Drivers of soil microbial activities and greenhouse gas emissions along an elevational gradient

Mountain ecosystems contribute substantially to global carbon and nitrogen biogeochemical cycles. Although soil respiration, and microbial biomass, activities and diversity have been extensively studied at different altitudes worldwide, little is known on causal link between environmental drivers, microbial functions and emissions of greenhouse gases (GHGs) in soils of different elevation. Here, by measuring in-situ GHG fluxes, soil properties, organic matter (OM) quality, microbial enzyme activities, biomass and gene abundances, we investigate factors that control long-term GHG fluxes (carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O)) in natural soils with an elevational gradient of ~2400 m across Switzerland with different vegetation covers. Results showed that CO₂ and N₂O fluxes increased significantly with elevation from top to the treeline, but slightly decreased from the treeline to bottom. Contrastingly, no significantly patterns of CH₄ fluxes across the whole elevation were observed. Spearman correlations revealed that the increased CO₂ and N₂O fluxes were highly correlated to the significant increases in soil temperature, moisture, organic matter (OM) quantity and quality (increases in the relative contribution of humic-like vs. fresh-like OM), bacterial and fungal biomass and gene abundances. Structural equation model, hierarchical partitioning and random forest regression further confirmed that, in addition to soil temperature and moisture, SOM quantity and quality are the most driving factors of microbial activity and respiration. Our study highlights the importance of OM quality as a driving factor of soil microbial metabolic activities in Alpine soils across the elevation, and predicts a potential increase in GHG emissions in high-altitudinal soils with the expected upwarding-shifting treeline under climate warming.