Long-term warming of a forest soil reduces microbial biomass and its carbon and nitrogen use efficiencies

Global warming impacts biogeochemical cycles in terrestrial ecosystems, but it is still unclear how the simul-taneous cycling of carbon (C) and nitrogen (N) in soils could be affected in the longer-term. Here, we evaluated how 14 years of soil warming (+4 degrees C) affected the soil C and N cycle across different soil depths and seasons in a temperate mountain forest. We used H218O incorporation into DNA and 15N isotope pool dilution techniques to determine gross rates of C and N transformation processes. Our data showed different warming effects on soil C and N cycling, and these were consistent across soil depths and seasons. Warming decreased microbial biomass C (-22%), but at the same time increased microbial biomass-specific growth (+25%) and respiration (+39%), the potential activity of & beta;-glucosidase (+31%), and microbial turnover (+14%). Warming reduced gross rates of protein depolymerization (-19%), but stimulated gross N mineralization (+63%) and the potential activities of N-acetylglucosaminidase (+106%) and leucine-aminopeptidase (+46%), and had no impact on gross nitrification (+1%). Microbial C and N use efficiencies were both lower in the warming treatment (-15% and-17%, respectively). Overall, our results suggest that long-term warming drives soil microbes to incorporate less C and N into their biomass (and necromass), and to release more inorganic C and N to the environment, causing lower soil C and N storage in this forest, as indicated by lower soil C and total N contents. The decreases in microbial CUE and NUE were likely triggered by increasing microbial P constraints in warmed soils, limiting anabolic processes and microbial growth and promoting pervasive losses of C and N from the soil.

Automated summary

Global warming has different effects on soil carbon (C) and nitrogen (N) cycling in a temperate mountain forest. Soil warming decreases microbial biomass C but increases microbial biomass-specific growth, respiration, and enzyme activities. It also stimulates gross N mineralization but reduces protein depolymerization. Overall, long-term warming leads to lower soil C and N storage due to decreased microbial C and N incorporation into biomass and increased release of inorganic C and N into the environment.