−1) and nitrification (0.5 g N m−2 d−1), exceeding those of UF and LP soils by an order of magnitude. This was accompanied by substantial N 2 O emissions comparable to temperate agricultural systems or tropical forests. The increase in gross soil microbial ammonification and nitrification was most pronounced in the rooted soil layer, where N inputs from biological N fixation almost doubled TN concentrations and halved SOC:TN ratios. The frozen ground of LA contained strongly increased ammonium concentrations that might be prone to release upon thaw via subsequent nitrification. This study shows that alder forests that further expand on permafrost-affected peatlands with global change create hot spots of soil mineral N turnover, thereby potentially enhancing permafrost N climate feedbacks via N 2 O emissions. • Boreal permafrost ecosystems of NE China show large gross nitrogen mineralization. • Nitrogen limitation is released by alder-associated biological nitrogen fixation. • Alder forests are hot spots of nitrification and N 2 O emissions. • Results suggest increasing N 2 O-induced permafrost nitrogen climate feedbacks. [ABSTRACT FROM AUTHOR]

Microscale processes occurring at biogeochemical interfaces in soils and sediments have fundamental impacts on phenomena at larger scales. To obtain the organo-mineral associations necessary for the study of biogeochemical interfaces, bulk samples are usually fractionated into microaggregates or micrometer-sized single particles. Such fine-grained mineral particles are often prepared for nanoscale secondary ion mass spectroscopy (NanoSIMS) investigations by depositing them on a carrier. This introduces topographic differences, which can strongly affect local sputtering efficiencies. Embedding in resin causes undesired C impurities. We present a novel method for preparing polished cross-sections of micrometer-sized primary soil particles that overcomes the problems of topography and C contamination. The particles are coated with a marker layer, embedded, and well-polished. The interpretation of NanoSIMS data is assisted by energy-dispersive X-ray spectroscopy on cross-sections prepared by a focused ion beam. In the cross-sections, organic assemblages on the primary soil particles become visible. This novel method significantly improves the quality of NanoSIMS measurements on grainy mineral samples, enabling better characterization of soil biogeochemical interfaces. In addition, this sample preparation technique may also improve results from other (spectro-) microscopic techniques. [ABSTRACT FROM AUTHOR]

Soil microbial communities play a crucial role in the accumulation and stabilization of soil organic carbon (SOC) through complex processes involving plant residue transformation and mineral interactions. These processes are influenced by plant inputs and modulated by soil properties that are mostly determined by the parent material. However, our understanding is limited regarding the manner in which vegetation and parent material affect microbial community structure, necromass accumulation, and their subsequent impact on SOC storage. To bridge this knowledge gap, we conducted an in-depth investigation focusing on the top-down influence of vegetation type and the bottom-up effect of parent material on microbial-mediated carbon transformation across soil profiles in a tropical region. Our study encompassed 42 sites on three parent materials (basalt, granite, and marine sediments) and four vegetation types (rubber, banana, areca plantations and uncultivated grassland). Soil samples were collected at 0–20, 20–40, 40–80, and 80–100 cm depth. Microbial community structure and necromass were quantified using microbial biomarkers of phospholipid fatty acids and amino sugars, respectively. In rubber plantations, we observed a trend toward higher microbial biomass that, though not significant when compared to other vegetation types, transformed to a significantly higher accumulation of microbial necromass. This increase in microbial necromass was linked to the accumulation of SOC facilitated by the presence of clay size minerals in clayey soils developed from basalt. In particular, basaltic soils were dominated by bacteria, which facilitated the accumulation of bacterial necromass that significantly bolstered its contribution to SOC. In contrast, in sandier soils developed from granite and marine sediments, fungal communities and necromass dominated due to the propensity of fungi for coarser soil environments. Overall, the main impact of vegetation on microbial communities and necromass accumulation was primarily demonstrated for the topsoil. Differences in soil texture arising from different parent materials exert significant effects on the fungal-to bacterial-biomass and necromass ratios, consequently influencing the contribution of fungal and bacterial necromass carbon to SOC across soil profiles. Our study underscores the pivotal role of parent material in governing tropical profile-scale soil carbon storage by shaping the structure of microbial communities and influencing the retention of microbial necromass. • Soils in rubber plantations showed higher microbial necromass content than other vegetation types. • Parent material had a greater influence on microbial necromass retention than vegetation. • Clayey basaltic soils retain more microbial necromass and SOC, dominated by G+ bacteria. • Bacteria dominate basaltic soils, while fungi prevail in sandy granite and marine sediment soils. [ABSTRACT FROM AUTHOR]

Rivers regulate the global carbon cycle by transferring particulate organic carbon (POC) from terrestrial landscapes to marine sedimentary basins, but the processes controlling the amount and composition of fluvially exported POC are poorly understood. We propose that hydrodynamic sorting processes modify POC fluxes during fluvial transit. We test this hypothesis by studying POC transported along a ∼1,200 km reach of the Rio Bermejo, Argentina. Nanoscale secondary ion mass spectrometry revealed that POC was either fine, mineral‐associated organic matter, or coarse discrete organic particles. Mineral‐associated POC is more resistant to oxidation and has a lower particle settling velocity than discrete POC. Consequently, hydraulic sorting and downstream fining amplify the proportion of fine, mineral‐associated POC from ∼55% to ∼78% over 1,220 km of downstream transit. This suggests that mineral‐associated POC has a greater probability of export and preservation in marine basins than plant detritus, which may be oxidized to CO2 during transit. Plain Language Summary: The carbon cycle regulates Earth's climate by determining the amount of carbon in our atmosphere. Rivers are important for the carbon cycle because they deliver large amounts of organic carbon from mountains to the oceans. If this carbon is quickly buried in ocean sediment, atmospheric CO2 concentrations decrease. In order to predict the fluxes of carbon to the oceans, we need to understand the processes controlling organic carbon transport from source to sink. In a 1,300‐km‐long river in Argentina, we measured the amount of sediment and carbon in river water at multiple stations along the source‐to‐sink pathway. Using geochemical methods, we identified two types of organic carbon: (a) plant fragments, which are easily oxidized to CO2, and (b) organic carbon attached to minerals, which is resistant to oxidation. We calculated the mass fluxes of these two types of organic carbon and found that export of mineral‐protected carbon is greater than the export of plant fragments. This suggests that most riverine organic carbon may be preserved upon delivery to the ocean. Our data show that we need to differentiate between plant fragments and mineral‐attached organic carbon to predict the amount of organic carbon moving between rivers, the oceans, and the atmosphere. Key Points: In lowland meandering rivers, downstream particle size fining and hydraulic sorting can modulate particulate organic carbon (POC) fluxesNano‐scale secondary ion mass spectrometry and biomarker isotope data show that mineral‐associated and discrete POC differ in composition and particle settling velocityDownstream particle size fining enhances fluvial export of recalcitrant, mineral‐associated POC that is more resistant to oxidation [ABSTRACT FROM AUTHOR]

[Display omitted] • Higher proportion of added C remained in soils with higher C loading. • Adding fresh OM to soil with lower C loading led to higher CO 2 release. • Plant litter rich in N promotes C retention in soils. • Plant litter amendments caused a higher fungi-to-bacteria ratio. It is crucial to promote soil carbon sequestration while reducing CO 2 emissions to mitigate climate change. However, the extent of increasing actual soil carbon storage depends on the amount and composition of organic matter input, including its fate during decomposition and soil organic matter (SOM) formation via microbial transformation. With respect to the need to increase carbon sequestration in soil and sustain soil fertility, it is of great interest to better understand how soils with different organic matter content react to amendment with fresh organic matter. Here, we incubated three agricultural soils representing a gradient in C content, adding two different 13C labeled plant residues varying in carbon-to-nitrogen ratio. Carbon mineralization was monitored together with the analysis of the 13CO 2 signatures. After the incubation, 13C compound-specific PLFAs, microbial necromass, and enzyme activities were analyzed. This study demonstrates that the carbon return on investment, thus the amount of retained fresh carbon in relation to the amount of added organic matter, clearly depends on the amount of native soil carbon. Notably, the addition of fresh organic matter to carbon-deficient soils leads to a higher specific CO 2 release compared to soils with high carbon loading, which can be attributed to the differences in the soil microorganisms' response. The CO 2 release of the soil with the lowest C-content was 2.1 and 2.0 mg g−1 soil for treatment with oat and pea litter addition, respectively, whereas for the soil with the highest C-content, CO 2 release was 1.7 mg g−1 soil for oat treatment and 1.6 mg g−1 soil for pea treatment. Thus, higher SOC contents sustain a higher 'return on investment' for the fresh carbon that is amended to soils. With plant litter amendments the microbial community shifted towards a higher fungi-to-bacteria ratio (F/B). This shift in the microbial community was more pronounced (F/B ranging from 0.04 to 0.11) with the addition of oat litter (low quality) compared to pea litter (high quality). Hence, it is important to consider the fate of organic amendments with different N availability when aiming to rebuild soil carbon stocks in degraded soils. Soil management should focus on sustaining soil carbon in balance with current carbon stocks to avoid the vicious circle of soils losing carbon in conjunction with increased greenhouse gas release. [ABSTRACT FROM AUTHOR]

• The retention of net rhizodeposition in the soil organic matter fractions was investigated. • Net rhizodeposition was predominantly retained as mineral-associated organic matter (MAOM). • Al and Fe reactive phases retained a substantial portion of the net rhizodeposition within MAOM but not the majority of it. • Rhizodeposition increased microbial PLFAs in the root vicinity, with a prominent incorporation into the fungal community. Net rhizodeposition corresponds to the portion of living root carbon (C) that remains in the soil after microbial processing and partial decomposition. Although it is assumed that this C input exerts an important role in the formation of soil organic matter (SOM), its contribution to distinct SOM pools is still not fully understood. In this study, we aimed to (i) quantify the retention of net rhizodeposition C in the different SOM fractions and in reactive Al and Fe mineral phases and (ii) investigate how rhizodeposition drives the spatial distribution of microbial communities in the rhizosphere. To track the transfer of net rhizodeposition into the soil, we used artificially labeled eucalypt (Eucalyptus spp.) seedlings under a 13C-CO 2 atmosphere (multiple-pulse labeling). Combining physical SOM fractionation and the chemical extraction of aluminum (Al) and iron (Fe) reactive phases, we studied the distribution of net rhizodeposition into different soil fractions. We also assessed the 13C incorporation into microbial phospholipid fatty acids (PLFAs) at different distances from the roots. Our results show that 76 % of the net rhizodeposition 13C was retained within the mineral-associated organic matter (MAOM) fraction. About 28 % of net rhizodeposition 13C within the MAOM fraction was retained within the Al and Fe reactive phases, indicating that this is a sizeable mechanism for the retention of net rhizodeposition in soil. Rhizodeposition increased the abundance of microbial PLFAs exclusively in the soil close to the roots (0–4 mm), with prominent incorporation of net rhizodeposition 13C into fungal biomarkers. Overall, our findings underscore the importance of mineral associations for the retention of net rhizodeposition in the soil. We also highlight the role of fungi in transferring the root-derived C beyond the root vicinity and promoting the formation of occluded SOM. [ABSTRACT FROM AUTHOR]

Incomplete knowledge on the environmental factors linking litter decomposition and the formation of soil organic matter (SOM) hampers the sustainable management of soil as a carbon (C) sink. Here, we explored the effect of soil texture on the fate of C from decomposing litter (Indiangrass; Sorghastrum nutans (L.) Nash) and the concurrent formation of SOM in mineral soils of different textures (sand- and clay-rich) and forest floor material. We quantified the amount of litter C respired, C remaining in the litter, and litter C retained in the soil/forest floor in a 186-day incubation employing stable isotope analyses (13C). We complemented our isotopic approach with the extraction of microbial biomarkers from the litter and soils/forest floor material and spectroscopic studies into the compositional changes of the incubated materials. We found that soil texture affected both the decomposition of litter and the retention of litter-derived C in the soil. The soil rich in clay provided conditions favorable for a more efficient microbial utilization of the litter material (high pH and high C use efficiency) as compared to the sand-rich soil and the forest floor. This resulted in lower amounts of litter C respired as CO 2 (25.0%, vs. 55.6 and 56.1% in clay vs. sand and forest floor material, respectively) and higher amounts of litter C retained in the clay-rich soil (12.6% vs. 3.5 and 5.3% in clay vs. sand and forest floor material, respectively). High contents of silt- and clay-sized mineral particles in the clay-rich soil likely resulted in the ability to stabilize litter C in aggregates and organo-mineral associations, perhaps as microbial residues. This ability was low in the sand-rich soil and virtually absent in the forest floor, where the recalcitrance of the litter and native SOM was probably more relevant, and a larger portion of litter C may have been retained in the soil as relatively untransformed plant compounds. We emphasize that litter decomposition, the formation of SOM, and soil texture are tightly linked, such that any differences in soil texture alter litter decomposition and SOM formation patterns for the same litter. • Role of clay/sand/forest floor in litter decomposition and SOM formation evaluated. • Clay-rich soil retained most litter C, while litter in this soil decomposed slowest. • Favorable conditions for efficient microbial use of litter C in clay-rich soil. • High amount of fine particles resulted in high ability to retain new litter C. • Soil texture differences affect litter decomposition & soil C sequestration rates. [ABSTRACT FROM AUTHOR]

Clay minerals and pedogenic metal (oxyhydr)oxides are the most reactive soil mineral constituents controlling the long-term persistence of organic carbon (OC) in terrestrial ecosystems. However, their co-occurrence in most soils complicates direct assessment of their individual contribution to OC persistence. Making use of unique mineralogical combinations in soils located in the East Usambara Mountains of Tanzania, we disentangled the contribution of clay-sized aluminous minerals (kaolinite, gibbsite) and pedogenic Fe (oxyhydr)oxides (predominant goethite and hematite) on OC storage and stabilization under natural forests and croplands. Topsoil samples, varying in contents but not types of aluminous clays and pedogenic Fe (oxyhydr)oxides, were identified by selective extractions, X-ray diffraction, and Mössbauer spectroscopy. Associated abundance of particulate and mineral-associated organic matter (OM) was quantified by density fractionation and their changes during land-use conversion were determined as a measure of OC persistence. Additionally, we assessed the resistance of OC to chemical oxidation as well as microbial decomposition in a 50-day laboratory incubation. We found that the ratio of pedogenic Fe to aluminous clay is more consequential for OC storage and stabilization than their individual contents, despite the fact that Fe (oxyhydr)oxides generally exert a stronger impact on OC than aluminous clays. Conjunction of large amounts of Fe (oxyhydr)oxides with low aluminous clay contents caused the strongest accumulation of mineral-associated OC, a low soil respiration, high OC stability against chemical oxidation, and high OC persistence during land-use change. Our study suggests that certain mineralogical combinations in the humid tropics alleviate OM losses during land conversion because of the strong and selective mineral control on OC stabilization, particular if the weight ratio of pedogenic Fe to aluminous clay exceeds the threshold range of 0.44‒0.56. [ABSTRACT FROM AUTHOR]

Clay minerals and pedogenic metal (oxyhydr)oxides are the most reactive soil mineral constituents controlling the long-term persistence of organic carbon (OC) in terrestrial ecosystems. However, their co-occurrence in most soils complicates direct assessment of their individual contribution to OC persistence. Making use of unique mineralogical combinations in soils located in the East Usambara Mountains of Tanzania, we disentangled the contribution of clay-sized aluminous minerals (kaolinite, gibbsite) and pedogenic Fe (oxyhydr)oxides (predominant goethite and hematite) on OC storage and stabilization under natural forests and croplands. Topsoil samples, varying in contents but not types of aluminous clays and pedogenic Fe (oxyhydr)oxides, were identified by selective extractions, X-ray diffraction, and Mössbauer spectroscopy. Associated abundance of particulate and mineral-associated organic matter (OM) was quantified by density fractionation and their changes during land-use conversion were determined as a measure of OC persistence. Additionally, we assessed the resistance of OC to chemical oxidation as well as microbial decomposition in a 50-day laboratory incubation. We found that the ratio of pedogenic Fe to aluminous clay is more consequential for OC storage and stabilization than their individual contents, despite the fact that Fe (oxyhydr)oxides generally exert a stronger impact on OC than aluminous clays. Conjunction of large amounts of Fe (oxyhydr)oxides with low aluminous clay contents caused the strongest accumulation of mineral-associated OC, a low soil respiration, high OC stability against chemical oxidation, and high OC persistence during land-use change. Our study suggests that certain mineralogical combinations in the humid tropics alleviate OM losses during land conversion because of the strong and selective mineral control on OC stabilization, particular if the weight ratio of pedogenic Fe to aluminous clay exceeds the threshold range of 0.44‒0.56. [ABSTRACT FROM AUTHOR]

Climate and litter quality drive litter decomposition, but there is currently little consensus on their relative importance, likely because studies differ in the duration, the climatic gradients and variability in litter‐trait values. Understanding these drivers is important because they determine the direct and indirect (via vegetation composition) effects of climate change on decomposition and thereby on carbon and nutrient cycling.We studied how microclimate (soil moisture and temperature) and litter traits interactively affect litter mass loss, by using a reciprocal litter translocation experiment along a large climatic gradient in Chile. We followed decomposition for 2 years and used 30 plant species with a wide spectrum of functional‐trait values.Litter traits had a strong impact on litter decomposition across the gradient, while an increase in decomposition with soil moisture was observed only in the wettest climates. Overall, soil moisture increased considerably in importance, relative to trait effects, at later decomposition stages, from c. 15% of the importance of traits after 3 and 6 months to c. 110% after 24 months. Moreover, analysing subsets of the 30 species showed that trait effects on litter decomposition gained in importance when including a greater variation in trait values.Synthesis. The relative effects of litter traits and climate on decomposition depend on the ranges in climate and litter traits considered and change with time. Our study emphasizes the critical role of representative ranges in climate and functional trait values for understanding the drivers of litter decomposition and for improving predictions of climate‐change effects on this important ecosystem process. [ABSTRACT FROM AUTHOR]

Here, we present our collective musings on soil research challenges and opportunities and, in particular, the points raised by Philippe Baveye (Baveye, 2020a, 2020b) and Johan Bouma (Bouma, 2020) on I bypass i and I hyperbole i in soil science. Furthermore, developing a healthy and constructive post-publication peer-review system, where bypasses and hyperbolic approaches can be identified and discussed, would ultimately boost publication quality and contribute to a more open discourse in soil science. [Extracted from the article]

Tree roots penetrate the soil to several meters depth, but the role of subsoils for the supply of nutrient elements such as phosphorus (P) to the trees is poorly understood. Here, we tested the hypothesis that increased P deficiency in the topsoil results in an increased microbial recycling of P from the forest subsoil. We sampled soils from four German temperate forest sites representing a gradient in total P stocks. We analyzed the oxygen isotopic composition of HCl-extractable phosphate (δ18OP) and identified differences in P speciation with increasing soil depth using X-ray absorption near-edge structure (XANES) spectroscopy. We further determined microbial oxygen demand with and without nutrient supply at different soil depths to analyse nutrient limitation of microbial growth and used nanoscale secondary ion mass spectrometry (NanoSIMS) to visualize spatial P gradients in the rhizosphere. We found that δ18OP values in the topsoil of all sites were close to the isotopic signal imparted by biological cycling when oxygen isotopes in phosphate are exchanged by enzymatic activity. However, with increasing soil depth and increasing HCl-P concentrations, δ18Ο values continuously decreased towards values expected for primary minerals in parent material at depths below 60 cm at sites with high subsoil P stocks and below more than 2 m at sites with low subsoil P stocks, respectively. For these depths, XANES spectra also indicated the presence of apatite. NanoSIMS images showed an enrichment of P in the rhizosphere in the topsoil of a site with high P stocks, while this P enrichment was absent at a site with low P stocks and in both subsoils. Addition of C, N and P alone or in combination revealed that microbial activity in subsoils of sites with low P stocks was mostly P limited, whereas sites with high P stocks indicated N limitation or N and P co-limitation. We conclude that subsoil P resources are recycled by trees and soil microorganisms. With continued weathering of the bedrock and mobilisation of P from the weathered rocks, P cycling will proceed to greater depths, especially at sites characterised by P limitation. [ABSTRACT FROM AUTHOR]

Dark, that is, nonphototrophic, microbial CO2 fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO2 fixation substantially contributes to the C balance of soils and what factors control this process. Therefore, the objective of this study was to quantitate dark microbial CO2 fixation in temperate forest soils, to determine the relationship between the soil CO2 concentration and dark microbial CO2 fixation, and to estimate the relative contribution of different microbial groups to dark CO2 fixation. For this purpose, we conducted a 13C‐CO2 labeling experiment. We found that the rates of dark microbial CO2 fixation were positively correlated with the CO2 concentration in all soils. Dark microbial CO2 fixation amounted to up to 320 µg C kg−1 soil day−1 in the Ah horizon. The fixation rates were 2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the respiration rate), our findings suggest that organic matter formed by microorganisms from CO2 contributes to the soil organic matter pool, especially given that microbial detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses indicated that CO2 was mostly fixed by gram‐positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO2 fixation rate in temperate forest soils increases in periods of high CO2 concentrations, that dark microbial CO2 fixation is mostly accomplished by gram‐positive bacteria, and that dark microbial CO2 fixation contributes to the formation of soil organic matter. [ABSTRACT FROM AUTHOR]

Climate differences can induce profound changes in organo-mineral associations in soils. However, the magnitude of these modifications, whether as a direct effect of climate conditions or an indirect effect through changes in soil mineralogy, are still not fully understood. In this study, we aimed to improve understanding of how climate and resultant changes in soil mineralogy affect subsoil (i.e., 0.4–0.9 m) organo-mineral interactions at the macro- and microscale. A set of subsoil samples were collected throughout an elevation gradient (approximately 1800–2400 mm precipitation year−1 and 15–24 °C) on Kohala Mountain, Hawaii. We carried out a combined approach of bulk soil analyses with mineral extractions and spectroscopic and spectromicroscopic analyses. Significant positive correlations (p < 0.05) between soil organic carbon (SOC) with extracted Fe and Al (dithionite citrate bicarbonate – DCB and ammonium oxalate – OX) at the bulk soil scale supported prior research showing concurrent decline of subsoil Fe, Al and SOC above a precipitation level of ∼2000 to ∼2200 mm year−1. However, divergence in microscale organo-mineral associations identified using NanoSIMS allowed us to discern the relative roles of Fe and Al in promoting organo-mineral associations. At the lower precipitation range (∼1800 mm year−1), the clay fraction < 2 µm showed higher amounts of organic matter (OM) co-localized with Fe & Al compared with the higher precipitation level (∼2300 mm year−1), where OM was mostly unassociated or only associated with Al. While Fe contributed to approximately 40% of the microscale organo-mineral associations in the lower precipitation site (quantified by co-localizations with OM segments), this contribution at the higher rainfall regime was only 5%. In contrast, the contribution of Al was approximately the same in both rainfall levels (approximately 30%). Therefore, associations with Al may be more important than Fe for OM stabilization under reducing climate conditions. The normalized CN:C ratio based on individual pixels was found to be higher when co-localized with Al, Fe, or both, especially under the high precipitation regime. This fact points towards the importance of Fe and Al to stabilize more N-rich OM, especially at high rainfall levels. In addition, subsoil from higher rainfall conditions exhibited more reduced forms of Fe (assessed by Fe K-edge XANES) and lower proportions of carboxyl-C (5% lower in the relative abundance) as well as higher alkyl/O-alkyl ratios determined by CP-MAS 13C NMR. Such differences in composition may directly influence the organo-mineral associations at both locations, as differences in Fe reduction and the presence of carboxyl-C groups are recognized to play a role in OM stabilization. We conclude that spatial relationships between Fe and Al with SOC at the microscale show a shift towards Al-dominated SOC associations at higher precipitation that could not be ascertained from bulk measurements alone. Therefore, they are of fundamental importance to understand the impact of climate change on SOC stabilization. [ABSTRACT FROM AUTHOR]

Various organic amendments are scrutinized as potential agricultural management strategies to ensure soil productivity while mitigating climate change due to the accumulation of soil organic matter (OM). The objectives of this experiment were to study the effects of biochar and biogas digestate versus mineral fertilizer on crop aboveground biomass as well as fractions and mineralization of soil organic carbon (SOC). Samples of a sandy Cambisol were taken 14 months after establishment of a field experiment in Germany. Treatments included application of equal nitrogen in the form of mineral fertilizer or liquid biogas digestate without biochar (B 0), with 1 Mg biochar ha−1season−1 for two growing seasons (B 2), or with 40 Mg biochar ha−1 application (B 40). Soil fractionation in water separated water-extractable and free particulate (fPOM) OM, followed by sonification and sieving to isolate occluded particulate (oPOM) and < 20 μm aggregate-occluded and mineral-associated OM. CO 2 emissions were measured during 92-day laboratory incubations at 10 and 20 °C. Analysis of variance found digestate lowered (p < 0.05) rye aboveground biomass compared to mineral fertilizer (9.3 vs. 10.6 Mg ha−1), while biochar had no effect. B 40 treatments increased C mineralization during incubation by 16% and contained 3.8 times more SOC than B 0 treatments. This additional SOC was allocated to fPOM (52%), oPOM (22%), and the <20 μm fraction (26%). Digestate application increased SOC content of oPOM by 11% compared to mineral fertilizer. Furthermore, combined application of 40 Mg biochar ha−1 with digestate resulted in 20% more SOC in the <20 μm fraction than biochar with mineral fertilizer. The lack of a significant fertilizer or biochar-fertilizer interaction effect on C mineralization during incubation demonstrates the stability of SOC from digestate alone or in combination with biochar. The absence of significant differences in SOC content between B 0 and B 2 treatments demonstrates the difficulty of documenting SOC sequestration in the field at low biochar application rates. Unlabelled Image • Compared to mineral fertilizer, digestate increased physically stabilized soil C. • 14 months after biochar application, 52% of additional soil C was not occluded. • Biochar increased CO 2 emissions during laboratory incubation, while digestate did not. • Biogas digestate resulted in lower aboveground crop biomass than mineral fertilizer. • 40 Mg biochar ha−1 applied to a sandy soil had no effect on aboveground crop biomass. [ABSTRACT FROM AUTHOR]

Nitrogen availability often restricts primary productivity in terrestrial ecosystems. Arbuscular mycorrhizal fungi are ubiquitous symbionts of terrestrial plants and can improve plant nitrogen acquisition, but have a limited ability to access organic nitrogen. Although other soil biota mineralize organic nitrogen into bioavailable forms, they may simultaneously compete for nitrogen, with unknown consequences for plant nutrition. Here, we show that synergies between the mycorrhizal fungus Rhizophagus irregularis and soil microbial communities have a highly non-additive effect on nitrogen acquisition by the model grass Brachypodium distachyon. These multipartite microbial synergies result in a doubling of the nitrogen that mycorrhizal plants acquire from organic matter and a tenfold increase in nitrogen acquisition compared to non-mycorrhizal plants grown in the absence of soil microbial communities. This previously unquantified multipartite relationship may contribute to more than 70 Tg of annually assimilated plant nitrogen, thereby playing a critical role in global nutrient cycling and ecosystem function. Rachel Hestrin et al. examine the synergies between mycorrhizal fungi and soil microbes that mediate plant nitrogen acquisition. They show that non-additive effects of fungi and soil microbes may be responsible for more than half of the nitrogen that arbuscular mycorrhizal plants derive from organic matter. [ABSTRACT FROM AUTHOR]

The physical protection of carbon (C) within aggregates contributes to the total C storage capability of soils. Changes in land use may have an immense effect especially in the topsoil where most of the disturbances (e.g. organic matter input, soil cultivation, surface temperature) intertwine. In strongly weathered and acidic tropical soils, pedogenic metal oxides and low-activity clay minerals should be the most important determinants of aggregation in addition to organic matter. However, it is unknown how different contents of these oxides and clay minerals affect the storage of organic C in aggregates. Moreover, we do not know whether effects of land use change on aggregate related C storage depend on the content of pedogenic oxides and clay in these tropical soils. To answer these questions, we selected an area located in the East Usambara Mountains, Tanzania, with abundant highly leached, acidic Acrisols. We selected each six sites under natural forest and annual cropping, having a similar plant species composition at each land use. Site selection at each land use was done based on strongly different ratios between the contents of pedogenic Fe oxides and oxide-free clay. The long soil development (East Usambara Mountains formed more than 25 million years ago) under these tropical climate conditions resulted in a similar mineral composition, consisting mainly of hematite, gibbsite, and kaolinite. Topsoil material was sampled (0-5 and 5-10 cm) at three slope positions and subjected to an aggregate fractionation by dry sieving. Bulk soils were analyzed for total element contents as well as contents of pedogenic metal oxides, soil texture (including oxide-free clay), pH, soil organic C, and total N. The obtained five aggregate size fractions (>4000 to <250 µm) were analyzed for their C and N contents. The unique characteristics of our gradient enable us to elucidate the influence of contrasting contents in pedogenic oxides and clay minerals on soil aggregation and the related C storage. Forest topsoils contained significantly more C than topsoils under cropland (4.4 vs. 3.1 kg/m²), whereas C storage under cropland was better related to mineralogical proxies. Therefore, we assume that mineral-organic associations are relatively more important for C storage under croplands, whereas labile particulate C contributes more to C storage in forest topsoils. Using all of our results, we will be able to estimate the impact of human disturbance on soil organic C storage along the gradient of pedogenic metal oxides and clay mineral contents. Thus, we will be able to identify the combination of these two factors resulting in the largest aggregate stability and the smallest land use-induced loss in organic C. [ABSTRACT FROM AUTHOR]

Mixed eucalypt forests and old growth rainforests represent two prevalent and interchangeable forest types covering large areas in Tasmania, Australia. Thereby, former consists of a eucalypt (fire tolerant) overstory undergrown by a developing (fire intolerant) rainforest understory. Pure rainforest is thus seen as the rare end of the natural succession evolving due to the absence of wildfires, which eucalypt trees need for germination. Recently, both forest types received attention in their role as an appreciable carbon (C) sink. Regarding their above-ground biomass, the mixed eucalypt forest stored more than twice as much C (378 ± 173 Mg C ha-1) compared to the old growth rainforest (171 ± 71 Mg C ha-1). Current studies on below ground soil C stocks, however, revealed no significant differences.Based on the sparse research available on soils in these unique ecosystems, this study focuses on topsoil and subsoil organic matter (OM) quality. Therefore, 14 study sites across Tasmania were grouped in soil pairs of geographic proximity and opposing forest types such that seven different parent materials (one for each soil pair) are covered ranging from Precambrian silt stone to Quaternary aeolian sediments. All sites were sampled according to their identified horizons to a depth of 100 cm. Laboratory analysis included pH, total soil organic carbon (SOC) and nitrogen as well as iron (oxalate and dithionite extractable) for each horizon. In order to determine the distribution and chemical composition of differently stabilized SOM fractions, the soils were fractionated into free, occluded and mineral-associated OM. Selected samples were analysed with 13C-CPMAS-NMR spectroscopy.At certain sites, relatively high amounts of SOC were detected in the subsoil for both forest types. This is in line with low bulk densities observed by previous studies in respective areas, pointing towards an important stabilization mechanism in such environments: through wildfires and subsequent erosion events, subsoil OM is rendered stable through burial with topsoil at foot slopes. Presumably due to regular occurrence of such incidents in the past, a less pronounced SOC-decrease with depth is observed compared to forests experiencing less or no wildfires. In the topsoils, mainly mixed eucalypt forests and partly old-growth rainforests show high amounts of black C representing another artefact from preceding wildfires as well as another rather stable SOM pool. This was supported by 13C-CPMAS-NMR spectroscopy demonstrating high amounts of aromatic C especially in the light particulate fractions.Apart from outlined findings, we will demonstrate differences with respect to distribution and composition of particulate vs. organo-mineral associated OM fractions in top- and subsoils as determined by the different parent materials. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40 year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15N and ∆14C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change. We investigated effects of tree species planted in replicated plots of a common garden on five indices of soil organic matter (SOM) stability (heterotrophic soil respiration, bulk soil δ15N and ∆14C, and C in particulate and mineral associated organic matter). Our results suggest tree species regulate SOM stability via the chemical composition of their tissues, especially roots. Some of our stability indices (C in mineral associated SOM and bulk soil ∆14C), though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change. [ABSTRACT FROM AUTHOR]

Abstract Phosphorus (P) and sulfur (S) are essential elements for life, and peculiarities of Antarctic soils regarding their S and P status likely affect terrestrial and aquatic ecosystems in Antarctica. Current climate warming is accompanied by deglaciation, increased availability of water, and vegetation establishment in many regions of Antarctica. The effect of these changes on soil P and S availability, ecosystem succession, mineral weathering, and important processes of soil formation and geochemistry (e.g. organic carbon sequestration, formation of secondary minerals) are largely unknown. Detailed knowledge on different P and S pools in soils of Antarctica is required for sound predictions of climate warming effects on important (bio)geochemical processes. We quantified the P and S concentration and speciation for various Antarctic soils using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy and related the soil P and S status to important soil and site properties. The soils, which have mostly remained at an initial stage of pedogenesis, differed considerably with respect to their P and S status. The P stock of most soils was dominated by Ca-bound (apatite) P (60–70% of total P). Some soils showed a marked accumulation of either Al-bound P (up to 60% of total P), Fe-bound P (up to 57%), Na- and K-bound P (up to 37%), or organic P (up to 54%). The S speciation also differed considerably among soils. Similar to other terrestrial environments, we observed a transformation of lithogenic apatite P and sulfide S into Al- and Fe-bound phosphate and sulfate as well as organic S and P. The transformation proceeded much more slowly than in soils with temperate humid climate, reflecting the cold and arid climate as well as the absence of vascular plants. The different P and S status of the soils could be related to the following site factors: (i) Parent material – soils formed from basalt had larger contents of total P as well as Al- and Fe-bound P compared to soils formed from mixture of basalt and sandstone. (ii) Sea spray influence – An upland soil affected by eolian deposition of sea spray showed topsoil enrichment of alkali-bound P, Ca-bound P, and CaSO 4. (iii) Inundation by seawater – Soils inundated by seawater had higher total P and S concentrations and higher concentrations of inorganic sulfide than upland soils. (iv) Soil moisture – Soils with increased soil moisture or wet conditions showed increased concentrations of P bound to pedogenic Al and Fe minerals and smaller contributions of Ca-bound (apatite) P to total P than soils under permanently arid conditions. (v) Colonization by mosses – Moss establishment resulted in topsoil P depletion and transformation of apatite P into Al- and Fe-bound P, but also in topsoil S accumulation and conversion of sulfide S into organic S forms. According to our study, the ongoing climate warming in Antarctica at many places will likely change soil P and S speciation as well as P and S availability to terrestrial and aquatic ecosystems, thus altering key biogeochemical processes like mineral weathering, secondary mineral formation, and soil C sequestration. [ABSTRACT FROM AUTHOR]