Awakening of dormant soil microbes: Implications for soil CO2 emissions

Background/Question/Methods

Transitions between active and dormant physiological states of soil microbial communities are an important, and often ignored, mechanism to understand soil respiration responses to environmental conditions. Current approaches to analyze microbial control over soil CO₂ production are mainly focused on total microbial biomass (TMB) and do not directly consider its active and dormant fractions. In general, more than 80-90 % of soil microbes are dormant and do not significantly contribute to soil CO₂ emissions. Since only active microbes are capable to drive soil biogeochemical processes, we tested the hypothesis that soil respiration responses to environmental conditions are better explained by active microbial biomass (AMB) than by TMB. We used a kinetic approach based on the Substrate Induced Growth Response (SIGR) methodology to measure TMB, AMB, and other microbial parameters (i.e. microbial specific growth rate and t_lag, the lag-time before microbial exponential growth starts in response to substrate input) that give us a more mechanistic understanding of the ways soil microbes control soil carbon cycling. For this, we used forest soil that was incubated for 1 week at different temperature (24 and 33 °C) and soil moisture (10 and 20% wt) conditions. 

Results/Conclusions

Soil respiration responses to warming and increased soil moisture were explained by changes in the active and dormant fractions of soil microbial communities rather than by changes in TMB. As expected, soil basal respiration (SBR) was more strongly correlated with AMB (R²=0.55, P=0.006) than with TMB (R²=0.04, P=0.51). This correlation was positive and indicates an approximate increase in SBR of 0.04 μg CO₂-C g^-1 soil h^-1 per μg active biomass C g^-1 soil. Also, microbial specific growth rate was poorly influenced by environmental conditions demonstrating intrinsic feature of dominating populations, whereas t_lag was significantly reduced by warming and increased soil moisture demonstrating an activity state of soil microorganisms. Overall, our findings suggest that SBR responses to environmental factors such as temperature and soil moisture are better explained by AMB than by TMB. Further research is needed to test our hypotheses at large spatio-temporal scales. If climatic changes such as those predicted for this century (e.g. warming and increased precipitation variability) increase AMB globally, estimations of future soil CO₂ production based on TMB may underestimate future soil CO₂ emissions.