Soil microbes are the planet's most abundant, diverse, and functionally vital organisms, yet only a small portion of these microbes actively drive soil processes. While resource availability is known to influence microbial physiological traits under multiple soil processes, how aboveground resource input structures the spatial distribution of the soil active microbiome remains virtually unknown. Here, we report the results from a continental standardized soil sampling at 601 sites across major biomes in China. We measured the proportion of the active microbiome (SAM%) using 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) staining by flow cytometry and simultaneously evaluated their main environmental drivers. On average, < 2% of all microbes constitute the active soil microbiome. Forests supported the most active soil microbiomes (> 2%), while cropland harbored the lowest (< 1%). Aboveground productivity, peaking in tropical warmer and wetter regions, was the major environmental factor explaining variation in the active soil microbiome. Our study suggests that a less productive planet may result in drastic reductions in the active soil microbiome with consequences for supporting ecosystem function and biogeochemical cycles under climate change.

Mineral protection mechanisms are important in determining the response of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) to temperature changes. However, the underlying mechanisms for how POC and MAOC respond to temperature changes are remain unclear. By translocating soils across 1304 m, 1425 m and 2202 m elevation gradient in a temperate forest, simulate nine months of warming (with soil temperature change of +1.41 degrees C and +3.91 degrees C) and cooling (with soil temperature change of-1.86 degrees C and-4.20 degrees C), we found that warming translocation significantly decreased POC by an average of 10.84 %, but increased MAOC by an average of 4.25 %. Conversely, cooling translocation led to an average increase of 8.64 % in POC and 13.48 % in MAOC. Exchangeable calcium (Caexe) exe ) had a significant positive correlation with POC and MAOC during temperature changes, and Fe/Al-(hydr)oxides had no significant correlation or a significant negative correlation with POC and MAOC. Our results showed that POC was more sensitive than MAOC to temperature changes. Ca exe mediated the stability of POC and MAOC under temperature changes, and Fe/Al(hydr)oxides had no obvious protective effect on POC and MAOC. Our results support the role of mineral protection in the stabilization mechanism of POC and MAOC in response to climate change and are critical for understanding the consequences of global change on soil organic carbon (SOC) dynamics.

The life strategies of bacterial communities determine their structure and function and are an important driver of biogeochemical cycling. However, the variations in these strategies under different soil resource conditions remain largely unknown. We explored the bacterial life strategies and changes in structure and functions between Antarctic soils and forest (temperate, subtropical, and tropical) soils. The results showed that the weighted mean rRNA operon copy number in temperate soils was 19.5% lower than that in Antarctic soils, whereas no significant differences were observed among Antarctic, subtropical, and tropical soils. An unexpected result was that bacterial communities in Antarctic soils tended to be copiotrophs, such as Actinobacteriota and Bacteroidota, whereas those in temperate soils tended to be oligotrophs, such as Acidobacteriota and Chloroflexi. Functional predictions showed that in comparison to copiotrophs in Antarctic soils, temperate-inhabiting oligotrophic bacteria exhibited an 84.2-91.1% lower abundance of labile C decomposition genes (hemicellulose, cellulose, monosaccharides, and disaccharides), whereas a 74.4% higher abundance of stable C decomposition (lignin). Genes involved in N cycling (nitrogen fixation, assimilatory nitrate reduction, and denitrification) were 24.3-64.4% lower in temperate soils than in Antarctic soils. Collectively, our study provides a framework for describing the life strategies of soil bacteria, which are crucial to global biogeochemical cycles.

Understanding the large-scale pattern of soil microbial carbon use efficiency (CUE) and its temperature sensitivity (CUET) is critical for understanding soil carbon-climate feedback. We used the 18O-H2O tracer method to quantify CUE and CUET along a north-south forest transect. Climate was the primary factor that affected CUE and CUET, predominantly through direct pathways, then by altering soil properties, carbon fractions, microbial structure and functions. Negative CUET (CUE decreases with measuring temperature) in cold forests (mean annual temperature lower than 10 °C) and positive CUET (CUE increases with measuring temperature) in warm forests (mean annual temperature greater than 10 °C) suggest that microbial CUE optimally operates at their adapted temperature. Overall, the plasticity of microbial CUE and its temperature sensitivity alter the feedback of soil carbon to climate warming; that is, a climate-adaptive microbial community has the capacity to reduce carbon loss from soil matrices under corresponding favorable climate conditions.

• CH 4 emission rates followed an increased pattern during the growing season at Tibetan Plateau. • Unique genes carried by abundant species were positively correlated with CH 4 emission rates. • Climate factors influenced CH 4 emission rates by regulating microbial community and their genes. Microorganisms play pivotal roles in soil methane (CH 4 ) emissions and their functional genes are origins of a key mechanism for soil CH 4 -cycling. However, understanding of the roles of specific genes (e.g., unique or shared genes carried by species) underlying CH 4 -cycling remains elusive. Here, we measured CH 4 emission rates and investigated variations in microbial community and the abundance of genes carried by species during the growing season in alpine meadow on the Tibetan Plateau. We discovered that CH 4 emission rates increased from 394.4, 745.9, and 1 092.7 µg CH 4 m −2 h −1 , in April, June, and August, respectively, and had a positive correlation with unique genes carried by abundant species during the growing season. Moreover, we found that unique genes carried by abundant species involved in methanogenesis processes have a higher abundance than methanotrophic processes. Further analysis indicated that climate factors (i.e., mean monthly temperature (MMT) and mean monthly precipitation (MMP)) influenced microbial community and their functional genes, and therefore affected the CH 4 emission rates. Overall, the present study provides a novel insight into the variation of soil CH 4 emissions from a functional gene perspective, highlighting the important roles of unique genes carried by abundant species in CH 4 emissions in the Tibetan Plateau under seasonal variation.

Carbohydrate-active enzymes (CAZymes) mediate carbohydrate turnover and play vital roles in plant- and microbial-derived carbon decomposition. However, the changes of genes that encoding enzymes for plant- and microbial-derived carbon decomposition along environmental gradients remains unclear. We used metagenomic sequencing to explore changes in genes encoding enzymes for carbon decomposition in five forest sites along an elevational gradient (1503–3182 m) on Qinling Mountain, China. The genes encoding CAZymes showed various patterns along the elevational gradient. In particular, the abundance of genes encoding auxiliary enzymes and glycoside hydrolases decreased with increasing elevation. The abundance of genes encoding enzymes for plant- and fungi-derived carbon decomposition was higher at low elevations than at high elevations, whereas the abundance of genes encoding enzymes for bacteria-derived carbon decomposition was higher at high elevations than at low elevations. The results indicate contrasting patterns of fungal- and bacterial-derived carbon decomposition with elevation. Proteobacteria and Acidobacteria were the dominant species that decomposed dead plant and microbial biomass. Moreover, our results reveal that soil properties (i.e., ammonium nitrogen and bulk density) and vegetation properties dominated the CAZymes gene distribution along the elevational gradient. Bacteria- and fungi-derived carbon decomposition potentials show contrasting elevational patterns in forest soils; soil and vegetation properties are common controls for the elevational patterns.

Microbial nitrogen transformation processes in grasslands are important nitrous oxide (N2O) sources on the Tibetan Plateau. Different microorganisms carrying various functional nitrogen (N) cycling genes colonize different soil types and have varying impacts on N2O emissions. However, the role of specific microbial com-ponents such as unique and shared microbial species, in N2O flux in alpine grassland soils remains understudied. Thus, we combined gas chromatography-based static opaque chamber methods with metagenomic sequencing to investigate the role of unique and shared microbial species carrying N-cycling genes in N2O emissions from two alpine grassland types on the Tibetan Plateau. Our results showed that the N2O flux in the alpine meadows (9.48 x 10-9 g m- 2 h-1) was higher than that in the alpine steppes (4.21 x 10-9 g m- 2 h-1). Furthermore, our study revealed a significant negative correlation between the abundance of unique microbial species and the N2O flux in alpine steppes. In contrast, the abundance of shared microbial species was significantly and positively correlated with N2O flux. The unique microbial species in the phyla Chloroflexi and Actinobacteria carried the ureC, nasA and nirB genes, which could consume nitrogen substrates in forms other than producing N2O, which could explain the N2O reduction seen in the alpine steppes. In addition, the higher abundance of shared microbial species in the phyla Proteobacteria, Actinobacteria, Thaumarchaeota and Acidobacteria carried genes encoding denitrification pathways (nirK, narG, and norB) could interpret the higher N2O emissions in the alpine meadows. In addition, our study revealed that the soil substrate contents (e.g., soil organic carbon and total nitrogen) and soil environment factors (e.g., soil pH and soil moisture) were potentially important drivers of N2O emissions in soil via their regulation of the composition of unique and shared microbial communities carrying N-cycling genes. Collectively, our results suggested that identifying the unique and shared microbial species carrying different N-cycling genes could provide potential biological strategies for mitigating N2O emissions in alpine grasslands, such as blocking the growth of shared microbial species. Future management practices for mitigating N2O emissions in alpine grassland should also focus on adopting appropriate nutrient management strategies, such as balanced fertilization, to prevent excessive nutrient. Our findings provide insights for further elucidating the different potential regulatory effects of unique and shared microbial species on emissions under the control of the soil environment in different types of grasslands on the Tibetan Plateau.

Highly variable soil microbial respiration among grasslands has been identified as a major cause of uncertainty in regional carbon (C) budget estimation in the Qinghai-Tibetan Plateau; microbial metabolism mechanisms might explain this variation, but remain elusive. Therefore, we investigated soil CO2 production in incubated soils and detected the associated functional genes at four sampling sites from two major alpine grasslands on the Qinghai-Tibetan Plateau. The results showed that the cumulative CO2 emissions from alpine meadow soils were 71 %-83 % lower than those from alpine steppe soils. Both the enriched genes abundance encoding fermentation and glycolysis (Embden-Meyerhof pathway (EMP)) and the diminished genes encoding tricarboxylic acid cycle (TCA) and phosphate pentose pathway (PPP) explained the CO2 emission reduction in the alpine meadow soils. The EMP: PPP and fermentation: TCA cycle ratios in alpine meadow soils were 1.45- and 1.50-fold higher than those in alpine steppe soils, respectively. Such shifts in metabolic pathways were primarily caused by the increasing dominance of an unknown species of Desulfobacteraceae with high glycolytic potential, carrying a higher abundance of ldh genes during fermentation. These unknown species were promoted by warmer temperatures and higher precipitation in the alpine meadows. Further studies on the unknown species would enhance our understanding and predictability of C cycling in alpine grasslands.

The alpine grasslands account for approximately 54.5% of the total carbon in China’s grasslands, and carbohydrate-active enzymes (CAZymes) play key roles in the turnover of carbon. However, the variation and factors influencing gene-encoding enzymes for plant- and microbial-derived carbon decomposition in alpine steppes and alpine meadows remain unclear. Here, the trends in microbial carbohydrate-active enzymes (CAZymes) and their responses to the decomposition of biomass of different origins were studied using metagenomics in the alpine steppes and alpine meadows on the Tibetan Plateau. Our results revealed the abundance of GTs and CBMs was higher in the alpine steppes than in the alpine meadows, whereas AAs were higher in the alpine steppes than in the alpine meadows. Soil properties (i.e., soil water content, soil ammonium nitrogen, and nitrate nitrogen) highly related to CAZyme genes (GTs, CBMs, and AAs) showed an abundant pattern between the alpine steppes and alpine meadows. Moreover, our results indicated that the relative abundance of genes encoding CAZymes involved in the decomposition of plant- (indicated by cellulose, hemicellulose, and lignin) and fungal-derived carbon (indicated by chitin and glucans) was higher by 8.7% and 10.1%, respectively, in the alpine steppes than in the alpine meadows, whereas bacterial-derived carbon (indicated by peptidoglycan) was lower by 7.9% in the alpine steppes than in the alpine meadows. Soil water content (SWC), nitrate nitrogen (NO3−), and pH influenced on the abundance of CAZyme genes involved in the decomposition of plant-, fungal-, bacterial-derived carbon. In addition, the dominant microbial phyla (Actinobacteria, Protebacteria, and Acidobacteria) mineralized carbon sources from plant- and microbial-derived carbon through their corresponding CAZyme families. In conclusion, our study compared plant- and microbial-derived carbon decomposition potentials and influencing factors to illustrate the contribution of dead biomass to carbon accumulation in alpine grasslands.

Aims Under the background of changing carbon cycle process in forest ecosystems caused by global environmental change, the microbial carbon use efficiency（CUE） in forest rhizosphere soil is critical to determine the strength of microbial anabolism and catabolism in forest ecosystems. However, the variation and influencing factors of microbial CUE in rhizosphere soils at different altitudes remain undetermined.Methods Rhizosphere soil at six different altitudes spanning four forest belts in Taibai Mountain was sampled to determine the physical and chemical properties, extracellular enzyme activity, and characteristics of microbial community and vegetation. Based on the stoichiometric ratio, the soil microbial CUE was estimated. Furthermore,the variation in microbial CUE of rhizosphere soil along the altitude gradient was analyzed to quantify the influencing factors of microbial CUE.Important findings The results showed that the microbial CUE of rhizosphere soil exhibited an overall upward trend with the increase in altitude. The microbial CUE increased by 4.36% from 0.505 at the lowest altitude to 0.527 at the highest altitude, but decreased at 1 603 and 2 405 m. Based on the Mantel analysis, we identified four categories of factors（i.e., altitude, soil matrix, vegetation and microbe） that related to microbial CUE in rhizosphere soil. The variations of microbial CUE in rhizosphere soil are affected by multiple environmental factors, with the dominant factor being soil matrix（such as dissolved organic carbon（DOC） content, ammonium nitrogen（NH 4 + -N） content）, followed by vegetation. Furthermore, the altitude factor and the microbial factor explained 2.6% and 3.1% of the CUE change, respectively. Although the microbial factors exerted no significant impact on microbial CUE, soil matrix, vegetation and microbe jointly explained 47.0% of the microbial CUE change. The variance partitioning analysis（VPA） quantitatively revealed the contribution of environmental factors to the change of microbial CUE, where soil matrix and vegetation explained 17.0% and 5.7% of the variation, respectively. While the interaction between soil matrix and vegetation accounted for 31.9% of the changes in microbial CUE. The above results indicated that the high-altitude rhizosphere soil in Taibai Mountain has a high carbon sequestration potential, and the carbon sequestration of forest rhizosphere soil may decrease with the intensification of global warming. The vertical temperature difference and the vertical differentiation of the vegetation belt induced by altitude gradient will alter the growth and metabolism environment of microorganisms in the rhizosphere soil. The comprehensive effect of multiple environmental factors dominated by soil matrix impacts the CUE of soil microorganisms, and ultimately changes the assimilation and catabolism processes of soil carbon. The results of this study can provide a scientific basis for the carbon assimilation capacity and carbon sequestration potential of forest soil microorganisms in Qinling Mountains, as well as the forest soil carbon cycle under the background of global change.

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 (eCO(2)), and N addition. Both warming and eCO(2) 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 eCO(2). Furthermore, in warming experiments, as long as the warming exceeded 4 degrees 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. (C) 2021 Elsevier B.V. All rights reserved.

Aim Fresh carbon (C) inputs to the soil can have important consequences for the decomposition rates of soil organic matter (priming effect), thereby impacting the delicate global C balance at the soil-atmosphere interface. Yet, the environmental factors that control soil priming effect intensity remain poorly understood at a global scale. Location Global. Time period 1980-2020. Major taxa studied Soil priming effect intensity. Methods We conducted a global dataset of CO2 effluxes in 711 pairwise soils with C-13 or C-14 simple C sources inputs and without C inputs from incubation experiments in which isotope-labelled C was used to quantify fresh C-induced rather than exudate-induced priming. Results Soil priming effect intensity is predominantly positive. Soil texture and C content were identified as the most important factors associated with priming effects, with sandy soils from tropical and mid-latitudes supporting the highest soil priming effect intensity, and soils with greater C content and fine textures from high latitudes maintaining the lowest soil priming effects. The negative association between C content and soil priming effect intensity was also indirectly driven by changing mean annual temperature, net primary productivity, and fungi : bacteria ratio. Using this information, we generated a global map of soil priming effect intensity, and found that the priming was lower at high latitudes and higher at lower latitudes. Main conclusions Global patterns of soil priming effect intensity can be predicted using environmental data, with soil texture and C content playing a predominant role in explaining in priming effects. These effects were also indirectly driven by climate, vegetation and soil microbial properties. We present the first global atlas of soil priming effect intensity and advance our knowledge on the potential mechanisms underlying soil priming effect intensity, which are integral to improving the climate change and soil C dynamics components of Earth System models.

Microorganisms play important roles in phosphorus (P) cycling via their regulation of P uptake and transport, P mineralization and solubilization, and the mediation of P deficiency in forest biomes. However, the dynamics of microbial P functional genes and the underlying regulatory mechanisms in different forest biomes (e.g., temperate vs. subtropical) have yet to be sufficiently clarified. In this study, we applied a metagenomics approach to investigate changes in the abundance of three microbial P functional gene groups (P-starvation response regulation genes, P-uptake and transport genes, and P-solubilization and mineralization genes) along a subtropical–temperate gradient of forest biomes (23°N–45°N) in China. Our results revealed that the abundances of P-starvation response regulation genes in temperate forest biomes were significantly higher than those in the subtropics (p < 0.05), although not in the cases of the other two P functional gene types (p > 0.05). Moreover, in both temperate and subtropical forests, Acidobacteria, Actinobacteria, Proteobacteria, and Verrucomicrobia were identified as key phyla associated with P cycling. Furthermore, in both forest biomes, microbial community α-diversity was significantly positively correlated with P-starvation response regulation genes (p < 0.05), whereas α-diversity was significantly positively related to P-uptake and transport genes in temperate forest biomes (p < 0.001), although not in subtropical forests (p > 0.05). In addition, we found that whereas soil substrates showed significant negative relationships with P-solubilization and mineralization genes in temperate forest biomes (p < 0.05), this was not the case in subtropical forests. Collectively, these findings indicate that the responses of microbial P functional genes to the environmental variation in temperate forests are more sensitive than those in subtropical forests, thereby providing a theoretical foundation for further elucidation of the differential regulatory roles of these genes in different forest biomes.

Microbes play an integral role in forest soil phosphorus (P) cycling. However, the variation of microbial P-cycling functional genes and their controlling factors in forest soils is unclearly. We used metagenomics to investigate changes in the abundance of genes involved in P-starvation response regulation, P-uptake and transport, and P-solubilization and mineralization along the five elevational gradients. Our results showed the abundance of three P cycling gene groups increasing along the elevational gradient. Acidobacteria and Proteobacteria were the dominant microbial phyla determining the turnover of soil P-solubilization and immobilization. Along the elevational gradient, soil substrates are the major factor explaining variation in P-starvation response regulation genes. Soil environment is the main driver of P-uptake and transport and P-solubilization and mineralization genes. This study provided insights into the regulation of P-cycling from a microbial functional profile perspective, highlighting the importance of substrate and environmental factors for P-cycling genes in forest soils.

Microorganisms play important roles in phosphorus (P) cycling via their regulation of P uptake and transport, P mineralization and solubilization, and the mediation of P deficiency in forest biomes. However, the dynamics of microbial P functional genes and the underlying regulatory mechanisms in different forest biomes (e.g., temperate vs. subtropical) have yet to be sufficiently clarified. In this study, we applied a metagenomics approach to investigate changes in the abundance of three microbial P functional gene groups (P starvation response regulation genes, P uptake and transport genes, and P solubilization and mineralization genes) along a subtropical–temperate gradient of forest biomes (23° N–45° N) in China. Our results revealed that the abundances of P starvation response regulation genes in temperate forest biomes were significantly higher than those in the subtropics (p < 0.05), although not in the cases of the other two P functional gene types (p > 0.05). Moreover, in both temperate and subtropical forests, Acidobacteria, Actinobacteria, Proteobacteria, and Verrucomicrobia were identified as key phyla associated with P cycling; moreover, we found dominate species of Acidobacteria and Proteobacteria at genus level were higher in subtropical zones than that of temperate zones, in most cases. Furthermore, our results showed that significant correlation was found between P functional genes and microbial α-diversity along latitude gradient. Furthermore, in both forest biomes, microbial community α-diversity was significantly positively correlated with P starvation response regulation genes (p < 0.05), whereas α-diversity was significantly positively related to P uptake and transport genes in temperate forest biomes (p < 0.001), although not in subtropical forests (p > 0.05). In addition, we found that whereas soil substrates showed significant negative relationships with P solubilization and mineralization genes in temperate forest biomes (p < 0.05), this was not the case in subtropical forests. Collectively, these findings indicate that the responses of microbial P functional genes to the environmental variation in temperate forests are more sensitive than those in subtropical forests, thereby providing a theoretical foundation for further elucidation of the differential regulatory roles of these genes in different forest biomes.

Soil microbes ultimately drive the mineralization of soil organic carbon and thus ecosystem functions. We compiled a dataset of the seasonality of microbial biomass carbon (MBC) and developed a semi-mechanistic model to map monthly MBC across the globe. MBC exhibits an equatorially symmetric seasonality between the Northern and Southern Hemispheres. In the Northern Hemisphere, MBC peaks in autumn and is minimal in spring at low latitudes (<25°N), peaks in the spring and is minimal in autumn at mid-latitudes (25°N to 50°N), while peaks in autumn and is minimal in spring at high latitudes (>50°N). This latitudinal shift of MBC seasonality is attributed to an interaction of soil temperature, soil moisture, and substrate availability. The MBC seasonality is inconsistent with patterns of heterotrophic respiration, indicating that MBC as a proxy for microbial activity is inappropriate at this resolution. This study highlights the need to explicitly represent microbial physiology in microbial models. The interactive controls of environments and substrate on microbial seasonality provide insights for better representing microbial mechanisms in simulating ecosystem functions at the seasonal scale.

Soil net nitrogen (N) mineralisation, the difference between organic nitrogen mineralisation and mineral nitrogen immobilisation, changes with elevation, thereby determining plant productivity and soil N cycling along elevation gradients. However, it has yet to be established how different microbial functional genes influence the rate of soil net N mineralisation along such gradients. To address this deficiency in our current knowledge, we performed metagenomic sequencing to identify soil microbial functional genes encoding enzymes involved in N cycling at five forest sites along an elevational gradient spanning a range from 1503 to 3182 m above sea level. Our results indicate that the rate of net N mineralisation follows a unimodal pattern with increasing elevation, with a peak (0.18 mg kg(-1) d(-1)) being detected at the mid-high elevation site. Further, we detected a significant correlation between the abundance of genes encoding enzymes involved in denitrification and ammonia assimilation pathways and net N mineralisation rate (p < 0.05). Moreover, we established that microbial species in the phyla Cyanobacteria, Acidobacteria and Planctomycetes, harbouring keystone functional genes, play a predominant role in determining the rate of net N mineralisation. Our findings also revealed soil substrate content (ammonium nitrogen and nitrite nitrogen, soil organic carbon, and C:N ratio) and soil environment (soil temperature and soil moisture) to be the major drivers of net N mineralisation in soil via their regulatory effects on the composition of microbial communities and functional genes. Our characterisation of the microbial metagenomic basis of net N mineralisation in forest soils accordingly highlights the importance of the combined contributions of soil microbial functional genes, soil substrate, and environmental factors in determining the cycling of N in forest soils at different elevations. Highlights Net N mineralisation rate followed a unimodal pattern along an elevational gradient. Denitrification and ammonia assimilation genes were correlated with net N mineralisation rate. Cyanobacteria play a dominant role in determining net N mineralisation rate. Soil substrate and environment determine microbial functional gene distribution.

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 20year-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.

Soil enzymes are the most potent bioactive components in forest ecosystems. Cellulases and ligninases are vital carbon (C)-degrading enzymes that target different C pools. The ratio of ligninase-to-cellulase activity is good indicator for microbial soil C preference, play an important role in soil C cycling. However, our understanding of enzyme ratios and their drivers across forest ecosystems remains unclear. In this study, we hypothesized that (i) the ligninase-to-cellulase ratio increased from temperate forests to tropical forest ecosystems, and (ii) the dominant factors would be microbial abundances. About 2-3 kg of topsoil (0-10 cm) from each of the ten forest ecosystems were collected across a 3425 km gradient in China between July and August 2019. We analyzed the biogeographic patterns of ligninase and cellulase activities and the ratio of ligninase-to-cellulase activities to determine how this ratio responded to climatic factors, soil properties and substrates, and microbial abundances across the forest ecosystems along the latitudinal gradient. Our findings showed that the average soil ligninase activity was 3.49 nmol h(-1) g(-1), whereas the average soil cellulase activity was 525.26 nmol h(-1) g(-1) across the forest ecosystems sampled. The average activity ratio of ligninase-to-cellulase in tropical forest ecosystems was 27.9% higher than that in subtropical forests and 64.2% higher than that in temperate forest ecosystems. The partial least squares path model demonstrated that the ligninase-to-cellulase activity ratio was significantly negatively correlated with soil substrates (r =-0.94, p < 0.001) and significantly positively correlated with microbial abundances (r = 0.38, p < 0.01). The variation partitioning analysis further revealed that soil sub-strates explained 19.4% variation regarding ligninase-to-cellulase ratio, whereas microbial abundance (fungal abundance) contributed 2.8%. This study provides crucial information about the distribution of enzyme ratios along the latitude gradient, highlights the microbial utilization of recalcitrant C pools in tropical forests, and provides an insight into the response of the global C cycle under a changing climate.

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.