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Aims: The improvement of nitrogen use efficiency (NUE) of crops allows crop nitrogen (N) demands to be met while reducing N supply, and so reducing excess N which has potential negative environmental implications. NUE is often determined destructively at the end of crop growth-cycles without considering temporal variability. Here we present a methodological study which optimises the determination of NUE throughout the sugarcane growth-cycle using minimally destructive methods, and suggest the use of these methods for non-destructive NUE determination over the growth-cycle of other giant C4 crops. Methods and results: The determination of the NUE relied on the optimisation of three methods: the estimation of aboveground biomass, N content and N derived from fertiliser (NdfF). First, the ability of different allometric relationships to estimate sugarcane biomass was investigated by selecting a relationship based on height and diameter to estimate aboveground biomass along the crop growth-cycle. Secondly, we assessed the minimum number of harvested sugarcane required to construct a dilution curve to predict N content from biomass and found that a sampling of 5 sugarcane at 3 dates was sufficient to represent aboveground N content over the growth-cycle. Finally, the ability of 15N content of individual leaves to represent the NdfF in 15N-fertilised cane was tested. The first and second leaf below the top visible dewlap were the most representative. Based on a variance analysis, we assessed the level of influence of each method on the NUE calculation. Crop age accounted for 54% of the variance of NUE, the choice of 15N leaf 13%, with the choice of model to estimate biomass and the number of plants harvested for the N dilution curve, each accounting for less than 2% over the four sampling dates. Conclusions: This study highlighted the importance of evaluating NUE not only at the point of harvest. We propose, therefore, a set of methods to study NUE throughout the sugarcane growth-cycle by using minimally destructive sampling. [ABSTRACT FROM AUTHOR]

Purpose: Understanding of the role of low molecular weight organic anions (OAs) in structuring rhizosphere microbial communities in situ is limited due to challenges associated with sampling. Improved techniques are needed for such studies. Methods: This study used in situ and destructive sampling techniques and compared two exudate extraction methods [anion exchange membrane (AEM) capturing and water extraction] from rhizosphere and non-rhizosphere samples of genetically modified (GM) and control Pinus radiata D. Don trees grown in large-scale rhizotrons for ~10 months. Metabolically active soil microbial communities were analysed using rRNA-DGGE. Results: Recovery of eight out of 12 anions was influenced by extraction methods, and in situ sampling using AEM was shown to be the most efficient method. Only minor differences were detected in OAs in root exudates collected from the GM and control trees. Significant differences in α-Proteobacterial and Pseudomonas communities were associated with the two tree lines in the topsoil at both sampling events. Additional differences in β-Proteobacterial and fungal communities between tree lines were detected in the rhizosphere using destructive sampling. Conclusion: This study demonstrated that in situ sampling was superior to destructive sampling for the efficient collection of root exudates and analysis of associated rhizosphere microbial communities. [ABSTRACT FROM AUTHOR]

Aims: Growers in northern U.S. regions rely on winter annual legume cover crop symbiotic nitrogen fixation (SNF) to reduce the need for spring fertilization and build soil organic matter. However, cold transitional fall temperatures potentially limit SNF. This study examined the effects of cold temperatures on nodulation and SNF in legume cover crops. Methods: In a growth chamber experiment, hairy vetch (Vicia villosa Roth), Austrian winter pea (Pisum sativum L.) and crimson clover (Trifolium incarnatum L.) were sown in growth pouches, inoculated with rhizobia, and placed in growth chambers ranging from 5℃ to 20℃ for six weeks. In a field experiment, the same legumes were sown in fall, with destructive sampling at four time points from November to May. In both experiments, plant biomass, nodulation, and SNF was assessed. Results: In the controlled environment, incrementally lower temperatures negatively affected legume cover crop biomass and nodulation parameters across all species, especially at 5 °C (41°F). Controlled environment results also suggest that optimum temperatures for SNF in cover crop legumes falls in the range of 15 °C to 20 °C. In the field, winter conditions negatively affected legume cover crop productivity, with plant biomass, nodule number, and nodule mass greatest in fall and spring, and lowest in winter. Hairy vetch had the best growth response to spring warming, suggesting it is well-suited to over-winter protected environment production. SNF also decreased during winter and failed to recover by spring cover crop termination. Conclusions: Winter annual legume cover crops may not experience reduced growth, nodulation, or SNF until temperatures fall below 10℃; this estimate is lower than previously reported for cover crop legumes. These temperatures are typical of cover crop establishment periods, suggesting winter cover crop species selection is critical; the over-wintering performance of hairy vetch makes it a promising option for temperate regions. [ABSTRACT FROM AUTHOR]

Purpose: Stable isotope tracer experiments provide a powerful tool for understanding plant root distributions, resource uptake, niche partitioning and water cycling. Plant water is typically collected from pre-transpiring tissues to avoid the effects of evaporative isotope enrichment at the leaf surface, but extracting water from these plant samples is difficult and expensive. The purpose of this study was to test a simple transpiration bagging approach for measuring hydrologic tracer uptake. Methods: Sampling was performed as part of a depth-specific tracer experiment in which 2H2O was injected to target depths (5, 15, 30, 60, or 150 cm) in different replicated plots. One day following injections, leaves from three species were sealed in bags for 16 h and transpired water was collected. Water from pre-transpiring stem tissue was then collected in a separate set of samples and extracted using cryogenic distillation. Results: Deuterium concentrations from the two techniques were correlated (R2 = 0.84) and both approaches produced similar descriptions of vertical root distributions for three dominant plant species. 18O concentrations from the two techniques were not correlated. Conclusion: Bagging transpired water produced similar estimates of 2H tracer uptake as the standard sampling technique. Bagging requires no destructive sampling, specialized laboratory equipment, training or consumables and is expected to halve sampling costs. While effective in this tracer experiment, bagging may be not be effective in natural abundance experiments, or tracer experiments with very small plants or small transpiration rates (i.e., early-season or arid sites). [ABSTRACT FROM AUTHOR]

Aims: Root-specific responses to stress are not well-known, and have been largely based on indirect measurements of bulk soil water extraction, which limits mechanistic modeling of root function.Methods: Here, we used neutron radiography to examine in situ root-soil water dynamics of a previously droughted black cottonwood (Populus trichocarpa) seedling, contrasting water uptake by the two major components of the root system that differed in initial recovery rate as apparent by ‘new’ (whiter, thinner), or ‘old’ (darker, thicker) parts of the fine root system.Results: The smaller diameter ‘new’ roots had greater water uptake per unit surface area than the larger diameter ‘old’ roots, but they had less total surface area leading to less total water extraction; rates ranged from 0.0027-0.0116 g cm−2 h−1. The finest most-active roots were not visible in the radiographs, indicating the need to include destructive sampling. Analysis based on root-free bulk soil hydraulic properties indicated substantial redistribution of water via saturated/unsaturated flow and capillary wicking across the layers - suggesting water uptake dynamics following an infiltration event may be more complex than approximated by common soil hydraulic or root surface area modeling approaches.Conclusions: Our results highlight the need for continued exploration of root-trait specific water uptake rates in situ, and impacts of roots on soil hydraulic properties - both critical components for mechanistic modeling of root function. [ABSTRACT FROM AUTHOR]

Aims: Although soil water deficit is the primary constraint on transpiration globally, the mechanisms by which soil drying and soil properties impact transpiration and stomatal regulation remain elusive. This work aimed to investigate how soil textures and vapor pressure deficit (VPD) impact the relationship between transpiration rate, canopy conductance, and leaf water potential of maize (Zea mays L.) during soil drying. We hypothesize that the decrease in soil–plant hydraulic conductance (Ksp) triggers stomatal closure and the latter is soil specific. Methods: Plants were grown in two contrasting soil textures (sand and loam) and exposed to two consecutive VPD levels (1.8 and 2.8 kPa). We measured transpiration rate, canopy conductance, soil and leaf water potentials during soil drying. Results: Transpiration rate decreased at higher soil matric potential in sand than in loam at both VPD levels. In sand, high VPD generated a steeper drop in canopy conductance with decreasing leaf water potential. The decrease in canopy conductance was well correlated with the drop in Ksp, which was significantly affected by soil texture. Conclusions: Our results demonstrated that variations in canopy conductance were not simply a function of leaf water potential but largely affected by soil hydraulics. These results reinforce a model of stomatal closure driven by a loss in soil hydraulic conductivity. Further studies will determine if soil-specific stomatal regulation exists among species. [ABSTRACT FROM AUTHOR]

Background and aims: Peatland moss communities play an important role in ecosystem function. Drivers such as fire and atmospheric pollution have the capacity to influence mosses via multiple pathways. Here, we investigate physical and chemical processes which may influence establishment and growth of three key moss species in peatlands. Methods: A controlled factorial experiment investigated the effects of different peat bulk density, ash deposition and rainwater chemistry treatments on the growth of Sphagnum capillifolium, S. fallax and Campylopus introflexus. Results: Higher peat bulk density limited growth of both Sphagnum species. S. capillifolium and C. introflexus responded positively to ash deposition. Less polluted rain limited growth of C. introflexus. Biomass was well correlated with percentage cover in all three species. Conclusions: Peat bulk density increases caused by fire or drainage can limit Sphagnum establishment and growth, potentially threatening peatland function. Ash inputs may have direct benefits for some Sphagnum species, but are also likely to increase competition from other bryophytes and vascular plants which may offset positive effects. Rainwater pollution may similarly increase competition to Sphagnum, and could enhance positive effects of ash addition on C. introflexus growth. Finally, cover can provide a useful approximation of biomass where destructive sampling is undesirable. [ABSTRACT FROM AUTHOR]

Background and Aims: Selection effect (SE) and complementarity effect (CE) are often considered as two key indicators to mediate the positive diversity-productivity relationship in the maize-legume intercropping system, as intercropping generally enables crops to use the available resources more efficiently than monoculture. Yet, it is unclear whether soil moisture drives the SE and CE transformation, plant-plant interaction, and accordingly affect the productivity. Methods: A two-year field experiment was conducted to explore the impacts of soil water status (with and without plastic mulching) on interspecific interaction and productivity and its key mechanism in the maize-faba bean intercropping system. Results: Low soil moisture reduced the net effect (NE) and land equivalent ratio (LER) with evident yield loss (NE < 0 and LER = 1.05), whereas the improved soil moisture reversed this trend and resulted in overyielding (NE > 0 and LER = 1.13). Regardless of soil moisture, faba bean was the dominant species and maize was the inferior one. Low soil moisture promoted the contribution of SE to productivity (P < 0.05). In contrast, high moisture turned to lower the competition intensity of faba bean against maize and accordingly promote the transformation of biodiversity effect from SE to CE. The main mechanism was that the water competition imposed by faba bean strips on maize ones decreased the NE through reducing photosynthetic rate and water use efficiency of maize. Conclusion: The adaptive transitions of interspecific relationship and the biodiversity effect were driven by soil water availability. This novel phenomenon should be emphasized in the crop diversity management of intercropping system in the rainfed region. [ABSTRACT FROM AUTHOR]

Aim: Incorporating non-bloat legumes into grass pastures can reduce enteric methane and alter cattle urinary urea N output by increasing protein intake. Deposition of high urea N urine influences soil N-cycling microbes and potentially N2O production. We studied how urine urea N concentration affects soil nitrifier and denitrifier abundances, activities, and N2O production. Methods: 15N13C-labelled urea dissolved in cattle urine was added at 3.5 and 7.0 g L−1 to soils from a grazed, non-bloat legume pasture and incubated under controlled conditions. CO2, N2O, 13C-CO2, and 15N-N2O production were quantified over 240 h, along with nitrifier and denitrifier N-cycling genes and mRNA transcripts. Results: High urea urine increased total N2O relative to the control; low urea was not significantly different from the control or the high urea treatment. As a result, N2O-N emission factors were not significantly different between the low urea treatment (1.17%) and high urea treatment (0.94%). Doubling urea concentration doubled CO2-Curea and N2O-Nurea but not total N2O-N. Urine addition initially inhibited then increased AOB transcripts and gene abundances. nirK and nirS transcript abundances indicated that denitrification by ammonia oxidizers and/or heterotrophic denitrifiers dominated N2O production. Urine addition increased nosZ-II vs. nosZ-I transcripts, improving soil N2O reduction potential. Conclusion: Characterizing this interplay between nitrifiers and denitrifiers improves the understanding of urine patch N2O sinks and source dynamics. This mechanistic information helps to explain the constrained short-term N2O emissions observed in response to excess urine N excretion from cattle consuming high protein diets, e.g. non-bloat legumes. [ABSTRACT FROM AUTHOR]

Aims: Terrestrial carbon (C) sequestration is derived mainly from plant photosysthetically-fixed C deposition but soil organic C (SOC) content in saline soils is generally low due to low deposition of C from restricted plant growth. It is important to explore the effects of soil salinity on the allocation of photosynthetically-fixed C to better understand C sequestration in saline wetland soils. Methods: We conducted a pot experiment in which reed ( Phragmites australis) was grown in a low salinity (LS) soil and a high salinity (HS) soil from the Yellow River Delta under flooded conditions. The allocation of photosynthetically-fixed C into plant tissues, SOC, dissolved organic C (DOC), microbial biomass C (MBC), particulate organic C (POC), and mineral-associated organic C (MAOC) was determined using a C pulse-labeling method after four labeling events during the 125-day-long reed growing season and destructive sampling immediately at the end of six hours of pulse labeling (end 6-h) and on the final harvest day (final day). Results: In most cases soil salinity, reed growth stage, or reed biomass significantly ( P < 0.05) affected the deposition of photosynthetically-fixed C into the plant-soil system. At all four pulses at end 6-h the high salinity soil had significantly ( P < 0.05) lower percentage net assimilated C in the roots and significantly higher ( P < 0.05) percentage net assimilated C in the soil than did the low salinity soil. At both end 6-h and on the final day the high salinity soil had significantly ( P < 0.05) lower SOC, and significantly ( P < 0.05) higher DOC/SOC ratio than the low salinity soil except for pulses 3 and 4 on the final day. The majority of photosynthetically-fixed C in soil was deposited into MAOC pools and >80 % of deposited SOC was present as MAOC in the high salinity soil due to its significantly ( P < 0.05) higher clay content compared with the low salinity soil. Conclusions: Soil salinity affected the allocation of photosynthetically-fixed C in the plant-soil system, and soil texture altered the allocation of rhizodeposition C in different soil particles. [ABSTRACT FROM AUTHOR]

Background and Aims: Biomass carbon (C) and soil organic carbon (SOC) stocks in three indigenous agroforestry (AF) systems (Enset [ Ensete ventricosum, Musaceae], Enset-Coffee and Fruit-Coffee) practiced on the Rift Valley escarpment of Ethiopia are presented and evaluated. These unique AF systems are characterised by a high proportion and diversity of trees. Methods: Data were collected from six smallholdings per AF system. Above- and belowground biomass of trees (fruit and non-fruit), enset and coffee plants were calculated using allometric equations while the biomasses of herbs, litter and fine roots (<2 cm) were determined by destructive sampling techniques. SOC stocks (0-30 and 30-60 cm) were calculated from measured C contents and bulk density values. Results: Smallholding total biomass C stocks averaged 67 Mg ha with trees accounting for 39-93 %. Herbs accounted for <4 % of aboveground biomass. Coffee accounted for 11 % and enset 9 % of total biomass C on average. SOC stocks (0-60 cm) were 109-253 Mg ha (52-91 % of total C stocks) and uncorrelated to biomass C stocks. Biomass C or SOC stocks did not significantly differ between AF systems but biomass C stocks were significantly correlated to elevation. Conclusions: The C stocks of the three studied AF systems were found to be amongst the highest reported for tropical forests and agroforestry systems. These unique AF systems are therefore well suited to REDD+ projects. However, the C stocks were more determined by individual smallholder management practice than by AF system or climate (elevation). [ABSTRACT FROM AUTHOR]

Background and Aims: Crop responses to elevated atmospheric CO are likely to be different in semi-arid cropping systems of Australia. This experiment aimed to investigate the interactive effects of atmospheric CO and nitrogen (N) fertiliser on carbon (C) and N partitioning in the soil-plant system of Wheat ( Triticum aestivum L.) and field pea ( Pisum sativum L.). Methods: Plants were grown with 40 or 100 mg N kg under ambient CO (390 ppm) or elevated CO (eCO; 550 ppm) using free-air CO enrichment (SoilFACE). Repeated CO pulse labelling was used to quantify C transfer via plant to the soil. Destructive sampling was performed at grain filling and maturity. Results: eCO increased shoot biomass of field pea (36 %) and wheat (55 %) but only increased root biomass of wheat (13.5 %) in the 25-50 cm soil layer. Total N content of both species was greater under eCO, and for field pea it indicated enhanced biological N fixation. However, eCO increased the C:N ratio of wheat even at the high N level. Greater C in soil of wheat grown under eCO indicated a minor increase in soil C via rhizodeposition. Conclusions: Increased biomass and C:N ratio of wheat could have implications for residue decomposition. eCO and low N tended to increase grain yield but the increase was highly variable and not significant. Additional N content of field pea under eCO exceeded the N that would be removed in wheat grain, albeit with lower than expected grain yield due to dry conditions. [ABSTRACT FROM AUTHOR]

Background and aims: Changes in water availability during the growing season are becoming more frequent due to climate change. Our study aimed to compare the fine-root acclimation capacity (plasticity) of six temperate tree species aged six years and exposed to high or low growing season soil water availability over five years. Methods: Root samples were collected from the five upper strata of mineral soil to a total soil depth of 30 cm in monoculture plots of Acer saccharum Marsh., Betula papyrifera Marsh., Larix laricina K. Koch, Pinus strobus L., Picea glauca (Moench) Voss and Quercus rubra L. established at the International Diversity Experiment Network with Trees (IDENT) field experiment in Sault Ste. Marie, Ontario, Canada. Four replicates of each monoculture were subjected to high or low water availability treatments. Results: Absorptive fine root density increased by 67% for Larix laricina, and 90% for Picea glauca, under the high-water availability treatment at 0–5 cm soil depth. The two late successional, slower growing tree species, Acer saccharum and Picea glauca, showed higher plasticity in absorptive fine root biomass in the upper 5 cm of soil (PIv = 0.36 & 0.54 respectively), and lower plasticity in fine root depth over the entire 30 cm soil profile compared to the early successional, faster growing tree species Betula papyrifera and Larix laricina. Conclusion: Temperate tree species show contrasting acclimation responses in absorptive fine root biomass and rooting depth to differences in water availability. Some of these responses vary with tree species successional status and seem to benefit both early and late successional tree species. [ABSTRACT FROM AUTHOR]

Aims: Phosphorus (P) deficiency-induced mobilization of rare earth elements (REEs) in the rhizosphere contributes to REE accumulation in the hyperaccumulator Phytolacca americana, but a lack of in situ methods for visualization of the root-soil interface limits our understanding of the underlying processes. Methods: Diffusive gradients in thin-films (DGT) devices were used for probing root exudates, REEs and P in the P. americana rhizosphere. Desorption electrospray ionization mass spectrometry and laser-ablation inductively coupled mass spectrometry were used for in situ imaging of root exudates, REEs and P sorbed on the DGT. Results: The novel approach demonstrated here is capable of synchronously and quantitatively characterizing the distribution of root exudates and labile elements in the rhizosphere. The secretion fluxes of citrate and oxalate in the rhizosphere under P deficiency were three times higher than under P sufficient condition; and the lanthanum (La) fluxes in the rhizosphere under P deficiency were ten times greater than at P sufficiency condition. The enrichment of P and La under P deficiency and depletion under P sufficient conditions in the rhizosphere suggests that P deficiency-induced organic acid secretion is crucial for the mobilization of soil REEs and subsequent REE accumulation in P. americana. Conclusion: The combination of DGT devices with mass spectrometry imaging is technically feasible for in situ synchronous imaging of root exudates, REEs and labile elements at the root-soil interface. Our study shed light on processes of mobilization of mineral elements in the rhizosphere induced as a side-effect of the P acquisition mechanism. [ABSTRACT FROM AUTHOR]

Aims: Recent research has suggested that microbial necromass has a disproportionate influence on soil organic C accumulation. But few field studies have followed the bacterial and fungal necromass changes during plant residue decomposition. How bacterial and fungal necromass drives soil C fractions; what role of soil C fractions play; and the critical factors which influence their formation have remained poorly understood. Methods: In a 512-days culture experiment with a perennial C3 herb (St.B, S. bungeana) we traced the formation of muramic acid (MurA) vs. Glucosamine (GluN), and investigated the relationships between MurA, GluN and soil C fractions. Results: The results showed that the bacteria community dominates the decomposition process due to soil pH (> 7) and microbial metabolic C-, P-limitations. The dynamics of bacterial necromass over the time-course of the experiment changed from fluctuating variations to a significant increase. This means that the bacterial necromass has been in a balance of accumulation and decomposition at early and middle periods. Significant accumulation only occurred in the later stages, which was attributed to the involvement of bacteria in more microbial necromass degradation. In this process, the entombing effect of bacterial necromass was more dramatic than their metabolic consumption. While in the case of microbial metabolism limitation, fungal necromass will lose its physicochemical protection of the mineral particles, and thus be degraded and utilized. The priming effecting caused by the one-time input of plant residues with high C/N ratios and soil mineralization resulted in the absence of SOC accumulation in the short term. Microorganisms regulate the turnover of POC, MAOC and MBC by microbial biomass and necromass. The utilization of soil C fractions is the direct cause of SOC decline, while the turnover of microbial necromass only plays an indirect role. Soil pH and microbial biomass stoichiometry as the critical factors in changes in bacterial and fungal necromass. Lower living microbial biomass production and the dynamics of microbial necromass indicate that microbial necromass is constantly broken down as an available C source. Conclusions: With the synergistic effect of soil C fractions, the production of microbial biomass and the degradation of cellular residues maintain microbial stoichiometric homeostasis. In addition to soil pH, microbial biomass stoichiometry co-determines microbial necromass formation. [ABSTRACT FROM AUTHOR]

Background and Aims: Rocket salad ( Eruca sativa Mills) is one of the major leafy vegetables produced worldwide and has been characterized as a rich source of chemoprotective glucosinolates (GSL). The relationship between N fertilization and the resulting plant biomass and N status with GSL quantity and quality in rocket leaves was examined. Methods: A pot experiment was conducted, applying ten different N-rates and destructive sampling was carried out 15, 30 and 45 days after transplanting (DAT). The Mitscherlich equation was used to establish NO-N critical levels at each growth stage and as an indicator of N demand for relative maximum dry matter accumulation and glucosinolate content and composition was determined. Results: Glucosinolate content was significantly influenced by N rate, growth stage and their interaction. Different GSL types showed dissimilar responses to N fertilization: aliphatic GSLs were significantly reduced under increased N rates whereas indole GSL showed the reverse. Under excess N fertilization (>1.04 g/plant), dry matter accumulation remained constant, NO-N was significantly increased and total GSL content was significantly reduced, factors that could lead to an anticipated product quality decline. Conclusions: The application of the critical NO-N level approach used to identify optimal N fertilization rates for plant growth could serve as means to obtain optimized GSL content in the edible plant parts. [ABSTRACT FROM AUTHOR]

Aims: Conventional methodology using destructive sampling, which is laborious and has poor spatial and temporal resolution, has limited our understanding of soil-plant interactions. New non-invasive tomographic techniques have the potential to significantly improve our knowledge. In this study we demonstrated the simultaneous use of PET (positron emission tomography) and CT (X-ray computed tomography) to (a) non-destructively image a whole plant growing in sand, and (b) to link the observed morphology with recently assimilated C. The PET scanner was used to detect and visualize the location of the short-lived radioisotope C (with a half-life of 20.4 min) taken up by the plant through C-labelled CO. This provided information on carbon translocation and the metabolism of photo-assimilates in the plant as well as root structure. The CT scanners yielded data on soil and root structure. Methods: A medical PET/CT scanner was used to scan a fodder radish plant growing in a pot with test soil composed of homogenous sand. We constructed an air-plant-soil controller system (APS) to control the environmental conditions, such as CO, temperature and light during the experiment. The plant was allowed to assimilate CO for 90 min before PET scanning was initiated. We carried out PET scanning for 60 min. Subsequently, the aerial parts of the plant was cut off and the pot was rescanned using a micro-CT scanner to obtain more detailed information on structure of the root system and the growth medium structure. Results: The acquired PET and CT images gave images clearly visualizing the architecture and morphology of root and soil. Using a CT scanner, we were able to detect the main taproot located at 0 to 30 mm depth. With the PET scanner, we were able to measure a signal down to 82 mm below the surface of the sand. We found the highest concentration of C at the position of the main root. The PET images, at different time intervals, showed the translocation and metabolisation of photo-assimilates from top to root. Using the micro-CT scanner (voxel size of 90 μm), we were able to detect roots down to 100 mm depth. These findings correlated the PET signals measured down to 82 mm depth. Conclusions: We conclude that the simultaneous use of PET and CT technologies was successfully applied for soil-plant studies. The combined PET/CT technology has potential to provide new fundamental insight into soil-plant interactions and especially into the effect of abiotic stresses in spite of the limitation due to spatial resolution. [ABSTRACT FROM AUTHOR]

Aims: X-ray computed tomography (CT) is widely recognized as a powerful tool for in-situ quantification of root system architecture (RSA) in soil. However, employing X-ray CT to identify the spatio-temporal dynamics of RSA still remains a challenge due to non-automatic, time-consuming image processing protocols and their poor recovery of fine roots in soil. Methods: Here we present a new protocol (Rootine) to segment roots rapidly and precisely down to fine roots with two voxels in diameter (90 μm in pots with 70 mm in diameter). This is facilitated by feature detection of the tubular shape of roots, an approach that was originally developed for detecting blood vessels in medical imaging. Results: In comparison to established root segmentation methods, Rootine produced a more accurate root network, i.e. more roots and less over-segmentation. Root length quantified by X-ray CT showed high correlation with results by root washing combined with 2D light scanning (R2 = 0.92). Tests with different soil materials showed that the recovery of roots depends on signal-to-noise ratio but can be up to 99% for a favorable contrast between fine roots and background. Conclusions: This new protocol provides great efficiency to study RSA in undisturbed soil. As it is fully automated it has the potential for high-throughput root phenotyping and related modelling. [ABSTRACT FROM AUTHOR]

Tea ( Camellia sinensis L.) is a perennial evergreen shrub managed intensively for continuous growth of young shoots. Most tea plantations were established at the expense of native forest. Change in carbon (C) and nutrient (nitrogen, phosphorus, potassium (NPK)) accumulation in forests over time has been intensively studied, but how C and NPK stocks in tea plantations are associated with age, genotype and plant density have not been reported yet in literature. To get a better insight in the effects of age and the associated change in genotype (clone or seedling) and density on C and NPK stocks data were collected in a chronosequence of tea plantations by destructive sampling of whole tea bushes. Four plantations differing in age (76-, 43-, 29- and 14-year old) and genotype–density combinations were selected. The 76- and 43-year old plantations were composed of seedlings planted at a relative low density: a spacing of 1.52 × 0.91 m and 1.22 × 1.22 m, respectively, whereas in the 29- and 14-year old plantations an improved clonal cultivar was grown at higher plant densities: a spacing of 1.22 × 0.61 m and 1.22 × 0.76 m, respectively. Total dry matter weights (DM) of tea bushes increased with age from 9.0 to 11.5 kg in clonal bushes and from 13.5 and 19.9 kg in seedling bushes for the 14- and 29-year, and the 43- and 76-year old plantations, respectively. Most DM was stored in the woody tissue of the tea bush frame. Total C- stocks expressed per unit area (ha) amounted to 44 and 72, and 43 and 69 t C ha−1 for clonal and seedling bushes, respectively. Total N stocks ranged from 732 to 995 and from 734 to 1,200 kg ha−1, P stocks from 78 to 120 and from 67 to 92 kg ha−1, and K stocks from 775 to 901 and from 646 to 1,120 kg ha−1 for clonal and seedling bushes, respectively. It is concluded that older stands of seedling and clonal tea plantations result in higher C and NPK stocks. Higher stocks may be functional in maintaining yield stability under adverse weather conditions. Plantations with higher nutrient stocks are less responsive to nutrient supply and therefore will require lower fertilizer applications. [ABSTRACT FROM AUTHOR]

The regrowth capacity after pollarding of a short-rotation plantation of Acacia saligna (Labill.) H. Wendl. was investigated in a field trial. This shrub has been proposed as a provider of biomass (fuelwood and fodder) in an arid environment, using local marginal water resources such as surface runoff and brackish groundwater. The specific objective of this study was to examine the effects of water quality, irrigation frequency and annual runoff flooding onthe above- and belowground development of the pollarded shrubs. Treatments consisted of drip-irrigation with freshwater or brackish water, at low (twice a month) or high (weekly) frequency, with or without annual freshwater flooding, and on a well-watered basis (twice a week) without flooding. Each 15 × 5 m2 plot contained four rows of four shrubs. After 5 years of growth, the shrubs were pollarded to a height of 1.5 m and during the subsequent year of regrowth, root development was monitored non-destructively using the minirhizotron, shoot growth was estimated from trunk cross-sectional area and allometric equations (obtained at the end of the measuring period by measurements and destructive sampling), and plant water status was monitored by measuring pre-dawn leaf water potential. Dry fodder (leaves and thin branches) production was between 3.50 and 9.75 t ha−1 and dry wood was between 3.50 and 15.50 t ha−1. The highest biomass production was obtained in the well-watered freshwater treatment, which also had the highest number of roots and highest predawn leaf water potential throughout the year. Shrubs irrigated with brackish water at low frequency without supplemental flooding produced the lowest yields. Water quality significantly affected shoot development only in the well-watered treatments although root development was reduced wherever brackish water was applied. Flooding the plots with freshwater once a year led to an increase in the number of roots outside the drip-irrigation zone, especially in brackish water treatments. A continued root growth with time was observed in all treatments even though the shoots were pollarded. In fact total root increments and aboveground biomass production were positively linearly related. Moreover the linear response of shoot and root increments to increasing water availability and not to water quality suggests that irrigation frequency was the main factor determining the regrowth capacity and amount of above- and belowground biomass production. Based on the above, runoff water and brackish groundwater could be used in a complementary manner for the sustainable production of fuelwood and fodder in a short-rotation plantation of this shrub. [ABSTRACT FROM AUTHOR]

A modified root ingrowth method was developed to minimize destructive sampling in experiments with limited space, and used to estimate belowground net primary production and root tissue quality in a native semiarid grassland exposed to elevated CO2 for five years. Increases in root production of over 60% were observed with elevated CO2 during years of intermediate levels of precipitation, with smaller effects in a very wet year and no effects in a very dry year. Aboveground to belowground production ratios, and the depth distribution of root production, did not differ between ambient and elevated CO2 treatments. Root soluble concentrations increased an average of 11% and lignin concentrations decreased an average of 6% with elevated CO2, while nitrogen concentrations decreased an average of 21%. However, most tissue quality responses to CO2 varied greatly among years, and C:N ratios were higher in only one year (22 ambient vs. 33 elevated). Among years, root nitrogen concentrations declined with increasing aboveground plant nitrogen yield, and increased over the study period. Estimates of root production by the ingrowth donut method were much lower than previous estimates in the shortgrass steppe based on14C decay. We discuss reasons why all ingrowth methods will always result in relative rather than absolute estimates of root production. [ABSTRACT FROM AUTHOR]

Aims: Incorporation of flower strips is an agricultural measure to increase aboveground biodiversity and ecosystem services. Although soil communities are key components of terrestrial biodiversity and drive important ecosystem services, their abundance, diversity, and composition in flower strips remain largely unexplored. Here, we shed light on earthworms and soil microorganisms in flower strips. Methods: We sowed a grassy field margin vegetation as well as two annual and two perennial flower strip mixtures in fully randomized plots of 9 × 28 m in three different types of soil in Germany. Two years following sowing, we determined earthworm communities using chemical extraction and investigated the soil microbiome using real-time PCR (archaea, bacteria, fungi, and soil-N-cycling genes) and amplicon sequencing (bacteria and fungi). Results: Different plant mixtures (i.e. field margin, annual and perennial flower strips) harbored distinct earthworm and soil microbial communities. Earthworm density and biomass declined or remained unaffected in annual flower strips but increased in perennial flower strips as compared to the field margins. Arbuscular mycorrhizal fungi showed greater diversity and relative abundance in non-tilled (i.e. field margin and perennial flower strips) than in tilled plant mixtures (i.e. annual flower strips). Conclusions: We attribute changes in earthworm and microbial communities mainly to the effect of tillage and plant diversity. Overall, we suggest that perennial flower strips serve as refugia. Future studies should compare soil biota in perennial flower strips to those in adjacent fields and investigate whether the promotion of soil communities extends into adjacent fields ('spillover'). [ABSTRACT FROM AUTHOR]

Purpose: Plants and soil microbes both influence how ecosystems respond to environmental change. Yet, we lack the ability to generalize how plants and soil microbes influence each other in the same or varying soil conditions. This limitation thwarts ecologists' ability to understand and predict effects of environmental changes such as elevated anthropogenic nitrogen (N) deposition. Accordingly, we examined the specificity of plant species' influence on soil microbial community composition. Methods: We tested (1) whether congeneric grass species have unique effects on soil microbial communities, (2) how relative abundances of microbial taxa can be explained by Poa phylogeny, plant traits, and range-wide traits (annual temperature and soil pH), (3) whether N addition alters associations between Poa species and soil microbes, and (4) whether the magnitude of microbial community change in response to elevated N can be explained by plant growth responses to N. We conducted a greenhouse experiment with seven Poa species and native soils. Results: We found that individual Poa species were associated with different soil fungal and bacterial assemblages. Differences in microbial composition were not attributable to Poa phylogeny, the plant traits we tested, nor range-wide species traits we tested. Nitrogen addition enhanced the unique effects of Poa species on fungal and bacterial community compositions. Conclusion: These results demonstrate how ecological interactions of related plant species vary depending on resource supply, revealing important context dependency for accurately predicting microbially mediated nutrient cycling and ecosystem responses to changes in nutrient availability. [ABSTRACT FROM AUTHOR]

Progress in understanding allelopathic interactions among plants has long been hampered by the complexity of the many direct and indirect interactions involved. Plant processes and growth are not only affected by allelochemicals but by resource limitations, pathogens, herbivores, and microbial interactions. Interference mechanisms frequently interact, and the magnitude of effects can depend on the plant's biotic and abiotic environment. The rhizosphere is chemically complex, with thousands of potentially bioactive allelochemicals produced by plants, microorganisms and soil invertebrates. The rhizosphere is also dynamic, in that concentrations of these metabolites vary as pulses of allelochemicals are released by plant roots and other organisms, as they leach from decaying plant material, as microorganisms degrade and sometimes transform allelochemicals, and as allelochemicals are taken up by plants, bind to soil components, or leach from the root zone. Recent advancements in instrumentation and technologies for the analysis of trace levels of chemical substances in soil, as well as the development of genomics, proteomics, and transcriptomics approaches allow researchers to probe both the biosynthesis of allelochemicals and plant responses to allelochemical exposures. These new technologies will provide much more detailed information on rhizosphere chemistry and about the production and response to metabolites by individual cells. This review describes case studies and current examples that illustrate how these new approaches and tools can enhance our understanding of allelopathic interactions, and argues that to truly advance our understanding of allelopathic interactions, these must be applied in ecologically rigorous and meaningful ways. [ABSTRACT FROM AUTHOR]

Background: Soil microarthropods influence many soil processes that support plant growth and development. Scope: In this paper we review the current understanding of direct microarthropod-plant interactions, how microarthropod-microbe interactions indirectly impact plant growth, and key areas for future study. Conclusion: Microarthropod impacts on plants are primarily routed through their interactions with microbial communities, mediating organic matter decomposition, nutrient cycling and allocation, and plant-pathogen dynamics in soils. The research investigating how microarthropod-saprotrophic microbe interactions affect plants through decomposition and nutrient cycling indicates a generally positive relationship, though this relationship is influenced by the overall diversity or species richness observed in the microarthropod communities. The effects of microarthropod-plant symbionts interactions on plants are varied and there is no clear benefit or detriment to plants via this mechanism. The effects of microarthropod-plant pathogen interactions on plants suggest that, in most cases, microarthropods will reduce disease incidence and severity. The limited diversity of the study taxa in this area of research is a major limitation to our understanding of how microarthropods impact plant health. Our review revealed that while much is known about microarthropod impacts on the intermediate processes that influence plants, only a subset of studies have quantified plant responses to microarthropod activity. Overall, existing evidence indicates that the overall effects of microarthropods on plants is positive. Future research should aim to incorporate more plant metrics and consider both microarthropod and microbial community dynamics in designed and observational studies. [ABSTRACT FROM AUTHOR]

Aims: We aimed at assessing the influence of above- and below-ground environmental conditions over the performance of Centaurea jacea L., a drought-resistant grassland forb species. Methods: Transpiration rate, CO2 assimilation rate, leaf water potential, instantaneous and intrinsic water use efficiency, temperature, relative humidity, vapor pressure deficit and soil water content in one plant and root length density in four plants, all grown in custom-made columns, were monitored daily for 87 days in the lab. The soil water isotopic composition in eleven depths was recorded daily in a non-destructive manner. The isotopic composition of plant transpiration was inferred from gas chamber measurements. Vertical isotopic gradients in the soil column were created by adding labeled water. Daily root water uptake (RWU) profiles were computed using the multi-source mixing model Stable Isotope Analysis in R (Parnell et al. PLoS ONE 5(3):1–5, 2010). Results: RWU occurred mainly in soil layer 0–15 cm, ranging from 79 to 44%, even when water was more easily available in deeper layers. In wet soil, the transpiration rate was driven mainly by vapor pressure deficit and light intensity. Once soil water content was less than 0.12 cm3 cm− 3, the computed canopy conductance declined, which restricted leaf gas exchange. Leaf water potential dropped steeply to around − 3 MPa after soil water content was below 0.10 cm3 cm− 3. Conclusion: Our comprehensive data set contributes to a better understanding of the effects of drought on a grassland species and the limits of its acclimation in dry conditions. [ABSTRACT FROM AUTHOR]

Purpose: We sought to identify plant traits important for initial establishment of a globally invasive shrub, Prosopis spp. (mesquite), by quantifying morphological and ecophysiological responses of seedlings to contrasting moisture regimes. Our model plant was Prosopis velutina, a prominent invader of North American semi-desert grasslands. Methods: Seedlings received one of three watering levels corresponding to regional 'ambient' (100%), 'dry' (−65%; 65% reduction of ambient), and 'wet' (+165%; 65% increase above ambient) growing season precipitation regimes. A total of 32 plant performance (e.g., biomass and growth) and functional (e.g., morphological and physiological) traits were assessed when seedlings reached 11- and 22-days old. Results: Germination under dry conditions was high (72%) and only slightly reduced compared to ambient and wet conditions with subsequent seedling survival unaffected. High trait variation enabled early recruitment over the range of soil moisture conditions, including extreme drought. Root volume, surface area, tips, tap root length, and leaf length functional traits were top predictors of seedling performance, with fresh weight, absolute growth rate, and root mass secondarily contributing to establishment. Conclusion: No physiological functional traits were significant in predicting performance or discriminating between moisture regimes; suggesting morphological traits for maintaining metabolic and hydraulic function are key to early establishment. Dry conditions reduced germination and slowed P. velutina seedling growth but did not prevent early establishment. Thus, recruitment of certain dryland shrubs may not be episodic with respect to above-average rainfall. Indeed, early recruitment of P. velutina and functionally similar species can potentially occur even in moisture conditions well below-average. [ABSTRACT FROM AUTHOR]

Purpose: Fine roots play an essential role in global carbon cycles, but phenological variations in root function and metabolism are poorly understood. To illustrate the dynamics of fine root function and metabolism in the field, we partitioned root respiration (Rr) into growth (Rg), maintenance (Rm), and ion uptake (Rion) components using a modified traditional model. Methods: A year-round experiment was conducted in a young larch-dominated forest regrowing on bare soil. Soil respiration was measured with a chamber method and partitioned into Rr and heterotrophic respiration by trenching. Fine root biomass and production were measured simultaneously. Using the field data, the model was parameterized, and Rr was further partitioned. Results: Annually, Rr (210–253 g C m−2 yr−1) accounts for 45–47% of the total soil respiration. The contribution of fine root Rg, fine root Rm, coarse root Rm, and fine root Rion were 26–40, 46–51, 10–16, and 12%, respectively. The Rg contribution showed a clear seasonal variation, with a peak in mid-spring and a minimum in early fall, mainly because of different seasonality between fine root production and soil temperature. Conclusion: The model parameters were consistent with those from our previous study conducted by the same method in the same site. Thus, we believe that our approach was robust under a relatively simple condition. However, our growth respiration parameter resulting from only field data was much higher than those from laboratory experiments. To further improve our understanding of root respiration, more field data should be accumulated. [ABSTRACT FROM AUTHOR]

Aims: Biological nitrification inhibitors (BNIs) can play an important role in inhibiting nitrification and enhancing nitrogen use efficiency (NUE) in agriculture. However, most current BNI studies have been conducted under hydroponics. Genotypic differences in the inhibition of nitrification in soil are still largely unknown. Our main aim was to investigate the regulation of N transformation processes by different rice genotypes with different BNI release potentials on NUE and N loss. Methods: Two rice genotypes, i.e. Wuyujing 3 (WYJ3) and Wuyunjing 7 (WYJ7), reported to have weak and strong BNI capacity, respectively, under hydroponic conditions, and four soils with different pH (i.e. JX (pH 5.09), FJ (pH 6.00), SC1 (pH 7.96) and SC2 (pH 7.94)) were selected for this study. Plant N uptake rates (esp. NH4+ uptake, UNH4) and soil N transformation rates were quantified by 15 N tracing to assess the effects of rice genotypes on nitrification inhibition activity. NUE and N loss were quantified in a separate 15 N-urea labeling experiment. Results: The results showed that the rice genotype with high BNI exudation (i.e. WYJ7) had lower autotrophic nitrification rate (ONH4) and higher UNH4 compared to WYJ3. ONH4 in WYJ7 decreased by 0.05, 0.42, 1.14, and 0.48 mg N kg−1 d−1 compared to WYJ3 for JX, FJ, SC1 and SC2, respectively. The abundance of AOB in soils planted with WYJ7 was lower than in soils planted with WYJ3, which was the most important factor to explain the variation of ONH4. WYJ7 tends to have a relatively higher NUE than WYJ3 in JX, SC1 and SC2. NUE was negatively correlated with ONH4. Conclusion: Our results indicate that some rice genotypes can optimize their N acquisition by regulating soil N transformations (especially nitrification). The development of rice genotypes with strong BNI exudation capacity could be a suitable management practice to increase NUE and yield. [ABSTRACT FROM AUTHOR]

Purpose: Mucilage plays crucial roles in root-soil interactions. Collection systems for maize (Zea mays L.) use primary and seminal roots of aeroponically-grown seedlings (CSA), or brace roots of soil-grown plants (CSB). While each method represents specific plant developmental stages, and root types growing in specific (micro-)environments, these factors are rarely considered. It is unclear whether mucilage exhibits distinct physico-chemical properties related to collection system-inherent factors. Methods: Mucilage of maize genotype B73 was collected from systems CSA and CSB. Chemical composition was assessed by pH, nutrient contents, neutral sugar composition, and polysaccharide polymer length. Viscosity, surface tension and contact angle represented physical properties. Results: The share of hexoses among total polysaccharides was 11% higher in CSB than in CSA, whereas pentoses were predominant in CSA, together with higher nutrient concentrations and pH values. Mannose was detected only in CSB, which also exhibited higher surface tension, viscosity and contact angle compared to CSA. Conclusions: Physico-chemical differences between the two mucilages are related to root type functions, environmental root growth conditions, and plant developmental state. Higher fractions of pentoses in CSA mucilage seem related to semi-sterile system conditions. Higher viscosity of CSB mucilage might reflect the need for enhanced water holding capacity of brace roots growing in drier conditions. A strong influence of environmental factors on mucilage properties even for a single genotype might play additional roles e.g. in the attraction of microbiomes. These aspects are relevant when assessing the role of mucilage in the rhizosphere, or when developing models of rhizosphere processes. [ABSTRACT FROM AUTHOR]

Aims: Root hairs and lateral growth are root traits among many which enable plants to adapt to environmental conditions. How different traits are coordinated under local heterogeneity, especially when two or more environmental factors vary in space, is currently poorly understood. We investigated the effect of heterogeneity on root system architecture of maize in response to the presence of loamy macroaggregates, which come along with both, increased penetration resistance and nutrient availability, i.e., two important environmental factors shaping root system architecture. The comparison between a mutant with defective root hairs and the corresponding wild type made it possible to investigate the importance of root hairs in the adaptation strategies of plant roots to these factors. Methods: Changes in root growth and root distribution with respect to macroaggregates were investigated using X-ray computed tomography. The wild-type of Zea mays L. was compared with the root hair defective mutant (rth3) to investigate the importance of root hairs in addition to adaption of root architecture. Results: The presence of aggregates lead to increased root length and branch densities around aggregates, while only a few roots were able to grow into them. Thereby, wildtype and rth3 were influenced in the same way. Aboveground biomass, however, was not affected by the presence of macroaggregates, as compared to controls with homogenously distributed loam. Conclusions: Macroaggregation of loam in sandy soil shows little influence on maize growth, due to local adaptations of root architecture to the heterogeneity in nutrient availability and penetration resistance caused by the aggregates. [ABSTRACT FROM AUTHOR]

Background: Even with extensive root growth, plants may fail to access subsoil water and nutrients when root-restricting soil layers are present. Biopores, created from decaying roots or soil fauna, reduce penetration resistance and channel root growth into the deeper soil. Further positive effects on plants result from biopore traits, as the pore walls are enriched in nutrients, microbial abundance, and activity relative to bulk soil. However, negative effects on plant growth have also been observed due to root clumping in biopores, less root-soil contact than in the surrounding bulk soil and leaching of nutrients. Scope: We discuss methods for biopore research, properties of biopores and their impact plant performance based on a literature review and own data. We elucidate potential implications of altered root-soil contact for plant growth and the consequences of root growth in pores for the rhizosphere microbiome. Conclusions: Biopores play an important but ambiguous role in soils. The effects of biopores on plant growth depend on soil properties such as compaction and moisture in an as-yet-unresolved manner. However, pore properties and root-soil contact are key parameters affecting plant yield. Knowledge gaps exist on signaling pathways controlling root growth in pores and on mechanisms modifying rhizosphere properties inside biopores. The degree to which negative effects of biopores on plant growth are compensated in the bulk soil is also unclear. Answering these questions requires interdisciplinary research efforts and novel imaging methods to improve our dynamic understanding of root growth and rhizosphere processes within biopores and at the rhizosphere-biopore interface. [ABSTRACT FROM AUTHOR]

Purpose: The woolly beech aphid thrives on European beech leaves, which has complex direct and indirect impacts on above- and belowground processes. A mechanistic understanding of insect-mediated changes in organic carbon (OC) availability for microbial life and its implications for element cycling is still lacking. This study aims at disentangling aphid-induced effects on phyllosphere and rhizosphere bacterial communities, as well as investigating feedbacks to OC transfer from the canopy to the mineral soil. Methods: Following 2.5 months of infestation, we tracked the fate of OC (13CO2 pulse-labelling) in several compartments of beech sapling – soil mesocosms over 5 days. In ecosystem solutions, water extracts and soil/plant compartments we determined OC and N and solid δ13C. Bacterial community structure (16S rRNA gene targeted amplicon sequencing and quantitative PCR) and metabolite profiles (LC-qTOF-MS) were analysed. Results: We found significantly higher aphid-mediated inputs of OC within throughfall. Honeydew-derived C on infested leaves was inconsequential for total phyllosphere bacterial abundances, but verifiably affected the community structure. In all soil compartments, cold-water extractable OC pools declined significantly by frequent inputs of readily available OC. This pattern might relate to reductions in rhizodepositions and altered microbial processing by accelerated soil C-mineralization. As a result, the abundance of metabolites changed significantly in different ecosystem solutions. Conclusions: Our findings attest that insect infestations induce distinct direct and indirect effects on plant-insect-microbiome interactions leading to marked alterations in C dynamics. This integrated approach improves our understanding on microbial dynamics and biogeochemistry and evaluates the role of insects for ecosystem processes. [ABSTRACT FROM AUTHOR]

Aims: To study nitrogen contribution to cucumber derived from nitrogen fixation of Paenibacillus polymyxa WLY78. Methods: The nif gene cluster deletion mutant (ΔnifB-V) of Paenibacillus polymyxa WLY78 was constructed by a homologous recombination method. The effects of plant-growth promotion were investigated by greenhouse experiments. The nitrogen fixation contribution was estimated by 15N isotope dilution method (also being called the 15N natural abundance technique). Results: Deletion of nif gene cluster of P. polymyxa WLY78 resulted in complete loss of nitrogenase activity. Greenhouse experiments showed that inoculation with P. polymyxa WLY78 could significantly enhance the lengths and dry weights of cucumber roots and shoots, but inoculation with ΔnifB-V mutant could not. 15N isotope dilution experiments showed that cucumber plants derive 25.93% nitrogen from nitrogen fixation performed by P. polymyxa WLY78, but the ΔnifB-V mutant nearly could not provide nitrogen for plant growth. Conclusions: This present study demonstrated that nitrogen fixation performed by P. polymyxa WLY78 contributes to plant growth. [ABSTRACT FROM AUTHOR]

Aims: Perennial crops with more extensive and deep root systems could access deep stored water and build resilience to water shortage. In the context of human nutrition, perennial grain crops are very interesting. However, it is still questionable whether they are effective in using subsoil water. We compared intermediate wheatgrass (Kernza®) Thinopyrum intermedium, a perennial grain crop, to alfalfa Medicago sativa, a forage crop, for subsoil root growth and water uptake. Alfalfa was chosen because of its deep root system and agronomical interest as a companion crop. Methods: Using TDR sensors, deuterium tracer labelling, minirhizotrons and the Hydrus-1D model we characterised the root distribution and water uptake patterns of these two perennial crops during two cropping seasons under field conditions down to 2.5 m soil depth. Results: Both crops grew roots down to 2.0 m depth that were active in water uptake but alfalfa was deeper rooted than intermediate wheatgrass. All experimental methods concluded that alfalfa used more water from below 1.0 m depth than intermediate wheatgrass. However, simulations predicted that intermediate wheatgrass used more than 20 mm of water after anthesis from below 1 m soil depth. Simulations confirmed the advantage of deep roots in accessing deep soil water under drought. Conclusions: In regions with high groundwater recharge, growing deep-rooted perennial crops have great potential to exploit deep soil water that is often left unused. However, the road to a profitable perennial grain crop is still long and breeding intermediate wheatgrass (Kernza®) cultivars for increased root growth at depth seems to be a worthy investment for the development of more drought tolerant cultivars. [ABSTRACT FROM AUTHOR]

Aims: The rhizosphere and root detritusphere are hotspots of microbial activity, where root-derived inputs induce intensive priming effects (PE) on soil organic carbon (SOC) decomposition. These conditions for induced PE differ between rhizosphere and detritusphere and are modified by plant traits. Methods: Continuous labelling with 13C-depleted CO2 allowed for the partitioning of plant and soil C sources of CO2 efflux and the investigation of the PE in the rhizosphere and detritusphere of slow-growing conservative Carex acuta and fast-growing acquisitive Glyceria maxima. Results: Glyceria allocated more C into the soil, induced higher microbial activity and a larger portion of active microorganisms, and depleted mineral N stronger than Carex. Its rhizosphere PE was 2.5 times stronger than that of Carex. Root residues (detritusphere) induced negative PE at the early stage of decomposition (1–9 months). The depletion of available organic substances in the detritusphere of more easily decomposable Glyceria roots resulted in positive PE after 3 months. The PE in the detritusphere of N-poorer Carex roots was more intensive but started after 9 months. Conclusions: The rhizosphere PE was positive and stronger than the detritusphere PE, which switched from initially negative to positive PE after depletion of available substances during few months. More productive species with faster N-uptake and higher belowground C input (here Glyceria) induce larger rhizosphere PE than slower-growing species (here Carex). The N-rich Glyceria roots decompose faster than N-poor roots of Carex and, consequently, have a lower impact on SOC dynamics and induced a smaller positive detritusphere PE. [ABSTRACT FROM AUTHOR]

Aims: The retreat of glaciers is exposing new terrains to primary plant succession around the globe. To improve the understanding of vegetation development along a glacier retreat chronosequence, we (i) evaluated a possible link between base metal (Ca, Mg, K, Na) supply and vegetation establishment, (ii) determined the rates of the establishment of soil and plant base metal stocks, and (iii) estimated the size of the main base metal fluxes. Methods: We determined base metal stocks in the soil organic layer, the mineral topsoil (0–10 cm), and in leaves/needles, trunk, bark, branches and roots of the dominating shrub and tree species and estimated fluxes of atmospheric deposition, plant uptake and leaching losses along the 127-yr Hailuogou chronosequence. Results: Total ecosystem Ca and Mg stocks decreased along the chronosequence, while those of K and Na were unrelated with ecosystem age. Fortyfour and 30% of the initial stocks of Ca and Mg, respectively, were leached during the first 47 years, at rates of 130 ± 10.6 g m−2 year−1 Ca and 35 ± 3.1 g m−2 year−1 Mg. The organic layer accumulated at a mean rate of 288 g m−2 year−1 providing a bioavailable base metal stock, which was especially important for K cycling. Conclusions: We suggest that the initial high Ca bioavailability because of a moderately alkaline soil pH and carbonate depletion in 47 years, together with the dissolution of easily-weatherable silicates providing enough Mg and K to the pioneer vegetation, contributed to the establishment of the mature forest in ca. 80 years. [ABSTRACT FROM AUTHOR]

Aims: The rhizosphere microbiome plays an important role in plant growth and behavior during phytoremediation. This work aims to explore how hyperaccumulating plant performance (shoot biomass and Cd uptake) and rhizosphere bacterial community are driven by plant development under a Cd concentration gradient and their relationship during plant growth. Methods: A pot experiment was conducted using the Cd hyperaccumulator Sedum plumbizincicola grown in soil spiked with 0, 5 and 20 mg Cd kg−1 for 13 weeks. Plant performance (shoot biomass and Cd uptake) was determined and the rhizosphere bacterial community was examined by weekly Illumina sequencing. Biomarker genera were identified by the Random Forest model and the functions were predicted by PICRUSt2. Results: The rhizospheres bacterial community tended to stabilize from the 6th or 7th week regardless of the soil Cd concentration, coinciding with the vigorous growth stage of the hyperaccumulator. Biomarker genera Paenibacillus and Streptacidiphilus enriched in the stable stage showed significant positive correlations with Cd uptake. In addition, some predicted gene abundances of enzymes related to plant growth promotion by the rhizosphere bacterial community were influenced to various degrees by plant development and soil Cd concentration. Conclusion: There exists a stable bacterial community in the rhizosphere of S. plumbizincicola during phytoremediation and biomarker taxa enriched at this stage were positively correlated with Cd uptake. Selecting microorganisms that match the developmental stages might further enhance phytoremediation. This provides some theoretical basis for modulating rhizosphere microbes at the appropriate time during phytoextraction to optimize the phytoremediation technique. [ABSTRACT FROM AUTHOR]

Aims: Root hairs are one root trait among many which enables plants to adapt to environmental conditions. How different traits are coordinated and whether some are mutually exclusive is currently poorly understood. Comparing a root hair defective mutant with its corresponding wild-type, we explored if and how the mutant exhibited root growth adaptation strategies and how dependent this was on substrate. Methods: Zea mays root hair defective mutant (rth3) and the corresponding wild-type siblings were grown under well-watered conditions on two substrates with contrasting texture and hence nutrient mobility. Root system architecture was investigated over time using repeated X-ray computed tomography. Results: There was no plastic adaptation of root system architecture to the lack of root hairs, which resulted in lower uptake of nutrients especially in the substrate with high sorption capacity. The function of the root hairs for anchoring did not result in different root length density profiles between genotypes. Both maize genotypes showed a marked response to substrate. This was well reflected in the spatiotemporal development of rhizosphere volume fraction but especially in the highly significant response of root diameter to substrate, irrespective of genotype. Conclusions: The most salient root plasticity trait was root diameter in response to substrate. Coping mechanisms for missing root hairs were limited to a shift in root-shoot ratio in loam. Further experiments are required, to elucidate whether observed differences can be explained by mechanical properties beyond mechanical impedance, root or microbiome ethylene production or differences in diffusion processes within the root or the rhizosphere. [ABSTRACT FROM AUTHOR]

Background and aims: Nitrogen (N) and phosphorus (P) availabilities limit plant productivity, especially in primary succession; however, our understanding of species-specific strategies regarding their allocation and coordination with other functional traits remains limited. Methods: Community-weighted mean traits were compared to decipher the ecophysiological mechanisms of forest succession in nine dominant species along 120-y successional stages across a glacier-retreating chronosequence. Results: High foliar N and P concentrations in N2-fixing plants on a 12-year-old surface did not result in high photosynthetic capacity due to the inefficient allocation, as indicated by their low photosynthetic N- and P-use efficiencies. On a 40-year-old surface, exploitative strategies, manifested in a higher specific leaf area and greater N allocation to Rubisco, as well as quick-return energy economics, helped deciduous forests dominate. When P availability decreased on a 120-year-old surface, evergreens maintained high photosynthetic P-use efficiency, by reducing overall P concentration and its allocation to structural fraction. Efficient P allocation and a higher ratio of leaf lifespan to payback time facilitated the dominance of evergreens in low P-sites. Conclusions: Optimizing allocation of limiting N or P among foliar fractions and fast–slow economic strategies drive primary succession after glacier retreat. Integrating the above- and below-ground subsystems through food webs will provide further insights into ecosystem dynamics. [ABSTRACT FROM AUTHOR]

Background and aims: Plant-soil feedbacks may determine the long-term success of introduced species. Here we examined plant-soil feedbacks of a globally invasive shrub, Cytisus scoparius (hereafter Cytisus), which associates with multiple guilds of microbial mutualists and dominates harvested Douglas-fir forests in the Pacific Northwest. Methods: We studied Cytisus root nodulation, mycorrhizal colonization, and growth in two greenhouse experiments. First, we compared invaded to uninvaded field soils. Then we did a soil conditioning experiment with Cytisus (in both invaded and uninvaded field soil) compared to soil conditioned by Douglas-fir. Results: Cytisus grown in invaded soils had 67% more root nodules and 72% more AMF colonization than uninvaded soil. Conditioning uninvaded soil with Cytisus increased root nodules by 14% and mycorrhizal colonization by 55%, compared to Douglas-fir conditioned soil. Despite the increased abundance of mutualists, Cytisus grown in Cytisus-conditioned soils were 41% smaller than in uninvaded soil. Conclusions: Cytisus increased the abundance of its microbial mutualists in soils, but overall plant-soil feedback was still negative, likely driven by soilborne pathogens, nutrient depletion, and/ or reduced benefits of mutualists. Our results do not support the idea that the high densities reached by Cytisus in its invaded range are caused by positive plant-soil feedbacks. [ABSTRACT FROM AUTHOR]

Background and aims: Mined materials often require rehabilitation or ecological restoration through revegetation as part of mine closure and relinquishment practices, yet there is a widening gap between the expectations of recovery and what industry achieve. The edaphic conditions of post-mining substrates present a suite of potential limitations to plant growth and may constrain the establishment capability and development of native species. Methods: We assessed seedling emergence, relative growth rate and calculated standardised growth estimates using 10 measured root and shoot parameters for six locally-dominant native species from different families and nutrient-acquisition strategies in a range of representative mining restoration substrates (topsoil, tailings, capped tailings and waste rock), examining their suitability as pioneers for ecological restoration. Results: The establishment and growth of all six species in post-mining substrates were significantly compromised. Root development was significantly responsive to substrate, with measured root parameters on average 27% lower in capped tailings, 41% lower in waste rock and 67% lower for individuals grown in tailings compared with those grown in topsoil alone. Plant growth was compromised at different life cycle stages (seed germination, seedling establishment, early growth and development) and across a number of different traits, with primary edaphic constraints including high pH (>8.5) and insufficient available N. The highest-performing species on post-mining substrates was an N2-fixing legume, while lowest-performing species included those with ectomycorrhizal associations or no specific nutrient-acquisition strategy. Conclusions: Edaphic filters may be significant drivers of trajectory and success in rehabilitation and restoration projects at scales ranging from individuals (by limiting establishment or constraining growth and development) to communities (by causing species to assemble in a different manner than the desired reference community). If intractable edaphic parameters constraining plant establishment and early development such as extreme pH and a lack of available nutrients are not ameliorated, the restoration trajectory on post-mining landforms is likely unfavourable. Failure to adequately ameliorate post-mining substrates may represent a major liability for industry in meeting mine-closure requirements. [ABSTRACT FROM AUTHOR]

Aims: Fire regimes are key drivers of ecosystem dynamics and are changing worldwide. Uncertainty about how fire history affects responses to individual fires hampers predictions of fire impacts on important ecosystem functions such as C cycling. Thus, we assessed how fire and fire history affect soil CO2 flux and aboveground net primary production (ANPP). Methods: We utilized a 35-year fire frequency experiment in a mesic grassland to quantify how two aspects of fire history, long-term fire frequency (fire every one, two, or four years, or no fire) and number of years elapsed since the most recent fire, affect soil CO2 flux. We used long-term annual records from the same grassland to compare this to the effect of fire history on ANPP. Results: Historic fire frequency altered the soil CO2 flux response to fire, with greater post-fire stimulation in grassland burned annually than in grassland burned less frequently (~100% vs. ~44% increase over long-term unburned grassland). The flux increase persisted for up to two years after fire. Though fire also stimulated ANPP, this increase did not vary by long-term fire frequency and did not persist into later years. Conclusions: Fire history modifies the soil CO2 flux response to individual fires in this grassland. Predicting the dynamics of this important C flux will require considering not only the presence vs. absence of fire, but also fire history. [ABSTRACT FROM AUTHOR]

Aims: Belowground interactions are still poorly understood in mixed-species forests. We investigated the effects of soil fertility on belowground processes in mixed planted forests. Methods: The dynamics and traits of Eucalyptus and Acacia mangium fine roots (diameter < 2 mm) in plantations with 50% of each species were studied in a randomized block design established in a nutrient depleted soil. Stands with NPK fertilization applied at planting (F+) were compared to unfertilized stands (F-). Results: In the 0–15 cm soil layer, Eucalyptus root mass density (RMD) was higher than Acacia RMD by 50% in F+ and 10% in F-, considering both ages. At 34 months of age, Eucalyptus RMD was 94% higher near Acacia trees than near Eucalyptus trees in F-. In this layer, Eucalyptus specific root length (SRL) was 21% higher than Acacia SRL at 16 months of age and was 10% higher in F- than in F+ at 34 months of age. The cumulative Eucalyptus fine root length production between 16 and 34 months was 66% higher in F- than in F+ in the 0–1 m soil layer. Conclusions: Fertilization increased the competition between species and led to a partial exclusion of Acacia fine roots from the nutrient-rich topsoil. Soil exploration by Eucalyptus roots in the vicinity of Acacia trees was higher in F- than in F+, which suggests that unfertilized trees benefited from facilitation through higher soil N availability and direct N transfer from Acacia trees. [ABSTRACT FROM AUTHOR]

Aims: Heterogeneous canopies in temperate montane forests affect microclimate and soil characteristics, with important effects on soil microbial communities and related processes. Here, we studied the interactions between plant root traits and soil bacterial and fungal communities in closed forest and open gaps in a mixed forest along an elevational gradient in the French Alps (1400, 1700 and 2000 m). Methods: Samples were separated into three fractions (plant root, rhizosphere and bulk soil), to further investigate the influence of plant zones on microbial communities. Bacterial (16S) and fungal (ITS) biodiversity was determined using high throughput sequencing, along with standard measures of soil, litter and root traits. Results: We found that (i) microbial community diversity was higher in gaps than in closed forest because of increased root trait diversity and density; (ii) open versus closed forest patches affected phylogenetic dispersion despite differences in elevations with phylogenetic clustering in closed forest; (iii) the interaction between root traits and microbial communities was stronger for rhizosphere and root fractions than for bulk soil and (iv) bacterial community composition was better explained by root traits than for fungi. Conclusions: Our findings highlight the importance of open gaps versus closed forest patches and associated root traits affecting microbial community structure, particularly for bacterial assemblages that exhibited a stronger interaction with root traits than for fungi. [ABSTRACT FROM AUTHOR]

Background and aims: In this study, the potential of non-intrusive root electrical capacitance (CR) measurements for monitoring the seasonal changes and drought response of root activity was tested on two grassland species in a climate change experiment. Methods: CR was detected between a ground electrode inserted into the soil and a plant electrode attached to the stem of the perennial grass Stipa borysthenica and the biennial herb Crepis rhoeadifolia in control and drought plots throughout two growing seasons. A pilot study revealed that CR was strongly correlated with root biomass for a given time and soil water content. The effect of changing soil water content on the measured CR value was accounted for by means of species-specific experimental calibrations. Results: Root activity (CR) was found to peak at the flowering stage in late spring (S. borysthenica) or early summer (C. rhoeadifolia). Both the natural shortage of rainfall and the experimental summer drought reduced root activity in both species. Stipa borysthenica displayed great plasticity in root activity, including quick post-treatment recovery during the rainy autumn. The changes observed in root activity were similar to those previously recorded using conventional root investigation techniques (i.e. destructive, minirhizotron, ingrowth core) in temperate grasslands. Conclusions: Root capacitance measurements proved to be adequate for monitoring root activity in situ in natural grassland. The method could be particularly useful in studies where plant injury or soil disturbance need to be avoided. [ABSTRACT FROM AUTHOR]

Aims: Cultivar mixtures can increase productivity through complementarity in resource use, but reported results are often conflicting and the role of plasticity in shaping plant-plant interactions is poorly understood. We aim to determine if individual cultivars show different phenotypic responses when grown in a mixture, whether these responses depend on the neighboring cultivar identity, and how they contribute to variations in productivity and nitrogen (N) use. Methods: Five spring barley cultivars were field-grown in pure stands and in mixtures during 2 years. Plant traits related to development, growth, N use, and reproduction were measured to identify temporal patterns of plastic responses to neighboring plants. Results: Plants in mixtures were shorter and developed slower early in the season, but later on they grew faster and produced more grain than the corresponding pure stands. Some cultivars showed complementary N accumulation only when grown together with specific neighbors. Mechanisms of improved productivity differed between the individual mixtures. Conclusions: Plastic plant-plant interaction between cultivars is an important driver behind the variability in mixing effects. Results contribute to a better understanding of how productivity in cultivar mixtures is affected by plastic adaptation and differentiation of plant traits, depending on the environment created by neighboring genotypes. [ABSTRACT FROM AUTHOR]

Aims: Cover crops (CC) have been widely used to improve soil quality and sustain agricultural productivity. However, the turnover of carbon (C) and nitrogen (N) from different species and parts of CCs incorporated into the soil under Northern European conditions remains unclear. Methods: In this study, we examined mineralization of C and N from isolated shoots and roots of two leguminous and two non-leguminous CCs labeled with 15N tracers in a 100-day incubation experiment at 10 °C. Results: After 100 days incubation, the average net N mineralization was 33% from the leguminous CCs and 20% from the non-legumes. Net N mineralization from shoots was 1–15% (% of the N input) higher than that from the corresponding roots, and the net C mineralization accounted for 53–62% and 18–39% of the total C applied with CC shoots and roots, respectively. The C/N ratio of plant residues was a good predictor for net N mineralization, while the lignin concentration was a good predictor for C mineralization from both roots and shoots. Roots of both legume and non-legume CCs caused net immobilization of (unlabeled) soil N, whereas no significant soil N immobilization occurred from shoots with low C/N ratios. Conclusions: Our results indicate that different CC characteristics (C/N ratio and lignin) control the turnover of residue N and C, respectively, in the soil. Our results further showed very limited interactions with soil N turnover (no priming effects) when materials with low C/N ratios were applied to soil. [ABSTRACT FROM AUTHOR]

Aims: Previous studies suggest that organic carbon (C) and nitrogen (N) can stimulate soil nitrification, but whether autotrophic or heterotrophic nitrification is stimulated and who are active nitrifiers for the nitrification activity is still in debate. We elucidated which nitrification dominated and the active nitrifiers during the decomposition of rice callus. Methods: 15N-labeled callus and acetylene (C2H2) inhibition were used to explore the autotrophic or heterotrophic nitrification during the decomposition of callus and DNA-based stable isotope probing (SIP) and high-throughput sequencing were used to investigate the active nitrifiers. Results: Autotrophic nitrification dominated the nitrification activity, driven by oxidation of ammonia (NH3) produced from mineralization of the callus-derived organic N. Callus significantly stimulated nitrification activity, which was paralleled by changes in the abundance and community composition of AOA. DNA-SIP further demonstrated that the active AOA outnumbered their bacterial counterparts in the 13C-DNA from the soil with callus amendment. Phylogenetic analysis revealed the functional importance of soil fosmid 29i4-like and 54d9-like AOA within soil group 1.1b during the active nitrification with callus cells. Conclusions: NH3 released from the mineralization of callus was the main substrate for autotrophic nitrification and preferentially stimulated the growth of AOA within group 1.1b in the paddy soil. [ABSTRACT FROM AUTHOR]