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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]

Geogenic organic carbon (GOC) from sedimentary rocks is an overlooked fraction in soils that has not yet been quantified but influences the composition, age, and stability of total organic carbon (OC) in soils. In this context, GOC is the OC in bedrock deposited during sedimentation. The contribution of GOC to total soil OC may vary, depending on the type of bedrock. However, no studies have been carried out to investigate the contribution of GOC derived from different terrestrial sedimentary rocks to soil OC contents. In order to fill this knowledge gap, 10 m long sediment cores from three sites recovered from Pleistocene loess, Miocene sand, and Triassic Red Sandstone were analysed at 1 m depth intervals, and the amount of GOC was calculated based on 14 C measurements. The 14 C ages of bulk sedimentary OC revealed that OC is comprised of both biogenic and geogenic components. The biogenic component relates to OC that entered the sediments from plant sources since soil development started. Assuming an average age for this biogenic component ranging from 1000–4000 years BP (before present), we calculated average amounts of GOC in the sediments starting at 1.5 m depth, based on measured 14 C ages. The median amount of GOC in the sediments was then taken, and its proportion of soil mass (g GOC per kg -1 fine soil) was calculated in the soil profile. All the sediments contained considerable amounts of GOC (median amounts of 0.10 g kg -1 in Miocene sand, 0.27 g kg -1 in Pleistocene loess, and 0.17 g kg -1 in Red Sandstone) compared with subsoil OC contents (between 0.53 and 15.21 g kg -1). Long-term incubation experiments revealed that the GOC appeared comparatively stable against biodegradation. Its possible contribution to subsoil OC stocks (0.3–1.5 m depth) ranged from 1 % to 26 % in soil developed in the Miocene sand, from 16 % to 21 % in the loess soil, and from 6 % to 36 % at the Red Sandstone site. Thus, GOC with no detectable 14 C content influenced the 14 C ages of subsoil OC and may partly explain the strong increase in 14 C ages observed in many subsoils. This could be particularly important in young soils on terrestrial sediments with comparatively low amounts of OC, where GOC can make a large contribution to total OC stocks. [ABSTRACT FROM AUTHOR]

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]

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]

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]

Forest subsoils may represent an important C sink in a warming world, but rhizodeposition as the key biogeochemical process determining the C sink strength of mature forests has not yet been quantified in subsoils. According to studies conducted in topsoil or laboratory experiments, soil C inputs by root exudation are increasing with increasing temperature and decreasing nutrient availability. We examined whether these relationships apply to forest subsoil by analyzing the response of root exudation to increasing soil depth up to 130 cm in a mature European beech ( Fagus sylvatica L.) forest. In two subsequent growing seasons differing in temperature and precipitation, we investigated in situ root exudation with a cuvette-based method and analyzed root morphology, microbial biomass, and soil nutrient availability. We proved that root exudation greatly decreases with soil depth as a consequence of a significant decrease in root-mass specific exudation activity to nearly a fifth of topsoil activity. The decrease in specific metabolic activity from 312 mg C g −1 yr −1 in the topsoil to 80 mg C g −1 yr −1 at 130 cm depth was amplified by an exponential decrease in root biomass per soil volume, leading to a relative decrease in root exudation per volume in the deep subsoil to 2% of topsoil root exudation (1 g C 10 cm −1 m −2 yr −1 at 130 cm depth). Specific root area decreased and mean fine root diameter and root tissue density increased with soil depth, indicating a shift in primary root functionality from fibrous roots in the topsoil to pioneer roots in the subsoil. The decrease in root exudation was accompanied by decreases in soil microbial biomass, extractable organic C (EOC), and N and P availability and increases in the aromatic C portion in SOM, but it did not relate to seasonal differences in climatic conditions. More specifically, it responded positively to an increase in EOC and ETN in the topsoil, but remained at its minimum rate in the SOC-poor subsoil, probably due to a lower organic N and higher mineral N content. The vertical pattern of beech root exudation is in accordance with a strategy to maximize whole-tree carbon-use efficiency, as it reduces C loss by exudation in soil spots where positive priming effects are unlikely, but enhances C exudation where microbes can mine less bioavailable SOM. The exudation patterns further suggest that increased C allocation to root systems as a likely tree response to elevated atmospheric [CO 2 ] may not lead to enhanced soil C input by root exudation to subsoils poor in SOM. [ABSTRACT FROM AUTHOR]

The spatial distribution of organic substrates and microscale soil heterogeneity significantly influence organic matter (OM) persistence as constraints on OM accessibility to microorganisms. However, it is unclear how changes in OM spatial heterogeneity driven by factors such as soil depth affect the relative importance of substrate spatial distribution on OM persistence. This work evaluated the decomposition and persistence of 13C and 15N labeled water-extractable OM inputs over 50 days as either hotspot (i.e., pelleted in 1–2 mm-size pieces) or distributed (i.e., added as OM < 0.07 μm suspended in water) forms in topsoil (0–0.2 m) and subsoil (0.8–0.9 m) samples of an Andisol. We observed greater persistence of added C in the subsoil with distributed OM inputs relative to hotspot OM, indicated by a 17% reduction in cumulative mineralization of the added C and a 10% higher conversion to mineral-associated OM. A lower substrate availability potentially reduced mineralization due to OM dispersion throughout the soil. NanoSIMS (nanoscale secondary ion mass spectrometry) analysis identified organo-mineral associations on cross-sectioned aggregate interiors in the subsoil. On the other hand, in the topsoil, we did not observe significant differences in the persistence of OM, suggesting that the large amounts of particulate OM already present in the soil outweighed the influence of added OM spatial distribution. Here, we demonstrated under laboratory conditions that the spatial distribution of fresh OM input alone significantly affected the decomposition and persistence of OM inputs in the subsoil. On the other hand, spatial distribution seems to play a lower role in topsoils rich in particulate OM. The divergence in the influence of OM spatial distribution between the top and subsoil is likely driven by differences in soil mineralogy and OM composition. [Display omitted] • The spatial distribution of organic matter is evaluated under experimental conditions. • Higher spatial distribution can increase organic matter persistence in the subsoil. • The influence of spatial distribution diminishes in a highly organic topsoil. • We emphasize the role of substrate microscale spatial distribution on soil microbiota. [ABSTRACT FROM AUTHOR]

• Lime addition impacts organic matter (OM) preservation in both top- and sub-soil. • Lime addition destabilizes N-poor OM associated with minerals in the topsoil. • The effect of lime on OM decomposition with depth depends on the OM availability. The development of effective strategies to maintain/increase soil C is hindered by the poor process-level understanding of the impact of management practices on C preservation, particularly at different soil depths. Based on the distinct biogeochemistry existing across a soil profile, a depth-dependent response of organic matter (OM) to soil amendments was hypothesized. To test this, we investigated the effect on OM preservation of lime addition to the topsoil and the subsoil of a volcanic soil classified as Andosol – the mineral soil with the largest organic C content worldwide. We incubated samples from each soil layer with (or without) lime addition, in the presence or absence of 13C- and 15N-labeled plant residue (simulating plant C inputs to the soil). The fate of OM in bulk soil and OM-mineral associations of microaggregates was measured using conventional chemical analyses and nano-scale secondary ion mass spectrometry, respectively. The results revealed contrasting geochemical properties existing within the soil profile, i.e. pH and amount of allophane increasing with soil depth. Functional complexity of OM also increased with depth, as revealed by an increase in spatial and molecular heterogeneity of OM, i.e. a larger proportion of microbial-derived OM with high spatial separation found in the subsoil. Lime addition caused OM destabilization as denoted by the increase in the amount of water-extractable C in both the topsoil and the subsoil (P < 0.05). In the topsoil, OM coverage of mineral surfaces decreased with liming from 49% to 30% whereas the opposite was observed in the subsoil, where it increased from 23% to 29%. Thus, liming caused the disruption of OM-mineral associations within microaggregates in the topsoil, but not in the subsoil. We infer that, at depth, the OM destabilization involved the mobilization of OM previously protected within macroaggregates. This pulse of labile C generated an advantageous environment for microbial OM mineralization in the subsoil, but this effect was diluted in the topsoil, richer in OM and where OM functional complexity is smaller. Depth-dependent soil geochemical properties and functional complexity determine differences in the effect of liming on the fate of soil OM in top vs. subsoils. [ABSTRACT FROM AUTHOR]

Subsoils are characterized by low concentrations of organic carbon (OC). Nevertheless, they contain more than half of the global soil OC because of their large volume. This discrepancy suggests that subsoils might further sequester carbon (C), thus acting as potential sinks for atmospheric C. Plant roots and associated rhizodeposits are a major OC input source to subsoils. However, whether and how increased OC inputs via plant roots to subsoils affect soil C sequestration mechanisms remains unclear. Here we set up a pot experiment with European Beech (Fagus sylvatica L.) seedlings to investigate the effect of tree roots and associated rhizosphere development on soil aggregation and C allocation in topsoil vs. subsoil material collected from three forest sites of different parent materials. Over a 5-month growth period, the seedlings developed a dense root system transforming the whole soil volume into root-affected (i.e., rhizosphere) soil. We found that roots and the associated rhizosphere development increased the amount of macroaggregates in the two finest-textured subsoils. The most C-poor and fine-textured subsoil had a 15% increase in bulk OC concentration, indicating a potential for C sequestration in subsoils by enhanced macroaggregation. Across subsoils, rooting strongly enhanced microbial abundance and was especially correlated with fungal abundance and a shift in the fungal-to-bacterial- ratio. The strong fungal growth was likely the cause for the enhanced macroaggregation in these subsoils. In topsoils, however, rooting treatment decreased macroaggregate abundance, potentially through the disruption of preexisting aggregates, as indicated by the concomitant increase in microaggregates. Our study supports the growing awareness that OC dynamics may be governed by different mechanisms in top- and subsoils, respectively. It demonstrates that the enhanced addition of OM via plant roots to subsoils boosts fungal growth and thereby increases macroaggregate formation, potentially facilitating C sequestration by occlusion. • Effects of rhizosphere development vary between top- and subsoil. • Rooting decreased macroaggregation in topsoil and increased it in subsoils. • Subsoil macroaggregation triggered by root-induced increase in fungal abundance. • Effect of rooting on aggregation more pronounced in finer-textured subsoils. • Subsoil OC storage increased by up to 15% due to rooting. [ABSTRACT FROM AUTHOR]

Contrasting environmental conditions in topsoil and subsoil determine both abundance and function of soil microbial communities, affecting carbon (C) dynamics throughout the entire soil profile. Although the response of soil microorganisms to single factors such as substrate availability or micro-climatic conditions has been frequently studied, fewer studies have focused on complex interactions between substrate availability and environmental conditions. To address this, we employed vertical soil translocations between topsoil and subsoil horizons of an acid and sandy Dystric Cambisol under European beech forest in Lower Saxony, Germany, to investigate the impact of changing habitat conditions on microbial decomposer communities. To follow microbial substrate utilization at different soil depths, we created hot spots of fresh organic matter (OM) by adding 13C-labelled root litter. Soil samples were taken every three months over an experimental period of twelve months (June 2014 to June 2015). Generally, microbial biomass was strongly controlled by C availability throughout the profile. The importance of root litter as a microbial C source increased from topsoil to subsoil, but changes in available C sources affected fungi and bacteria differently. Fungi preferentially used root litter-derived C throughout the entire soil profile, demonstrating that limited access to preferred substrates, rather than micro-climatic conditions, was the main driver of decreasing fungal abundance with soil depth. In contrast, bacteria intensified utilization of root-derived C only in the absence of alternative C sources in the subsoil and were more strongly affected by spatial separation from C sources. Low soil moisture in combination with the highly sandy subsoil environment limited bacterial access to their substrates and, consequently, bacterial growth. In conclusion, fungal C utilization relies mainly on the quantity of recent plant-derived substrates, whereas bacterial access to substrates is additionally controlled by environmental conditions. This study indicates that limited microbial access to their heterogeneously distributed substrates may be an important factor for C accumulation and stabilization in subsoils. • Micro-climatic conditions inhibit bacterial carbon utilization in subsoil. • Substrate limitation is the main driver of low fungal abundance in subsoil. • Altered carbon sources with soil depth affect the microbial community structure. [ABSTRACT FROM AUTHOR]

Subsoils store up to 70 % of the global soil organic carbon (SOC) stocks and may act as key carbon (C) sinks given appropriate management. A major input pathway of organic matter (OM) to subsoils is rhizodeposition which triggers biological, chemical and physical processes in the surroundings of plant roots, leading to the formation of the so-called rhizosphere. However, knowledge on SOC dynamics and especially stabilisation mechanisms, such as soil aggregation, in the rhizosphere of subsoils is very limited.In this study, we planted European beech (Fagus sylvatica L.) seedlings in topsoil and subsoil material from three beech forest sites that developed on different parent materials (red sandstone, loess, pleistocene sands). During a growth period of five months, the seedlings developed a dense root system that affected the whole soil volume inside the pots. This controlled experimental setup enabled us to sample relatively large amounts of rhizosphere soil. For each soil, unplanted controls were established. Samples were analysed for organic carbon (OC) and N, phospholipid fatty acids (PLFA) were extracted to investigate the microbial community composition, the activities of eight extracellular enzymes were measured, and extracellular polymeric substances (EPS) were quantified. Furthermore, the soils were subjected to an aggregate fractionation, yielding macroaggregates (> 250 μm), as well as different size classes of microaggregates (250-53 μm, 53-20 μm, < 20 μm).First results indicate that rooting effects vary between different soil types and soil depths. In the red sandstone and the loess soil, soil structure (measured as aggregate distribution) was strongly affected by rooting. However, the effects were contrary for topsoil and subsoil. In the topsoil, relative distribution of macroaggregates was significantly lower in the rhizosphere samples as compared to the unplanted control pots. In contrast, macroaggregation increased strongly in the subsoil rhizosphere, potentially triggered by a microbial community shift towards fungi. This might enhance the protection of SOC from degradation and thereby increase subsoil C sequestration. For the loess site, an increase in subsoil OC concentrations due to rooting was already visible in this medium-term experiment. Moreover, microbial biomass and enzyme activities tend to increase with rooting in the subsoil of all sites.Our study shows that plant roots affect deep soil OC dynamics in a very different way than those of topsoils and thus may contribute to subsoil C sequestration via input of OM and associated shifts in microbial community and soil structure. [ABSTRACT FROM AUTHOR]

As the largest active terrestrial reservoir of organic carbon (OC), soils play an essential role inthe global carbon cycle. Their response to rising global temperatures is of uttermost interest,having the potential to move and store an extensive amount of carbon within the climatesystem. One of the critical aspects of soil OC is the turnover rate, which has been reported todecrease rapidly below the A horizon. While most studies have focused on processes thatpromote higher ages of organic matter including various stabilization mechanisms, recentobservations suggest that part of the soil OC might already have been deposited with thesediment parent material, i.e. geogenic carbon and thus has turnover rates of millennia orlonger. This could lead to underestimations of turnover rates and needs to be taken intoaccount in biogeochemical models. This study aims to characterize geogenic OC and quantifyits contribution to subsoils. Deep soil cores (10 m) were collected from three different parent materials incentral Germany: Pleistocene loess, Paleogene sand and red sandstone. These wereanalyzed using a set of geochemical methods, including bulk 14C, selected lipidbiomarkers, pyrolysis-GCMS and 13C-CPMAS spectroscopy in order to characterizeorganic matter at multiple depth intervals and identify contributions of geogenicorigin. In the Pleistocene loess, apparent bulk 14C ages increased with depth from 2,200 to30,700 years BP, reflecting the depositional history of this relatively young sediment. Thelipid analysis revealed multiple sections with high OC content and Carbon Preference Index(CPI) values (&#x003E;13), suggesting the presence of paleosols. The oldest part yielded lower CPIvalues (&#x003C;3.0), evidently phases where deposition was too rapid for pedologicalprocesses to take place. This was supported by 13C-CPMAS NMR spectroscopy,demonstrating a highly degraded (alkyl-C rich) but soil like chemical composition of theOM. In these sections the organic matter is scarce and characterized by aliphaticcompounds such as n-alkanes, possibly deposited along with the mineral material,and methyl ketones as well as monoaromatic compounds including benzene andtoluene. Apparent bulk 14C ages of the Paleogene sand varied between 6,750 and 12,000 years,significantly younger than the deposition of the sediment (23-34 Mio. years BP). The sampleswere absent of n-alkanes and only small traces of fatty acids (2.67-0.59 μg/g dry weight)were detected, dominated by even long chain homologues. Young ages were also displayedby the red sandstone (about 245 Mio. years old), ranging between 12,760 and 17,400 yearsBP. In summary, the hitherto investigations of the deep sediment cores of differentparent material show a significant variance in age and composition of the OC fordifferent parent material at various depths, owing to OC components deposited alongwith the sediment parent material mixed with more recently deposited biogeniccarbon. [ABSTRACT FROM AUTHOR]