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Analyses of diverse aboveground and belowground indicators should underpin assessments of ecosystem recovery, yet monitoring many indicators is costly and their integration is challenging. Our objective was to combine indicators through a Bayesian hierarchical model to provide a comprehensive assessment of ecosystem status and identify a cost‐effective subset of indicators to provide an accurate estimate of ecosystem recovery. We assessed 59 aboveground–belowground indicators, classified into nine components of composition, structure, and function, to estimate the ecosystem status of restored rock spoils and reference forests in south‐eastern Australia. Overall ecosystem status, which integrates across ecosystem components and supporting indicators, was lower within restored forests but positively correlated with forest age. Reference forests had greater aboveground and belowground biotic structure, organic matter supply, and soil carbon stability, and trends were consistent among all of their supporting indicators. A subset of organic matter quality and nutrient cycling indicators were greater within restored forests, suggesting high ecosystem process rates, but that soil carbon may be more vulnerable to loss. Aboveground biotic structure was correlated with organic matter supply and quality, stability of soil carbon, the cycling of nutrients, and belowground biotic structure, providing evidence of aboveground–belowground coupling. A combination of four indicators representing belowground biotic structure, soil carbon stability, organic matter supply, and aboveground composition, provided a good estimate of ecosystem status at a third of the cost. Although ecosystem status can be monitored with a small set of indicators, a diversity of aboveground–belowground indicators provide a robust and comprehensive assessment of recovery. [ABSTRACT FROM AUTHOR]

Accurate predictions of soil C feedbacks to climate change depend on an improved understanding of responses of soil C pools and C use by soil microbial groups. We assessed soil and microbial C in a 7‐year manipulation of CO2 and warming in a semi‐arid grassland. Continuous field isotopic labelling under elevated CO2 further allowed us to study the dynamics of the existing C (Old C) in soil and microbes as affected by warming. Warming reduced soil C under elevated CO2 but had no impact under ambient CO2. Loss of soil C under warming and elevated CO2 was attributed to increased proportional loss of Old C. Warming also reduced the proportion of Old C in microbes, specifically the bacteria, but not the fungi. These findings highlight that warming impacts are C pool and microbial taxa dependent and demonstrate interactive effects of warming and atmospheric CO2 on soil C. [ABSTRACT FROM AUTHOR]

It is becoming increasingly clear that plant roots can impact the decomposition of existing soil C in the rhizosphere. Studies under controlled conditions suggest this impact may be plant species dependent, but whether this is the case in natural conditions or what factors underlie this variation is mostly unknown., With a novel field-based isotopic approach combining 13C-enriched glucose and 5-bromo-2-deoxyuridine additions, we compared in situ C decomposition of added labile C and native soil C (priming) among eight semi-arid grassland species' rhizospheres to investigate the factors driving inter-species variation. We examined the influence of several rhizosphere factors related to soil chemistry, microbial activity, microbial community, microbial stoichiometry, plant chemistry and root morphology., Plant species generated distinct microbial and chemical rhizosphere environments, which translated into differences in the direction, magnitude and temporal dynamics of the soil C priming. Soil C decomposition was positively related to soil C/P and soil N/P (via its influence on the bacterial community), which in turn were positively related to plant N/P. Plant C/N was also a significant factor via its negative influence on soil N/P. In contrast, the main direct predictors of labile C decomposition were microbial biomass, microbial C/N and the C-degrading enzymes, which in turn were linked to root morphology and C chemistry., Synthesis. Within this community, plant species' rhizospheres can vary in their susceptibility to C loss in response to changes in C availability. Soil stoichiometry, driven by plant chemical traits, appeared to be the strongest driver of priming. Our study suggests that shifts in plant communities involving increases in N relative to P have the greatest potential to lead to C loss. We provide evidence of root morphology and C chemistry as drivers of labile C processing in soil, a novel empirical contribution to our understanding of the role of plant traits below-ground. The contrasting regulation of different pools of soil C suggests observations of the regulation of simple C compounds should not be extrapolated to the whole C pool. Our findings provide support for rhizosphere-driven mechanisms by which shifts in plant community composition could have implications on the ecosystem-level C balance. [ABSTRACT FROM AUTHOR]

Rising levels of atmospheric CO2 frequently stimulate plant inputs to soil, but the consequences of these changes for soil carbon (C) dynamics are poorly understood. Plant-derived inputs can accumulate in the soil and become part of the soil C pool ('new soil C'), or accelerate losses of pre-existing ('old') soil C. The dynamics of the new and old pools will likely differ and alter the long-term fate of soil C, but these separate pools, which can be distinguished through isotopic labeling, have not been considered in past syntheses. Using meta-analysis, we found that while elevated CO2 (ranging from 550 to 800 parts per million by volume) stimulates the accumulation of new soil C in the short term (<1 year), these effects do not persist in the longer term (1-4 years). Elevated CO2 does not affect the decomposition or the size of the old soil C pool over either temporal scale. Our results are inconsistent with predictions of conventional soil C models and suggest that elevated CO2 might increase turnover rates of new soil C. Because increased turnover rates of new soil C limit the potential for additional soil C sequestration, the capacity of land ecosystems to slow the rise in atmospheric CO2 concentrations may be smaller than previously assumed. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal (AMF) function has mostly been studied from the plant perspective, but there is a shortage of empirical assessments of their ecosystem level impacts on soil carbon (C). Our understanding of the role of AMF on C processing belowground has been restricted mostly to fresh plant residues, not stabilized soil organic matter. The mechanisms by which elevated CO (eCO) alter soil C remain an open question but AMF likely play a role via C and nutrients, which could in turn, be plant species dependent. We assessed AMF as mediators of C processing in the rhizosphere of two grasses under eCO. We exposed a C and a C grass to a combination of ambient and eCO with and without modification of the AMF communities and using stable isotopes quantified the respiration of native soil C (as rhizosphere priming), its contribution to dissolved and microbial C and the final remaining C pool. The AMF treatment impacted soil C respiration under the C-plant and only under eCO. eCO suppressed decomposition (negative priming) but this effect disappeared when the AMF community was reduced. In contrast to studies of fresh plant residues suggesting that AMF can enhance C loss, our observations indicate that AMF may promote C storage in the soil organic matter pool. Results support that AMF can mediate the effect of eCO on soil C in the rhizosphere of some plant species, a potential mechanism explaining variation in impacts of eCO on soil C storage and C balances across species and ecosystems. [ABSTRACT FROM AUTHOR]

The common notion for describing N mineralization in models is that it results from decomposer organisms trying to meet their stoichiometric demands based on their own elemental composition and that of the resource. However, in addition to influencing C and nutrient availability, plant litter also influences the composition of both the litter and mineral soil community e importantly not in the same manner e resulting in altered trophic interactions. Since decomposer groups and their consumers vary in their elemental composition and demands, a change in composition and abundance of soil functional groups may result in a change in the stoichiometry of the whole soil food web, thus altering their stoichiometric relations with the available resource with potential functional consequences. We use experimental data and quantitative food web modeling to investigate the impact of the changes in the litter and soil food webs brought about by the differing stoichiometry of plant litter on (a) N mineralization, (b) the contribution of different functional groups to mineralization, and (c) the stoichiometric flexibility of the system, assessed as the ability to mineralize materials with different stoichiometry. Our simulations suggested that the effects of litter stoichiometry on trophic interactions, their impacts on N mineralization and the relative contribution of functional groups may not behave as a continuum across the litter and soil interface. Further, changes in food webs associated with variation in plant stoichiometric traits can influence the relative importance of functional groups, which given their particular stoichiometric demands may affect ecosystem-level N cycling. Our results suggested that litter materials of intermediate N contents, or litter mixtures encompassing materials with different nutrient contents and thus resulting in mixtures of intermediate stoichiometry, may promote food webs that are better suited to deal with changing substrate stoichiometry. [ABSTRACT FROM AUTHOR]

Abstract: There is increasing awareness of the importance of ecological legacies in contemporary ecosystem processes. Decomposition is regulated by a set of interacting hierarchically organized factors. As spatial and temporal scales decrease, decomposition is largely dependent on the quality of resources and the decomposer community, but whether and how these factors manifest via historical legacy effects is not well understood. We tested whether the history of plant litter inputs had short-term legacy effects on contemporary litter and soil organic matter carbon (C) and nitrogen (N) mineralization. Using a field/laboratory microcosm approach, we exposed soils to two litters of contrasting chemistry and, after adding fresh substrates, we monitored C and N dynamics. In a parallel experiment, we manipulated the soil community to reduce litter-history impacts on its composition and size to investigate whether the soil community could be an important contributor to legacy effects We found strong short-term litter legacy effects on contemporary litter and soil N mineralization, the duration of which was dependent on the contemporary substrate for decomposition. These strong effects were not consistent with the home field advantage phenomenon, as exposure to a specific litter did not favor the decomposition of the same litter when it was applied as a contemporary substrate. Reduction of the litter-history effects on soil biota decreased the impact of litter history on N immobilization, suggesting that plant litter impacts on the soil community may be an important component of plant litter legacies on N decomposition. In contrast to N, litter legacies appeared to be much less important for C decomposition, suggesting that legacy effects might uncouple contemporary C and N dynamics. [Copyright &y& Elsevier]

There is increasing interest in the ecology of tree-based intercropping systems, particularly alley cropping, in temperate settings. Shoot pruning of the tree component is an integral practice to alley cropping. Shoot pruning likely results in an input of belowground C and N to soil, with potential, but unknown impacts on the time pattern of N availability. We evaluated the impacts of pruning of Amorpha fruticosa L. on (a) its root dynamics; (b) mineral N and microbial N, and (c) the fate of root-derived N and its importance in determining N availability. For this, we conducted repeated partial pruning and foliar labeling with N under field conditions in Georgia, USA. Pruning altered root dynamics by initially decreasing biomass and later preventing increases, likely due to root death. Pruning modified the time pattern of mineral N by substantially increasing concentrations during the 3 months following pruning. N labeling indicated that root-derived N was an important source of this extra N and that pruning caused the availability of root-derived N to microbes to be extended until the following growing season. Greater concentrations of mineral N, a greater pool of microbial N and prolonged microbial access and storage of root-derived N with pruning has the potential to positively impact system productivity. [ABSTRACT FROM AUTHOR]

The increasing prevalence of drought events in grasslands and shrublands worldwide potentially has impacts on soil organic carbon (SOC). We leveraged the International Drought Experiment to study how SOC, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) concentrations, responds to extreme drought treatments (1-in-100-year) for 1 to 5 years at 19 sites worldwide. In more mesic areas (aridity index > 0.65), SOC and POC concentrations decreased by 7.9% (±3.9) and 15.9% (±6.2) with drought, respectively, but there were no impacts on MAOC concentrations. However, drought had no impact on SOC, POC, or MAOC concentrations in drylands (aridity index < 0.65). The response of SOC to drought varied along an aridity gradient, concomitant with interannual precipitation variability and standing SOC concentration gradients. These findings highlight the differing response magnitudes of POC and MAOC concentrations to drought and the key regulating role of aridity. [ABSTRACT FROM AUTHOR]

Rising levels of atmospheric CO2 frequently stimulate plant inputs to soil, but the consequences of these changes for soil carbon (C) dynamics are poorly understood. Plant-derived inputs can accumulate in the soil and become part of the soil C pool ('new soil C'), or accelerate losses of pre-existing ('old') soil C. The dynamics of the new and old pools will likely differ and alter the long-term fate of soil C, but these separate pools, which can be distinguished through isotopic labeling, have not been considered in past syntheses. Using meta-analysis, we found that while elevated CO2 (ranging from 550 to 800 parts per million by volume) stimulates the accumulation of new soil C in the short term (<1 year), these effects do not persist in the longer term (1-4 years). Elevated CO2 does not affect the decomposition or the size of the old soil C pool over either temporal scale. Our results are inconsistent with predictions of conventional soil C models and suggest that elevated CO2 might increase turnover rates of new soil C. Because increased turnover rates of new soil C limit the potential for additional soil C sequestration, the capacity of land ecosystems to slow the rise in atmospheric CO2 concentrations may be smaller than previously assumed. [ABSTRACT FROM AUTHOR]

• Identifying common responses of microbial functional genes to elevated CO 2 (eCO 2) • Nitrogen (N) fixation, carbon fixation and decomposition genes increased under eCO 2. • Plant biomass, soil carbon (C) decomposition and soil moisture increased under eCO 2. • Potential molecular biomarkers of global change for predicting ecosystem functions. • Increasing soil C, soil N and net primary productivity in future eCO 2 environments. Atmospheric CO 2 concentration is increasing, largely due to anthropogenic activities. Previous studies of individual free-air CO 2 enrichment (FACE) experimental sites have shown significant impacts of elevated CO 2 (eCO 2) on soil microbial communities; however, no common microbial response patterns have yet emerged, challenging our ability to predict ecosystem functioning and sustainability in the future eCO 2 environment. Here we analyzed 66 soil microbial communities from five FACE sites, and showed common microbial response patterns to eCO 2 , especially for key functional genes involved in carbon and nitrogen fixation (e.g., pcc/acc for carbon fixation, nifH for nitrogen fixation), carbon decomposition (e.g., amyA and pulA for labile carbon decomposition, mnp and lcc for recalcitrant carbon decomposition), and greenhouse gas emissions (e.g., mcrA for methane production, norB for nitrous oxide production) across five FACE sites. Also, the relative abundance of those key genes was generally increased and directionally associated with increased biomass, soil carbon decomposition, and soil moisture. In addition, a further literature survey of more disparate FACE experimental sites indicated increased biomass, soil carbon decay, nitrogen fixation, methane and nitrous oxide emissions, plant and soil carbon and nitrogen under eCO 2. A conceptual framework was developed to link commonly responsive functional genes with ecosystem processes, such as pcc/acc vs. soil carbon storage , amyA/pulA/mnp/lcc vs. soil carbon decomposition, and nifH vs. nitrogen availability, suggesting that such common responses of microbial functional genes may have the potential to predict ecosystem functioning and sustainability in the future eCO 2 environment. [ABSTRACT FROM AUTHOR]