Effects Of Local Adaptation Of Microbial Community On Soil Carbon Turnover Along Elevation Gradients

Soil represents one of the most important carbon（C）pool in terrestrial ecosystems,and its turnover processes driven by soil microbial communities play a vital role in mediating C cycle-climate feedback.Soil C storage and release to the atmosphere is ultimately a consequence of microbial catabolism（respiration）and anabolism（biomass synthesis）,which will affect the direction and magnitude of future climate change.Elevation gradients exhibit dramatic changes in climatic,soil,and biotic conditions over short geographic distances.Especially,soil microbial communities and their metabolic activities generally vary with elevation,which may influence soil C turnover and its response to global climate change.However,the role and mechanism of microbial community in soil C turnover along elevation gradients is poorly understood.Therefore,we first conducted a global field survey to investigate the distribution patterns of soil C and influencing factors,and to explore the relative contribution of microbial communities in explaining soil C distribution along the global elevation gradient（0-3164 m）.Furthermore,we used the typical elevation gradient（i.e.,the Tibetan Plateau in China,2974-3558 m）as a system model to reveal the microbial ecological mechanism of soil C turnover along the elevation gradient.We evaluated the elevational patterns of soil microbial C metabolism processes（temperature sensitivity of microbial respiration,residual C and metabolic efficiency）,and the effects of microbial community composition and physiological attributes（life-style strategies and trade-offs in different microbial traits）on these soil C turnover processes through laboratory simulation experiments with high-throughput sequencing technology.The main results are as follows:（1）We found that elevation was a essential predictor of global soil organic carbon（SOC）distribution,and was highly positively correlated with SOC.However,elevation had the relatively weak explanatory power for global soil inorganic carbon（SIC）distribution.In addition,SOC and SIC varied significantly across climates and ecosystem types.Specifically,SOC in forest and grassland ecosystems under continental and temperate climatic conditions was significantly high,while SIC peaked in arid and temperate ecosystems,particularly in shrublands.There was no significant difference in SOC and SIC between different land uses（natural ecosystems vs.urban greenspaces）.Structural Equation modeling（SEM）further provided evidence that soil microbes had significant direct associations with the global elevational patterns of SOC and SIC.In particular,SOC was significantly positively correlated with bacterial biomass and negatively correlated with the fungal to bacterial ratio,and SIC was significantly positively correlated with fungal biomass.（2）The temperature sensitivity(Q₁₀)of soil microbial respiration increased significantly along the typical elevation gradient in the climate-sensitive Tibetan Plateau,indicating that soil microbial respiration is more sensitive to temperature changes in cold high-elevation regions.Both Random Forest analysis and correlation analysis consistently indicated that shifts in bacterial and fungal community composition were more closely related to Q₁₀of soil microbial respiration than other environmental factors such as soil p H,moisture,substrate quantity and quality,microbial biomass,diversity and enzyme activities.More importantly,we identified major microbial assemblies（ecological clusters or modules）with different trophic strategies that were significant predictors of Q₁₀of soil microbial respiration based on co-occurrence network and Random Forest analysis.Specifically,Q₁₀of soil microbial respiration were significantly negatively correlated with the relative abundance of module#1（dominated by taxa involved in labile C consumption or r-strategists）,but positively correlated with those of module#4（dominated by taxa involved in recalcitrant C consumption or K-strategists）.（3）Microbial residue carbon（MRC）and SOC increased significantly and had an extremely significant positive correlation along the two independent elevation gradients（i.e.,the Tibetan Plateau and Shennongjia Mountain）.Moreover,the contribution of MRC to SOC（MRC/SOC,26.60%-77.26%）decreased with the increasing elevation.Both Random Forest analysis and Variation partitioning analysis consistently indicated that MRC explained the highest proportion of variations in SOC along the elevation gradients.SEM further revealed that local temperature increases were negatively correlated with SOC via MRC.MRC was positively associated with the prevalence of microbial communities with K-strategists,characterized by high bacterial oligotroph/copiotroph ratios and ECM/saprotrophic fungi ratios,and also high ratios of recalcitrant/labile C degradation genes and enzyme activities,with a low weighted average r RNA operon copy number of bacterial communities.However,Random Forest analysis indicated that shifts in MAT,p H,and moisture were more important predictors of MRC,when we concurrently considered essential microbial traits including microbial biomass,metabolic efficiency and r/K strategy.（4）Soil microbial metabolic quotient（q CO₂）and specific P-acquiring enzyme activities（AP/MBC）increased,but carbon use efficiency（CUE）and ecoenzymatic vector length（Vlength）declined with increasing elevation in the Tibetan Plateau.Random Forest analysis indicated that both q CO₂and CUE were predominantly predicted by microbial resource acquisition strategies（including Vlength and AP/MBC）and microbial community composition.Regression analysis further indicated that q CO₂correlated negatively with Vlength,and CUE showed positive relationships with Vlength,but negative associations with AP/MBC.The key microbial assemblies（clusters of microbial phylotypes that highly correlated with each other）based on network analyses were essential predictors of the variations in Vlength,q CO₂and CUE.The proportion of B＿Mod#0 and F＿Mod#6（dominated by taxa with high investment into nutrient acquisition and cold-stress tolerance）were correlated negatively with Vlength and CUE,but correlated positively with q CO₂.The proportions of B＿Mod#2 and F＿Mod#1（dominated by taxa with low investment into nutrient acquisition and cold-stress tolerance）had opposite patterns along the elevation gradient.In this study,we comprehensively and systematically assessed the influence of local adaptation of microbial community on soil C and microbial C metabolism processes along the elevation gradients through the combination of global field surveys and sampling analysis of typical altitude gradients.The above findings showed that microbial communities significantly influenced the distribution of SOC along global elevation gradient.There were progressive shifts of microbial assemblies from labile to recalcitrant C consumption along the elevation gradient,resulting in a higher temperature sensitivity of microbial respiration in response to climate change in high-elevation regions.Rising temperature could reduce SOC by increasing the decomposition of MRC.In addition,microbial communities increased investment into nutrient（particularly for phosphorus）acquisition via enzymes,thus reducing metabolic efficiency along the elevation gradient.Our findings provide new insights into microbial-mediated C processes and its responses to global climate changes.