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Nitrogen availability in the rhizosphere relies on root‐microorganism interactions, where root exudates trigger soil organic matter (SOM) decomposition through the rhizosphere priming effect (RPE). Though microbial necromass contribute significantly to organically bound soil nitrogen (N), the role of RPEs in regulating necromass recycling and plant nitrogen acquisition has received limited attention. We used 15N natural abundance as a proxy for necromass‐N since necromass is enriched in 15N compared to other soil‐N forms. We combined studies using the same experimental design for continuous 13CO2 labelling of various plant species and the same soil type, but considering top‐ and subsoil. RPE were quantified as difference in SOM‐decomposition between planted and unplanted soils. Results showed higher plant N uptake as RPEs increased. The positive relationship between 15N‐enrichment of shoots and roots and RPEs indicated an enhanced necromass‐N turnover by RPE. Moreover, our data revealed that RPEs were saturated with increasing carbon (C) input via rhizodeposition in topsoil. In subsoil, RPEs increased linearly within a small range of C input indicating a strong effect of root‐released C on decomposition rates in deeper soil horizons. Overall, this study confirmed the functional importance of rhizosphere C input for plant N acquisition through enhanced necromass turnover by RPEs. Summary statement: Interactions between roots and microorganisms in the rhizosphere enhance nitrogen availability through the rhizosphere priming effect (RPE). Our study confirmed that increasing RPEs promote higher plant nitrogen uptake and indicate enhanced turnover of necromass‐nitrogen through rhizosphere priming. [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]

Arbuscular mycorrhizal fungi (AMF) influence plant nitrogen/phosphorus (N/P) by modifying plant N and P uptake, which further affects plant stoichiometric N/P homeostasis. Plant species and community stoichiometric N/P homeostasis can impact plant species and community stability, respectively, in response to variations of soil N and P availabilities. We investigated interspecific plant interactions via AMF in regard to plant and soil microbial N/P stoichiometry across different soil N and P availabilities induced by N and P addition (0 mg N kg−1, 25 mg N kg−1, 50 mg N kg−1, 30 mg P kg−1 and 100 mg P kg−1). We selected one dominant (Bothriochloa ischaemum; C4 grass) and one subordinate (Lespedeza davurica; legume) species in a natural grassland climax community. We examined how AMF influences stoichiometric N/P homeostasis in monoculture and mixed culture systems, and the resulting consequences for temporal stability of plant species and community in response to variations in soil N and P availability. The AMF mitigated the P limitation of soil microbial communities and decreased the degree of stoichiometric N/P homeostasis of host plants in monoculture. Through their resource‐scavenging and soil organic matter mineralisation functions, AMF enhances plant 'luxury consumption', promoting species stability in monoculture in response to soil N and P availability variations. Compared with plants in monoculture, the interaction between B. ischaemum and L. davurica via AMF increased shoot N/P under soil N‐poor conditions, leading to an enhanced degree of stoichiometric N/P homeostasis in both plant species, especially the legume. Our results suggest that interspecific plant interaction between C4 grass and legume mediated by AMF confers an advantage in complementarity in plant N acquisition under N‐poor conditions, leading to increased stability of plant communities and better maintenance of subordinate species (legume) in response to soil N deficiency. [ABSTRACT FROM AUTHOR]

Various studies showed a decrease of drought stress specific parameters of plants after silicon (Si) fertilization. But all studies differed in soil Si concentration between the control and Si treatments. As amorphous silica (ASi) was recently found to cause a strong increase of water holding capacity and plant available water in soils, a combined effect of soil moisture and plant response due to Si addition was assumed. In this study, the influence of the soil Si content was excluded by using the same Si enriched soil for treatments of two rice lines, lsi1 mutant defective in Si uptake and its wild‐type rice. Most plant parameters, such as nutrient contents, biomass, specific leaf area, specific root length, leaf water content and C allocation did not differ significantly between the genotypes neither under flooded conditions, nor under drought conditions. Only photosynthesis and stomatal conductance were slightly higher for the wild type in both drought and flooded treatments. Overall, our data showed that Si accumulation within the plant tissues has only a minor effect on plant performance under drought stress. Hence, existing studies should be reinterpreted in light of the fact that Si additions may increase soil water availability. Our data shows that the effect of Si‐fertilization on mitigating drought stress for plants may be less via effects in planta but rather by Si‐fertilization increasing soil water availability to plants. [ABSTRACT FROM AUTHOR]

Aims: There is a trend of increasing woody biomass in tropical savannas. Here we ask what effect this increase may have on soil carbon pools and fluxes. Methods: Using a field experiment we determine the amount of soil carbon directly under grasses, a juvenile tree among grasses and a juvenile tree with no grasses. We also measure CO2 efflux at the soil surface and use gas wells to extract CO2 from several soil depths. Results: Our results show that grasses contribute substantially more than trees to both soil carbon and soil respiration. Grasses also make a disproportionate contribution to the δ13C value of SOC in the tree only treatments. The surface gas efflux data show that soil respiration increased with an increase in volumetric soil moisture and temperature and plots with both grasses and trees had higher respiration rates than plots with trees only or with grasses only. Conclusions: The highest soil respiration is in the top 20 cm of the soil with grasses the primary contributors to both δ13CSOC and δ13CR. Any increase in woody biomass will result in a decline in SOM turnover and nitrogen mineralization rates resulting in higher SOC pools. The associated increases in SOC and above ground biomass will however be associated with negative economic and biodiversity impacts. [ABSTRACT FROM AUTHOR]

Most subsoil carbon (C) turnover occurs in biopore hotspots such as root channels and earthworm burrows. Biopores allocate large C amounts into the subsoil, where a vast capacity for long-term C sequestration is predicted. We hypothesise that organic matter (OM) cycling in biopores depends on their origin. Earthworm and root biopores were induced under field conditions and were sampled from the subsoil (45–75 and 75–105 cm) after two years of biopore formation. The effects of biopore formation on OM decomposition were studied by biomarkers: neutral sugars, cutin and suberin-derived lipids, lignin-derived phenols and free lipids. The degradation stage of OM was biopore type-specific but was only governed by the soil depth in root biopores. Degradation of OM increased from earthworm biopores to root biopores and bulk soil. Hemicelluloses (GM/AX ratio) were more strongly degraded than lignin side-chains (relative change from initial values). Two years of microbial processing during biopore formation increased the GM/AX ratio in earthworm biopores from 0.65 to 1.05 and in root biopores from 0.15 to 1.35 (both relative to source biomasses). Root biopores and bulk soil had the highest GM/AX ratios (1.2–1.3), hinting to rapid processing of plant residues and accumulation of microbial residues. The regular, frequent OM inputs by earthworms stimulated microbial growth and processing of mostly bioavailable OM and, thus, relatively enriched more persistent OM (e.g. lignin). Syringyl subunits of lignin underwent low (ratio changed from 0.35 to 0.55 relative to initial input) and vanillyl subunits underwent almost no processing in earthworm biopores indicating the preferential microbial utilisation of the easily available compounds frequently replenished by earthworm activity. After two years of decomposition of the root detritus, mainly structural plant material was enriched in root biopores. Short periods (6 months) of earthworm activity effectively recharged the highly processed OM in root biopores with fresh OM. In total, deep-rooting catch crops and short-term earthworm activities promote C accumulation in the subsoil followed by biopore-specific microbial processing predominantly governed by the C input frequency. As root biopores are up to 40 times more common than earthworm biopores, they dominate the OM input into subsoils. Such C inputs create several years lasting hotspots for preferential root growth and nutrient mobilisation in the subsoil. We conclude that root- and earthworm-derived biopores are vertical pathways for plant C from the soil surface into the subsoil and for intensive processing of litter C and sequestration of microbial necromass. [ABSTRACT FROM AUTHOR]

This study assessed the carbon (C) budget and the C stocks in major compartments of the soil food web (bacteria, fungi, protists, nematodes, meso- and macrofauna) in an arable field with/without litter addition. The C stocks in the food web were more than three times higher in topsoil (0–10 cm) compared to subsoil (>40 cm). Microorganisms contained over 95% of food web C, with similar contributions of bacteria and fungi in topsoil. Litter addition did not alter C pools of soil biota after one growing season, except for the increase of fungi and fungal feeding nematodes in the topsoil. However, the C budget for functional groups changed with depth, particularly in the microfauna. This suggests food web resilience to litter amendment in terms of C pool sizes after one growing season. In contrast, the distinct depth dependent pattern indicates specific metacommunities, likely shaped by dominant abiotic and biotic habitat properties. [ABSTRACT FROM AUTHOR]

Despite its fundamental role for carbon (C) and nutrient cycling, rhizodeposition remains 'the hidden half of the hidden half': it is highly dynamic and rhizodeposits are rapidly incorporated into microorganisms, soil organic matter, and decomposed to CO2. Therefore, rhizodeposition is rarely quantified and remains the most uncertain part of the soil C cycle and of C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands (281 data sets). The allocation dynamics of assimilated C (after 13C- CO2 or 14C- CO2 labeling of plants) were quantified within shoots, shoot respiration, roots, net rhizodeposition (i.e., C remaining in soil for longer periods), root-derived CO2, and microorganisms. Partitioning of C pools and fluxes were used to extrapolate belowground C inputs via rhizodeposition to ecosystem level. Allocation from shoots to roots reaches a maximum within the first day after C assimilation. Annual crops retained more C (45% of assimilated 13C or 14C) in shoots than grasses (34%), mainly perennials, and allocated 1.5 times less C belowground. For crops, belowground C allocation was maximal during the first 1-2 months of growth and decreased very fast thereafter. For grasses, it peaked after 2-4 months and remained very high within the second year causing much longer allocation periods. Despite higher belowground C allocation by grasses (33%) than crops (21%), its distribution between various belowground pools remains very similar. Hence, the total C allocated belowground depends on the plant species, but its further fate is species independent. This review demonstrates that C partitioning can be used in various approaches, e.g., root sampling, CO2 flux measurements, to assess rhizodeposits' pools and fluxes at pot, plot, field and ecosystem scale and so, to close the most uncertain gap of the terrestrial C cycle. [ABSTRACT FROM AUTHOR]

Biopores are pores or voids in soil produced by roots, by earthworms, or by the occupation of earthworms in root pores, which are considered important microbial hotspots, especially in subsoil. We hypothesized that earthworms ( Lumbricus terrestris L.) exert stronger effects on microbial activities than decaying plant roots (of Cichorium intybus L.) in the subsoil because of the addition of pre-digested organic material. We tested this hypothesis by analyzing microbial biomass (C), total organic C (C), and activities of eight enzymes (cellobiohydrolase, β-glucosidase, xylanase, acid phosphomonoesterase, leucine aminopeptidase, tyrosine aminopeptidase, chitotriosidase, and n-acetylglucosaminidase) down to 105-cm depth. The C increase was associated with a two- to threefold increase of C content in biopores as compared to bulk soil. The highest percentage of C-to-C (3.7 to 7.3 %) in the drilosphere demonstrated the enhancement of microbial efficiency for organic matter decomposition by earthworms. The availability of organic matter in biopores increased the activities of C- and N-targeting enzymes by 1.2-11.3 times, but reduced acid phosphomonoesterase activity by 10-40 % in biopores versus bulk soil. Introducing earthworms in root biopores caused 1.5-1.8 times higher microbial biomass and 1.2-1.9 times increased enzyme activities compared to the sole effect of earthworms. Soil depth showed a strong effect on the drilosphere, but only slight effects on the biochemical properties of root biopores and bulk soil. In conclusion, biopores are important microbial hotspots of C, N, and P transformations in subsoil. Earthworms exerted stronger effects on biochemical properties of biopores than decaying roots. [ABSTRACT FROM AUTHOR]

The influence of plant roots and the associated rhizosphere activities on decomposition of soil organic matter (SOM), the rhizosphere priming effect, has emerged as a crucial mechanism regulating global carbon (C) and nitrogen (N) cycles. However, the role of root morphology in controlling the rhizosphere priming effect remains largely unknown. To investigate the link between root hairs, a critical part of the entire root morphology, and the rhizosphere priming effect, we grew a barley wild type and a barley mutant without root hairs in a greenhouse and continuously labeled them with 13 C-depleted CO 2 . Soil CO 2 efflux was measured during tillering and head-emergence stages of plant growth. Based on its δ 13 C signature, total CO 2 was partitioned for root-derived and SOM-derived CO 2, and the SOM decomposition primed in the rhizosphere was calculated. Soil microbial biomass C and N, and the activities of six extracellular enzymes (β-cellobiohydrolase, β-glucosidase, acid phosphatase, β-xylosidase, leucin-aminopeptidase, and N-acetyl-β-glucosaminidase) were measured to test the effects of root hairs. During the early stage of development (tillering), when plants were sufficiently supplied with nutrients, the barley mutant without root hairs produced more shoot biomass. In contrast, high C costs for root-hair formation likely reduced the growth of the barley wild type. At this stage, the wild type with regular root hairs produced a positive rhizosphere priming effect (69% increase), but the mutant without root hairs produced a negative priming effect on SOM decomposition (28% decline). At the head-emergence stage, when nutrients were scarce, plant biomass production of the mutant was reduced, probably due to inefficient nutrient uptake in the absence of root hairs. At this stage, both barley types produced positive rhizosphere priming effects (72% and 209% increase for the wild type and the mutant, respectively) and the microbial biomass was higher for both planted soils compared to the tillering stage. Extracellular enzymes responsible for the decomposition of stable SOM had higher activities in cases of positive priming effects. Overall, root hairs played an important role in regulating rhizosphere priming. [ABSTRACT FROM AUTHOR]

Decomposition of soil organic matter (SOM) protected within aggregates can be accelerated via priming effect (PE) by the addition of fresh substrates. However, the knowledge of the sources of mineralization and PE in aggregate size classes is absent. We applied the three-source-partitioning isotopic ( 14 C + δ 13 C) approach to determine how aggregate size classes affect the contribution of three C sources (substrate added, recent and old SOM) to CO 2 efflux and PE depending on the amount of added primer. Soil from a field with 3 years of maize cropping (C 4 plants) after long-term C 3 vegetation was used to differentiate between recent C (C 4 C; < 3 years) and old C (C 3 C; >3 years). Soil samples were separated into three aggregate size classes (>2 mm, 2–0.25 mm macroaggregates and <0.25 mm microaggregates) and were incubated for 49 days after being amended with two levels of 14 C labeled glucose. The proportion of glucose mineralized to CO 2 increased with decreasing aggregate size, but 14 C incorporation into microbial biomass decreased, indicating higher C use efficiency in macroaggregates compared with microaggregates. The short-time PE was positive and was accompanied by a rapid reduction of dissolved organic C. After 49 days, the PE was higher in macro-versus microaggregates at both glucose levels. Positive PE induced by a low glucose level was observed only in large macroaggregates (>2 mm), but was observed in both macroaggregates (>0.25 mm) and microaggregates (<0.25 mm) after high glucose amendment. These results indicate that SOM pools are more decomposable in macro-versus microaggregates and that the SOM pools are involved in PE according to their biochemical availability. More primed CO 2 originated from recent C 4 C than old C 3 C in larger macroaggregates under a low glucose level. The relative contribution of recent C 4 C to primed CO 2 increased from macroaggregates (37.8%) to microaggregates (100%) after high glucose amendment. Therefore, increasing glucose addition stimulated the decomposition of old C 3 C in macroaggregates, but not in microaggregates. This indicates that microaggregates protect SOM against decomposition better than macroaggregates, and consequently, microaggregates can be considered as a potential reservoir for long-term C sequestration. Concluding, aggregate size is crucial for SOM decomposition, and it determines the source of PE and thus the protection of sequestrated C. The effects of the added primer on C sources involved in PE depend on the aggregate size. [ABSTRACT FROM AUTHOR]

The complexity of soil food webs and the cryptic habitat hamper the analyses of pools, fluxes and turnover rates of carbon (C) in organisms and the insight into their interactions. Stable isotope analysis has been increasingly used to disentangle soil food web structure, yet it has not been applied to quantitatively characterize C dynamics at the level of the entire soil food web., The present study employed 13CO2 pulse labelling to investigate the incorporation of maize root-derived C into major soil compartments and food web players in an arable field for 25 days. Bulk tissue and compound-specific (lipids) C isotope ratios were used to quantify pool sizes and 13C incorporation in bacteria and fungi as primary decomposers, nematodes as key drivers of the microfood web and decomposers and predators among the meso- and macrofauna., About 20% of the C assimilated by maize was transferred to below-ground pools. 13C was predominantly incorporated into rhizosphere micro-organisms rather than in those of the bulk soil. 13C in phospholipid fatty acid biomarkers revealed that root-derived C was incorporated into the soil food web mainly via saprotrophic fungi rather than via bacteria. Only small amounts of 13C were transferred to higher trophic levels, predominantly into fungal-feeding nematodes and macrofauna decomposers., Most importantly, C pool size and 13C incorporation did not match closely. Although the fungal C stock was less than half that of bacteria, C transfers from fungi into higher trophic levels of the fungal energy pathway, that is fungal-feeding nematodes and meso- and macrofauna decomposers, by far exceed that of bacterial C. This challenges previous views on the dominance of bacteria in root C dynamics and suggests saprotrophic fungi to function as major agents channelling recent photoassimilates into the soil food web. [ABSTRACT FROM AUTHOR]

Aims: Soil priming affects soil N transformation and plant N availability, but few studies have investigated these interactions to date. Methods: To address this, we reviewed the literature for studies quantifying soil priming, soil N transformation and plant N uptake. Results: Gross N mineralization was strongly controlled by soil priming in studies with plants, while abiotic factors had a minor influence on gross N mineralization. In contrast, soil priming was negatively related to gross N mineralization and had a low explanatory power in incubation studies where substrates are added as surrogates for root exudates. These results indicate that plants support increased N mineralization and that this is not adequately reflected in incubation studies. Additionally, we observed a positive relationship between soil priming and the % of Norg-derived N uptake as well as total N uptake, which demonstrates that priming enhances the availability of N that was previously organically bound and that at least part of the N mineralized during priming was available for plant uptake. Conclusion: Our results show that the effect of roots and rhizodeposition leads to a number of processes supporting N mineralization and availability through priming that are not well reflected in incubation studies. To fully capture the interactions between plant roots and their associated microbiota, we recommend focusing research on systems with plants. Additionally, the strong correlation between C and N transformation should be considered in biogeochemical modelling. [ABSTRACT FROM AUTHOR]

Soil organic matter (SOM) pools, allocated within various aggregates, are characterized by different degradability and turnover rates that depend on the spatial accessibility of organics and their recalcitrance. Hence, to understand the processes and mechanisms of SOM cycling and stability, the contribution of individual aggregate size classes to the total CO 2 efflux including extra mineralization via priming effect (PE) should be considered. In this study, we determined whether aggregate size classes and their disruption affected the mineralization of SOM and induced PE depending on the primer amount. Soil samples were separated into three aggregate size classes (>2 mm, 2–0.25 mm macroaggregates and <0.25 mm microaggregates). Half of the samples within each class were left intact, whereas half were crushed. After the addition of two levels of 14 C-labeled glucose, the amount of 14 C in CO 2 efflux and microbial biomass were measured several times during the 49-day incubation. Cumulative SOM-derived CO 2 production from the macroaggregates was 16–21% greater than the CO 2 production from the microaggregates after 49 days. The percentage of glucose mineralized to CO 2 increased with the level of glucose addition, but 14 C incorporation into microbial biomass decreased, indicating lower carbon (C) use efficiency at high substrate availability. Aggregate disruption had no effect on the cumulative total and SOM-derived CO 2 production, but it increased glucose mineralization up to 11.2% while the percentage of added glucose incorporated into microbial biomass in macroaggregates decreased. The PE increased with an increased glucose level for the intact aggregates. Aggregate disruption increased the PE in all aggregates sizes under low glucose level. In summary, our findings demonstrate that the aggregate size class has clear effects on C mineralization while their disruption affects the added labile C decomposition and transformation, indicating the relevance of soil structure for SOM cycling in terms of priming and C sequestration. [ABSTRACT FROM AUTHOR]

Background and Aims: Below-ground translocated carbon (C) released as rhizodeposits is an important driver for microbial mobilization of nitrogen (N) for plants. We investigated how a limited substrate supply due to reduced photoassimilation alters the allocation of recently assimilated C in plant and soil pools under legume and non-legume species. Methods: A non-legume ( Lolium perenne) and a legume ( Medicago sativa) were labelled with N before the plants were clipped or shaded, and labelled twice with CO thereafter. Ten days after clipping and shading, the N and C in shoots, roots, soil, dissolved organic nitrogen (DON) and carbon (DOC) and in microbial biomass, as well as the C in soil CO were analyzed. Results: After clipping, about 50 % more C was allocated to regrowing shoots, resulting in a lower translocation to roots compared to the unclipped control. Clipping also reduced the total soil CO efflux under both species and the C recovery of soil CO under L. perenne. The N recovery increased in the shoots of M. sativa after clipping, because storage compounds were remobilized from the roots and/or the N uptake from the soil increased. After shading, the assimilated C was preferentially retained in the shoots of both species. This caused a decreased C recovery in the roots of M. sativa. Similarly, the total soil CO efflux under M. sativa decreased more than 50 % after shading. The N recovery in plant and soil pools showed that shading has no effect on the N uptake and N remobilization for L. perenne, but, the N recovery increased in the shoot of M. sativa. Conclusions: The experiment showed that the dominating effect on C and N allocation after clipping is the need of C and N for shoot regrowth, whereas the dominating effect after shading is the reduced substrate supply for growth and respiration. Only slight differences could be observed between L. perenne and M. sativa in the C and N distribution after clipping or shading. [ABSTRACT FROM AUTHOR]

Background and aims: Rhizodeposition of plants is the most uncertain component of the carbon (C) cycle. By existing approaches the amount of rhizodeposition can only roughly be estimated since its persistence in soil is very short compared to other organic C pools. We suggest an approach to quantify rhizodeposition at the field scale by assuming a constant ratio between rhizodeposited-C to root-C. Methods: Maize plants were pulse-labeled with CO under controlled conditions and the soil CO efflux was separated into root and rhizomicrobial respiration. The latter and the C activity remaining in the soil corresponded to total rhizodeposition. By relating rhizodeposited-C to root-C a rhizodeposition-to-root ratio of 0.56 was calculated. This ratio was applied to the root biomass C measured in the field to estimate rhizodeposition under field conditions. Results: Maize allocated 298 kg C ha as root-C and 166 kg C ha as rhizodeposited-C belowground, 50 % of which were recovered in the upper 10 cm. The fate of rhizodeposits was estimated based on the C data, which showed that 62 % of total rhizodeposition was mineralized within 16 days, 7 % and 0.3 % was incorporated into microbial biomass and DOC, respectively, and 31 % was recovered in the soil. Conclusions: We conclude that the present approach allows for an improved estimation of total rhizodeposition, since it accounts not only for the fraction of rhizodeposits remaining in soil, but also for that decomposed by microorganisms and released from the soil as CO. [ABSTRACT FROM AUTHOR]

Abstract: Photosynthesis of higher plants drives carbon (C) allocation below-ground and controls the supply of assimilates to roots and to rhizosphere microorganisms. To investigate the effect of limited photosynthesis on C allocation, redistribution and reutilization in plant and soil microorganisms, perennial grass Lolium perenne and legume Medicago sativa were clipped or shaded. Plants were labelled with three 14C pulses to trace allocation and reutilization of C assimilated before clipping or shading. Five days after the last 14C pulse, plants were clipped or shaded and the total CO2 and 14CO2 efflux from the soil was measured. 14C in above- and below-ground plant biomass and bulk soil, rhizosphere soil and microorganisms was determined 10 days after clipping or shading. After clipping, 2% of the total assimilated 14C originating mainly from root reserves were detected in the newly grown shoots. This corresponded to a translocation of 5 and 8% of total 14C from reserve organs to new shoots of L. perenne and M. sativa, respectively. The total CO2 efflux from soil decreased after shading of both plant species, whereas after clipping, this was only true for L. perenne. The 14CO2 efflux from soil did not change after clipping of both species. An increased 14CO2 efflux from soil under shading for both plants indicated that lower assimilation was compensated by higher utilization of the reserve C for root and rhizomicrobial respiration. We conclude that C stored in roots is an important factor for plant recovery after limiting photosynthesis. This stored C is important for shoot regrowth after clipping, whereas after shading, it is utilized mainly for maintenance of root respiration. Based on these results as well as on a review of several studies on C reutilization for regrowth after clipping, we conclude that because of the high energy demand for nitrogen fixation, legumes use a higher portion (9–10%) of stored C for regrowth compared to grasses (5–7%). The effects of limited photosynthesis were of minor importance for the exudation of the reserve C and thus, have no effect on the uptake of this C by microorganisms. [Copyright &y& Elsevier]

Abstract: Living roots and their rhizodeposits can stimulate microbial activity and soil organic matter (SOM) decomposition up to several folds. This so-called rhizosphere priming effect (RPE) varies widely among plant species possibly due to species-specific differences in the quality and quantity of rhizodeposits and other root functions. However, whether the RPE is influenced by plant inter-species interactions remains largely unexplored, even though these interactions can fundamentally shape plant functions such as carbon allocation and nutrient uptake. In a 60-day greenhouse experiment, we continuously labeled monocultures and mixtures of sunflower, soybean and wheat with 13C-depleted CO2 and partitioned total CO2 efflux released from soil at two stages of plant development for SOM- and root-derived CO2. The RPE was calculated as the difference in SOM-derived CO2 between the planted and the unplanted soil, and was compared among the monocultures and mixtures. We found that the RPE was positive under all plants, ranging from 43% to 136% increase above the unplanted control. There were no significant differences in RPE at the vegetative stage. At the flowering stage however, the RPE in the soybean–wheat mixture was significantly higher than those in the sunflower monoculture, the sunflower–wheat mixture, and the sunflower–soybean mixture. These results indicated that the influence of plant inter-specific interactions on the RPE is case-specific and phenology-dependent. To evaluate the intensity of inter-specific effects on priming, we calculated an expected RPE for the mixtures based on the RPE of the monocultures weighted by their root biomass and compared it to the measured RPE under mixtures. At flowering, the measured RPE was significantly lower for the sunflower–wheat mixture than what can be expected from their monocultures, suggesting that RPE was significantly reduced by the inter-species effects of sunflower and wheat. In summary, our results clearly demonstrated that inter-species interactions can significantly modify rhizosphere priming on SOM decomposition. [Copyright &y& Elsevier]

Aims: Although water conservation in rice production has become increasingly important, the effects of water management on the allocation and dynamics of carbon (C) within the rice-soil system remain unknown. Methods: We compared the allocation and dynamics of C assimilated by rice under continuously flooded, non-flooded and alternate water regimes. Rice ( Oryza sative L. cv. Luliangyou 996) was labeled with CO and harvested 7 times within 45 days. Results: More C was released from roots into the soil in non-flooded and alternate water regimes treatments. Microbial C decreased with time after the labeling and was lowest under flooded condition. Roots and rhizomicrobial respiration followed the order of non-flooded > alternate water regimes > flooded treatment. Water management affected C distribution in aggregates with more C in macroaggregates in the non-flooded treatment. Estimated amounts of C transferred remaining belowground by rice 45 days after labeling were 1,986, 2,827 and 2,472 kg C ha, of which rhizodeposition accounted for about 41 %, 16 % and 30 % of C transferred belowground under non-flooded, flooded and alternate water regimes, respectively. Conclusions: Water management affected the allocation and dynamics of recently assimilated C within the rice-soil system and also changed the relative contribution of rhizodeposition to C transferred belowground. This study suggests the differences in the driving mechanisms of C sequestration under flooded vs. non-flooded and alternate water regimes. [ABSTRACT FROM AUTHOR]

Abstract: The production of CO2 in soil strongly depends on the availability of organic carbon (C) for microorganisms. It is obvious, that C that entered the soil recently is more easily available for microorganisms in comparison to older C. However, only very few approaches allow for a quantitative estimation of the availability of C in relation to the time it is entering the soil. We hypothesized that δ13C values of CO2 and of soil organic matter (SOM) after a C3 to C4 vegetation change will enable to calculate the relative availability of younger (C4-derived) and older C (C3-derived) sources for microorganisms. Soil CO2 was sampled over one vegetation period at depths of 10, 40–50 and 60–70 cm at three treatments: a C3 reference (wheat), a C4/fallow (fallow after one year of maize cropping), and a C4/C4 (two years of maize cropping). Based on the δ13C of CO2 purified from the admixture of atmospheric CO2 by the Miller/Tans model and on the δ13C values of SOM, the contributions of younger and older C sources to CO2 and SOM were assessed. Depending on the soil depth and the presence of living roots, the contribution of younger C to soil CO2 ranged from 16 to 50%, but that to SOM was less than 5%. By comparing the contributions of older and younger C to CO2 and SOM, we found that the relative availability of organics recently introduced into the soil (C4-derived) was about 7 times higher than the availability of C stabilized in soil for longer than one year (C3-derived). We concluded that simultaneous analysis of the δ13C values of both SOM and of CO2 allows not only for the quantification of the CO2 sources, but also for the estimation of the availability of soil C pools of different age for microorganisms. [Copyright &y& Elsevier]

Premise of the study: In addition to autotrophic and fully mycoheterotrophic representatives, the orchid family comprises species that at maturity obtain C and N partially from fungal sources. These partial mycoheterotrophs are often associated with fungi that simultaneously form ectomycorrhizas with trees. This study investigates mycorrhizal nutrition for orchids from the southwestern Australian biodiversity hotspot. Methods: The mycorrhizal fungi of 35 green and one achlorophyllous orchid species were analyzed using molecular methods. Nutritional mode was identified for 27 species by C and N isotope abundance analysis in comparison to non-orchids from the same habitat. As a complementary approach, 13CO2 pulse labeling was applied to a subset of six orchid species to measure photosynthetic capacity. Key results: Almost all orchids associated with rhizoctonia-forming fungi. Due to much higher than expected variation within the co-occurring nonorchid reference plants, the stable isotope approach proved challenging for assigning most orchids to a specialized nutritional mode; therefore, these orchids were classified as autotrophic at maturity. The 13CO2 pulse labeling confirmed full autotrophy for six selected species. Nonetheless, at least three orchid species (Gastrodia lacista, Prasophyllum elatum, Corybas recurvus) were identified as nutritionally distinctive from autotrophic orchids and reference plants. Conclusions: Despite the orchid-rich flora in southwestern Australia, partial mycoheterotrophy among these orchids is less common than in other parts of the world, most likely because most associate with saprotrophic fungi rather than ectomycorrhizal fungi. [ABSTRACT FROM AUTHOR]

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

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

Despite its fundamental role for carbon (C) and nutrient cycling, rhizodeposition remains 'the hidden half of the hidden half': it is highly dynamic and rhizodeposits are rapidly incorporated into microorganisms, soil organic matter, and decomposed to CO2. Therefore, rhizodeposition is rarely quantified and remains the most uncertain part of the soil C cycle and of C fluxes in terrestrial ecosystems. This review synthesizes and generalizes the literature on C inputs by rhizodeposition under crops and grasslands (281 data sets). The allocation dynamics of assimilated C (after 13C-CO2 or 14C-CO2 labeling of plants) were quantified within shoots, shoot respiration, roots, net rhizodeposition (i.e., C remaining in soil for longer periods), root-derived CO2, and microorganisms. Partitioning of C pools and fluxes were used to extrapolate belowground C inputs via rhizodeposition to ecosystem level. Allocation from shoots to roots reaches a maximum within the first day after C assimilation. Annual crops retained more C (45% of assimilated 13C or 14C) in shoots than grasses (34%), mainly perennials, and allocated 1.5 times less C belowground. For crops, belowground C allocation was maximal during the first 1-2 months of growth and decreased very fast thereafter. For grasses, it peaked after 2-4 months and remained very high within the second year causing much longer allocation periods.Despite higher belowground C allocation by grasses (33%) than crops (21%), its distribution between various belowground pools remain very similar. Hence, the total C allocated belowground depends on the plant species, but its further fate is species independent. This review demonstrates that C partitioning can be used in various approaches, e.g. root sampling, CO2 flux measurements, to assess rhizodeposits' pools and fluxes at pot, plot, field and ecosystem scale and so, to close the most uncertain gap of the terrestrial C cycle. [ABSTRACT FROM AUTHOR]

Summary: Stable isotope signatures of fungal sporocarps have been instrumental in identifying carbon gains of chlorophyllous orchids from a fungal source. Yet, not all mycorrhizal fungi produce macroscopic sporocarps and frequently fungi of different taxa occur in parallel in orchid roots.To overcome this obstacle, we investigated stable isotope signatures of fungal pelotons extracted from orchid roots and compared these data to the respective orchid and reference plant tissues. Anoectochilus sandvicensis and Epipactis palustris represented specialized or unspecialized rhizoctonia‐associated orchids. Epipactis atrorubens and Epipactis leptochila are orchids considered ectomycorrhiza‐associated with different preferences for Basidio‐ and Ascomycota.13C enrichment of rhizoctonia pelotons was minor compared with plant tissues and significantly lower than enrichments of pelotons from ectomycorrhizal Epipactis species. 15N values of pelotons from E. leptochila and E. atrorubens showed similar patterns as known for respective sporocarps of ectomycorrhizal Ascomycota and Basidiomycota, however, with an offset towards lower 15N enrichments and nitrogen concentrations.Our results suggest an explicit fungal nutrition source of orchids associated with ectomycorrhizal fungi, whereas the low 13C enrichment in rhizoctonia‐associated orchids and fungal pelotons hamper the detection of carbon gains from fungal partners. 15N isotopic pattern of orchids further suggests a selective transfer of 15N‐enriched protein‐nitrogen into orchids. [ABSTRACT FROM AUTHOR]

The study aimed to evaluate the effects of long‐term fertilisation on soil aggregation and the associated changes in soil organic carbon (SOC) and nitrogen (N) pools in aggregates. The combined application of mineral fertiliser and manure improved soil aggregation, SOC and N content in aggregates, compared to manure or mineral fertiliser alone, and thus proved to be a suitable fertilisation strategy to increase C sequestration in agroecosystems. [ABSTRACT FROM AUTHOR]

Kobresia grasslands on the Tibetan Plateau comprise the world’s largest pastoral alpine ecosystem. Overgrazing-driven degradation strongly proceeded on this vulnerable grassland, but the mechanisms behind are still unclear. Plants must balance the costs of releasing C to soil against the benefits of accelerated microbial nutrient mineralization, which increases their availability for root uptake. To achieve the effect of grazing on this C-N exchange mechanism, a 15NH4+ field labeling experiment was implemented at grazed and ungrazed sites, with additional treatments of clipping and shading to reduce belowground C input by manipulating photosynthesis. Grazing reduced gross N mineralization rates by 18.7%, similar to shading and clipping. This indicates that shoot removal by grazing decreased belowground C input, thereby suppressing microbial N mining and overall soil N availability. Nevertheless, NH4+ uptake rate by plants at the grazed site was 1.4 times higher than at the ungrazed site, because plants increased N acquisition to meet the high N demands of shoot regrowth (compensatory growth: grazed > ungrazed). To enable efficient N uptake and regrowth, Kobresia plants have developed specific traits (i.e., efficient above-belowground interactions). These traits reflect important mechanisms of resilience and ecosystem stability under long-term moderate grazing in an N-limited environment. However, excessive (over)grazing might imbalance such C-N exchange and amplify plant N limitation, hampering productivity and pasture recovery over the long term. In this context, a reduction in grazing pressure provides a sustainable way to maintain soil fertility, C sequestration, efficient nutrient recycling, and overall ecosystem stability. [ABSTRACT FROM AUTHOR]

One of the most prominent changes in the rhizospheric soil structure is associated with the formation of a strongly bound soil layer in the surroundings of the root, which is named rhizosheath. In this study, we investigated how root hair elongation, a ubiquitous root morphological trait, affect the stability of rhizosheath aggregates. Using 13CO2 pulse labeling, we tracked the fate of root-derived 13C inputted into the rhizosheath of two Zea mays L. genotypes with contrasting root hair elongation: a mutant with root hair defective elongation (rth3) and a corresponding wild type (WT). In addition, we also investigated the differences between two 13CO2 labeling approaches (single vs. multiple pulse labeling) in the distribution of 13C in the rhizosheath aggregates. We were able to demonstrate that the rhizosheath aggregate stability and the resulting aggregate size distribution follows the same mechanisms irrespective of the root hair elongation. This result reinforces the assumption that other soil properties are more decisive for the soil structure formation in the rhizosheath in comparison to root hair elongation. The majority of recently deposited root-derived C (57%) was found in the macroaggregates. Increasing the number of pulses (multiple pulse labeling approach) resulted in a higher 13C enrichment of the rhizosheath aggregates fractions in comparison to the application of a single pulse. While both labeling approaches resulted in a similar distribution of 13C in the rhizosheath aggregates, the higher enrichment given by multiple pulse labeling allowed the separation of significant differences between the genotypes in plant C allocation in the rhizosheath. [ABSTRACT FROM AUTHOR]

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

Biopores are hotspots of nutrient mobilisation and shortcuts for carbon (C) into subsoils. C processing relies on microbial community composition, which remains unexplored in subsoil biopores. Phospholipid fatty acids (PLFAs; markers for living microbial groups) and amino sugars (microbial necromass markers) were extracted from two subsoil depths (45-75 cm ; 75-105 cm) and three biopore types: (I) drilosphere of Lumbricus terrestris L., (II) 2-year-old root biopores and (III) 1.5-year-old root biopores plus six 6 months of L. terrestris activities. Biopore C contents were at least 2.5 times higher than in bulk soil, causing 26-35 times higher Σ PLFAs g soil. The highest Σ PLFAs were found in both earthworm biopore types; thus, the highest soil organic matter and nutrient turnover were assumed. Σ PLFAs was 33% lower in root pores than in earthworm pores. The treatment affected the microbial community composition more strongly than soil depth, hinting to similar C quality in biopores: Gram-positives including actinobacteria were more abundant in root pores than in earthworm pores, probably due to lower C bioavailability in the former. Both earthworm pore types featured fresh litter input, promoting growth of Gram-negatives and fungi. Earthworms in root pores shifted the composition of the microbial community heavily and turned root pores into earthworm pores within 6 months. Only recent communities were affected and they reflect a strong heterogeneity of microbial activity and functions in subsoil hotspots, whereas biopore-specific necromass accumulation from different microbial groups was absent. [ABSTRACT FROM AUTHOR]

The decomposition of soil organic matter depends strongly on its availability to microorganisms and their enzymes. The rhizosphere and detritusphere are microbial hot spots due to additional substrate input, leading to high abundance, specific species diversity and functional diversity of microbial communities. However, rhizosphere and detritusphere differ in substrate quality, localization, and duration of input. We hypothesized that the contrasting substrate availability between rhizosphere and detritusphere affects the activity of microorganisms and associated enzymes. Organic carbon (C) from the rhizosphere and detritusphere decreases with soil depth and, consequently, microbial hot spots become rarer and competition for C and nutrients increases. In deeper soil (>40 cm depth) the amount and quality of substrates is expected to decrease and, therefore, the effect of contrasting substrate input to disappear. Plant N uptake is expected to reduce N availability in the rhizosphere of maize compared to the detritusphere and bare fallow. These hypotheses were tested in a factorial field experiment with 1) maize-planted, 2) maize litter-amended, and 3) bare sites. Enzyme kinetic parameters (V max , K m , K a ), extractable organic C and microbial biomass C were compared in soil affected by rhizosphere and detritusphere throughout the profile to 70 cm depth, to assess microbial C and nutrient limitations. A decrease in enzyme activity with depth due to resource scarcity and lower substrate quality appeared in planted and litter-amended soil. N limitation in planted soil increased the activity and substrate affinity of proteolytic enzymes to provide for microbial N demand through SOM decomposition. This was in line with lower V max ratios (V max for C-cycling enzymes divided by V max for N-cycling enzymes) in planted relative to litter-amended topsoil. The catalytic efficiency of enzymes decreased 2- to 20-fold from top- (<40 cm) to subsoil (>40 cm), irrespective of the substrate input. Substrate quality in the rhizosphere and detritusphere affected enzyme activities only in the topsoil, whereas a sharp decline of C input with depth led to similar activities in the subsoil. Most of the enzyme indexes reflected shifts in allocation of C and nutrients in the rhizosphere and detritusphere. The presented results underline the role of microorganisms as critical links in the C and nutrient transfers in the rhizosphere and detritusphere. [ABSTRACT FROM AUTHOR]

Introduction: Root-mediated changes in soil organic matter (SOM) decomposition, termed rhizosphere priming effects (RPE), play crucial roles in the global carbon (C) cycle, but their mechanisms and field relevance remain ambiguous. We hypothesize that nitrogen (N) shortages may intensify SOM decomposition in the rhizosphere because of increase of fine roots and rhizodeposition. Methods: RPE and their dependence on N-fertilization were studied using a C-to-C vegetation change. N-fertilized and unfertilized soil cores, with and without maize, were incubated in the field for 50 days. Soil CO efflux was measured, partitioned for SOM- and root-derived CO, and RPE was calculated. Plant biomass, microbial biomass C (MBC) and N (MBN), and enzyme activities (β-1,4-glucosidase; N-acetylglucosaminidase; L-leucine aminopeptidase) were analyzed. Results: Roots enhanced SOM mineralization by 35 % and 126 % with and without N, respectively. This was accompanied by higher specific root-derived CO in unfertilized soils. MBC, MBN and enzyme activities increased in planted soils, indicating microbial activation, causing positive RPE. N-fertilization had minor effects on MBC and MBN, but it reduced β-1,4-glucosidase and L-leucine aminopeptidase activities under maize through lower root-exudation. In contrast, N-acetylglucosaminidase activity increased with N-fertilization in planted and unplanted soils. Conclusions: This study showed the field relevance of RPE and confirmed that, despite higher root biomass, N availability reduces RPE by lowering root and microbial activity. [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]

Agroforestry systems are characterized by a high complexity between vegetation components and niche partitioning. In a crop-grass-tree agroforestry system, rape, willow, and grasses were in situ pulse labeled separately with 13CO2 for 6 h, and 13C was traced in shoots, roots, topsoil (0–15 cm) and subsoil (15–30 cm), microbial biomass carbon (C), and dissolved organic C, as well as respiration losses (CO2) up to 28 days after labeling to investigate the effects of vegetation components on C allocation belowground. 13C recovery in roots after 28 days was 7.0% of total assimilated C for grassland, which was 3.5- and 5.2-fold higher than that for rape and willow, respectively. The larger C allocation belowground in grassland was ascribed to its higher root/shoot ratio compared to willow and rape. Grassland facilitated higher accumulation of root-derived C in soil compared to rape (9.2% of recovered 13C) and compared to willow (1.6% of 13C). Willow retained more photosynthetic C aboveground and less was allocated to roots compared to rape. Although the C allocated to the top 15-cm soil was similar between willow and rape, willow facilitated C allocation in deeper soil compared to rape (0.6% vs. 0.2%). This could be explained by the lower microbial activity and subsequent weaker decomposition of rhizodeposits in 15–30-cm depth under willow. The net belowground C inputs in grassland, willow, and rape were 0.53, 0.06, and 0.10 g C m−2 month−1 of vegetation period, including rhizodeposition of 0.24, 0.05, and 0.04 g C m−2 month−1, respectively. Overall, integrating trees and grassland within cropland facilitates higher root-derived C input into soil, thus contributing to the soil C sequestration in agroforestry systems. [ABSTRACT FROM AUTHOR]

Microbial and enzyme functioning depends on the quality of substrates, which strongly differ in bare soil and in the hotspots of microbial activity such as the rhizosphere and detritusphere. We established a field experiment to determine the effects of contrasting substrate quality, namely, soil organic matter, maize shoot litter (detritusphere) and maize rhizodeposits (rhizosphere) on microorganisms and their extracellular enzymes in an arable soil. Kinetic parameters (Vmax and Km) of four hydrolytic extracellular enzymes: β-cellobiohydrolase, β-glucosidase, acid phosphate and β-xylosidase were analyzed in 0e10 and 10e20 cm to elucidate the effects of substrate content on substrate affinity and catalytic efficiency (Vmax/Km). Living roots increased microbial biomass by 179% and microbial respiration by 100% compared to fallow soil. Lower enzyme affinities to substrates (e.g. 93% for β-glucosidase) in rooted soil pointed to the domination of r-strategists, which are favored in the decomposition of labile organics common in the rhizosphere. No differences in catalytic properties of cellulolytic enzymes were detected between bulk and litter-treated soil, indicating the recalcitrance of organics in both treatments. The rhizosphere and detritusphere effects on enzyme kinetics were negligible in 10e20 cm, except β-glucosidase. The reduction of Km of all enzymes in 10e20 cm versus the upper 10 cm indicated increasing substrate affinity with depth. Nonetheless, the catalytic efficiency increased from 0 to 10 to 10e20 cm (e.g. up to 420% for acid phosphatase), reflecting changes in properties and functioning of enzymatic systems. This pointed to a shift towards a more K-selected microbial community with higher affinity and more efficient substrate utilization. It also indicated the microbial adaptation to decreasing substrate contents with depth by altered enzyme functioning. Overall, the catalytic properties of cellulolytic enzymes were much more strongly affected by plants (substrate quality in the rhizosphere and detritusphere compared to bare fallow) than by depth (substrate content). [ABSTRACT FROM AUTHOR]

• High microbial activity in topsoil causes apparent PE shortly after substrate addition. • The input of C from fertilizers induces apparent PE at the 10–30 cm depth. • The release and decomposition of mineral-bound organic C shortly after substrate addition contributes to positive PE. • Negative PE in topsoil during later incubation stages is linked to reduced microbial biomass and substrate depletion. • In subsoil, positive PE and net C loss was caused by microbial N mining. Fertilization is crucial for increasing crop productivity and it alters soil microbial biomass and activities. These alterations exert implications for soil carbon (C) stocks, primarily through the priming effect (PE). Here, we investigated the mechanisms underlying PE and their impact on soil C stocks in paddy soils subjected to long-term (31 years) fertilization. Soils from four depths within the upper 40 cm layer, subjected to four fertilization types (no fertilizer, mineral fertilizer, mineral + straw, mineral + chicken manure), were incubated for 60 days with or without 13C-labeled glucose. Following substrate addition, an increase in isotopically unlabeled CO 2 efflux may stem from accelerated microbial turnover (apparent PE) or modified soil organic matter (SOM) mineralization (real PE). By analyzing microbial biomass (MBC), CO 2 and dissolved organic C (DOC) dynamics, we explored the partitioning of C from glucose into three pools. Following glucose addition (day 2), the SOM-derived MBC decreased by 4.7 % at 0–10 cm in unfertilized and by 6.3 % in mineral-fertilized soils. SOM-derived MBC also decreased in soils fertilized with straw- and chicken manure at 0–20 and 0–30 cm. Hence, the apparent PE closely correlated with fertilization types. At day 2, the SOM-derived DOC increased with the increase in extractable Fe2+ content, indicating high contribution of the released mineral-bound organic C in the DOC pool. Microbial decomposition of SOM-derived DOC and loss of unlabeled C through apparent PE increased the release of unlabeled CO 2 , resulting in a positive but low PE. After 60 days of incubation, the addition of glucose to the subsoils (30–40 cm) caused a net loss of C (positive PE) of 6.6–19 mg kg−1. In contrast, negative PEs were observed in the topsoil, indicating a decrease in microbial activity and C turnover. These findings suggested that fertilization regimes, whether involving long-term extra C input or not, can alter microbial biomass and turnover rates. Moreover, the C turnover mechanisms in paddy soils are complex, including apparent PE, release of mineral-bound organic C, and negative PE. Collectively, these processes can consequently change soil C stocks. [ABSTRACT FROM AUTHOR]

Carbon (C) cycling is crucial to agroecosystem functioning. Important determinants for the belowground C flow are soil food webs, with microorganisms and microfaunal grazers, i.e., nematodes, as key biota. The present study investigates the incorporation of plant-derived C into the nematode micro-food web under two different cropping systems, grassland (ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.)) and agroforest (willow (Salix schwerinii Wolf and Salix viminalis L)). To quantify the C flux from the plant into the soil micro-food web, grass and willow were pulse-labeled with 13CO2 and the incorporation of 13C into the nematode trophic groups was monitored 3, 7, 14 and 28 days after labeling. The natural stable isotope signals (13C/12C, 15N/14N) were analyzed to determine the structure of the nematode micro-food web. The natural isotopic δ15N signal revealed different trophic levels for omnivores and predators in grassland and agroforest soils. The incorporation of plant C into nematode tissue was detectable three days after 13CO2 labeling with the highest and fastest C allocation in plant feeders in grassland, and in fungal feeders in agroforest soil. C flux dynamics between the aboveground vegetation and belowground micro-food web varied with cropping system. This demonstrates that crop-specific translocation of C affects the multitrophic interactions in the root environment, which in turn can alter soil nutrient cycling. [ABSTRACT FROM AUTHOR]

Abstract: Rhizodeposit-carbon (rhizo-C) serves as a primary energy and C source for microorganisms in the rhizosphere. Despite important progress in understanding the fate of rhizo-C in upland soils, little is known about microbial community dynamics associated with rhizo-C in flooded soils, especially depending on water regimes in rice systems. In this study, rice grown under non-flooded, continuously flooded and alternating water regimes was pulse labeled with 13CO2 and the incorporation of rhizo-C into specific microbial groups was determined by 13C in phospholipid fatty acids (PLFAs) at day 2 and 14 after the labeling. A decreased C released from roots under continuously flooded condition was accompanied with lower total 13C incorporation into microorganisms compared to the non-flooded and alternating water regimes treatments. Continuous flooding caused a relative increase of 13C incorporation in Gram positive bacteria (i14:0, i15:0, a15:0, i16:0, i17:0, a17:0). In contrast, Gram negative bacteria (16:1ω7c, 18:1ω7c, cy17:0, cy 19:0) and fungi (18:2ω6, 9c, 18:1ω9c) showed greater rhizo-C incorporation coupled with a higher turnover under non-flooded and alternating water regimes treatments. These observations suggest that microbial groups processing rhizo-C differed among rice systems with varying water regimes. In contrast to non-flooded and alternating water regimes, there was little to no temporal 13C change in most microbial groups under continuous flooding condition between day 2 and 14 after the labeling, which may demonstrate slower microbial processing turnover. In summary, our findings indicate that belowground C input by rhizodeposition and its biological cycling was significantly influenced by water regimes in rice systems. [Copyright &y& Elsevier]

• Intact plant-soil mesocosms are translocated along an elevational gradient ranging from 1260 m to 350 m a.s.l. • Translocated microorganisms increased their active fraction, enzymatic activity and changed enzyme systems. • Warming in combination with decreased precipitation leads to shifts in microbial life strategies to K -strategists. • Microorganisms are C limited along the translocation from higher to lower elevations. • Climate change with warmer temperatures and periodically less precipitation will modify the microbial C and N demand. Soil microorganisms are the major players controlling the soil carbon (C) and nutrient cycling, however the extent to which the functional properties of soil microorganisms will alter with climate change in mountain grasslands is still unclear. To simulate future climatic conditions (higher temperature and less precipitation), intact plant-soil mesocosms of three mountain grassland sites, Esterberg (1260 m a.s.l.), Graswang (860 m a.s.l.), and Fendt 550 m a.s.l.), were translocated to the lowest elevation Bayreuth (350 m a.s.l.). After two years of translocation to warmer and drier environments, specific growth rates of soil microorganisms were 25 % slower than those at higher elevations. This indicated a shift towards to slow-growing K -strategists with increasing temperature and decreasing soil moisture, presumably attributed to the depletion of available C for microorganisms. Simultaneously, higher activity and faster substrate turnover time of C-acquiring enzymes in the warmer and drier soils implied a stronger C rather nutrient limitation. This was supported by the increased vector length, which was induced by decreased dissolved organic compounds at lower elevation. In contrast, microorganisms were limited by nitrogen (N) at higher elevation because of the stronger competition for N between plants and microorganisms. In short, translocated microorganisms increased their active fraction, enzymatic activity and altered enzyme systems but decreased specific growth rates. This indicated that soil microbial community adjusted to the changing climate resulting in faster substrate turnover, and in turn caused 15 % reduction in soil C content from Esterberg to Bayreuth. We therefore conclude that climate warming in combination with decreased precipitation increases microbial activity, leads to shifts in microbial life strategies and induces alterations in microbial nutrient limitation, i.e. shift from N- to C and P co-limitation with translocation to lower elevation sites. This in turn, is likely to result in C losses of mountain grassland soils under future climate change, which will amplify negative impacts on global warming. [ABSTRACT FROM AUTHOR]

Drought and the availability of mineable phosphorus minerals used for fertilization are two of the important issues agriculture is facing in the future. High phosphorus availability in soils is necessary to maintain high agricultural yields. Drought is one of the major threats for terrestrial ecosystem performance and crop production in future. Among the measures proposed to cope with the upcoming challenges of intensifying drought stress and to decrease the need for phosphorus fertilizer application is the fertilization with silica (Si). Here we tested the importance of soil Si fertilization on wheat phosphorus concentration as well as wheat performance during drought at the field scale. Our data clearly showed a higher soil moisture for the Si fertilized plots. This higher soil moisture contributes to a better plant performance in terms of higher photosynthetic activity and later senescence as well as faster stomata responses ensuring higher productivity during drought periods. The plant phosphorus concentration was also higher in Si fertilized compared to control plots. Overall, Si fertilization or management of the soil Si pools seem to be a promising tool to maintain crop production under predicted longer and more serve droughts in the future and reduces phosphorus fertilizer requirements. [ABSTRACT FROM AUTHOR]

Plant roots interact with rhizosphere microorganisms to accelerate soil organic matter (SOM) mineralization for nutrient acquisition. Root‐mediated changes in SOM mineralization largely depend on root‐derived carbon (root‐C) input and soil nutrient status. Hence, intraspecific competition over plant development and spatiotemporal variability in the root‐C input and nutrients uptake may modify SOM mineralization. To investigate the effect of intraspecific competition on SOM mineralization at three growth stages (heading, flowering, and ripening), we grew maize (C4 plant) under three planting densities on a C3 soil and determined in situ soil C‐ and N‐mineralization by 13C‐natural abundance and 15N‐pool dilution approaches. From heading to ripening, soil C‐ and N‐mineralization rates exhibit similar unimodal trends and were tightly coupled. The C‐to‐N‐mineralization ratio (0.6 to 2.6) increased with N availability, indicating that an increase in N‐mineralization with N depletion was driven by microorganisms mining N‐rich SOM. With the intraspecific competition, plants increased specific root lengths as an efficient strategy to compete for resources. Root morphologic traits rather than root biomass per se were positively related to C‐ and N‐mineralization. Overall, plant phenology and intraspecific competition controlled the intensity and mechanisms of soil C‐ and N‐ mineralization by the adaptation of root traits and nutrient mining. Soil C‐ and N‐mineralization rates are tightly coupled throughout maize growth stages and dependent on soil N availability. Plant phenology and intraspecific competition govern the intensity and mechanisms of the C‐ and N‐mineralization by the adaptation of root traits and nutrient mining. [ABSTRACT FROM AUTHOR]

Background and aims: Plant growth affects soil moisture, mineral N and organic C availability in soil, all of which influence denitrification. With increasing plant growth, root exudation may stimulate denitrification, while N uptake restricts nitrate availability. Methods: We conducted a double labeling pot experiment with either maize (Zea mays L.) or cup plant (Silphium perfoliatum L.) of the same age but differing in size of their shoot and root systems. The 15N gas flux method was applied to directly quantify N2O and N2 fluxes in situ. To link denitrification with available C in the rhizosphere, 13CO2 pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil. Results: Plant water and N uptake were the main factors controlling daily N2O + N2 fluxes, cumulative N emissions, and N2O production pathways. Accordingly, pool-derived N2O + N2 emissions were 30–40 times higher in the treatment with highest soil NO3− content and highest soil moisture. CO2 efflux from soil was positively correlated with root dry matter, but we could not detect any relationship between root-derived C and N2O + N2 emissions. Conclusions: Root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth. [ABSTRACT FROM AUTHOR]

As the dynamics and magnitude of rhizodeposition vary considerably among cropping systems, we investigated effects of cropping system on the incorporation of root-derived carbon (C) into Collembola, a dominant taxon of soil microarthropods. In the field, we used 13CO2 to pulse label a crop monoculture (oilseed rape, Brassica napus L.), a mixed-grass community (dominated by Lolium perenne L. mixed with clover Trifolium repens L.), and a tree plantation (willow, Salix schwerinii E.L. Wolf and Salix viminalis L.). During 28 days, the incorporation of 13C was traced in nine species of Collembola including epedaphic (surface-dwelling), hemiedaphic (litter-dwelling), and euedaphic (soil-dwelling) functional groups. Incorporation of 13C into Collembola reached a plateau before day 3 after the labeling in grass and willow, but increased up to day 14 in rape. While euedaphic Collembola incorporated less root-derived C than epedaphic and hemiedaphic Collembola in rape and willow, the incorporation of 13C was similar among functional groups in grass. Differential incorporation of 13C in euedaphic species points to niche differentiation within the same functional group. Our findings highlight that cropping system not only affects the flux of root C into soil mesofauna, being slower in rape than in grass and willow, but also the utilization of root-derived resources by functional groups and species of Collembola. The results indicate that pronounced differences in belowground C inputs between cropping systems affect microbivores as basal species and thereby soil food webs and their functioning and services. [ABSTRACT FROM AUTHOR]

To overcome soil nutrient limitation, many plants have developed complex nutrient acquisition strategies including altering root morphology, root hair formation or colonization by arbuscular mycorrhizal fungi (AMF). The interactions of these strategies and their plasticity are, however, affected by soil nutrient status throughout plant growth. Such plasticity is decisive for plant phosphorus (P) acquisition in P‐limited soils. We investigated the P acquisition strategies and their plasticity of two maize genotypes characterized by the presence or absence of root hairs. We hypothesized that in the absence of root hairs plant growth is facilitated by traits with complementary functions, e.g., by higher root mycorrhizal colonization. This dependence on complementary traits will decrease in P fertilized soils. At early growth stages, root hairs are of little benefit for nutrient uptake. Regardless of the presence or absence of root hairs, plants produced average root biomass of 0.14 g per plant and exhibited 23% root mycorrhizal colonization. At later growth stages of maize, contrasting mechanisms with functional complementarity explained similar plant biomass production under P limitation: the presence of root hairs versus higher root mycorrhizal colonization (67%) favored by increased fine root diameter in absence of root hairs. P fertilization decreased the dependence of plant on specific root traits for nutrient acquisition. Through root trait plasticity, plants can minimize trade‐offs for developing and maintaining functional traits, while increasing the benefit in terms of nutrient acquisition and plant growth. The present study highlights the plasticity of functional root traits for efficient nutrient acquisition strategies in agricultural systems with low nutrient availability. [ABSTRACT FROM AUTHOR]

Aims: Microbial turnover processes are typically restricted by low substrate availability in the subsoil. We hypothesized that SOM decomposition increases with plant density and decreases with N fertilization: We expected a greater rate of C allocation to the rhizosphere in the topsoil than in the subsoil treatments. Methods: In order to simulate different degrees of rhizodeposition, wheat was planted in pots at four different densities. The plants were continuously labeled with 13C-depleted CO2. Soil CO2 efflux was partitioned for root- and SOM-derived CO2. Moreover, we determined the enzyme kinetics by measuring catalytic efficiency and enzyme stoichiometry in both topsoil and subsoil. Results: Shoot biomass and the shoot to root ratio were significantly higher for plants grown in the topsoil compared with the subsoil, which demonstrated higher relative C allocation to root biomass in the subsoil treatment. Despite the similar size of the rhizosphere, root-derived CO2 was always higher in the topsoil compared with the subsoil treatment, indicating enhanced root exudation. Effect sizes of all enzyme activities showed stronger magnitudes for the subsoil treatments. This was in line with a two-times increase of the effect size of SOM decomposition in the subsoil relative to topsoil. Conclusions: Overall, the plants in the subsoil treatments allocated more C to root biomass, less C to shoot biomass, and substantially less C to root exudates. However, the effect sizes of both SOM decomposition and enzyme activities were higher in the subsoil than in the topsoil, reflecting a stronger sensitivity to C inputs. [ABSTRACT FROM AUTHOR]

Meadows and forests are the main vegetation types in temperate terrestrial ecosystems, and largely contribute to soil carbon (C) stock. Bioavailable C inputs can accelerate microbial decomposition of soil organic matter (SOM), which is known as "priming effect". However, it is still unclear how priming effect, as an important mechanism influencing soil C sequestration, is influenced by spatial transition of vegetation from meadow to forest. To investigate the mechanism of priming effect along a spatial transition gradient of vegetation, a soil incubation experiment with 14C labeled glucose was combined with microbial rDNA sequencing and gene composition prediction. The results showed that with the vegetation transition from meadow to forest soil available phosphorus (P) significantly increased, in contrast to dissolved nitrogen (N) and C which remained unaffected. Moreover, the soil microbial community composition shifted towards a higher relative abundance of K-strategists (Acidobacteria) and a lower abundance of r-strategists (Actinobacteriota) along the vegetation transition from meadow to forest. In the meadow, the microbial community consumed more of the added glucose and increased priming effect. This was accompanied by lower available P but higher soil bacterial gene function encoding P cycling. In contrast, increased soil P availability in forest soils caused a decelerated microbial metabolism of phosphorylated organic compounds within microbial biomass due to decreased microbial demand for P acquisition from SOM, and thus resulted in suppression of the priming effect. Our study showed that P availability and microbial community shifts in spatial transition zones between meadows and forests are important drivers for the priming effect on SOM decomposition. • 14C radioactive isotope labeling was applied in combination with microbial rDNA sequencing. • Soil microbial community in meadow consumed more of the added glucose than that in forest. • Vegetation transition from meadow to forest suppressed the priming effect. • P availability and microbial community shifts are important drivers for the priming effect. [ABSTRACT FROM AUTHOR]

Abstract Numerous 13CO 2 labeling studies have traced the flow of carbon from fresh plant exudates into rhizosphere bacterial communities. However, the succession of the uptake of carbon leaving the roots by distinct rhizosphere microbiota has rarely been resolved between microbial kingdoms. This can provide valuable insights on the niche partitioning of primary rhizodeposit consumption, as well as on community interactions in plant-derived carbon flows in soil. Here, we have traced the flow of fresh plant assimilates to rhizosphere microbiota of maize (Zea mays L.) by rRNA-stable isotope probing (SIP). Carbon flows involving bacteria, unicellular fungi, as well as protists were observed over 5 and 8 days. Surprisingly, labeling of Paraglomerales and several bacteria including Opitutus, Mucliaginibacter and Massilia spp. was especially apparent in soil surrounding the strict rhizosphere after 5 d. This highlights the central role of arbuscular mycorrhizal fungi (AMF) as a shunt for fresh plant assimilates to soil microbes not directly influenced by root exudation. Distinct trophic webs involving different flagellates, amoeba and ciliates were also observed in rhizosphere and surrounding soil, while labeling of filamentous saprotrophic Ascomycota or Basidiomycota was not apparent. This challenges the proposed "sapro-rhizosphere" concept and demonstrates the utility of rRNA-SIP to disentangle inter-kingdom microbial relationships in the rhizosphere. Graphical abstract Image 1 Highlights • rRNA-SIP was used to trace plant-derived carbon flows in the rhizosphere of maize. • 13C-labeling of mycorrhiza was stronger in surrounding soil than in the rhizosphere. • Labelling of bacteria incl. Opitutus spp. was also stronger in surrounding soil. • AMF are a shunt of plant assimilates to microbes outside the strict rhizosphere. [ABSTRACT FROM AUTHOR]

Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Instead of "Institute of Soil Science and Soil Conservation, Justus Liebig University Giessen, Giessen, Germany", the right affiliation should have been "Institute of Soil Science, Leibniz University of Hannover, Germany". The original article can be found online at https://doi.org/10.1007/s00374-023-01708-6 B Correction to: Biology and Fertility of Soils (2023) 59:351-361 b https://doi.org/10.1007/s00374-023-01708-6 The above article's initial published version contained an error regarding the co-author Vincent J. M. N. L. Felde's affiliation. [Extracted from the article]