Differential responses of carbon-degrading enzyme activities to warming: Implications for soil respiration

Impact of warming and nitrogen addition on soil greenhouse gas fluxes: A global perspective

Global warming and nitrogen (N) deposition have a profound impact on greenhouse gas (GHG) fluxes and consequently, they also affect climate change. However, the global combined effects of warming and N addition on GHG fluxes remain to be fully understood. To address this knowledge gap, a global meta-analysis of 197 datasets was performed to assess the response of GHG fluxes to warming and N addition and their interactions under various climate and experimental conditions. The results indicate that warming significantly increased CO2 emissions, while N addition and the combined warming and N addition treatments had no impact on CO2 emissions. Moreover, both warming and N addition and their interactions exhibited positive effects on N2O emissions. Under the combined warming and N addition treatments, warming was observed to exert a positive main effect on CO2 emissions, while N addition had a positive main effect on N2O emissions. The interactive effects of warming and N addition exhibited antagonistic effects on CO2, N2O, and CH4 emissions, with CH4 uptake dominated by additive effects. Furthermore, we identified biome and climate factors as the two treatments. These findings indicate that both warming and N addition substantially impact soil GHG fluxes and highlight the urgent need to investigate the influence of the combination of warming and N addition on terrestrial carbon and N cycling under ongoing global change.

Two pathways for reducing soil aggregate organic carbon mineralisation via minimum tillage under a long-term field experiment

Soil aggregates are the basic units of soil organic carbon (SOC) mineralisation. However, it is unclear through which pathways soil aggregates and their hydrolytic activities affect SOC mineralisation under minimum tillage. For this purpose, we conducted incubation experiments. Soil samples of conventional (CTS) and minimal tillage [subsoiling (SSS) and no tillage (NTS)] with straw return to the field were collected from a long-term tillage experimental field. Soil aggregates with column sizes of 2-5 mm (large macro-aggregates; LMA), 0.25-2 mm (small macro-aggregates; SMA) and 0.053-0.25 mm (micro-aggregates; MA) were prepared, with bulk soil serving as a control. Results indicated that minimum tillage increased macro-aggregate (>0.25 mm) content (12.9 %-17.7 %) and soil carbon fraction content (2.7 %-46.9 %) but decreased soil hydrolase activity (5.0 %-45.8 %) and CO2 cumulative emissions (8.1 %-28.0 %) compared with CTS. Minimum tillage reduced the SOC mineralisation contribution of SMA compared with CTS (66.6 %-70.3 %). Partial least squares path model analysis showed that there are two pathways by which the proportions of soil aggregate-size classes (ASC) and hydrolase activity affect SOC mineralisation in soil aggregates. First, minimum tillage increased the ASC of LMA, which negatively regulated the hydrolase activity of macro-aggregates, and then indirectly reduced SOC mineralisation by negatively regulating the content of soil carbon fractions. Second, minimum tillage decreased the ASC of MA, which positively regulated soil hydrolase activity and reduced SOC mineralisation. This study provides new insights into reducing GHG emissions from agricultural production from a soil aggregation perspective.

Increased temperature enhances microbial-mediated lignin decomposition in river sediment

BACKGROUND

Lignin, as the most abundant recalcitrant organic carbon in terrestrial ecosystems, plays a crucial role in the Earth's carbon cycle. After lignin entering aquatic environments, portion of it tends to accumulate in sediments, forming a stable carbon relatively reservoir. However, the increasing temperature caused by human activities may impact microbial-mediated lignin decomposition, thereby affecting sedimentary carbon reservoirs. Therefore, revealing how temperature affects microbial-mediated lignin decomposition in river sediment, a topic that remains elusive, is essential for comprehending the feedbacks between river carbon reservoirs and climate. To address this, we conducted stable isotope probing of river surface sediment using 13C-lignin and 13C-vanillin, and utilized a series of techniques, including CO2 production analysis, 16S rRNA gene amplicon sequencing, metagenomics, and metatranscriptomics, to identify the lignin-decomposing microbes and the effects of temperature on microbial-mediated lignin decomposition.

RESULTS

We found that elevated temperatures not only increased the total sediment respiration (total CO2) and the CO2 emissions from lignin/vanillin decomposition, but also enhanced priming effects. The 13C-labled taxa, including Burkholderiales, Sphingomonadales, and Pseudomonadales, were identified as the main potential lignin/vanillin decomposers, and their abundances and activity significantly increased as temperature increased. Furthermore, we observed that increasing temperature significantly increased the activity of lignin decomposing pathways, including β-aryl ether fragments and 4,5-PDOG pathway. Additionally, as temperature increases, the transcriptional abundances of other carbon cycling related genes, such as pulA (starch decomposition) and xyla (hemicellulose decomposition), also exhibited increasing trends. Overall, our study elucidated the potential lignin-decomposing microbes and pathways in river sediment and their responses to temperature increasing.

CONCLUSIONS

Our study demonstrated that the temperature increasing can increase the rate of lignin/vanillin decomposition via affecting the activity of lignin-decomposing microbes. This finding indicates that the ongoing intensification of global warming may enhance the decomposition of recalcitrant organic carbon in river sediment, thereby impacting global carbon cycling. Video Abstract.

Altered precipitation affects soil enzyme activity related to nitrogen and phosphorous but not carbon cycling: A meta-analysis

Altered precipitation significantly influences soil function in terrestrial ecosystems. As a bioindicator of soil function, soil extracellular enzyme activity (EEA) plays a crucial role in mediating ecosystem responses to altered precipitation. However, the global patterns and regulatory mechanisms of altered precipitation impacts on soil EEAs remain unclear. We conducted hierarchical mixed-effects meta-analyses to explore the responses and regulators of carbon, nitrogen, phosphorus hydrolytic EEAs, and carbon oxidative EEA to changes in precipitation, using the largest dataset to date, comprising 1185 observations of 14 soil EEAs from 73 publications. The results indicated that soil nitrogen hydrolytic EEA increased by 14.3% under increased precipitation, while phosphorus hydrolytic EEA decreased by 8.8% under decreased precipitation, showing higher sensitivity to altered precipitation compared to carbon-degrading EEAs. These responses varied across ecosystem types and depended on the magnitude of precipitation manipulation (MPM). Specially, decreased precipitation significantly reduced phosphorus hydrolytic EEA in forests, while increased precipitation enhanced nitrogen hydrolytic EEA in grasslands. Furthermore, these effects were linearly correlated with MPM, deviating from the expected nonlinear double asymmetric model. The response of soil hydrolytic EEAs was predominately regulated by soil water content, organic carbon, and microbial biomass. These findings underscore the higher sensitivity of nitrogen and phosphorus cycling EEAs to altered precipitation compared to carbon cycling EEAs and extend the application of the double asymmetric model for understanding soil EEAs' responses to precipitation changes. This synthesis provides essential insights for predicting biogeochemical cycling and improving ecosystem models to evaluate ecosystem functions under altered precipitation.

Microbial mechanisms of interactive climate-driven changes in soil N2O and CH4 fluxes: A global meta-analysis

Soils represent both a source of and sink for greenhouse gases (GHG). Elevated temperature (eT) affects both the physical and biological factors that drive GHG emissions from soil and thus understanding the effects of rising global temperatures on terrestrial GHG emission is needed to predict future GHG emissions, and to identify mitigation strategies. However, uncertainty remains about the interactive effects of multiple climate factors across different ecosystems, complicating our ability to develop robust climate change projections. Therefore, a global meta-analysis of 1337 pairwise observations from 150 peer-reviewed publications (1990-2023) was conducted to assess the individual effect of eT and its combined effects with eCO2 (eT + eCO2), drought (eT + drought) and increased precipitation (eT + ePPT) on soil N2O and CH4 fluxes, microbial functional genes, and soil extracellular enzyme activities across grassland, cropland, and forestland ecosystems. Across the dataset, eT significantly increased N2O emissions (21%) and CH4 uptake (36%). Nitrogen cycling was consistently stimulated by eT, with NO3- and NH4+ and the abundance of amoA-AOB gene increasing by 6%, 10%, and 18%, respectively. Soil water content (SWC) was reduced, whereas increases of 9% in soil organic carbon (SOC), 14% in microbial biomass carbon (MBC), and 10% in total plant biomass were found under eT. The stimulation of soil N2O emissions by eT was maintained for all ecosystems when combined with other global change factors (ie., eT + eCO2, eT + ePPT, and eT + drought). By contrast, effects of eT on CH4 uptake and emissions were more variable when combined with other factors; for instance, eT + eCO2 and eT + ePPT suppressed CH4 uptake in grasslands. This study highlights the urgent need to study the microbial mechanisms responsible for combined global change effects on N2O and especially CH4 fluxes.

Minor Effects of Warming on Soil Microbial Diversity, Richness and Community Structure

Climate warming has caused widespread global concern. However, how warming affects soil microbial diversity, richness, and community structure on a global scale remains poorly understood. Here we conduct a meta-analysis of 945 observations from 100 publications by collecting relevant data. The results show that field warming experiments significantly modify soil temperature (+1.8°C), soil water content (-3.2%), and soil pH (-0.04). However, field warming does not significantly alter the diversity, richness, and community structure of soil bacteria and fungi. Warming-induced changes in soil variables (i.e., ΔSoil water content, ΔpH), ΔTemperature and experimental duration are important factors influencing the microbial responses to warming. In addition, soil bacterial α-diversity (Shannon index) decreases significantly (-3.4%) when the warming duration is 3-6 years, and bacterial β-diversity increases significantly (35.2%) when warming exceeds 6 years. Meta-regression analysis reveals a positive correlation between the change of bacterial Shannon index and ΔpH. Moreover, warming produces more pronounced effects on fungal Shannon index and β-diversity in experimental sites with moderate mean annual temperature (MAT, 0°C-10°C) than in higher (> 10°C) or lower (< 0°C) MAT. Overall, this study provides a global perspective on the response of soil microorganisms to climate warming and improves our knowledge of the factors influencing the response of soil microorganisms to warming.

Heating up the roof of the world: tracing the impacts of in-situ warming on carbon cycle in alpine grasslands on the Tibetan Plateau

Climate warming may induce substantial changes in the ecosystem carbon cycle, particularly for those climate-sensitive regions, such as alpine grasslands on the Tibetan Plateau. By synthesizing findings from in-situ warming experiments, this review elucidates the mechanisms underlying the impacts of experimental warming on carbon cycle dynamics within these ecosystems. Generally, alterations in vegetation structure and prolonged growing season favor strategies for enhanced ecosystem carbon sequestration under warming conditions. Whilst warming modifies soil microbial communities and their carbon-related functions, its effects on soil carbon release fall behind the increased vegetation carbon uptake. Despite the fact that no significant accumulation of soil carbon stock has been detected upon warming, notable changes in its fractions indicate potential shifts in carbon stability. Future studies should prioritize deep soil carbon dynamics, the interactions of carbon, nitrogen, and phosphorus cycles under warming scenarios, and the underlying biological mechanisms behind these responses. Furthermore, the integration of long-term warming experiments with Earth system models is essential for reducing the uncertainties of model predictions regarding future carbon-climate feedback in these climate-sensitive ecosystems.

Changes in enzyme activity, structure and growth strategies of the rhizosphere microbiome influenced by elevated temperature and CO2

The impacts of global warming and increased CO2 levels on soil processes and crop growth are concerning. Soil enzymes in the rhizosphere, produced mainly by microbes, play a vital role in nutrients mobilization for plants. Nevertheless, a comprehensive understanding of how microbial communities in the rhizosphere respond to increased temperatures and CO2 levels, particularly in relation to nutrient acquisition, is still lacking. Addressing this problem, we grew soybeans under elevated temperature (ET, +2 °C) and CO2 levels (eCO2, +300 ppm), both individually and in combination (eCO2 + eT), in rhizobox mesocosms. Enzyme activity and microbial communities in soybean rhizospheres were investigated using soil zymography. eCO2 increased enzyme activity by 2.5 % to 8.7 %, while eT expanded the hotspot area from 1.8 % to 3.3 %. The combined factors amplified both the hotspot area by 5.3 % to 10.1 % and enzyme activity by 35.4 % to 67.3 %. Compared to ambient conditions, rhizosphere communities under eCO2 were predominantly comprised of r-strategist keystone taxa, including Acidobacteria, Proteobacteria, and Ascomycota. On the contrary, eT induced a shift in the microbial community towards K-selected taxa, characterized by an increased relative abundance of Basidiomycota and Actinobacteria. Furthermore, the combination of eCO2 and eT led to an increase in the relative abundance of key bacterial species (Acidobacteria, Proteobacteria, and Actinobacteria) as well as fungi (Ascomycota and Basidiomycota). These findings indicate the potential significance of enzyme hotspots in modulating responses to climate change. Changes in enzyme activity and hotspot area could indicate the alteration in microbial growth strategies. The treatments exhibited distinct changes in the composition of microbial communities, in network organization, and in the proportion of species designated as r or K-strategists. Overall, these findings highlight the combined effects of global change factors on bacterial and fungal communities, providing insights into their growth strategies and nutrient mobilization under climate change scenarios.

Role of methanogenesis and methanotrophy in CH4 fluxes from rice paddies under elevated CO2 concentration and elevated temperature

The differential responses of methanogenesis and methanotrophy to elevated carbon dioxide concentrations ([CO2]) (e[CO2]) and elevated temperature ([T]) (e[T]) may lead to dramatic changes in the response of CH4 emissions from rice paddies to global warming. In this study, we systematically investigated the responses and mechanisms of CH4 flux from rice paddies to e[CO2] and e[T] based on the production and oxidation of CH4. The CH4 flux, soil properties, and soil methanogenesis and methanotrophy were observed under CK (ambient [CO2] + ambient [T]), EC (e[CO2] by 200 μmol mol-1 + ambient [T]), ET (ambient [CO2] + e[T] by 2 °C), and ECT (e[CO2] by 200 μmol mol-1 + e[T] by 2 °C) treatments. The results revealed that EC, ET, and ECT significantly increased the cumulative amount of CH4 (CAC) in the rice paddies by 10.63, 15.20, and 11.77 kg ha-1, respectively, compared with CK. ECT increased the CAC in the rice paddies by 1.14 kg ha-1 compared with EC. Moreover, EC, ET, and ECT significantly enhanced the methane production potential (MPP) and methane oxidation potential (MOP) and tended to increase the mcrA gene abundance of the methanogens. EC tended to prompt the pmoA gene abundance of the methanotrophs, but the effect of ET on the pmoA gene abundance was less consistent across the growth stages. ECT significantly decreased the relative abundances of Methanosarcina and Methylocystis (Type II) by 4.9 % and 14.2 %, respectively, while it increased the relative abundance of Methylosarcina (Type I) by 24.0 % compared with CK. Overall, the increased MPP/MOP, mcrA/pmoA, and microbial biomass carbon under climate change increased the CH4 flux from the rice paddies. The contribution of e[CO2] to the CH4 flux was significantly enhanced by e[T], which could further exacerbate the risk of global climate change induced by e[CO2].

Asymmetric winter warming reduces microbial carbon use efficiency and growth more than symmetric year-round warming in alpine soils

Asymmetric seasonal warming trends are evident across terrestrial ecosystems, with winter temperatures rising more than summer ones. Yet, the impact of such asymmetric seasonal warming on soil microbial carbon metabolism and growth remains poorly understood. Using 18O isotope labeling, we examined the effects of a decade-long experimental seasonal warming on microbial carbon use efficiency (CUE) and growth in alpine grassland ecosystems. Moreover, the quantitative stable isotope probing with 18O-H2O was employed to evaluate taxon-specific bacterial growth in these ecosystems. Results show that symmetric year-round warming decreased microbial growth rate by 31% and CUE by 22%. Asymmetric winter warming resulted in a further decrease in microbial growth rate of 27% and microbial CUE of 59% compared to symmetric year-round warming. Long-term warming increased microbial carbon limitations, especially under asymmetric winter warming. Long-term warming suppressed the growth rates of most bacterial genera, with asymmetric winter warming having a stronger inhibition on the growth rates of specific genera (e.g., Gp10, Actinomarinicola, Bosea, Acidibacter, and Gemmata) compared to symmetric year-round warming. Bacterial growth was phylogenetically conserved, but this conservation diminished under warming conditions, primarily due to shifts in bacterial physiological states rather than the number of bacterial species and community composition. Overall, long-term warming escalated microbial carbon limitations, decreased microbial growth and CUE, with asymmetric winter warming having a more pronounced effect. Understanding these impacts is crucial for predicting soil carbon cycling as global warming progresses.

Contrasting carbon cycle responses of semiarid abandoned farmland to simulated warmer-drier and warmer-wetter climates

Rewilding abandoned farmlands provides a nature-based climate solution via carbon (C) offsetting; however, the C-cycle-climate feedback in such restored ecosystems is poorly understood. Therefore, we conducted a 2-year field experiment in Loess Plateau, China, to determine the impacts of warming (∼1.4 °C) and altered precipitation (±25 %, ±50 %, and ambient), alone or in concert on soil C pools and associated C fluxes. Experimental warming significantly enhanced soil respiration without affecting the ecosystem net C uptake and soil C storage; these variables tended to increase along the manipulated precipitation gradient. Their interactions increased ecosystem net C uptake (synergism) but decreased soil respiration and soil C accumulation (antagonism) compared with a single warming or altered precipitation. Additionally, most variables related to the C cycle tended to be more responsive to increased precipitation, but the ecosystem net C uptake responded intensely to warming and decreased precipitation. Overall, ecosystem net C uptake and soil C storage increased by 94.4 % and 8.2 %, respectively, under the warmer-wetter scenario; however, phosphorus deficiency restricted soil C accumulation under these climatic conditions. By contrast, ecosystem net C uptake and soil C storage decreased by 56.6 % and 13.6 %, respectively, when exposed to the warmer-drier climate, intensifying its tendency toward a C source. Therefore, the C sink function of semiarid abandoned farmland was unsustainable. Our findings emphasize the need for management of post-abandonment regeneration to sustain ecosystem C sequestration in the context of climate change, aiding policymakers in the development of C-neutral routes in abandoned regions.

Initial soil condition, stand age, and aridity alter the pathways for modifying the soil carbon under afforestation

Afforestation is a crucial pathway for ecological restoration and has the potential to modify soil microbial community, thereby impacting the cycling and accumulation of carbon in soil across diverse patterns. However, the overall patterns of how afforestation impacts below-ground carbon cycling processes remain uncertain. In this comprehensive meta-analysis, we systematically evaluated 7045 observations from 210 studies worldwide to evaluate the influence of afforestation on microbial communities, enzyme activities, microbial functions, and associated physicochemical properties of soils. Afforestation increases microbial biomass, carbon and nitrogen hydrolase activities, and microbial respiration, but not carbon oxidase activity and nitrogen decomposition rate. Conversely, afforestation leads to a reduction in the metabolic quotient, with significant alteration of bacterial and fungal community structures and positive effects on the fungi: bacteria ratio rather than alpha and beta diversity metrics. We found a total 77 % increase in soil organic carbon (SOC) content after afforestation, which varied depending on initial SOC content before afforestation, afforestation stand age, and aridity index of afforestation sites. The modified SOC is associated with bacterial community composition along with intracellular metabolic quotient and extracellular carbon degrading enzyme activity playing a role. These findings provide insights into the pathways through which afforestation affects carbon cycling via microorganisms, thus improving our knowledge of soil carbon reservoir's responses to afforestation under global climate change.

Carbon availability and microbial activity manipulate the temperature sensitivity of anaerobic degradation in a paddy soil profile

Soil contains a substantial amount of organic carbon, and its feedback to global warming has garnered widespread attention due to its potential to modulate atmospheric carbon (C) storage. Temperature sensitivity (Q10) has been widely utilized as a measure of the temperature-induced enhancement in soil organic carbon (SOC) decomposition. It is currently rare to incorporate Q10 of CO2 and CH4 into the study of waterlogged soil profiles and explore the possibility of artificially reducing Q10 in rice fields. To investigate the key drivers of Q10, we collected 0-1 m paddy soil profiles, and stratified the soil for submerged anaerobic incubation. The relationship between SOC availability, microbial activity, and the Q10 of CO2 and CH4 emissions was examined. Our findings indicate that as the soil layer deepens, soil C availability and microbial activity declined, and the Q10 of anaerobic degradation increased. Warming increased C availability and microbial activity, accompanied by weakened temperature sensitivity. The Q10 of CO2 correlated strongly with soil resistant C components, while the Q10 of CH4 was significantly influenced by labile substrates. The temperature sensitivity of CH4 (Q10 = 3.99) was higher than CO2 emissions (Q10 = 1.78), indicating the need for greater attention of CH4 in predicting warming's impact on anaerobic degradation in rice fields. Comprehensively assessing CO2 and CH4 emissions, the 20-40 cm subsurface soil is the most temperature-sensitive. Despite being a high-risk area for C loss and CH4 emissions, management of this soil layer in agriculture has the potential to reduce the threat of global warming. This study underscores the importance of subsurface soil in paddy fields, advocating greater attention in scientific simulations and predictions of climate change.

Long-term vegetation restoration promotes lignin phenol preservation and microbial anabolism in forest plantations: Implications for soil organic carbon dynamics

Vegetation restoration contributes to soil organic carbon (C; SOC) sequestration through the accumulation of plant and microbial residues, but the mechanisms underlying this microbially mediated process are not well resolved. To depict the dynamics of plant- and microbial-derived C in restored forest ecosystems, soil samples were collected from Robinia pseudoacacia plantations of different stand ages (15, 25, 35, 45 years old) established on degraded wheat fields. The results showed that the degree of lignin phenol oxidation decreased with increasing stand age (P < 0.05), and hemicellulose-degrading genes were detected at higher relative abundances than other functional gene categories, indicating selective preservation of recalcitrant lignin phenols. Despite both glucosamine (R2 = 0.61, P < 0.001) and muramic acid (R2 = 0.37, P < 0.001) contents trending upward over time, fungal residual C accounted for a greater proportion of SOC compared with bacterial residual C. Accordingly, fungal residual C, which exhibited a similar response pattern as total microbial residual C to vegetation restoration, was considered a major contributor to the SOC pool. These results provided evidence that long-term vegetation restoration enhanced SOC sequestration in R. pseudoacacia forest by promoting the preservation of plant-derived lignin phenols and concomitant microbial anabolism. Partial least squares-discriminant analysis identified two important ecological clusters (i.e., modules) in the fungal network that profoundly influenced lignin phenol oxidation (P < 0.05) and microbial residual C accumulation (P < 0.01). Among the dominant taxa in microbial networks, the bacterial phyla Proteobacteria and Acidobacteriota had potential to degrade recalcitrant C compounds (e.g., cellulose, lignin), whereas the fungal phylum Ascomycota could outcompete for labile C fractions (e.g., dissolved organic C). Findings of this study can enable a mechanistic understanding of SOC stability driven by microbial turnover in restored forest ecosystems.

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.

Effects of warming on soil organic carbon pools mediated by mycorrhizae and hyphae on the Eastern Tibetan Plateau, China

Mycorrhizae and their hyphae play critical roles in soil organic carbon (SOC) accumulation. However, their individual contributions to SOC components and stability under climate warming conditions remain unclear. This study investigated the effects of warming on the SOC pools of Picea asperata (an ectomycorrhizal plant) and Fargesia nitida (an arbuscular mycorrhizal plant) mycorrhizae/hyphae on the eastern Tibetan Plateau. The results indicated that mycorrhizae made greater contributions to SOC accumulation than hyphae did by increasing labile organic carbon (LOC) components, such as particle organic carbon (POC), easily oxidizable organic carbon, and microbial biomass carbon, especially under warming conditions. Plant species also had different effects on SOC composition, resulting in higher mineral-associated organic carbon (MAOC) contents in F. nitida plots than in P. asperata plots; consequently, the former favored SOC stability more than the latter, with a lower POC/MAOC. Partial least-squares path modelling further indicated that mycorrhizae/hyphae indirectly affected LOC pools, mainly by changing soil pH and enzyme activities. Warming had no significant effect on SOC content but did change SOC composition by reducing LOC through affecting soil pH and iron oxides and ultimately increasing SOC stability in the presence of mycorrhizae for both plants. Therefore, the mycorrhizae of both plants are major contributors to the variation of SOC components and stability under warming conditions.

Warming enhances the negative effects of shrub removal on phosphorus mineralization potential

Shrubs have developed various mechanisms for soil phosphorus utilization. Shrub encroachment caused by climate warming alters organic phosphorus mineralization capability by promoting available phosphorus absorption and mediating root exudates. However, few studies have explored how warming regulates the effects of dominant shrubs on soil organic phosphorus mineralization capability. We provide insights into warming, dominant shrub removal, and their interactive effects on the soil organic phosphorus mineralization potential in the Qinghai-Tibetan Plateau. Real-time polymerase chain reaction was used to quantify the soil microbial phosphatase genes (phoC and phoD), which can characterize the soil organic phosphate mineralization potential. We found that warming had no significant effect on the soil organic phosphate-mineralized components (total phosphate, organic phosphate, and available phosphate), genes (phoC and phoD), or enzymes (acid and alkaline phosphatases). Shrub removal negatively influenced the organic phosphate-mineralized components and genes. It significantly decreased soil organic phosphate mineralization gene copy numbers only under warming conditions. Warming increased fungal richness and buffered the effects of shrub removal on bacterial richness and gene copy numbers. However, the change in the microbial community was not the main factor affecting organic phosphate mineralization. We found only phoC copy number had significant correlation to AP. Structural equation modelling revealed that shrub removal and the interaction between warming and shrub removal had a negative direct effect on phoC copy numbers. We concluded that warming increases the negative effect of shrub removal on phosphorus mineralization potential, providing a theoretical basis for shrub encroachment on soil phosphate mineralization under warming conditions.

Grazing alters the soil nematode communities in grasslands: A meta-analysis

Grazing causes great disturbances in grassland ecosystems and may change the abundance, diversity, and ecological function of soil biota. Because of their important role in nutrient cycling and as good environmental indicators, nematodes are very representative soil organisms. However, the mechanisms by which grazing intensity, livestock type, duration, and environmental factors (e.g., climate and edaphic factors) affect soil nematodes remain poorly understood. In this study, we collected 1964 paired observations all over the world from 53 studies to clarify the grazing response patterns of soil nematodes and their potential mechanisms. Overall, grazing significantly decreased the abundance of bacterial-feeding (BF) nematodes (-16.54%) and omnivorous-predatory (OP) nematodes (-36.81%), and decreased nematode community diversity indices (Shannon-Weiner index: -4.33%, evenness index: -9.22%, species richness: -5.35%), but had no effect on ecological indices under a global regional scale. The response of soil nematodes to grazing varied by grazing intensity, animals, and duration. Heavy grazing decreased OP nematode abundance, but had no effect on the abundance of other trophic groups, or on diversity or ecological indices. Grazing by small animals had stronger effects than that by large animals and mixed-size animals on BF, fungal-feeding (FF), plant-feeding (PF) and OP nematodes, the Shannon-Wiener index, and the species richness index. The abundance of FF and OP nematodes influenced significantly under short-term grazing. The evenness index decreased significantly under long-term grazing (>10 years). Climate and edaphic factors impacted the effects of grazing on nematode abundance, diversity, and ecological indices. When resources (i.e., rain, heat, and soil nutrients) were abundant, the negative effects of grazing on nematodes were reduced; under sufficiently abundant resources, grazing even had positive effects on soil nematode communities. Thus, the influence of grazing on soil nematode communities is resource-dependent. Our study provides decision makers with grazing strategies based on the resource abundance. Resource-poor areas should have less grazing, while resource-rich areas should have more grazing to conserve soil biodiversity and maintain soil health.

A meta-analysis reveals increases in soil organic carbon following the restoration and recovery of croplands in Southwest China

In China, the Grain for Green Program (GGP) is an ambitious project to convert croplands into natural vegetation, but exactly how changes in vegetation translate into changes in soil organic carbon remains less clear. Here we conducted a meta-analysis using 734 observations to explore the effects of land recovery on soil organic carbon and nutrients in four provinces in Southwest China. Following GGP, the soil organic carbon content (SOCc) and soil organic carbon stock (SOCs) increased by 33.73% and 22.39%, respectively, compared with the surrounding croplands. Similarly, soil nitrogen increased, while phosphorus decreased. Outcomes were heterogeneous, but depended on variations in soil and environmental characteristics. Both the regional land use and cover change indicated by the landscape type transfer matrix and net primary production from 2000 to 2020 further confirmed that the GGP promoted the forest area and regional mean net primary production. Our findings suggest that the GGP could enhance soil and vegetation carbon sequestration in Southwest China and help to develop a carbon-neutral strategy.

Mechanisms of microbial-driven changes in soil ecological stoichiometry around gold mines

In this study, we used soils with different pollution and nutrient levels (non-polluted S1, highly polluted low-nutrient S2, and highly polluted high nutrient S3) around the gold mine tailing ponds, and combined with metabolic limitation modeling and macro-genomics approaches, aiming to investigate the relationship between soil microbial composition and soil eco-chemometrics characteristics under heavy metal stress. The results showed that heavy pollution resulted in reduced SOC, TN, microbial biomass, and with C- and P- acquisition (BG, CBH, ALP) as well as nitrogen limitation of soil microbial metabolism in soils (S2, S3). Further analysis by macrogenomics showed that heavy metal contamination led to an increase in α-microbial diversity and altered the composition of microbial communities in the soil. The cycling of C, N, and P nutrients was altered by affecting the relative abundance of Anaeromyxobacter, Steroidobacter, Bradyrhizobium, Acidobacterium, Limnochorda (predominantly in the Ascomycetes and Acidobacteria phyla), with the most pronounced effect on the composition of microorganisms synthesizing C-acquiring enzymes, and heavy metals and pH were the main influences on ecological stoichiometry. The results of this study are useful for understanding the sustainability of ecological remediation in heavy metal contaminated areas and for developing ecological restoration strategies.

Microbial-driven mechanisms for the effects of heavy metals on soil organic carbon storage: A global analysis

Heavy metal (HM) enrichment is closely related to soil organic carbon (SOC) pools in terrestrial ecosystems, which are deeply intertwined with soil microbial processes. However, the influence of HMs on SOC remains contentious in terms of magnitude and direction. A global analysis of 155 publications was conducted to integrate the synergistic responses of SOC and microorganisms to HM enrichment. A significant increase of 13.6 % in SOC content was observed in soils exposed to HMs. The response of SOC to HMs primarily depends on soil properties and habitat conditions, particularly the initial SOC content, mean annual precipitation (MAP), initial soil pH, and mean annual temperature (MAT). The presence of HMs resulted in significant decreases in the activities of key soil enzymes, including 31.9 % for soil dehydrogenase, 24.8 % for β-glucosidase, 35.8 % for invertase, and 24.3 % for cellulose. HMs also exerted inhibitory effects on microbial biomass carbon (MBC) (26.6 %), microbial respiration (MR) (19.7 %), and the bacterial Shannon index (3.13 %) but elevated the microbial metabolic quotient (qCO2) (20.6 %). The HM enrichment-induced changes in SOC exhibited positive correlations with the response of MBC (r = 0.70, p < 0.01) and qCO2 (r = 0.50, p < 0.01), while it was negatively associated with β-glucosidase activity (r = 0.72, p < 0.01) and MR (r = 0.39, p < 0.01). These findings suggest that the increase in SOC storage is mainly attributable to the inhibition of soil enzymes and microorganisms under HM enrichment. Overall, this meta-analysis highlights the habitat-dependent responses of SOC to HM enrichment and provides a comprehensive evaluation of soil carbon dynamics in an HM-rich environment.

Unveiling the impacts moso bamboo invasion on litter and soil properties: A meta-analysis

Moso bamboo invasion potentially alters litter, soil properties and soil microbial communities in forest ecosystems. However, the overall direction and magnitude of this alteration at a large spatial scale remain unclear. Here, we conducted a meta-analysis of 72 experimental studies on the impact of moso bamboo invasion on litter, soil physicochemical properties, and soil microbial communities. Overall, the moso bamboo invasion increased litter decomposition, soil pH, and NH4+-N, while concurrently leading to a decrease in soil bulk density, soil electrical conductivity, soil TN: TP ratio, soil NO3--N, and available potassium. Moreover, we observed that the invasion significantly enhanced soil microbial biomass nitrogen, fungal ACE diversity index, fungal biomass, and bacterial Shannon diversity index, while decreasing the ratio of Gram-positive to Gram-negative bacteria and the biomass of Gram-positive bacteria. Furthermore, we identified the primary factors influencing specific soil properties and microbial community responses to moso bamboo invasion. Specifically, the response of NH4+-N, NO3--N, soil bulk density, fungal diversity and pH were found to be primarily influenced by climatic factors (mean annual temperature, mean annual precipitation), topographic factors (aspect), and invasion stage, respectively. In addition, we further revealed a close relationship between soil physicochemical properties and microbial communities during moso bamboo invasion. Specifically, the response of soil microbial biomass nitrogen was positively correlated with the responses of soil organic nitrogen and total nitrogen content, Gram-positive bacteria biomass was positively correlated with soil total nitrogen but negatively correlated with soil pH. Meanwhile, soil bacterial diversity showed a significant positive correlation with soil pH but exhibited a negative correlation with soil SOC. Our study suggests that macro-climatic conditions, local microenvironment, and invasion stage co-regulate the important effects of moso bamboo invasion on litter, soil physicochemical properties, and microbial communities.

Warming promotes accumulation of microbial- and plant-derived carbon in terrestrial ecosystems

The impact of global warming on soil carbon pools has been extensively investigated, however, there is still a lack of understanding regarding the specific response of microbial- and plant-derived carbon to warming. To address this knowledge gap, we conducted a comprehensive meta-analysis of 142 studies and evaluated 986 observations comparisons of different carbon source responses to warming. Our results revealed several key insights. Firstly, climate warming resulted in an average increase of 5.46 % in the terrestrial soil carbon pool. Specifically, microbial-derived carbon showed an average increase of 6.32 %, while plant-derived carbon exhibited an average increase of 3.70 %. Secondly, while warming duration and magnitude do not significantly affect the response of microbial-derived carbon to warming, they did impact the response of plant-derived carbon. Lastly, we observed that the response of different carbon sources to warming was affected by the specific environmental backgrounds:ecosystem and climatic zone types affect the response of warming to microbial-derived carbon, while differences in climatic region affect response of warming to plant-derived carbon. The variations in the response of different soil carbon sources to warming can be attributed to the nature of the carbon source themselves, as well as the complex transformations that occur between them through microbial metabolic processes and their interactions with soil mineral particles. We suggest that interactions at the soil-plant-microbe interface should be considered more carefully, and the response of ecosystems to warming should be observed from the perspective of soil organic carbon sources, so as to better understand the response of terrestrial ecosystems carbon cycle to global warming.

Macrofungi promote SOC decomposition and weaken sequestration by modulating soil microbial function in temperate steppe

Soil organic carbon (SOC) sequestration is a key grassland ecosystem function, and the magnitude of SOC reservoirs depends on microbial involvement, especially that of fungi. Mycelia developed by macrofungi potentially influence carbon (C) fixation and decomposition; however, the mechanisms underlying their effects on SOC storage in grassland ecosystems remain poorly understood. The fairy rings formed by macrofungi in grasslands are natural platform for exploring macrofungal effects on SOC. In this study, we collected topsoil (0-10 cm) from four different fairy ring zones in a temperate steppe to reveal the macrofungal effects on SOC fractions, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), and the SOC storage microbial mechanism using metagenomic sequencing technology. Both POC and MAOC decreased after macrofungal passage, resulting in a 7.37 % reduction in SOC. Macrofungal presence reduced microbial biomass carbon (MBC), but significantly enhanced the β-1,4-glucosidase (BG) activity, which increased dissolved organic carbon (DOC). In addition, the abundance of copiotrophs (Proteobacteria and Bacteroidetes) with lower C metabolic rates increased, and that of oligotrophs (Actinobacteria, Acidobacteria, Chloroflexi, and Verrucomicrobia) with higher substrate utilization efficiency decreased in the presence of macrofungi. This may further promote SOC decomposition. Correspondingly, there was a lower abundance of C-fixation genes but more C-degradation genes (especially hemicellulosic degradation genes) during macrofungal passage. Our results indicate that the presence of macrofungi can modulate the soil microbial community and functional genes to reduce SOC storage by inhibiting microbial C sequestration while promoting C decomposition in grassland ecosystems. These findings refine our mechanistic understanding of SOC persistence through the interactions between macrofungi and other microbes.

Dominant influence of plants on soil microbial carbon cycling functions during natural restoration of degraded karst vegetation

The impacts of natural restoration projects on soil microbial carbon (C) cycling functions have not been well recognized despite their wide implementation in the degraded karst areas of southwest China. In this study, metagenomic sequencing assays were conducted on functional genes and microorganisms related to soil C-cycling at three natural restoration stages (shrubbery, TG; secondary forest, SG; old-growth forest, OG) in the southeast of Guizhou Province, China. The aims were to investigate the changes in microbial potentials responsible for soil C cycling and the underlying driving forces. The natural restoration resulted in vegetation establishment at all three restoration stages, rendering alterations of soil microbial C cycle functions as indicated by metagenomic gene assays. When TG was restored into OG, the number and diversity of genes and microorganisms involved in soil C cycling remained unchanged, but their composition underwent significant shifts. Specifically, microbial potentials for soil C decomposition exhibited an increase driven by the collaborative efforts of plants and soils, while microbial potentials for soil C biosynthesis displayed an initial upswing followed by a subsequent decline which was primarily influenced by plants alone. In comparison to soil nutrients, it was determined that plant diversities served as the primary driving factor for the alterations in microbial carbon cycle potentials. Soil microbial communities involved in C cycling were predominantly attributed to Proteobacteria (31.87%-40.25%) and Actinobacteria (11.29%-26.07%), although their contributions varied across the three restoration stages. The natural restoration of degraded karst vegetation thus influences soil microbial C cycle functions by enhancing C decomposition potentials and displaying a nuanced pattern of biosynthesis potentials, primarily influenced by above-ground plants. These results provide valuable new insights into the regulation of soil C cycling during the restoration of degraded karst vegetation from genetic and microbial perspectives.

Trade-offs in carbon-degrading enzyme activities limit long-term soil carbon sequestration with biochar addition

Biochar amendment is one of the most promising agricultural approaches to tackle climate change by enhancing soil carbon (C) sequestration. Microbial-mediated decomposition processes are fundamental for the fate and persistence of sequestered C in soil, but the underlying mechanisms are uncertain. Here, we synthesise 923 observations regarding the effects of biochar addition (over periods ranging from several weeks to several years) on soil C-degrading enzyme activities from 130 articles across five continents worldwide. Our results showed that biochar addition increased soil ligninase activity targeting complex phenolic macromolecules by 7.1%, but suppressed cellulase activity degrading simpler polysaccharides by 8.3%. These shifts in enzyme activities explained the most variation of changes in soil C sequestration across a wide range of climatic, edaphic and experimental conditions, with biochar-induced shift in ligninase:cellulase ratio correlating negatively with soil C sequestration. Specifically, short-term (<1 year) biochar addition significantly reduced cellulase activity by 4.6% and enhanced soil organic C sequestration by 87.5%, whereas no significant responses were observed for ligninase activity and ligninase:cellulase ratio. However, long-term (≥1 year) biochar addition significantly enhanced ligninase activity by 5.2% and ligninase:cellulase ratio by 36.1%, leading to a smaller increase in soil organic C sequestration (25.1%). These results suggest that shifts in enzyme activities increased ligninase:cellulase ratio with time after biochar addition, limiting long-term soil C sequestration with biochar addition. Our work provides novel evidence to explain the diminished soil C sequestration with long-term biochar addition and suggests that earlier studies may have overestimated soil C sequestration with biochar addition by failing to consider the physiological acclimation of soil microorganisms over time.

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.

Tidal organic input restricts CO2 sequestration capacity of estuarine wetlands

The inland and estuary wetlands that characterized by different natural environment perform distinctly in soil carbon (C) sink. It was deemed that estuary wetland has a higher organic C accumulation rate than inland wetland, due to its higher primary production and tidal organics input, thus having higher organic C sink capacity. While from CO₂ budge in view, whether does the large organic input from tide restrict CO₂ sequestration capacity of estuary wetland has not been discussed comparing with inland wetland. In this study, inland and estuary wetlands were selected to study the potential of CO₂ sequestration capacity. It was found that inland wetland had most of soil organic carbon (SOC) derived from plant C, which brought remarkable organic C content and nourished higher microbial biomass, dehydrogenase, and β_glucosidase than estuary wetland. The estuary wetland instead accumulated less SOC, a considerable proportion of which came from tidal waters, therefore supporting lower microbial biomass and enzyme activities than that in inland wetland. However, estuary wetland was evaluated having higher capability in SOC mineralization than inland wetland in consideration of soil respiration (SR) and SR quotient. It was concluded that tidal organic C accelerated the SOC mineralization in estuarine wetland, thus weakening the CO₂ sequestration. These results implied the importance of pollution control for reservation CO₂ sink function in estuarine wetland.

Linking between soil properties, bacterial communities, enzyme activities, and soil organic carbon mineralization under ecological restoration in an alpine degraded grassland

Soil organic carbon (SOC) mineralization is affected by ecological restoration and plays an important role in the soil C cycle. However, the mechanism of ecological restoration on SOC mineralization remains unclear. Here, we collected soils from the degraded grassland that have undergone 14 years of ecological restoration by planting shrubs with Salix cupularis alone (SA) and, planting shrubs with Salix cupularis plus planting mixed grasses (SG), with the extremely degraded grassland underwent natural restoration as control (CK). We aimed to investigate the effect of ecological restoration on SOC mineralization at different soil depths, and to address the relative importance of biotic and abiotic drivers of SOC mineralization. Our results documented the statistically significant impacts of restoration mode and its interaction with soil depth on SOC mineralization. Compared with CK, the SA and SG increased the cumulative SOC mineralization but decreased C mineralization efficiency at the 0-20 and 20-40 cm soil depths. Random Forest analyses showed that soil depth, microbial biomass C (MBC), hot-water extractable organic C (HWEOC), and bacterial community composition were important indicators that predicted SOC mineralization. Structural equal modeling indicated that MBC, SOC, and C-cycling enzymes had positive effects on SOC mineralization. Bacterial community composition regulated SOC mineralization via controlling microbial biomass production and C-cycling enzyme activities. Overall, our study provides insights into soil biotic and abiotic factors in association with SOC mineralization, and contributes to understanding the effect and mechanism of ecological restoration on SOC mineralization in a degraded grassland in an alpine region.

Nitrogen and water availability control plant carbon storage with warming

Plants may slow global warming through enhanced growth, because increased levels of photosynthesis stimulate the land carbon (C) sink. However, how climate warming affects plant C storage globally and key drivers determining the response of plant C storage to climate warming remains unclear, causing uncertainty in climate projections. We performed a comprehensive meta-analysis, compiling 393 observations from 99 warming studies to examine the global patterns of plant C storage responses to climate warming and explore the key drivers. Warming significantly increased total biomass (+8.4 %), aboveground biomass (+12.6 %) and belowground biomass (+10.1 %). The effect of experimental warming on plant biomass was best explained by the availability of soil nitrogen (N) and water. Across the entire dataset, warming-induced changes in total, aboveground and belowground biomass all positively correlated with soil C:N ratio, an indicator of soil N availability. In addition, warming stimulated plant biomass more strongly in humid than in dry ecosystems, and warming tended to decrease root:shoot ratios at high soil C:N ratios. Together, these results suggest dual controls of warming effects on plant C storage; warming increases plant growth in ecosystems where N is limiting plant growth, but it reduces plant growth where water availability is limiting plant growth.

Warming Enhances the Co-Metabolism Effect During the Decomposition of Sediment Organic Carbon in Eutrophic Lakes

Warming has been affecting carbon cycling in freshwater ecosystems throughout recent decades. However, how the co-metabolism effect (CE) during the decomposition of sediment organic carbon (SOC) in eutrophic lakes responds to warming remains understudied. A 33-day experiment was conducted to examine the mechanisms that underpin the CE in lacustrine sediments. The results indicated that warming increased the co-metabolism intensity of sedimentary organic matter. At the beginning of the experiment (0-9 d), the co-metabolism intensity increased rapidly at both 25℃ and 35℃. However, at the end of the experiment (33 d), the cumulative co-metabolism intensity was highest at 25℃, which was 33.75% and 153.74% higher than the intensities at 15℃ and 35℃, respectively. By enhancing the co-metabolism intensity of the SOC, warming would weaken lakes "carbon sink" functions. Thus, our study provides novel evidence that microorganisms regulate SOC turnover and effectively maintain a balance between resources and microbial requirements.

Linking Microbial Functional Gene Abundance and Daqu Extracellular Enzyme Activity: Implications for Carbon Metabolism during Fermentation

Daqu is the starter of Baijiu, it provides the microbes and enzymes necessary for fermentation. Studies have already established carbohydrate metabolism as the primary functional module in Daqu fermentation. The present study investigated the changes in microbial functions and the relationship between carbohydrate metabolism-related functional genes and extracellular enzyme activity during the Daqu fermentation. Amplicon sequencing identified 38 bacterial and 10 fungal phyla in Daqu samples, while shotgun metagenomic sequencing classified and annotated 40.66% of the individual features, of which 40.48% were prokaryotes. KEGG annotation showed that the pathways related to metabolites were less in the early fermentation stage, but higher in the middle and late stages. The functional genes related to pyruvate metabolism, glyoxylate and dicarboxylate metabolism, and propanoate metabolism were relatively high in the early and late stages of fermentation, while that for start and cross metabolism was relatively low. The study also found that amino sugar and nucleoside sugar metabolism were dominant in the middle stage of fermentation. Finally, the correlation network analysis showed that amylase activity positively correlated with many carbon metabolism-related pathways, while liquefaction activity negatively correlated with these pathways. In conclusion, the present study provides a theoretical basis for improving and stabilizing the quality of Daqu.

Salt-tolerant plant moderates the effect of salinity on soil organic carbon mineralization in a subtropical tidal wetland

Although salinization is widely known to affect cycling of soil carbon (C) in tidal freshwater wetlands, the role of the presence or absence of plants in mediating the responses of soil organic carbon (SOC) mineralization to salinization is poorly understood. In this study, we translocated soils collected from a tidal freshwater wetland to sites with varying salinities along a subtropical estuarine gradient and established unplanted and planted (with the salt-tolerant plant Cyperus malaccensis Lam.) mesocosms at each site. We simultaneously investigated cumulative soil CO₂ emissions, C-acquiring enzyme activities, availability of labile organic C (LOC), and structures of bacterial and fungal communities. Overall, in the planted mesocosm, the soil LOC content and the activities of β-1,4-glucosidase, cellobiohydrolase, phenol oxidase, and peroxidase increased with salinization. However, in the unplanted mesocosm, soil LOC content decreased with increasing salinity, whereas all the C-acquiring enzyme activities did not change. In addition, salinization favored the dominance of bacterial and fungal copiotrophs (e.g., γ-Proteobacteria, Bacteroidetes, Firmicutes, and Ascomycota) in the planted mesocosms. Contrarily, in the unplanted mesocosms salinization favored bacterial and fungal oligotrophs (e.g., α-Proteobacteria, Chloroflexi, Acidobacteria, and Basidiomycota). In both planted and unplanted mesocosms, cumulative soil CO₂ emissions were affected by soil LOC content, activities of C-acquiring enzymes, and microbial C-use trophic strategies. Overall, cumulative soil CO₂ emissions increased by 35% with increasing salinity in the planted mesocosm but decreased by 37% as salinity increased in the unplanted mesocosm. Our results demonstrate that the presence or absence of salt-tolerant plants can moderate the effect of salinity on SOC mineralization in tidal wetland soils. Future C prediction models should embed both planted and unplanted modules to accurately simulate cycling of soil C in tidal wetlands under sea level rise.

Sedimentary organic carbon storage of thermokarst lakes and ponds across Tibetan permafrost region

Sedimentary soil organic carbon (SOC) stored in thermokarst lakes and ponds (hereafter referred to as thaw lakes) across high-latitude/altitude permafrost areas is of global significance due to increasing thaw lake numbers and their high C vulnerability under climate warming. However, to date, little is known about the SOC storage in these lakes, which limits our better understanding of the fate of these active carbon in a warming future. Here, by combining large-scale field observation data and published deep (e.g., 0-300 cm) permafrost SOC data with a random forest (RF) machine learning technique, we provided the first comprehensive estimation of thaw lake SOC stocks to 3 m depth on the Tibetan Plateau. This study demonstrated that combining multiple environmental factors with the RF model could effectively predict the spatial distributions of the thaw lake SOC density values (SOCDs). The model results revealed that the soil respiration, normalized difference vegetation index (NDVI), and mean annual precipitation (MAP) were the most influential factors for predicting thaw lake SOCDs. In total, the sedimentary SOC stocks in the thaw lakes were approximately 52.62 Tg in the top 3 m, with 53% of the SOC stored in the upper layers (0-100 cm). The SOCDs generally exhibited high values in eastern Tibetan Plateau, and low values in mid- and western Tibetan Plateau, which were similar to the patterns of the land cover types that affected the SOCDs. We further found that the SOCDs of thaw lakes were generally higher than those of their surrounding permafrost soils at different layer depths, which could be ascribed to the erosion of soil particles or leaching solution from the thawing permafrost soils to lakes and/or enhanced vegetation growth at the lake bottom. This research highlights the necessity of explicitly considering the thaw lake SOC stocks in Earth system models for more comprehensive future projections of the carbon dynamics on the plateau.

Assessing the quality of the soil around a shale gas development site in a subtropical karst region in southwest China

The construction of shale gas facilities disturbs large areas of land and affects soil quality and function. In this study, we investigated the properties (including physical, chemical, and microbiological indicators) of soil at three different distances from a shale gas development site (<30 m, 30-50 m, and 50-100 m) in a karst area in 2017 and 2020. Our results showed that the soil water content; available carbon, nitrogen, and phosphorus concentrations; total nitrogen and total phosphorus concentrations; microbial biomass, and enzyme activities increased (P < 0.05) as the distance from the well pad increased, and the total carbon content, pH, electrical conductivity, and some ions (magnesium, sodium, and potassium) decreased with distance from the well pad (P < 0.05). The differences in the soil properties were most noticeable in 2017. The increases in the available nutrients were greater than in the total nutrients. The overall soil quality after the shale gas well pad construction was limited by the microbial biomass and sodium contents. The soil properties recovered most quickly at 30-50 m from the well pad, because of local farmland management practices that improved the soil properties and microbial biomass, and reduced the microbial stress. Therefore, we recommend planting sodium-tolerant crops on the land closest to the well pads, to facilitate restoration of the soil that was disturbed during the construction period.

Soil enzyme activity and stoichiometry in secondary grasslands along a climatic gradient of subtropical China

Soil extracellular enzymes plays key roles in ecosystem carbon (C), nitrogen (N), and phosphorus (P) cycling, and are very sensitive to climatic, plant, and edaphic factors. However, the interactive effects of these factors on soil enzyme activities at large spatial scales remain unclear. Here, we investigated the spatial pattern of the activities of five soil hydrolyzing enzymes [β-D-cellobiohydrolase (CB), β-1,4-glucosidase (BG), β-1,4-N-acetyl-glucosaminidase (NAG), L-leucine aminopeptidase (LAP), and acid phosphatase (AP)], and their C:N:P acquisition ratios in relation to plant inputs and edaphic properties across a 600-km climatic gradient in secondary grasslands of subtropical China. The activities of CB, BG, and NAG decreased while that of LAP increased with the increasing mean annual temperature (MAT). The activities of all enzymes did not significantly vary with the mean annual precipitation (MAP). We found that the activities of BG, NAG, and AP were predominately dependent on plant N contents, while the soil LAP activity was tightly related to soil recalcitrant C and N contents. In contrast, the ecoenzymatic C:nutrient (N and P) acquisition ratios increased with increasing MAP and decreasing MAT, primarily due to the increase in plant input at warmer and wetter sites. In addition to climates, plant C inputs, C use efficiency, soil pH, soil organic C, soil C:P, and N:P ratios explained 79% and 72% of the overall variation in ecoenzymatic C:nutrient and P:N acquisition ratios, respectively. The pattern of ecoenzymatic C:N:P acquisition ratios also revealed unexpected N limitation in subtropical grasslands. Overall, our study highlighted the importance of climate in controlling soil biological C, N, and P acquisition activities through its direct and indirect effects on plant inputs and soil edaphic factors, thereby providing useful information for better understanding and predictions of soil C and nutrient cycling in grassland ecosystems at regional scales.

Microbial community structure and functional genes drive soil priming effect following afforestation

Afforestation substantially modifies native soil organic carbon (SOC) decomposition via plant carbon inputs (the priming effect), and in turn, triggers vital biogeochemical processes that influence the regulation of soil carbon dynamics. Soil microbes are crucial in regulating the direction and magnitude of the priming effect. In the present study, we performed metagenomic sequencing and ¹³C-glucose labeling analyses of microbial communities and priming effects across a Robinia pseudoacacia afforestation chronosequence (14-, 20-, 30-, and 45-year-old stands) in the Loess Plateau in China, with adjacent farmland being selected as a control. Our results revealed that the cumulative priming effect across five sites along the afforestation chronosequence initially increased and approached a peak value in the 20-year-old stand, after which it declined. The priming effect was predominantly driven by the microbial community structure (i.e., the fungal-to-bacterial ratios and relative abundances of Proteobacteria and Actinobacteria), and stable C decomposition genes and C-degrading enzymes. Specifically, among the key functional genes correlated with priming effect, which were identified in orders Rhizobiales and Pseudonocardiales, considerably promoted SOC priming. Overall, our findings indicate that afforestation alters soil microbial community structure and function, particularly with respect to enhancing stable soil C decomposition genes, which may promote SOC priming. The findings of the present study could enhance our understanding of fresh C input-induced changes associated with C mineralization in the context of the revegetation of ecologically fragile areas.

Microbial assemblies associated with temperature sensitivity of soil respiration along an altitudinal gradient

Identifying the drivers of the response of soil microbial respiration to warming is integral to accurately forecasting the carbon-climate feedbacks in terrestrial ecosystems. Microorganisms are the fundamental drivers of soil microbial respiration and its response to warming; however, the specific microbial communities and properties involved in the process remain largely undetermined. Here, we identified the associations between microbial community and temperature sensitivity (Q₁₀) of soil microbial respiration in alpine forests along an altitudinal gradient (from 2974 to 3558 m) from the climate-sensitive Tibetan Plateau. Our results showed that changes in microbial community composition accounted for more variations of Q₁₀ values than a wide range of other factors, including soil pH, moisture, substrate quantity and quality, microbial biomass, diversity and enzyme activities. Specifically, co-occurring microbial assemblies (i.e., ecological clusters or modules) targeting labile carbon consumption were negatively correlated with Q₁₀ of soil microbial respiration, whereas microbial assemblies associated with recalcitrant carbon decomposition were positively correlated with Q₁₀ of soil microbial respiration. Furthermore, there were progressive shifts of microbial assemblies from labile to recalcitrant carbon consumption along the altitudinal gradient, supporting relatively high Q₁₀ values in high-altitude regions. Our results provide new insights into the link between changes in major microbial assemblies with different trophic strategies and Q₁₀ of soil microbial respiration along an altitudinal gradient, highlighting that warming could have stronger effects on microbially-mediated soil organic matter decomposition in high-altitude regions than previously thought.

Impact of zinc and plant growth-promoting bacteria on soil health as well as aboveground biomass of desi and kabuli chickpea under arid conditions

BACKGROUND

Zinc (Zn) deficiency and low soil fertility are the major factors responsible for low yield in chickpea. This study was conducted to evaluate the effect of Zn application and plant growth-promoting bacteria (PGPB) (endophyte Enterobacter sp. MN17) on soil health and aboveground biomass of desi and kabuli chickpea under natural field conditions. Zn was applied as seed priming (0.001 mol L^-1 ) and soil application (10 kg Zn ha^-1 ) with and without PGPB. To determine the impacts of Zn and PGPB on soil biological health, soil microbial biomass carbon (MBC) and soil extracellular enzyme activities were analyzed at two growth stages: vegetative (90 days after sowing) and maturity (163 days after sowing).

RESULTS

The highest aboveground biomass (5.1 t ha^-1 ) was recorded with Zn seed priming + PGPB in kabuli chickpea and in desi chickpea (4.8 t ha^-1 ) with Zn seed priming only. The application of Zn significantly increased soil MBC, which was higher in kabuli (795 and 731 μg C g^-1 ) compared to desi chickpea (655 and 533 μg C g^-1 ) at both vegetative and reproductive growth stages, respectively. The highest extracellular soil enzyme activities, - β-glucosidase (4758 nmol g^-1  h^-1 ), acid phosphatase (5508 nmol g^-1  h^-1 ), chitinase (5997 nmol g^-1  h^-1 ) and leucine aminopeptidase (993 nmol g^-1  h^-1 ) - were recorded with Zn seed priming. Of the chickpea types, kabuli chickpea had higher soil extracellular enzyme activities in the rhizosphere than desi chickpea.

CONCLUSION

Zn seed priming along with PGPB application may improve soil health and chickpea biomass in marginal soils. © 2021 Society of Chemical Industry.

Land-Use Change Enhanced SOC Mineralization but Did Not Significantly Affect Its Storage in the Surface Layer

To achieve carbon (C) neutrality and mitigate climate change, it is crucial to understand how converting natural forests to agricultural plantations influences soil organic C (SOC) mineralization. In this study, we investigated the impact of converting evergreen broadleaf forests (EBF) to extensively managed Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau) plantations (MBP) in subtropical China on SOC mineralization rate; the concentrations of labile SOC fractions such as dissolved organic C (DOC), microbial biomass C (MBC), and readily oxidizable C (ROC); the activities of C-degrading enzymes (cellobiohydrolase and phenol oxidase); and the abundance of C-degrading enzyme-encoding genes (cbhI and lcc). Three paired soil samples were taken from the surface layer (0–20 cm) of adjacent EBF-MBP sites in Anji County, Zhejiang province. Results showed that converting EBF to MBP significantly increased the SOC mineralization rate as well as soil pH, MBC, cellobiohydrolase, and phenol oxidase activities, and cbhI gene abundance, but did not change other soil properties described above. In addition, structural equation modelling (SEM) showed that the conversion increased SOC mineralization rate through increasing soil pH, cbhI gene abundance, MBC, and cellobiohydrolase and phenol oxidase activities. Our novel finding that converting EBF to extensively managed MBP enhanced SOC mineralization via increasing the activities of C-degrading enzymes suggests that C-degrading enzymes were a key factor regulating SOC mineralization in the extensively managed subtropical bamboo plantations.

Microbial traits determine soil C emission in response to fresh carbon inputs in forests across biomes

Soil priming is a microbial-driven process, which determines key soil-climate feedbacks in response to fresh carbon inputs. Despite its importance, the microbial traits behind this process are largely undetermined. Knowledge of the role of these traits is integral to advance our understanding of how soil microbes regulate carbon (C) emissions in forests, which support the largest soil carbon stocks globally. Using metagenomic sequencing and ¹³ C-glucose, we provide unprecedented evidence that microbial traits explain a unique portion of the variation in soil priming across forest biomes from tropical to cold temperature regions. We show that microbial functional profiles associated with the degradation of labile C, especially rapid simple sugar metabolism, drive soil priming in different forests. Genes involved in the degradation of lignin and aromatic compounds were negatively associated with priming effects in temperate forests, whereas the highest level of soil priming was associated with β-glucosidase genes in tropical/subtropical forests. Moreover, we reconstructed, for the first time, 42 whole bacterial genomes associated with the soil priming effect and found that these organisms support important gene machinery involved in priming effect. Collectively, our work demonstrates the importance of microbial traits to explain soil priming across forest biomes and suggests that rapid carbon metabolism is responsible for priming effects in forests. This knowledge is important because it advances our understanding on the microbial mechanisms mediating soil-climate feedbacks at a continental scale.

Responses of AM fungal abundance to the drivers of global climate change: A meta-analysis

Arbuscular mycorrhizal fungi (AMF), playing critical roles in carbon cycling, are vulnerable to climate change. However, the responses of AM fungal abundance to climate change are unclear. A global-scale meta-analysis was conducted to investigate the response patterns of AM fungal abundance to warming, elevated CO₂ concentration (eCO₂), and N addition. Both warming and eCO₂ significantly stimulated AM fungal abundance by 18.6% (95%CI: 5.9%-32.8%) and 21.4% (15.1%-28.1%) on a global scale, respectively. However, the response ratios (RR) of AM fungal abundance decreased with the degree of warming while increased with the degree of eCO₂. Furthermore, in warming experiments, as long as the warming exceeded 4 °C, its effects on AM fungal abundance changed from positive to negative regardless of the experimental durations, methods, periods, and ecosystem types. The effects of N addition on AM fungal abundance are -5.4% (-10.6%-0.2%), and related to the nitrogen fertilizer input rate and ecosystem type. The RR of AM fungal abundance is negative in grasslands and farmlands when the degree of N addition exceeds 33.85 and 67.64 kg N ha^-1 yr^-1, respectively; however, N addition decreases AM fungal abundance in forests only when the degree of N addition exceeds 871.31 kg N ha^-1 yr^-1. The above results provide an insight into predicting ecological functions of AM fungal abundance under global changes.

Enzymic moderations of bacterial and fungal communities on short- and long-term warming impacts on soil organic carbon

Microbial communities play critical roles in soil carbon-warming feedback, but our understanding of their linkages to soil carbon (C) pools in response to short- and long-term warming is deficient. Here, by conducting a meta-analysis of 150 studies, we show that short-term (<5 years) warming mainly affects soil labile carbon (LC) pools by changing bacterial community structure, while long-term (≥5 years) warming promotes the decomposition of recalcitrant C (RC) pools by increasing fungal biomass and decreasing actinobacterial biomass. Specifically, under short-term warming, significant increases in actinobacterial biomass (+15.9%) and the G+/G- ratio (+8.0%) were accompanied by an increase in carbon-degrading enzyme activities and a decrease in LC (-5.9%). Under long-term warming, the fungal biomass (+20.4%) and related POX (phenol oxidase) activity (+34.9%) increased significantly, while actinobacterial biomass (-20.1%), RC (-18.8%) and SOC (-6.7%) decreased. Meanwhile, we observed that warming impacts on soil microbial communities can be predicted by ecosystem type, the magnitude of warming, pH and elevation. Latitude and warming duration contributed the most to explaining the responses of LC and RC, respectively, across studies. Given that RC accounts for a substantial fraction of global soil C pools, the decline in RC pools greatly contributes to soil C degradation. Our findings suggest that different microbial groups may mediate the temporal dynamics of the decomposition of different soil C components and highlight that incorporating the temporal responses of soil microorganisms will improve predictions of the long-term dynamics of soil C pools in a warmer world.

Plant and Soil Enzyme Activities Regulate CO2 Efflux in Alpine Peatlands After 5 Years of Simulated Extreme Drought

Increasing attention has been given to the impact of extreme drought stress on ecosystem ecological processes. Ecosystem respiration (Re) and soil respiration (Rs) play a significant role in the regulation of the carbon (C) balance because they are two of the largest terrestrial C fluxes in the atmosphere. However, the responses of Re and Rs to extreme drought in alpine regions are still unclear, particularly with respect to the driver mechanism in plant and soil extracellular enzyme activities. In this study, we imposed three periods of extreme drought events based on field experiments on an alpine peatland: (1) early drought, in which the early stage of plant growth occurred from June 18 to July 20; (2) midterm drought, in which the peak growth period occurred from July 20 to August 23; and (3) late drought, in which the wilting period of plants occurred from August 23 to September 25. After 5 years of continuous extreme drought events, Re exhibited a consistent decreasing trend under the three periods of extreme drought, while Rs exhibited a non-significant decreasing trend in the early and midterm drought but increased significantly by 58.48% (p < 0.05) during the late drought compared with the ambient control. Plant coverage significantly increased by 79.3% (p < 0.05) in the early drought, and standing biomass significantly decreased by 18.33% (p < 0.05) in the midterm drought. Alkaline phosphatase, polyphenol oxidase, and peroxidase increased significantly by 76.46, 77.66, and 109.60% (p < 0.05), respectively, under late drought. Structural equation models demonstrated that soil water content (SWC), pH, plant coverage, plant standing biomass, soil β-D-cellobiosidase, and β-1,4-N-acetyl-glucosaminidase were crucial impact factors that eventually led to a decreasing trend in Re, and SWC, pH, β-1,4-glucosidase (BG), β-1,4-xylosidase (BX), polyphenol oxidase, soil organic carbon, microbial biomass carbon, and dissolved organic carbon were crucial impact factors that resulted in changes in Rs. Our results emphasize the key roles of plant and soil extracellular enzyme activities in regulating the different responses of Re and Rs under extreme drought events occurring at different plant growth stages.

Long-term warming and nitrogen fertilization affect C-, N- and P-acquiring hydrolase and oxidase activities in winter wheat monocropping soil

The enzymatic activities and ratios are critical indicators for organic matter decomposition and provide potentially positive feedback to carbon (C) loss under global warming. For agricultural soils under climate change, the effect of long-term warming on the activities of oxidases and hydrolases targeting C, nitrogen (N) and phosphorus (P) and their ratios is unclear, as well as whether and to what extend the response is modulated by long-term fertilization. A 9-year field experiment in the North China Plain, including an untreated control, warming, N fertilization, and combined (WN) treatment plots, compared the factorial effect of warming and fertilization. Long-term warming interacted with fertilization to stimulate the highest activities of C, N, and P hydrolases. Activities of C and P hydrolase increased from 8 to 69% by N fertilization, 9 to 53% by warming, and 28 to 130% by WN treatment compared to control, whereas the activities of oxidase increased from 4 to 16% in the WN soils. Both the warming and the WN treatments significantly increased the enzymatic C:N ratio from 0.06 to 0.16 and the vector length from 0.04 to 0.12 compared to the control soil, indicating higher energy and resource limitation for the soil microorganisms. Compared to WN, the warming induced similar ratio of oxidase to C hydrolase, showing a comparable ability of different microbial communities to utilize lignin substrates. The relationship analyses showed mineralization of organic N to mediate the decomposition of lignin and enzyme ratio in the long-term warming soil, while N and P hydrolases cooperatively benefited to induce more oxidase productions in the soil subject to both warming and N fertilization. We conclude that coupled resource limitations induced microbial acclimation to long-term warming in the agricultural soils experiencing high N fertilizer inputs.