Consistent responses of microbial C and N metabolic processes to elevated CO2 across global terrestrial ecosystems

Purpose Elevated CO2 contributes greatly to global warming, playing a pivotal role in terrestrial ecosystem processes, in particular microbially regulated C and N cycling. However, the responses of microbial C and N anabolic and metabolic processes to elevated CO2 are unclear. Methods This study used a meta-analysis based on a global dataset (i.e., 312 observations from 66 studies) to calculate the effect size (i.e., natural log response ratio) of soil microbial C and N metabolic processes and relevant soil C and N concentrations under elevated CO2. Results Results showed that elevated CO2 increased soil total C concentrations by 5.3% and total N concentrations by 4.8%, and decreased soil dissolved organic N and NO3- concentrations by 4.4% and 9.4%, respectively, but did not affect dissolved organic C or C:N ratios across global terrestrial ecosystems. Elevated CO2 significantly increased soil CO2 emissions and microbial biomass C by 19.3% and 13.3%, respectively, indicating that elevated CO2 increased both microbial anabolic and catabolic processes in soil. Similarly, elevated CO2 significantly increased soil N2O emissions and microbial biomass N by 18.7% and 9.0%, respectively. Microbial C cycling processes were associated with microbial N cycling processes under elevated CO2. Specifically, CO2 and N2O emissions were highest in soils with moisture contents of 40-60% and 60-80%, respectively, and microbial biomass C was largest in soils with pH values of 6.5-7.5. Conclusion Our findings demonstrated the profound impacts of elevated CO2 on microbially regulated C and N metabolic processes and the close linkage between soil microbial C and N cycling under global warming.