Soil organic matter properties drive microbial enzyme activities and greenhouse gas fluxes along an elevational gradient

Abstract

Mountain ecosystems, contributing substantially to the global carbon (C) and nitrogen (N) biogeochemical cycles, are heavily impacted by global changes. Although soil respiration and microbial activities have been extensively studied at different elevation, little is known on the relationships between environmental drivers, microbial functions, and greenhouse gas fluxes (GHGs; carbon dioxide [CO₂], methane [CH₄] and nitrous oxide [N₂O]) in soils of different elevation. Here, we measured how in situ GHG fluxes were linked to soil properties, soil organic matter (SOM) quantity and composition (the proportion of humic-like vs. protein-like OM), microbial biomass, enzyme activities and functional gene abundances in natural soils spanning an elevational gradient of ∼2400 m in Switzerland. Soil CO₂ fluxes did not significantly vary from low (lowland zone) to higher (montane and subalpine zones) elevation forests, but decreased significantly (P<0.001) from the treeline to the mountain summit. Multivariate analyses revealed that CO₂ fluxes were controlled by C-acquiring enzymatic activities which were mainly controlled by air mean annual temperature (MAT) and SOM quantity and composition. CH₄ fluxes were characterized by uptake of atmospheric CH₄, but no trend was observed along the elevation. N₂O fluxes were also dominated by uptake of atmospheric N₂O. The flux rates remained stable with increasing elevation below the treeline, but decreased significantly (P<0.001) from the treeline to the summit. N₂O fluxes were driven by specific nitrifying and denitrifying microbial genes (ammonia-oxidizing amoA and N₂O-producing norB), which were again controlled by SOM quantity and composition. Our study indicates the treeline as a demarcation point changing the patterns of CO₂ and N₂O fluxes along the elevation, highlighting the importance of SOM quantity and composition in controlling microbial enzyme activities and GHG fluxes.