Soil viral-host interactions regulate microplastic-dependent carbon storage.

Microplastic is globally regarded as an important factor impacting biogeochemical cycles, yet our understanding of such influences is limited by the uncertainties of intricate microbial processes. By multiomics analysis, coupled with soil chemodiversity characterization and microbial carbon use efficiency (CUE), we investigated how microbial responses to microplastics impacted soil carbon cycling in a long-term field experiment. We showed that biodegradable microplastics promoted soil organic carbon accrual by an average of 2.47%, while nondegradable microplastics inhibited it by 17.4%, as a consequence of the virus-bacteria coadaptations to the microplastics disturbance. In the relevant functional pathways, nondegradable microplastics significantly (P < 0.05) enhanced the abundance and transcriptional activity related to complex carbohydrate metabolism, whereas biodegradable microplastics significantly (P < 0.05) promoted functions involved in amino acid metabolism and glycolysis. Accordingly, viral lysis enhanced in nondegradable microplastics treatments to introduce more complex organic compounds to soil dissolved organic matters, thus benefiting the oligotrophs with high carbon metabolic capabilities in exploitation competition. In contrast, biodegradable microplastics enriched viral auxiliary metabolic genes of carbon metabolism through "piggyback-the-winner" strategy, conferring to dominant copiotrophs, enhanced substrate utilization capabilities. These virus-host interactions were also demonstrated in the corresponding soil plastisphere, which would alter microbial resource allocation and metabolism via CUE, affecting carbon storage consequently. Overall, our results underscore the importance of viral-host interactions in understanding the microplastics-dependent carbon storage in the soil ecosystem.

Whole-soil-profile warming does not change microbial carbon use efficiency in surface and deep soils.

The paucity of investigations of carbon (C) dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Soil microbes are important engines driving terrestrial biogeochemical cycles; their carbon use efficiency (CUE), defined as the proportion of metabolized organic C allocated to microbial biomass, is a key regulator controlling the fate of soil C. It has been theorized that microbial CUE should decline with warming; however, empirical evidence for this response is scarce, and data from deeper soils are particularly scarce. Here, based on soil samples from a whole-soil-profile warming experiment (0 to 1 m, +4 °C) and 18O tracing approach, we examined the vertical variation of microbial CUE and its response to ~3.3-y experimental warming in an alpine grassland on the Qinghai-Tibetan Plateau. Microbial CUE decreased with soil depth, a trend that was primarily controlled by soil C availability. However, warming had limited effects on microbial CUE regardless of soil depth. Similarly, warming had no significant effect on soil C availability, as characterized by extractable organic C, enzyme-based lignocellulose index, and lignin phenol-based ratios of vanillyls, syringyls, and cinnamyls. Collectively, our work suggests that short-term warming does not alter microbial CUE in either surface or deep soils, and emphasizes the regulatory role of soil C availability on microbial CUE.

Microbial central carbon metabolism in a tidal freshwater marsh and an upland mixed conifer soil under oxic and anoxic conditions.

The central carbon (C) metabolic network is responsible for most of the production of energy and biosynthesis in microorganisms and is therefore key to a mechanistic understanding of microbial life in soil communities. Many upland soil communities have shown a relatively high C flux through the pentose phosphate (PP) or the Entner-Doudoroff (ED) pathway, thought to be related to oxidative damage control. We tested the hypothesis that the metabolic organization of the central C metabolic network differed between two ecosystems, an anoxic marsh soil and oxic upland soil, and would be affected by altering oxygen concentrations. We expected there to be high PP/ED pathway activity under high oxygen concentrations and in oxic soils and low PP/ED activity in reduced oxygen concentrations and in marsh soil. Although we found high PP/ED activity in the upland soil and low activity in the marsh soil, lowering the oxygen concentration for the upland soil did not reduce the relative PP/ED pathway activity as hypothesized, nor did increasing the oxygen concentration in the marsh soil increase the PP/ED pathway activity. We speculate that the high PP/ED activity in the upland soil, even when exposed to low oxygen concentrations, was related to a high demand for NADPH for biosynthesis, thus reflecting higher microbial growth rates in C-rich soils than in C-poor sediments. Further studies are needed to explain the observed metabolic diversity among soil ecosystems and determine whether it is related to microbial growth rates.IMPORTANCEWe observed that the organization of the central carbon (C) metabolic processes differed between oxic and anoxic soil. However, we also found that the pentose phosphate pathway/Entner-Doudoroff (PP/ED) pathway activity remained high after reducing the oxygen concentration for the upland soil and did not increase in response to an increase in oxygen concentration in the marsh soil. These observations contradicted the hypothesis that oxidative stress is a main driver for high PP/ED activity in soil communities. We suggest that the high PP/ED activity and NADPH production reflect higher anabolic activities and growth rates in the upland soil compared to the anaerobic marsh soil. A greater understanding of the molecular and biochemical processes in soil communities is needed to develop a mechanistic perspective on microbial activities and their relationship to soil C and nutrient cycling. Such an increased mechanistic perspective is ecologically relevant, given that the central carbon metabolic network is intimately tied to the energy metabolism of microbes, the efficiency of new microbial biomass production, and soil organic matter formation.

Recovery of Soil Microbial Metabolism After Rewetting Depends on Interacting Environmental Conditions and Changes in Functional Groups and Life History Strategies.

Climate change is causing an intensification of soil drying and rewetting events, altering microbial functioning and potentially destabilizing soil organic carbon. After rewetting, changes in microbial community carbon use efficiency (CUE), investment in life history strategies, and fungal to bacterial dominance co-occur. Still, we have yet to generalize what drives these dynamic responses. Here, we collated 123 time series of microbial community growth (G, sum of fungal and bacterial growth, evaluated by leucine and acetate incorporation, respectively) and respiration (R) after rewetting and calculated CUE = G/(G + R). First, we characterized CUE recovery by two metrics: maximum CUE and time to maximum CUE. Second, we translated microbial growth and respiration data into microbial investments in life history strategies (high yield (Y), resource acquisition (A), and stress tolerance (S)). Third, we characterized the temporal change in fungal to bacterial dominance. Finally, the metrics describing the CUE recovery, investment in life history strategies, and fungal to bacterial dominance after rewetting were explained by environmental factors and microbial properties. CUE increased after rewetting as fungal dominance declined, but the maximum CUE was explained by the CUE under moist conditions, rather than specific environmental factors. In contrast, higher soil pH and carbon availability accelerated the decline of microbial investment in stress tolerance and fungal dominance. We conclude that microbial CUE recovery is mostly driven by the shifting microbial community composition and the metabolic capacity of the community, whereas changes in microbial investment in life history strategies and fungal versus bacterial dominance depend on soil pH and carbon availability.

Temperature controls the relation between soil organic carbon and microbial carbon use efficiency.

Microbial carbon use efficiency (CUE) is an important variable mediating microbial effects on soil organic carbon (SOC) since it summarizes how much carbon is used for microbial growth or is respired. Yet, the role of CUE in regulating SOC storage remains debated, with evidence for both positive and negative SOC-CUE relations. Here, we use a combination of measured data around the world and numerical simulations to explore SOC-CUE relations accounting for temperature (T) effects on CUE. Results reveal that the sign of the CUE-T relation controls the direction of the SOC-CUE relations. A negative CUE-T relation leads to a positive SOC-CUE relation and vice versa, highlighting that CUE-T patterns significantly affect how organic carbon is used by microbes and hence SOC-CUE relations. Numerical results also confirm the observed negative SOC-T relation, regardless of the CUE-T patterns, implying that temperature plays a more dominant role than CUE in controlling SOC storage. The SOC-CUE relation is usually negative when temperature effects are isolated, even though it can become positive when nonlinear microbial turnover is considered. These results indicate a dominant role of CUE-T patterns in controlling the SOC-CUE relation. Our findings help to better understand SOC and microbial responses to a warming climate.

Heat wave-induced microbial thermal trait adaptation and its reversal in the Subarctic.

Climate change predictions suggest that arctic and subarctic ecosystems will be particularly affected by rising temperatures and extreme weather events, including severe heat waves. Temperature is one of the most important environmental factors controlling and regulating microbial decomposition in soils; therefore, it is critical to understand its impact on soil microorganisms and their feedback to climate warming. We conducted a warming experiment in a subarctic birch forest in North Sweden to test the effects of summer heat waves on the thermal trait distributions that define the temperature dependences for microbial growth and respiration. We also determined the microbial temperature dependences 10 and 12 months after the heat wave simulation had ended to investigate the persistence of the thermal trait shifts. As a result of warming, the bacterial growth temperature dependence shifted to become warm-adapted, with a similar trend for fungal growth. For respiration, there was no shift in the temperature dependence. The shifts in thermal traits were not accompanied by changes in α- or ß-diversity of the microbial community. Warming increased the fungal-to-bacterial growth ratio by 33% and decreased the microbial carbon use efficiency by 35%, and both these effects were caused by the reduction in moisture the warming treatments caused, while there was no evidence that substrate depletion had altered microbial processes. The warm-shifted bacterial thermal traits were partially restored within one winter but only fully recovered to match ambient conditions after 1 year. To conclude, a summer heat wave in the Subarctic resulted in (i) shifts in microbial thermal trait distributions; (ii) lower microbial process rates caused by decreased moisture, not substrate depletion; and (iii) no detectable link between the microbial thermal trait shifts and community composition changes.

"Energy and enthalpy" for microbial energetics in soil.

Energy is the driver of all microbial processes in soil. The changes in Gibbs energy are equal to the enthalpy changes during all processes in soil because these processes are ongoing under constant pressure and volume-without work generation. The enthalpy change by transformation of individual organic compounds or of complex organic matter in soil can be exactly quantified by the nominal oxidation state of carbon changes. Consequently, microbial energy use efficiency can be assessed by the complete combustion enthalpy of organic compounds when microorganisms use O2 as the terminal electron acceptor for microbial processes under aerobic conditions.

Long-term soil warming decreases soil microbial necromass carbon by adversely affecting its production and decomposition.

Microbial necromass carbon (MNC) accounts for a large fraction of soil organic carbon (SOC) in terrestrial ecosystems. Yet our understanding of the fate of this large carbon pool under long-term warming is uncertain. Here, we show that 14 years of soil warming (+4°C) in a temperate forest resulted in a reduction in MNC by 11% (0-10 cm) and 33% (10-20 cm). Warming caused a decrease in the content of MNC due to a decline in microbial biomass carbon and reduced microbial carbon use efficiency. This reduction was primarily caused by warming-induced limitations in available soil phosphorus, which, in turn, constrained the production of microbial biomass. Conversely, warming increased the activity of soil extracellular enzymes, specifically N-acetylglucosaminidase and leucine aminopeptidase, which accelerated the decomposition of MNC. These findings collectively demonstrate that decoupling of MNC formation and decomposition underlie the observed MNC loss under climate warming, which could affect SOC content in temperate forest ecosystems more widespread.

Environment and microbiome drive different microbial traits and functions in the macroscale soil organic carbon cycle.

Soil microbial traits and functions play a central role in soil organic carbon (SOC) dynamics. However, at the macroscale (regional to global) it is still unresolved whether (i) specific environmental attributes (e.g., climate, geology, soil types) or (ii) microbial community composition drive key microbial traits and functions directly. To address this knowledge gap, we used 33 grassland topsoils (0-10 cm) from a geoclimatic gradient in Chile. First, we incubated the soils for 1 week in favorable standardized conditions and quantified a wide range of soil microbial traits and functions such as microbial biomass carbon (MBC), enzyme kinetics, microbial respiration, growth rates as well as carbon use efficiency (CUE). Second, we characterized climatic and physicochemical properties as well as bacterial and fungal community composition of the soils. We then applied regression analysis to investigate how strongly the measured microbial traits and functions were linked with the environmental setting versus microbial community composition. We show that environmental attributes (predominantly the amount of soil organic matter) determined patterns of MBC along the gradient, which in turn explained microbial respiration and growth rates. However, respiration and growth normalized for MBC (i.e., specific respiration and growth) were more linked to microbial community composition than environmental attributes. Notably, both specific respiration and growth followed distinct trends and were related to different parts of the microbial community, which in turn resulted in strong effects on microbial CUE. We conclude that even at the macroscale, CUE is the result of physiologically decoupled aspects of microbial metabolism, which in turn is partially determined by microbial community composition. The environmental setting and microbial community composition affect different microbial traits and functions, and therefore both factors need to be considered in the context of macroscale SOC dynamics.

Nitrogen availability and summer drought, but not N:P imbalance, drive carbon use efficiency of a Mediterranean tree-grass ecosystem.

All ecosystems contain both sources and sinks for atmospheric carbon (C). A change in their balance of net and gross ecosystem carbon uptake, ecosystem-scale carbon use efficiency (CUEECO), is a change in their ability to buffer climate change. However, anthropogenic nitrogen (N) deposition is increasing N availability, potentially shifting terrestrial ecosystem stoichiometry towards phosphorus (P) limitation. Depending on how gross primary production (GPP, plants alone) and ecosystem respiration (RECO, plants and heterotrophs) are limited by N, P or associated changes in other biogeochemical cycles, CUEECO may change. Seasonally, CUEECO also varies as the multiple processes that control GPP and respiration and their limitations shift in time. We worked in a Mediterranean tree-grass ecosystem (locally called 'dehesa') characterized by mild, wet winters and summer droughts. We examined CUEECO from eddy covariance fluxes over 6 years under control, +N and + NP fertilized treatments on three timescales: annual, seasonal (determined by vegetation phenological phases) and 14-day aggregations. Finer aggregation allowed consideration of responses to specific patterns in vegetation activity and meteorological conditions. We predicted that CUEECO should be increased by wetter conditions, and successively by N and NP fertilization. Milder and wetter years with proportionally longer growing seasons increased CUEECO, as did N fertilization, regardless of whether P was added. Using a generalized additive model, whole ecosystem phenological status and water deficit indicators, which both varied with treatment, were the main determinants of 14-day differences in CUEECO. The direction of water effects depended on the timescale considered and occurred alongside treatment-dependent water depletion. Overall, future regional trends of longer dry summers may push these systems towards lower CUEECO.

Disproportionate Carbon Dioxide Efflux in Bacterial Metabolic Pathways for Different Organic Substrates Leads to Variable Contribution to Carbon-Use Efficiency.

Microbial organic matter turnover is an important contributor to the terrestrial carbon dioxide (CO2) budget. Partitioning of organic carbons into biomass relative to CO2 efflux, termed carbon-use efficiency (CUE), is widely used to characterize organic carbon cycling by soil microorganisms. Recent studies challenge proposals of CUE dependence on the oxidation state of the substrate carbon and implicate instead metabolic strategies. Still unknown are the metabolic mechanisms underlying variability in CUE. We performed a multiomics investigation of these mechanisms in Pseudomonas putida, a versatile soil bacterium of the Gammaproteobacteria, processing a mixture of plant matter derivatives. Our 13C-metabolomics data captured substrate carbons into different metabolic pathways: cellulose-derived sugar carbons in glycolytic and pentose-phosphate pathways; lignin-related aromatic carbons in the tricarboxylic acid cycle. Subsequent 13C-metabolic flux analysis revealed a 3-fold lower investment of sugar carbons in CO2 efflux compared to aromatic carbons, in agreement with reported substrate-dependent CUE. Proteomics analysis revealed enzyme-level regulation only for substrate uptake and initial catabolism, which dictated downstream fluxes through CO2-producing versus biomass-synthesizing reactions. Metabolic partitioning as shown here explained the substrate-dependent CUE calculated from reported metabolic flux analyses of other bacteria, further supporting a metabolism-guided perspective for predicting the microbial conversion of accessible organic matter to CO2 efflux.

New insights into the prediction for the potential of soil organic carbon accumulation: From the perspective of non-equilibrium statistical mechanics.

The accumulation of soil organic carbon (SOC) is significant for soil health and ecosystem services. Numerous studies have assessed the dynamic changes of SOC by considering the microbial system as an equilibrium system. However, they failed to reveal the complexity of the SOC accumulation/loss process, as the microbial system is a non-equilibrium system affected by stochastic fluctuations from the external environment. This study is the first to explore the complex non-equilibrium relationship between microbial carbon use efficiency (CUE) and SOC by using potential landscape and flux in non-equilibrium statistical mechanics. Nitrogen (N) was identified as the most critical environmental factor influencing CUE on a global scale, with the transition between the carbon loss state and the carbon sequestration state observed along N gradients. Random perturbations of other environmental factors could also trigger transition. Non-equilibrium thermodynamic quantities indicated that carbon sequestration had the potential to be achieved when N = 0.5 g/kg, where active soil management measures should be taken. Furthermore, the non-equilibrium relationship between CUE and SOC was clarified through potential energy analysis, where the average deviation between predictions and actual observations of SOC is about 1.9792 g/kg. This study provides an effective framework for predicting SOC accumulation.

Microbial nutrient limitation and carbon use efficiency changes under different degrees of litter decomposition.

Alpine ecosystems are important terrestrial carbon (C) pools, and microbial decomposers play a key role in litter decomposition. Microbial metabolic limitations in these ecosystems, however, remain unclear. The objectives of this study aim to elucidate the characteristics of microbial nutrient limitation and their C use efficiency (CUE), and to evaluate their response to environmental factors. Five ecological indicators were utilized to assess and compare the degree of microbial elemental homeostasis and the nutrient limitations of the microbial communities among varying stages of litter decomposition (L, F, and H horizon) along an altitudinal gradient (2800, 3000, 3250, and 3500 m) under uniform vegetation (Abies fabri) on Gongga Mountain, eastern Tibetan Plateau. In this study, microorganisms in the litter reached a strictly homeostatic of C content exclusively during the middle stage of litter decomposition (F horizon). Based on the stoichiometry of soil enzymes, we observed that microbial N- and P-limitation increased during litter degradation, but that P-limitation was stronger than N-limitation at the late stages of degradation (H horizon). Furthermore, an increase in microbial CUE corresponded with a reduction in microbial C-limitation. Additionally, redundancy analysis (RDA) based on forward selection further showed that microbial biomass C (MBC) is closely associated with the enzyme activities and their ratios, and MBC was also an important factor in characterizing changes in microbial nutrient limitation and CUE. Our findings suggest that variations in MBC, rather than N- and P-related components, predominantly influence microbial metabolic processes during litter decomposition on Gongga Mountain, eastern Tibetan Plateau.

Components explain, but do eddy fluxes constrain? Carbon budget of a nitrogen-fertilized boreal Scots pine forest.

Nitrogen (N) fertilization increases biomass and soil organic carbon (SOC) accumulation in boreal pine forests, but the underlying mechanisms remain uncertain. At two Scots pine sites, one undergoing annual N fertilization and the other a reference, we sought to explain these responses. We measured component fluxes, including biomass production, SOC accumulation, and respiration, and summed them into carbon budgets. We compared the resulting summations to ecosystem fluxes measured by eddy covariance. N fertilization increased most component fluxes (P < 0.05), especially SOC accumulation (20×). Only fine-root, mycorrhiza, and exudate production decreased, by 237 (SD = 28) g C m-2 yr-1 . Stemwood production increases were ascribed to this partitioning shift, gross primary production (GPP), and carbon-use efficiency, in that order. The methods agreed in their estimates of GPP in both stands (P > 0.05), but the components detected an increase in net ecosystem production (NEP) (190 (54) g C m-2 yr-1 ; P < 0.01) that eddy covariance did not (19 (62) g C m-2 yr-1 ; ns). The pairing of plots, the simplicity of the sites, and the strength of response provide a compelling description of N effects on the C budget. However, the disagreement between methods calls for further paired tests of N fertilization effects in simple forest ecosystems.

Microbial carbon use efficiency in different ecosystems: A meta-analysis based on a biogeochemical equilibrium model.

Soil microbial carbon use efficiency (CUE) is a crucial parameter that can be used to evaluate the partitioning of soil carbon (C) between microbial growth and respiration. However, general patterns of microbial CUE among terrestrial ecosystems (e.g., farmland, grassland, and forest) remain controversial. To address this knowledge gap, data from 41 study sites (n = 197 soil samples) including 58 farmlands, 95 forests, and 44 grasslands were collected and analyzed to estimate microbial CUEs using a biogeochemical equilibrium model. We also evaluated the metabolic limitations of microbial growth using an enzyme vector model and the drivers of CUE across different ecosystems. The CUEs obtained from soils of farmland, forest, and grassland ecosystems were significantly different with means of 0.39, 0.33, and 0.42, respectively, illustrating that grassland soils exhibited higher microbial C sequestration potentials (p < .05). Microbial metabolic limitations were also distinct in these ecosystems, and carbon limitation was dominant exhibiting strong negative effects on CUE. Exoenzyme stoichiometry played a greater role in impacting CUE values than soil elemental stoichiometry within each ecosystem. Specifically, soil exoenzymatic ratios of C:phosphorus (P) acquisition activities (EEAC:P ) and the exoenzymatic ratio of C:nitrogen (N) acquisition activities (EEAC:N ) imparted strong negative effects on soil microbial CUE in grassland and forest ecosystems, respectively. But in farmland soils, EEAC:P exhibited greater positive effects, showing that resource constraints could regulate microbial resource allocation with discriminating patterns across terrestrial ecosystems. Furthermore, mean annual temperature (MAT) rather than mean annual precipitation (MAP) was a critical climate factor affecting CUE, and soil pH as a major factor remained positive to drive the changes in microbial CUE within ecosystems. This research illustrates a conceptual framework of microbial CUEs in terrestrial ecosystems and provides the theoretical evidence to improve soil microbial C sequestration capacity in response to global change.

Functional substitutability of native herbivores by livestock for soil carbon stock is mediated by microbial decomposers.

Grazing by large mammalian herbivores impacts climate as it can favor the size and stability of a large carbon (C) pool in the soils of grazing ecosystems. As native herbivores in the world's grasslands, steppes, and savannas are progressively being displaced by livestock, it is important to ask whether livestock can emulate the functional roles of their native counterparts. While livestock and native herbivores can have remarkable similarity in their traits, they can differ greatly in their impacts on vegetation composition which can affect soil-C. It is uncertain how these similarities and differences impact soil-C via their influence on microbial decomposers. We test competing alternative hypotheses with a replicated, long-term, landscape-level, grazing-exclusion experiment to ask whether livestock in the Trans-Himalayan ecosystem of northern India can match decadal-scale (2005-2016) soil-C stocks under native herbivores. We evaluate multiple lines of evidence from 17 variables that influence soil-C (quantity and quality of C-input from plants, microbial biomass and metabolism, microbial community composition, eDNA, veterinary antibiotics in soil), and assess their inter-relationships. Livestock and native herbivores differed in their effects on several soil microbial processes. Microbial carbon use efficiency (CUE) was 19% lower in soils under livestock. Compared to native herbivores, areas used by livestock contained 1.5 kg C m-2 less soil-C. Structural equation models showed that alongside the effects arising from plants, livestock alter soil microbial communities which is detrimental for CUE, and ultimately also for soil-C. Supporting evidence pointed toward a link between veterinary antibiotics used on livestock, microbial communities, and soil-C. Overcoming the challenges of sequestering antibiotics to minimize their potential impacts on climate, alongside microbial rewilding under livestock, may reconcile the conflicting demands from food-security and ecosystem services. Conservation of native herbivores and alternative management of livestock is crucial for soil-C stewardship to envision and achieve natural climate solutions.

Substrate availability and not thermal acclimation controls microbial temperature sensitivity response to long-term warming.

Microbes are responsible for cycling carbon (C) through soils, and predicted changes in soil C stocks under climate change are highly sensitive to shifts in the mechanisms assumed to control the microbial physiological response to warming. Two mechanisms have been suggested to explain the long-term warming impact on microbial physiology: microbial thermal acclimation and changes in the quantity and quality of substrates available for microbial metabolism. Yet studies disentangling these two mechanisms are lacking. To resolve the drivers of changes in microbial physiology in response to long-term warming, we sampled soils from 13- and 28-year-old soil warming experiments in different seasons. We performed short-term laboratory incubations across a range of temperatures to measure the relationships between temperature sensitivity of physiology (growth, respiration, carbon use efficiency, and extracellular enzyme activity) and the chemical composition of soil organic matter. We observed apparent thermal acclimation of microbial respiration, but only in summer, when warming had exacerbated the seasonally-induced, already small dissolved organic matter pools. Irrespective of warming, greater quantity and quality of soil carbon increased the extracellular enzymatic pool and its temperature sensitivity. We propose that fresh litter input into the system seasonally cancels apparent thermal acclimation of C-cycling processes to decadal warming. Our findings reveal that long-term warming has indirectly affected microbial physiology via reduced C availability in this system, implying that earth system models including these negative feedbacks may be best suited to describe long-term warming effects on these soils.

Low soil moisture suppresses the thermal compensatory response of microbial respiration.

The thermal compensatory response of microbial respiration contributes to a decrease in warming-induced enhancement of soil respiration over time, which could weaken the positive feedback between the carbon cycle and climate warming. Climate warming is also predicted to cause a worldwide decrease in soil moisture, which has an effect on the microbial metabolism of soil carbon. However, whether and how changes in moisture affect the thermal compensatory response of microbial respiration are unexplored. Here, using soils from an 8-year warming experiment in an alpine grassland, we assayed the thermal response of microbial respiration rates at different soil moisture levels. The results showed that relatively low soil moisture suppressed the thermal compensatory response of microbial respiration, leading to an enhanced response to warming. A subsequent moisture incubation experiment involving off-plot soils also showed that the response of microbial respiration to 100 d warming shifted from a slight compensatory response to an enhanced response with decreasing incubation moisture. Further analysis revealed that such respiration regulation by moisture was associated with shifts in enzymatic activities and carbon use efficiency. Our findings suggest that future drought induced by climate warming might weaken the thermal compensatory capacity of microbial respiration, with important consequences for carbon-climate feedback.

Effects of experimental nitrogen deposition on soil organic carbon storage in Southern California drylands.

Atmospheric nitrogen (N) deposition is enriching soils with N across biomes. Soil N enrichment can increase plant productivity and affect microbial activity, thereby increasing soil organic carbon (SOC), but such responses vary across biomes. Drylands cover ~45% of Earth's land area and store ~33% of global SOC contained in the top 1 m of soil. Nitrogen fertilization could, therefore, disproportionately impact carbon (C) cycling, yet whether dryland SOC storage increases with N remains unclear. To understand how N enrichment may change SOC storage, we separated SOC into plant-derived, particulate organic C (POC), and largely microbially derived, mineral-associated organic C (MAOC) at four N deposition experimental sites in Southern California. Theory suggests that N enrichment increases the efficiency by which microbes build MAOC (C stabilization efficiency) if soil pH stays constant. But if soils acidify, a common response to N enrichment, then microbial biomass and enzymatic organic matter decay may decrease, increasing POC but not MAOC. We found that N enrichment had no effect on C fractions except for a decrease in MAOC at one site. Specifically, despite reported increases in plant biomass in three sites and decreases in microbial biomass and extracellular enzyme activities in two sites that acidified, POC did not increase. Furthermore, microbial C use and stabilization efficiency increased in a non-acidified site, but without increasing MAOC. Instead, MAOC decreased by 16% at one of the sites that acidified, likely because it lost 47% of the exchangeable calcium (Ca) relative to controls. Indeed, MAOC was strongly and positively affected by Ca, which directly and, through its positive effect on microbial biomass, explained 58% of variation in MAOC. Long-term effects of N fertilization on dryland SOC storage appear abiotic in nature, such that drylands where Ca-stabilization of SOC is prevalent and soils acidify, are most at risk for significant C loss.

Assessment of water and carbon use efficiency in the SAARC region for ecological resilience under changing climate.

According to the United Nations International Children's Emergency Fund (UNICEF) assessment report released in 2021, South Asian countries were among the most vulnerable in the world to the effects of climate change on future generations. Hence it is become crucial to assess how resilient the ecosystems are to these changes. The current study incorporated a novel approach, the Combined Ecological Resiliency Indices Approach (CERIA), to assess ecological resiliency status at various scales during hydroclimatic disturbances. Water and carbon use efficiency (WUE and CUE, respectively) were used as indicators for the examination of ecological resilience. The standardized Precipitation Index (SPI) was adopted to assess the initial stage of hydroclimatic disturbances (meteorological drought). A resiliency analysis based on combined Rd and Rd' indices (derived from WUE and CUE, respectively) revealed that just 1.87% land cover area of the entire SAARC (South Asian Association for Regional Cooperation) region's total 17 land cover classes was resilient to meteorological drought. At the river basin scale, only 16.58% of the total 62 river basins were found resilient. Only 11 (27.46%) of the 21 climate classes on the Koppen climate classification scale were resilient to the hydro-climatic disturbance period. To achieve the United Nations sustainable development goals (SDGs goal-2 and goal-13) of 'No Hunger' and 'Protect the Planet', the Joint Ecosystem Resiliency Enhancement Programme (JEREP) should be adopted in land cover, river basins, or climatic classes of the SAARC region that were highly affected.