Asymmetric responses of microbial respiration to extreme drought and wetting

Background/Question/Methods: Earth system models predict that there will be more frequent and severe drought and extreme precipitation events under climate change. Globally, soils store over twice as much carbon (C) as the atmosphere. As such, a small change in soil C content may have a large impact on the magnitude of atmospheric CO₂ concentrations and therefore climate change. Soil microbial respiration, one of the most important CO₂ effluxes from soils to the atmosphere, is dependent on soil moisture conditions which impact microbial decomposers, the diffusivity of soluble substrates and oxygen in the soil matrix. However, the magnitude of microbial respiration in response to the intensified moisture extremes is still unclear. Here, we conducted a theoretical analysis of the response of soil microbial respiration to intensified soil moisture extremes by the end of the 21^st century using two different soil carbon models, ELM-CN and MEND. ELM-CN is a conventional model used widely in the current generation of Earth system models, while MEND includes emerging microbial mechanisms in soil organic C dynamics.

Results/Conclusions: Although the two models represented soil C dynamics differently, they provided similar results in terms of the responses of soil microbial respiration to the intensified soil moisture extremes. Results showed that the magnitude of reduced microbial respiration by drought was generally greater than that of increased microbial respiration by wetting, suggesting an asymmetric response of microbial respiration to soil moisture variations. Further analysis indicated that the asymmetric response of microbial respiration was due to greater responses of active microbial biomass and substrate acquisition rates to drought than to wetting. As a consequence, increased frequency and severity of soil moisture extremes reduced soil C loss through microbial respiration. This study emphasizes the non-linear response of soil microbial response to moisture variations. Currently, most soil C studies are focused on temperature responses, while this study points to the importance of the sensitivity of soil CO₂ fluxes to moisture.