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Abstract: Microbial utilization is a key transformation process of soil organic matter (SOM). For the first time, position-specific 13C labeling was combined with compound-specific 13C-PLFA analysis to trace metabolites of two amino acids in microbial groups and to reconstruct detailed biochemical pathways. Short-term transformation was assessed by applying position-specifically 13C labeled alanine and glutamic acid to soil in a field experiment. Microbial utilization of the amino acids' functional groups was quantified by 13C incorporation in total microbial biomass and in distinct microbial groups classified by 13C-PLFA. Loss from PLFAs was fastest for the highly oxidized carboxyl group of both amino acids, whereas the reduced C positions, e.g. C3–5, were preferentially incorporated into microorganisms and their PLFAs. The incorporation of C from alanines' C2 position into the cell membrane of gram negative bacteria was higher by more than one order of magnitude than into all other microbial groups. Whereas C2 of alanine was still bound to C3 at day 3, the C2 and C3 positions were partially split at day 10. In contrast, the C2 of glutamic acid was lost faster from PLFAs of all microbial groups. The divergence index, which reflects relative incorporation of one position to the incorporation of C from all positions in a molecule, revealed that discrimination between positions is highest in the initial reactions and decreases with time. Reconstruction of microbial transformation pathways showed that the C2 position of alanine is lost faster than its C3 position regardless of whether the molecule is used ana- or catabolically. Glutamic acid C2 is incorporated into PLFAs only by two out of eight microbial groups (fungi and part of gram positive prokaryotes). Its incorporation in PLFA can only be explained by either the utilization of the glyoxolate bypass or the transformation of glutamic acid into aspartate prior to being fed into the citric acid cycle. During these pathways, no C is lost as CO2 but neither is energy produced, making them typical C deficiency pathways. Glutamic acid is therefore a promising metabolic tracer in regard to ecophysiology of cells and therefore changing environmental conditions. Analyzing the fate of individual C atoms by position-specific labeling allows insight into the mechanisms and kinetics of microbial utilization by various microbial groups. This approach will strongly improve our understanding of soil C fluxes. [Copyright &y& Elsevier]

The high relevance of microbial necromass as source of long-term stable C has become evident in the past decade. However, microbial cell death releases nutrient-rich organic matter (OM) with unique properties: 1) Its narrow C:N:P stoichiometry makes it a valuable nutrient source. 2) Its location in patchy microbial hotspots makes it easily exploitable once the hotspot is reached by roots. 3) Its structural location embedded in biofilms or hyphal structures minimizes the risk of nutrient-leaching. 4) Depending on C availability and microbial growth dynamics, microbial necromass is a continuously renewing pool with seasonal dynamic. 5) The binding forms of nutrients cover a wide range of molecular compounds from easily available forms (free amino acids, free nucleotides) to medium (peptides, DNA, RNA, phospholipids) and complex forms (e.g. chitin or peptidoglycane). Such a wide compositional range provides a set of successively exploitable nutrients depending on the ecosystems' nutrient limitation status. Although systematic studies are still lacking, such properties suggest a key role of microbial necromass for ecosystem nutrition. To identify the significance of necromass P and N for plant nutrition in forest ecosystems we established a series of experiments with soils from four contrasting beech forests sites. The use of T-shaped pots allowed to create i) an intact rhizosphere and ii) a hyphosphere by using PTFE gauze not permeable by plant roots but by mycorrhizal hyphae. Thus, beech P and N uptake directly via roots incl. mycorrhizal associations or via mycorrhizal association only could be distinguished. 14C-, 15N-, and 33P-labelled microbial necromass was applied on both ends of the side compartments of the T-shaped pots. Gradients of necromass OM depolymerizing exoenzymes (phosphatases, chitinases, etc.) were analyzed along the lateral axis from the beech stem to the necromass hotspot. Generally, the hyphosphere alone was as potent as the combination of roots and their mycorrhizal partners in mobilizing necromass nutrients and providing them for plant nutrition demonstrating the key role of ectomycorrhizal fungi in necromass nutrient utilization in forests. Uptake of microbial necromass-derived P by beech trees increased with decreasing soil P availability, suggesting a higher relevance of microbial necromass-derived nutrients in ecosystems with closed nutrient (re-)cycling. In these ecosystems, the activity of exoenzyms releasing necromass nutrients (e.g. chitinase or phosphatase) was factor 2 to 20 higher in close proximity to the necromass hotspot compared to the middle soil compartments. This shows that triggering the activity of necromass decomposing enzymes is a key trait of the rhizo-hyphosphere of trees to access the necromass nutrient reservoir, especially in recycling ecosystems.Our results reveal that forest ecosystems have defined strategies to exploit the intermediate nutrient reservoir of the microbial necromass by the interaction of roots, mycorrhiza and activation of microbial functions. This suggests that the role of microbial necromass as dynamic nutrient reservoir for ecosystem nutrition was strongly underestimated up to now and that the ecosystem's demand on exploiting this nutrient reservoir can be a triggering factor for its potential to function as C sink in soils. [ABSTRACT FROM AUTHOR]

A 163-day decomposition experiment with 13C-enriched leaf litter of Populus davidiana (low quality, with low N content, high C:N and high lignin content) and Quercus wutaishanica (high quality, with high N content, low C:N and low lignin content) was conducted to investigate the effects of litter quality on the microbial contribution to soil organic C (SOC). We used stable isotope probing (SIP) technology of phospholipid fatty acid (PLFA) and amino sugar, determined soil enzyme activities, and microbial C use efficiency (CUE) to study the microbial contribution to SOC formation as affected by litter quality. Gram-positive (G +) and Gram-negative (G −) bacteria rapidly assimilated the readily available C of high- and low-quality litter, whereas fungi selectively utilized more recalcitrant compounds. The ratio of 13C-fungal to 13C-bacterial necromass increased and then leveled off until the end of the incubation for both litters. Therefore, litter-derived C was first utilized by bacteria, then allocated presumably by the consumption of bacterial necromass to fungi, and, at the end, the litter C was mainly stabilized as fungal necromass. The addition of high-quality litter led to higher total necromass and SOC in comparison to the addition of low-quality litter. Likely this difference depended on the higher availability of easily available C compounds in the Q. wutaishanica than in P. davidiana litters. The efficiency of SOC formation, determined by the percentage of SOC content gain divided by the litter C content loss, correlated with the microbial incorporation of P. davidiana litter-derived 13C into PLFAs and amino sugars. However, it increased sharply in the late phases of Q. wutaishanica litter decomposition despite the decreased 13C incorporation in PLFAs and amino sugars, suggesting the dominance of physical litter C stabilization. Compared to the high-quality litter, the low-quality litter induced lower but steadier necromass accumulation, thus increasing the SOC content in the long term. Litter quality, litter-derived 13C in PLFAs, and microbial CUE are the main drivers of litter-derived C use pathways. Our findings underpin the microbial C pump-regulated SOC formation, whereby differences in litter quality shape the composition of main microbial groups, leading to differences in enzyme activities and CUE, which determine necromass turnover and thus SOC formation. [ABSTRACT FROM AUTHOR]

When multiple metabolic pathways lead to the same product, compound-specific isotope analysis may not provide enough information to quantify the activities of the contributing pathways. Instead, identification of where in the molecule the 13C is incorporated is required. Here we show how knowledge of position-specific 13C incorporation in fatty acids (FA) and FA fragments can be used to quantitatively estimate the fluxes through the central C metabolic network. We developed a method to measure 13C enrichment of FA and FA fragments (ethanoate, propionate) using electron impact GC–MS. We tested the accuracy and repeatability of the measurements using natural abundance and position-specific 13C labelled standards and FA extracted from Bacillus licheniformis and Pseudomonas fluorescens grown with labelled and unlabelled glucose. The molecular ions of FA generally reflected theoretical predictions of mass isotopomer distributions for natural abundance values, but that of the associated FA fragments deviated from expected values, likely associated with McLafferty rearrangements of hydrogen. After correction for naturally occurring isotopes, 13C enrichments of FA and FA fragments showed good agreement with expected isotope composition of FA standards (root mean square error < 0.044 at%; δ13C of ∼ 40‰), natural abundance and labelled glucose. The unsaturated FA extracted from P. fluorescens deviated from expected values likely associated with problems of co-elution and ion suppression and were excluded from analysis. The ratio of glucose-1-13C to glucose-3-13C incorporation into FA fragments was high for B. licheniformis, but low for P. fluorescens. Metabolic flux modelling based on the 13C enrichment of ethanoate and propionate fragments showed that B. licheniformis used Embden-Meyerhof-Parnas and pentose phosphate pathway (66% and 30%, respectively), whereas P. fluorescens utilized Entner-Doudoroff and pentose phosphate pathway (72% and 27%, respectively). FA fragment analysis is therefore a promising tool to study central C metabolic network activities of co-occurring groups of microbes in intact and complex environmental communities. [ABSTRACT FROM AUTHOR]

Aims: As soil dries, the loss of soil hydraulic conductivity limits water supply to the leaves, which is expected to generate a nonlinear relationship between leaf water potential (ψleaf) and transpiration (E). The effect of soil drying and root properties on ψleaf and E remains elusive. Methods: We measured E and ψleaf of pearl millet for varying E and soil moisture using a root pressure chamber. A model of water flow in soil and plant was used to fit the ψleaf (E) relationship. Results: The relation between ψleaf and E was linear at all soil moistures. The slope of ψleaf (E) increased with decreasing soil moisture due to the decreasing soil-root conductance. The fact that the relation remained linear also in dry soils and high E is surprising. Indeed, it indicates that the gradients in soil water potential (ψsoil) were small, probably because of the large root surface (13.5 cm cm−3) active in water uptake. ψleaf at E = 0 was less negative than ψsoil, indicating a more negative osmotic potential in the xylem than in the soil. Conclusions: We propose that the linearity between ψleaf and E and the high ψleaf (E = 0) compared to ψsoil support transpiration in drying soils. [ABSTRACT FROM AUTHOR]

Abstract Microbial activity and functioning in soils are strongly limited by carbon (C) availability, of which a great proportion is released by living roots. Rhizodeposition and especially root exudates stimulate microbial activity and growth, and may shift the stoichiometric balance between C, N, and P. Thereby, exudates heighten microbial nutrient demand and acquisition of N and P from organic matter, leading to an increase in enzyme production. Aim of this study was to determine environmental controls of extracellular enzyme production, and hence on potential enzyme activities (V max) and substrate affinities (K m). To determine the controlling factors, we worked on four spatial scales from the microscale (i.e. rhizosphere) through the mesoscale (i.e. soil depth) and landscape scale (relief positions), and finally to the continental scale (1200 km transect within the Coastal Cordillera of Chile). Kinetics of seven hydrolyzing enzymes of the C, N, and P cycles (cellobiohydrolase, β‑glucosidase, β‑xylosidase, β‑ N ‑acetylglucosaminidase, leucine‑aminopeptidase, tyrosine‑aminopeptidase, and acid phosphatase) were related to soil texture, C and N contents, pH, and soil moisture via redundancy analysis (RDA). Potential activities of C, N, and P acquiring enzymes increased up to 7-times on the continental scale with rising humidity of sites and C and N contents, while substrate affinities simultaneously declined. On the landscape scale, neither V max nor K m of any enzyme differed between north and south slopes. From top- to subsoil (down to 120 cm depth) potential activities decreased (strongest of aminopeptidases under humid temperate conditions with up to 90%). Substrate affinities, however, increased with soil depth only for N and P acquiring enzymes. Affinities of cellobiohydrolase and β‑xylosidase, on the contrary, were 1.5- to 3-times higher in top- than in subsoil. Potential activities of N and P acquiring enzymes and β‑glucosidase increased form bulk to roots. Simultaneously, substrate affinities of N and P acquiring enzymes declined, whereas affinities of β‑glucosidase increased. These trends of activities and affinities in the rhizosphere were significant only for acid phosphatase. The RDA displayed a strong relation of potential activities of C and P acquiring enzymes and β‑ N ‑acetylglucosaminidase to C and N contents in soil as well as to the silt and clay contents. Aminopeptidase activity was mainly dependent on soil moisture and pH. We conclude that substrate availability for microorganisms mainly determined enzyme activity patterns on the continental scale by the humidity gradient. Patterns on the meso- and microscale are primarily controlled by nutrient limitation, which is induced by a shift of the stoichiometric balance due to input of easily available C by roots in the rhizosphere. Highlights • Subsoil enzyme systems show low K m as adaptation to low substrate availabilities. • Stoichiometric balance in root proximity was maintained by high enzyme activities. • Chitin-based N nutrition got more relevant in the humid South. [ABSTRACT FROM AUTHOR]

Aim: The Paris Climate Agreement is pursuing efforts to limit global warming to less than 2 °C in this century, but increasing evidence shows that temperatures are likely to rise up to 4.8 °C by 2100. This points to an urgent need to investigate how the temperature impact on microbial regulation will endanger soil organic matter stability under warming approaching or exceeding 2 °C. Location: Southern Germany and the globe. Time Period: 2010–2018. Major Taxa Studied: Soil organic matter stability and microbial adaptations. Methods: We analyzed soil properties and enzyme activities within (a) a long‐term field experiment with soil warming to below and above 2 °C (+1.6 vs. +3.2 °C), and (b) a literature review of 213 comparable studies globally (+0–2 vs. +2–4 °C). Results: The soil organic C (SOC) stock remained unchanged after 8 years under both warming magnitudes, whereas the labile C pool increased by 10% under >2 °C. Unlike the SOC pool, total N (TN) content increased by 20% under >2 °C as compared to ambient. A potential explanation for the increased TN content is linked to unbalanced processes of necromass formation and enzymatic decomposition. Warming induced faster microbial growth and turnover, but reduced catalytic efficiency and the enzyme‐mediated decomposition of oligosaccharides and polypeptides. This consequently caused N accumulation in microbial necromass. Although microbial regulation can maintain SOC at stable levels, warming exceeding 2 °C will change the projected effects of temperature on soil TN pools in the future. Early action to accomplish the 2 °C temperature goal can therefore markedly reduce the likelihood that large regions will face substantial increase of SOC availability, N accumulation and related climate impacts on C and N cycling. Main Conclusions: It is crucial to include microbial metabolic responses (i.e., faster microbial growth and turnover) to warming in global C and N cycle models to improve the prediction of climate warming scenarios. [ABSTRACT FROM AUTHOR]

Deadwood is a key component of forest ecosystems, but there is limited information on how it influences forest soils. Moreover, studies on the effect of thinning-derived deadwood logs on forest soil properties are lacking. This study aimed to investigate the impact of thinning-derived deadwood logs on the soil chemical and microbial properties of a managed spruce forest on a loamy sand Podzol in Bavaria, Germany, after about 15 years. Deadwood increased the soil organic carbon contents by 59% and 56% at 0–4 cm and 8–12 cm depths, respectively. Under deadwood, the soil dissolved organic carbon and carbon to nitrogen ratio increased by 66% and 15% at 0–4 cm depth and by 55% and 28% at 8–12 cm depth, respectively. Deadwood also induced 71% and 92% higher microbial biomass carbon, 106% and 125% higher microbial biomass nitrogen, and 136% and 44% higher β-glucosidase activity in the soil at 0–4 cm and 8–12 cm depths, respectively. Many of the measured variables significantly correlated with soil organic carbon suggesting that deadwood modified the soil biochemical processes by altering soil carbon storage. Our results indicate the potential of thinned spruce deadwood logs to sequester carbon and improve the fertility of Podzol soils. This could be associated with the slow decay rate of spruce deadwood logs and low biological activity of Podzols that promote the accumulation of soil carbon. We propose that leaving thinning-derived deadwood on the forest floor can support soil and forest sustainability as well as carbon sequestration. [ABSTRACT FROM AUTHOR]

Antarctic King George Island is the fastest-warming area in the Southern Hemisphere. Organic matter inputs are scarce in this area, as they are derived from lichens, mosses, avian faeces, and minor inputs from two vascular plant species, Deschampsia antarctica É. Desv. and Colobanthus quitensis (Kunth) Bartl. Here, we examined the effects of freezing and thawing (FT) cycles on the priming effect (PE). We hypothesised that soil microorganisms preferentially use freeze-preserved soil organic carbon (SOC) exposed after thawing as an important energy source, resulting in intense PE. Two soils with contrasting clay contents were characterised by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and incubated with and without 13C-glucose for 21 d. CO2 and 13CO2 were recorded from soil (i) without FT, (ii) one FT, and (iii) three FT cycles (− 18/12 °C). SOC exhibited low aromaticity stretching at 920 cm−1 and 1650 cm−1. Glucose-derived CO2 was maximal (26 ± 2.2 mg g−1 C) in the control soil without FT and decreased to 8.6 ± 0.1 mg g−1 C after three cycles. Glucose induced an intensely positive PE, 41–64% of basal respiration for a single FT cycle and 72–76% for no cycles. However, after three FT cycles, there was null or negative PE (− 9.5–0.4%). On average, the SOC content after net C balance increased with freezing frequency from 103 ± 14 to 212 ± 7. mg C kg−1 in low clay forming soil and from 129 ± 14 to 156 ± 2 mg C kg−1 in high clay forming soil and declined with increasing PE in both soils (R2 = 0.87, p < 0.01). Diminution in freezing frequency because of global warming will increase the positive PE, affecting the C sequestration of incipient SOC formation in maritime Antarctic soils. [ABSTRACT FROM AUTHOR]

Short rotation coppices (SRCs) are a promising alternative for environmental-friendly biomass production. However, profound understanding of nitrogen (N) uptake and allocation dynamics and their interaction with biomass production of individual tree species is required for their sustainable management. In-situ 15 N soil pulse labeling of the widely applied willow cv. Tordis and poplar cv. Max 1 allowed tracing their uptake of N min and to evaluate the effect of N nutrition on their growth. A pulse of either 15 NH 4 NO 3 or NH 4 15 NO 3 was applied to the soil of four replicate trees of each species in a pot experiment. Leaf, twigs, stem, root were analyzed to quantify the uptake and allocation of 15 N after labeling. Summarizing all compartments of poplar, almost all of 15 NO 3 − (97%) from the N soil pool could be recovered, but only a third of the 15 NH 4 + (34%). In contrast, willow incorporated exactly the same amount of 15 N (49%) from both tracers i.e. showing no preference for a certain N min species. Poplar did not only have the higher N min uptake but also showed a higher total biomass (12.2 g·tree −1 ) production than willow (10.2 g·tree −1 ) in first 56 days, which goes along which its higher allocation of N into leaves. We conclude that the poplar cv. Max 1 might be a better choice for biomass production, especially at arable sites with high N contents as well as for protecting from all negative impacts of non-closed N cycles as typical for classical agricultural managed sites (e.g. nitrate leaching or N 2 O emissions). [ABSTRACT FROM AUTHOR]

Root morphology reflects plant adaptations to phosphorus (P) deficiency. We hypothesized that changes in rice root morphology reflect P deficiency decrease after ferric iron (Fe(III))-bound phosphate (Fe–P) dissolution in low-redox paddy soils. We developed a novel in-situ 32P phosphor-imaging approach under flooding to estimate P uptake by rice roots released from Fe–P dissolution. 32P-labeled ferrihydrite (31 mg P kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots but not microorganisms from direct Fe–P mobilization, or (2) directly mixed with soil to enable roots and microorganisms unrestricted access to the Fe–P. The establishment of low redox conditions (E h values between −176 and −224 mV) drove the reductive dissolution of Fe–P. Rice root-derived organic acids alone were unable to control Fe–P dissolution, and Fe(III) reduction is predominately a microbially-mediated process. Direct root access to Fe–P raised both the number and mean diameter of crown roots and root tips, and increased P uptake by 149–231%. Crown root elongation rate, 32P activities along roots and root tips were 5–133% higher when roots directly accessed Fe–P compared to Fe–P excluded from roots in mesh bags. Iron accumulation on roots depended on the rice growth stage, but not on their access to Fe–P. Roots' access to Fe–P increased rice crown roots elongation and branching and increased P accessibility under P deficiency. [Display omitted] • Fe–P dissolution increased rice root growth and induced root morphological changes. • Increased root tip numbers raised P uptake by roots from the soil. • Organic acids exuded by rice roots were unable to control Fe–P dissolution. • Iron accumulation on root surface was independent of root's access to Fe–P. [ABSTRACT FROM AUTHOR]

Summary: Root hairs and soil water content are crucial in controlling the release and diffusion of root exudates and shaping profiles of biochemical properties in the rhizosphere. But whether root hairs can offset the negative impacts of drought on microbial activity remains unknown.Soil zymography, 14C imaging and neutron radiography were combined to identify how root hairs and soil moisture affect rhizosphere biochemical properties. To achieve this, we cultivated two maize genotypes (wild‐type and root‐hair‐defective rth3 mutant) under ambient and drought conditions.Root hairs and optimal soil moisture increased hotspot area, rhizosphere extent and kinetic parameters (Vmax and Km) of β‐glucosidase activities. Drought enlarged the rhizosphere extent of root exudates and water content. Colocalization analysis showed that enzymatic hotspots were more colocalized with root exudate hotspots under optimal moisture, whereas they showed higher dependency on water hotspots when soil water and carbon were scarce.We conclude that root hairs are essential in adapting rhizosphere properties under drought to maintain plant nutrition when a continuous mass flow of water transporting nutrients to the root is interrupted. In the rhizosphere, soil water was more important than root exudates for hydrolytic enzyme activities under water and carbon colimitation. See also the Commentary on this article by Zhang et al., 237: 707–709. [ABSTRACT FROM AUTHOR]

Aims: This study investigated the influence of climate and soil on the exudation rate and polysaccharide composition of aerial nodal root mucilage from drought-resistant and drought-susceptible maize varieties. Methods: Two maize varieties were grown in two different soils (sandy-clay loam Acrisol and loam Luvisol) under simulated climatic conditions of their agroecological zones of origin in Kenya and Germany. The exudation rate of mucilage from the aerial nodal roots was quantified as dry weight per root tip per day and the mucilage was characterized for its polysaccharide composition. Results: On average, the mucilage exudation rate was 35.8% higher under the Kenyan semi-arid tropical than under the German humid temperate climatic conditions. However, cultivation in the loam Luvisol soil from Germany led to 73.7% higher mucilage exudation rate than cultivation in the sandy-clay loam Acrisol soil from Kenya, plausibly due to its higher microbial biomass and nutrient availability. The drought-resistant Kenyan maize variety exuded 58.2% more mucilage than the drought-susceptible German variety. On average, mucilage polysaccharides were composed of 40.6% galactose, 26.2% fucose, 13.1% mannose, 11% arabinose, 3.5% glucose, 3.2% xylose, 1.3% glucuronic acid, and 1% an unknown uronic acid. Overall, significantly higher proportions of the uronic acids were found in the mucilage of the plants grown in the Kenyan sandy-clay loam soil and under the Kenyan semi-arid tropical climatic conditions. Conclusions: Maize is able to enhance its mucilage exudation rate under warm climatic conditions and in soils of high microbial activity to mitigate water stress and support the rhizosphere microbiome, respectively. [ABSTRACT FROM AUTHOR]

The transformation and turnover time of medium- to long-chain dicarboxylic acids (DCA) in soil is regulated by microbial uptake and mineralization. However, the chain length of n-alkyl lipids may have a remarkable influence on its microbial utilization and mineralization and therefore on the formation of stable soil organic carbon from e.g. leave- needle- and root-derived organic matter during decomposition. To investigate their size dependent mineralization and microbial incorporation, four DCA of different chain lengths (12–30 carbon atoms), that were 13C labeled at each of their terminal carboxylic groups, were applied to the Ah horizon of a Fluvic Gleysol. Incorporation of 13C into CO2 and in distinct microbial groups classified by phospholipid fatty acid (PLFA) analysis was investigated. Mineralization of DCA and incorporation into PLFA decreased with increasing chain length, and the mineralization rate was highest during the first days of incubation. Half-life time of DCA carbon in soil increased from 7.6 days for C12 DCA to 86.6 days for C18 DCA and decreased again to 46.2 days for C22 DCA, whereas C30 DCA had the longest half-life time. Rapid and efficient uptake of C12 DCA as an intact molecule was observable. Gram-negative bacteria incorporated higher amounts of DCA-derived 13C compared to other microbial groups, especially compared to actinomycetes and fungi during the first phase of incubation. However, the incorporation of C12 DCA derived 13C into the PLFA of actinomycetes, and fungi increased steadily during the entire incubation time, suggesting that those groups take up the 13C label from necromass of bacteria that used the C12 DCA for formation of their lipids before. [ABSTRACT FROM AUTHOR]

Consequences of interactions between ectomycorrhizal fungi (EcMF) and non‐mycorrhizal rhizosphere fungi (NMRF) for plant carbon (C) allocation belowground and nutrient cycling in soil remain unknown. To address this topic, we performed a mesocosm study with Norway spruce seedlings [Picea abies (L.) H. Karst] inoculated with EcMF, NMRF, or a mixture of both (MIX). 14CO2 pulse labelling of spruce was applied to trace and visualize the 14C incorporation into roots, rhizohyphosphere and hyphosphere. Activities and localization of enzymes involved in the C, nitrogen (N) and phosphorus (P) cycling were visualized using zymography. Spruce seedlings inoculated with EcMF and NMRF allocated more C to soils (EcMF: 10.7%; NMRF: 3.5% of total recovered C) compared to uninoculated control seedlings. The 14C activity in the hyphosphere was highest for EcMF and lowest for NMRF. In the presence of both, NMRF and EcMF (MIX), the 14C activity was 64% lower compared with EcMF inoculation alone. This suggests a suppressed C allocation via EcMF likely due to the competition between EcMF and NMRF for N and P. Furthermore, we observed 57% and 49% higher chitinase and leucine‐aminopeptidase activities in the rhizohyphosphere of EcMF compared to the uninoculated control, respectively. In contrast, β‐glucosidase activity (14.3 nmol cm−2 h−1) was highest in NMRF likely because NMRF consumed rhizodeposits efficiently. This was further supported by that enzyme stoichiometry in soil with EcMF shifted to a higher investment of nutrient acquisition enzymes (e.g., chitinase, leucine‐aminopeptidase, acid phosphatase) compared to NMRF inoculation, where investment in β‐glucosidase increased. In conclusion, the alleviation of EcMF from C limitation promotes higher activities of enzymes involved in the N and P cycle to cover the nutrient demand of EcMF and host seedlings. In contrast, C limitation of NMRF probably led to a shift in investment towards higher activities of enzymes involved in the C cycle. [ABSTRACT FROM AUTHOR]

Scope: Biochemistry is an essential yet undervalued aspect of soil ecology, especially when analyzing soil C cycling. We assume, based on tradition, intuition or hope, that the complexity of biochemistry is confined to the microscopic world, and can be ignored when dealing with whole soil systems. This opinion paper draws attention to patterns caused by basic biochemical processes that permeate the world of ecosystem processes. From these patterns, we can estimate activities of the biochemical reactions of the central C metabolic network and gain insights into the ecophysiology of microbial biosynthesis, growth, and maintenance energy requirements: important components of Carbon Use Efficiency (CUE). Observations: We show that glucose is processed via the Embden-Meyerhof-Parnas glycolysis in one soil, but via Pentose Phosphate or Entner-Doudoroff pathways in two other soils. However, notwithstanding this metabolic diversity, glucose use efficiency is high and thus substrate use for maintenance energy and overflow respiration is low in these soils. These results contradict current dogma, based on four decades of debate in soil ecology, that the maintenance energy demand in soil communities is a quantitative important, although variable, component of soil community energy metabolism. Conclusions: We identify three main shortcomings in our current understanding of substrate use efficiency: 1) in numeric and conceptual models, we lack appreciation of the strategies that microbes employ to quickly reduce energy needs in response to starvation; 2) in order to understand variation in CUE, we need to improve our understanding of the processes of exudation (including all changes in allocation of C from the cell soluble to insoluble fraction and extracellular environment) and microbial turnover; and 3) whether tracer experiments can be used to measure the long-term substrate use efficiency of soil microbial communities depends critically on the ability and speed with which non-growing cells take up tracer substrates and metabolism activates and subsequently de-activates in response to starvation, as well as on how cellular activities scale to the community level. To move the field of soil ecology forward, future research must consider the details of microbial ecophysiology and develop new tools that enable direct measurement of microbial functioning in intact soils. We submit that 13C metabolic flux analysis is one of those new tools. [ABSTRACT FROM AUTHOR]

During an archaeological excavation of a Slavic settlement (10th/11th C. A.D.) in Brünkendorf (Wendland region in Northern Germany), a thick black soil (Nordic Dark Earth) was discovered that resembled the famous terra preta phenomenon. For the humid tropics, terra preta could act as model for sustainable agricultural practices and for long-term CO 2 -sequestration into terrestrial ecosystems. The question was whether this Nordic Dark Earth had similar properties and genesis as the famous Amazonian Dark Earth in order to find a model for sustainable agricultural practices and long term CO 2 -sequestration in temperate zones. For this purpose, a multi-analytical approach was used to characterise the sandy-textured Nordic Dark Earth in comparison to less anthropogenically influenced soils in the adjacent area in respect of ecological conditions (pH, electric conductivity, cation exchange capacity, amino sugar) and input materials. Total element contents (C, N, P, Ca, Mg, K, Na, Fe, Cu, K, Zn, Mn and Ba) were highly enriched in the Nordic Dark Earth compared to the reference soil. Faecal biomarkers such as stanols and bile acids indicated animal manure from omnivores and herbivores but also human excrements. Amino sugar analyses showed that Nordic Dark Earth contained higher amounts of microbial residues being dominated by soil fungi. Black carbon content of about 30 Mg ha − 1 in the Nordic Dark Earth was about four times higher compared to the adjacent soil and in the same order of magnitude compared to terra preta . The input materials and resulting soil chemical characteristics of the Nordic Dark Earth were comparable to those of Amazonian Dark Earth suggesting that their genesis was also comparable. Amazonian Dark Earth and Nordic Dark Earth were created by surface deposition and/or shallow soil incorporation of waste materials including human and animal excrements together with charred organic matter. Over time, soil organisms degraded and metabolized these materials leaving behind deep black stable soil organic matter. The existence of the Nordic Dark Earth in the temperature zone of Europe demonstrates the capability of sandy-textured soils to maintain high soil organic matter contents and nutrient retention over hundreds of years. Deeper insights are needed urgently to understand soil organic matter stabilization mechanisms in this sandy soil to promote conceptual models for sustainable land use and long-term C sequestration. It is argued that the knowledge of Nordic Dark Earth probably was an important part of the Viking–Slavic subsistence agriculture system, which could have had a great impact on the development of the Viking age emporia in the 9th/10th C. A.D. [ABSTRACT FROM AUTHOR]

Enzymes produced by microorganisms and plants are very sensitive to variations in soil microclimate, yet most enzyme assays are conducted under oxic conditions irrespective of the origin of environmental samples. It remains unclear how short-term aeration (minutes to hours) affects the hydrolytic and oxidative enzymes in anoxic systems. This key gap in current methods was addressed by measuring the kinetics of hydrolytic phosphomonoesterase, β-glucosidase, and leucine aminopeptidase and the activities of oxidative phenol oxidases and peroxidases by fluorogenic substrates under oxic (+O 2) and anoxic conditions (–O 2). Aeration effects were tested in a flooded paddy soil with growing rice (research task 1: moderate O 2 limitation) and without rice (research task 2: strong O 2 limitation). We tested two hypotheses explaining possible effects of short-term aeration on hydrolytic versus oxidative enzymes. (1) Aeration promotes Fe(II) oxidation, which leads to the accumulation of phenolics through the "iron-gate" mechanism, thus suppressing the activities of hydrolytic enzymes compared to the anoxic conditions. (2) Aeration stimulates phenol oxidases that degrade phenolics according to the "enzyme latch" concept, thus eliminating the suppression of hydrolytic enzymes. The activities of hydrolytic enzymes were lower by 5–43% in both experiments under + O 2 compared to –O 2. In contrast, the activities of peroxidases and phenol oxidases were 2–14 times higher under + O 2 than under –O 2. Thus, the activation of oxidative enzymes under + O 2 was uncoupled from the hydrolytic activities. This contradicts both the "iron gate" and the "enzyme latch" mechanisms. We explain the short-term suppressive effect of O 2 in assays by increased concentrations of reactive oxygen species, which decreased microbial activity. We conclude that our modification of enzyme assays under anoxic conditions is required for samples taken from low-oxygen environments to avoid underestimation due to rapid suppression of hydrolytic enzyme activities by O 2. [Display omitted] • O 2 suppressed hydrolytic enzyme activities (V max) up to 43% during 2.5 h of assays. • Aeration increased the activities of phenol oxidases by 14 and peroxidases by 2 times. • Short-term suppressive effect of O 2 contradicted both "enzyme latch" and "iron gate". • Enzyme activities in low-O 2 ecosystems should be assayed under anoxic conditions. [ABSTRACT FROM AUTHOR]

Biogeochemical cycles of phosphorus (P) and iron (Fe) are tightly interlinked, especially in highly weathered acidic subtropical and tropical soils rich in iron (oxyhydr)oxides (Fe(III)). The quantitative contribution of the reductive dissolution of Fe(III)-bound inorganic P (P i) (Fe–P) in low-redox paddy soils may cover the demands of rice plants (Oryza sativa L.) and microorganisms. We hypothesized that microbially-driven Fe(III) reduction and dissolution can cover the P demand of microorganisms but not of the young rice plants when the plants' P demand is high but their root systems are not sufficiently developed. We grew pre-germinated rice plants for 33 days in flooded rhizoboxes filled with a paddy soil of low P availability. 32P-labeled ferrihydrite (30.8 mg kg−1) was supplied either (1) in polyamide mesh bags (30 μm mesh size) to prevent roots from directly mobilizing Fe–P (pellets-in-mesh bag treatment), or (2) in the same pellet form but without a mesh bag to enable roots accessing the Fe–P (pellets-no-mesh bag treatment). Without mesh bags, P i was more available leading to increases in microbial biomass carbon (MBC) by 18–55% and nitrogen (MBN) by 4–108% in rooted soil as compared to P i pellets not directly available to roots. The maximum enzyme activities (V max) of phosphomonoesterase and β-glucosidase followed this pattern. During rice root growth, day 10 to day 33, MBC and microbial biomass phosphorus (MBP) contents in both rooted and bottom bulk (15–18 cm) soil gradually decreased by 28–56% and 47–49%, respectively. In contrast to our hypothesis, the contribution of Fe–P to MBP remarkably decreased from 4.5% to almost zero from 10 to 33 days after rice transplantation, while Fe–P compensated up to 16% of the plant P uptake at 33 days after rice transplantation, thus outcompeting microorganisms. Although both plants and microorganisms obtained P i released by Fe(III) reductive dissolution, this mechanism was not sufficient for the demand of either organism groups. [Display omitted] • 32P application can assess plant-microbial competitive dynamics for P absorbed by Fe. • Contribution of Fe–P to MBP decreased from 4.5% to almost zero with rice growth. • Fe–P compensated up to 16% of rice P uptake during 33 days after rice transplantation. • Rice plants outcompeted microorganisms for P at the tillering stage. [ABSTRACT FROM AUTHOR]

Comprehensive climate change mitigation necessitates soil carbon (C) storage in cultivated terrestrial ecosystems. Deep-rooted perennial crops may help to turn agricultural soils into efficient C sinks, especially in deeper soil layers. Here, we compared C allocation and potential stabilization to 150 cm depth from two functionally distinct deep-rooted perennials, i.e., lucerne (Medicago sativa L.) and intermediate wheatgrass (kernza; Thinopyrum intermedium), representing legume and non-legume crops, respectively. Belowground C input and stabilization was decoupled from nitrogen (N) fertilizer rate in kernza (100 and 200 kg mineral N ha−1), with no direct link between increasing mineral N fertilization, rhizodeposited C, and microbial C stabilization. Further, both crops displayed a high ability to bring C to deeper soil layers and remarkably, the N2-fixing lucerne showed greater potential to induce microbial C stabilization than the non-legume kernza. Lucerne stimulated greater microbial biomass and abundance of N cycling genes in rhizosphere soil, likely linked to greater amino acid rhizodeposition, hence underlining the importance of coupled C and N for microbial C stabilization efficiency. Inclusion of legumes in perennial cropping systems is not only key for improved productivity at low fertilizer N inputs, but also appears critical for enhancing soil C stabilization, in particular in N limited deep subsoils. [ABSTRACT FROM AUTHOR]

Input of organic carbon (C) to the soil stimulates soil microbial activity leading to changes in turnover of soil organic matter, a phenomenon referred to as priming effect (PE). However, contribution of various management forms in tropical drylands, the role of land use conversion and the soil depth at which such management induces altered PEs remain largely unclear. In this study, we quantified respiration and PE in semi-arid soils of northern Ethiopia. Soils from three depths (0–30, 30–60 and 60–90 cm) in forest, croplands, exclosure and grazing land use types were sampled. The soil samples were incubated for 23 days and PE and respiration quantified after addition of 14C labeled glucose corresponding to 50% of initial microbial biomass carbon (MBC). Generally, CO2 respired was 30–63% lower in sub than in topsoil with most expressed depth gradients in croplands. The weak negative PEs in subsoil is an indication of highly stabilized C. Contrary, glucose addition induced stronger positive PEs in topsoils collected from forest, exclosure and grazing land. The temporal dynamics of PEs involved a strong positive peak for the first five days after glucose addition and a second smaller peak 10 days after glucose addition in natural ecosystem, corresponding to apparent and real PE, respectively. Lack of positive correlation between PEs and C/N ratio ruled out the N-mining hypothesis, but a positive correlation between PE and MBC suggests co-metabolism as possible mechanism behind the real PE. Higher priming in natural ecosystem compared to cropland is an indication that conversion of natural ecosystem to continuous cropping system leads to depletion of the "primable" C pool in dryland soils. Additionally, this land use conversion negatively affects biogeochemical C cycling by an altered response of soil microbes to C input. [ABSTRACT FROM AUTHOR]

[Display omitted] • C cycling from microbial biomass as driven by microbial catabolism and anabolism. • Lower CO 2 efflux with high-quality litter than with low-quality litter. • Higher necromass formation of high-quality litter than low-quality litter. • Litter in low-quality is readily respired, in high-quality is mainly formed necromass. • The soil living microbes, remaining litter and q CO 2 regulate the C cycling. Litter input regulates the mineralization and accrual of soil organic carbon (SOC), depending on microbial groups and litter quality. Stable isotope probing was used after the addition of 13C-labeled litter of different recalcitrance, including Populus davidiana (characterized by low quality, high C:N, and lignin content) and Quercus wutaishanica (characterized by high quality, low C:N, and lignin content). The flow of litter C to microbial biomass and necromass was followed by quantification of 13C incorporation into phospholipid fatty acids (13C-PLFAs) and amino sugars (13C-AS), and their respiration. The CO 2 efflux and positive priming effects (PE) were higher, but the 13C-necromass was lower (8.5% vs. 10%) in the soil with P. davidiana than with Q. wutaishanica. The 13C-necromass continuously increased with P. davidiana litter decomposition, but reached a peak at day 14 after Q. wutaishanica litter addition. These results indicated that the decomposition of resistance compounds mainly resulted in necromass formation after low-quality litter addition. The P. davidiana- derived 13C in fungal PLFAs increased with decomposition, suggesting the importance of these microorganisms in decomposing resistance compounds in low-quality litter despite the bacterial-dominated decomposition process of both litters. Bacteria frequently exhibit versatility in utilizing C from different sources, whereas a larger proportion of fungi display specialization. SOC mineralization increased with the remaining litter but decreased with 13C-G- bacterial PLFAs and 13C-actinobacterial PLFAs after both types of litter addition because starving microbes need to invest more soil C in maintenance respiration vs. biomass formation. 13C-necromass increased with 13C-PLFA because microbial biomass is the primary precursor of necromass. Altogether, low-quality compounds are more readily respired, whereas C from high-quality biopolymers is relatively more assimilated into the necromass. This study further explains the effect of soil microbial groups on SOC under different qualities of litter decomposition and deepens our understanding of the mechanism of SOC sequestration. [ABSTRACT FROM AUTHOR]

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochemistry,https://doi.org/10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENIP) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-Porg precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition. [ABSTRACT FROM AUTHOR]

Abstract: We propose a novel pragmatic approach of in situ 15N and 13C isotope labelling of trees for subsequent litter decomposition and turnover studies under field conditions. Using this method the labelling of even large trees under natural conditions is possible and compared to tree labelling under artificial conditions in greenhouses the in situ approach is less expensive. 13C and 15N labelling were carried out simultaneously via photosynthesis by tree gassing with 13CO2 and by stem injection of 15NH4 15NO3. The aims of this study were: (i) to produce a sufficient quantity of labelled plant material for subsequent field incubation studies and (ii) to investigate the effectiveness and distribution of in situ 15N (15NH4 15NO3) and 13C (13CO2) labelling of Podocarpus falcatus, Croton macrostachys, Prunus africana and Cupressus lusitanica. The following targets need to be achieved: (i) Assuming almost natural litter fall conditions, enough labelled plant material must be produced in situ for the turnover experiment; (ii) intra-plant tracer enrichment shall be homogeneous; (iii) tracer enrichment should be comparable for different tree species; and (iv) tracer enrichment must be sufficient for subsequent litter turnover studies using the stable isotope approach. Our results clearly demonstrated that several kilograms of labelled plant material can be produced in situ. For many ecosystems, this amount is sufficient for a long term litter turnover experiment on a field scale under almost natural litter fall conditions. However, intra-plant label uptake of 13C and 15N was heterogeneous so that only leaves (litter) should be used for the turnover study. It could be shown that only a part of the labile C and N fraction in the leaves was labelled. Nevertheless, label uptake was sufficient for subsequent litter turnover studies. [Copyright &y& Elsevier]

positive plant diversity to plant aboveground productivity relation has been shown to alter carbon and nitrogen fluxes in soils. Thus, most investigations focussed on the C- and N-input via litter fall, widely neglecting the importance of root exudation. As microbes, which are known to be important drivers of matter fluxes in soil, feed on these root exudates, increased knowledge on the availability of these compounds in soil might help to understand biodiversity effects on soil. We therefore investigated the effect of plant diversity on size and composition of the free soil amino acid pool in a grassland experiment, as amino acids are an important C- as well as N-source for microbes. Despite a positive diversity effect on plant productivity, we only found an insignificant increase of the size of the free amino acid pool in soil. This was most likely caused by an increase of the microbial population and thus an increased amino acid mineralization. At the same time the composition of the amino acid pool changed significantly between plant diversity levels. This most likely reflects differences in plant input as well as differences in microbial mineralization and enabled us to separate diversity levels by means of discriminant analysis. [ABSTRACT FROM AUTHOR]

The area under oil palm cultivation has expanded importantly in South-East Asia. The resulting land-use transformations are associated with environmental degradation such as substantial carbon dioxide emissions. Soil conservation practices can contribute to more sustainable oil palm systems by increasing soil organic carbon (SOC) contents and yields. One soil conservation practice in oil palm plantations is the mulching with empty fruit bunches (EFB). We estimated the relationship between EFB mulching and, respectively, yields, SOC and total nitrogen contents, the carbon‑nitrogen ratio and the average bulk density in smallholder oil palm plantations. We further analyzed the determinants of smallholders' EFB application taking into account regional market, farm, and plot level aspects, as well as farmers' perceptions. We estimated the relationship between mulching and yields with panel data, and between mulching and soil properties with cross-sectional data. We accounted for the possible selection bias of the decision to apply EFB mulching with instrumental variable estimations. Descriptive analysis was used to analyze the determinants of smallholder EFB use. Our results indicated that in particular the regional supply in EFB and farmers' wealth levels were correlated with smallholder EFB mulching. EFB mulching was associated with 39% higher yields and 19% higher SOC contents. We attribute the higher yields to potentially improved soil chemical functions linked to higher SOC contents, such cation exchange capacity effects, and to an increased K and potentially Mg availability. Since EFB is not available in sufficient quantity to be applied to all plantations, policy makers could identify user groups with the greatest potentials for environmental and income improvements. Site-specific life cycle assessments and yield effects can help to inform such identification. Despite their growing importance in the oil palm sector, evidence on the effect of EFB mulching was missing for smallholder farmers. Because of associated higher SOC contents and yields, our findings indicate that the promotion of EFB mulching as best management practice could potentially contribute to more sustainable oil palm systems. Strengthening inclusive value chains for smallholders, information provision and investments in road infrastructure represent potential policy instruments to increase smallholder EFB application. The identification and promotion of best management practices are important with view to the still ongoing oil palm expansion in South-East Asia, and progressively in other parts of the world. [Display omitted] • Empty fruit bunch mulching could be a means to increase the sustainability of oil palm systems, which are expanding globally. • We explored whether empty fruit bunch mulching is associated with improved yields and soil properties in oil palm smallholdings. • Mulching was associated with 39% higher yields and 19% higher soil organic carbon content, but not with improved other soil properties. • Smallholders' links to palm oil mills correlated with mulching so that strengthening inclusive value chains could support wider adoption. • Promotion of empty fruit bunch mulching can improve smallholder oil palm systems, but supply constraints need to be taken into account. [ABSTRACT FROM AUTHOR]

Severe droughts endanger ecosystem functioning worldwide. We investigated how drought affects carbon and water fluxes as well as soil-plant-atmosphere interactions by tracing 13CO2 and deep water 2H2O label pulses and volatile organic compounds (VOCs) in an enclosed experimental rainforest. Ecosystem dynamics were driven by different plant functional group responses to drought. Drought-sensitive canopy trees dominated total fluxes but also exhibited the strongest response to topsoil drying. Although all canopy-forming trees had access to deep water, these reserves were spared until late in the drought. Belowground carbon transport was slowed, yet allocation of fresh carbon to VOCs remained high. Atmospheric VOC composition reflected increasing stress responses and dynamic soil-plant-atmosphere interactions, potentially affecting atmospheric chemistry and climate feedbacks. These interactions and distinct functional group strategies thus modulate drought impacts and ecosystem susceptibility to climate change. [ABSTRACT FROM AUTHOR]

The phospholipid fatty acid (PLFA) composition of soils was analysed at three poplar-based silvo-arable systems and at one willow-based silvo-grassland alley agroforestry system in Central Germany. The objective was to analyse tree row effects on the PLFA composition of main fungal and main bacterial groups. The fungal groups were BAM (Basidiomycota + Ascomycota + Mucoromycota) and AMF (Arbuscular Mycorrhizal Fungi). The bacterial groups were Gram-negative, Firmicutes, and Actinobacteria. The total PLFA content varied between 53 and 170 nmol g−1 soil. Total PLFA and microbial biomass carbon (MBC) showed a strong linear relationship, which resulted in a mean MBC/total PLFA ratio of 4.2 μg nmol−1. AMF contributed on average 4 mol% and the fungal BAM group 10 % to total PLFA. Gram-negative bacteria contributed on average 37 mol%, Firmicutes 23 mol%, and Actinobacteria 6 mol% to total PLFA. The presence of poplar or willow trees increased the mean total PLFA content in comparison with the alleyways by 30 %. Especially the mean contribution of fungal PLFA to total PLFA showed a significant +7.0 mol% increase in the tree row compared with the alleyways, exclusively caused by the BAM group (+7.6 mol%), whereas the contribution of the AMF PLFA linearly decreased from the middle of the alleyway to the tree row at all sites. Within the alleyways, the Gram-negative/Firmicutes PLFA ratio showed a significant decline from the 1 m up to the 24 m distance samples at sites Dornburg and Forst. Despite a decrease of AMF in tree rows, agroforestry tree rows led to a rapid increase in fungi, most likely due to the promotion of ecto-mycorrhizal fungi. • Tree rows shifted the PLFA composition to more fungal biomarkers. • Bacterial PLFA diversity was higher in the arable alleyways than in the tree rows. • PLFA composition contrasts bacterial gene abundance, especially for Actinobacteria. • Agroforestry generally supports the C sequestration potential of mycorrhizal fungi. [ABSTRACT FROM AUTHOR]

Multipurpose cropping is promoted to provide biomass to meet the growing demand for biorefining, but excessive biomass removal may decrease soil C stocks and ecosystem multifunctionality. Here, soil C stocks across 0–200 cm depth and soil ecosystem multifunctionality at 0–40 cm were evaluated based on a 19–year field experiment, including three multipurpose cropping systems as continuous cotton (CC), ryegrass–sweet sorghum double cropping (RS) and perennial switchgrass (SG). Compared to CC, RS and SG increased the soil C stocks by 19% and 28%, and N stocks by 19% and 23% at 0–200 cm, respectively. The soil C stocks were positively correlated with belowground C input and negatively associated with C mineralization. RS and SG increased soil ecosystem multifunctionality by 16.8 and 15.7 folds at 0–20 cm and 0.34 and 0.61 folds at 20–40 cm relative to CC, respectively. The benefit of soil C stocks and ecosystem multifunctionality was more pronounced in SG than in RS due to the deeper root system. In conclusion, relative to continuous monoculture, rotational and perennial cropping can enhance soil C sequestration, maintain soil function, and provide feedstock for bioenergy. [Display omitted] • Rotational and perennial cropping increased soil carbon (C) stocks at 0–2 m depth. • Rotational and perennial cropping boost soil ecosystem multifunctionality (SEMF). • Higher C stocks under perennials are due to higher C input and lower mineralization. • Perennial overperforms rotational cropping in enhancing soil C stocks and SEMF. [ABSTRACT FROM AUTHOR]

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

In the original version of this article, equations 4 and 9 unfortunately contained errors [ABSTRACT FROM AUTHOR]

• Warming facilitates the consumption of labile organics because of the faster microbial growth. • Microbial respiration and enzyme activities acclimates to 8-year warming. • The increased labile organics weakens the memory effect of soil microbial functions. • Microbial memory to warming is strongly dependent on microbial physiological state. The feedback between soil carbon (C) and climate change has the potential to decrease over time, which is mainly due to the thermal acclimation of microbial decomposition of soil organic matter (SOM). The adaption and stabilization mechanisms of microbial functions in response to long-term warming in agroecosystems, however, remain intensive debate. Therefore, we explored the controls of thermal acclimation based on an 8-year field warming (ambient, +1.6 °C, +3.2 °C) in agroecosystem and a short-term incubation at five constant temperature levels (from 5 to 25 °C with 5 °C intervals) under microbial steady-state (without glucose, non-activated microbial community) and activate microbial growth induced by sufficient glucose addition. At the steady-state, field warming (+3.2 °C) increased the specific growth rate whilst decreased the portion of growing microbial biomass as compared to ambient soils. This indicated that 8-year field warming facilitated the consumption of labile organics due to faster microbial growth. Consequently, microbial growth, activities of β-glucosidase and leucine aminopeptidase, as well as microbial respiration acclimated to field warming (+1.6 °C, +3.2 °C) over 8-year, responded up to 40% weaker to further short-term temperature increase (above 15 °C). Also, the temperature sensitivity of enzyme activity decreased with field warming magnitude under higher incubation temperatures, which further indicated enzyme acclimation. Under activation mode, however, the higher enzyme activities suggested that the increased labile C remarkably relieved the substrate limitation of microbial growth and activated dormant microorganisms in field warmed soils, and weaken the thermal acclimation of soil microbial functions. We thus found experimental evidence that the transition between microbial physiological states (i.e., dormant vs. active) owing to variations in C availability is the most plausible explanation for the alterations in temperature sensitivity of enzyme activities and microbial respiration. The compensating effect of the increased labile substrate under climate warming on the thermal acclimation of soil microbe-driven CO 2 may thus be larger than that currently predicted, with important consequences for atmospheric CO 2 concentrations. [ABSTRACT FROM AUTHOR]