As the Earth warms, the respiration of soil microorganisms increases, which could be a signal of a carbon-climate feedback. Soil is the largest terrestrial carbon (C) pool and the rate at which soil C is released into the atmosphere is highly dependent on microbial processes. When the environment changes from stressful to favorable for microbial processes (e.g. wetting dry soil), rapid activation of dormant microbes can contribute to pulses of soil heterotrophic respiration (Rh) over very short time scales (hours). Because the importance of microbial mechanisms can change across temporal scales, in this study we tested whether dormancy and reactivation also contribute meaningfully to seasonal changes in Rh. We periodically measured Rh in an old-field ecosystem at the Boston Area Climate Experiment (BACE). We hypothesized that changes in Rh at the seasonal scale would be linked not just to activation of dormant cells, but also to net changes in total biomass and to changes in community composition (i.e. decreases of fungi:bacteria ratio due to growth of the fast-responding bacterial communities during the growing season). We used a flow-cytometric single-cell metabolic assay to quantify microbial activity/dormancy, and the PLFA method for measuring microbial biomass and composition.
Seasonal Rh changed with the amount of active microbes in soil, but not with total microbial biomass or community composition. Rh was higher in the warm (22.8±0.31 °C), dry (0.06±0.01% vol) summer (June; 2.87±0.30 μmol m-2s-1), than in the colder (16.8±0.34 °C), less dry (0.09±0.01% vol) fall (October; 1.16±0.05 μmol m-2s-1). Although total microbial biomass was less than two thirds as much in the summer (0.15±0.01 PLPO4 nmol g-1 soil) than in the fall (0.24±0.02 PLPO4 nmol g-1 soil), there was a larger proportion (and amount) of actively respiring cells in the summer (23.4±2.8 %) than in the fall (9.0±1.5 %). Overall, this suggests that seasonal changes in Rh in temperate ecosystems such as the one considered in this study can be more strongly linked to switches in microbial metabolic state than to changes in total biomass or community composition. This further supports the idea that microbial dormancy needs to be taken into account when predicting future carbon-climate feedbacks, and poses the question whether further warming could enhance Rh via activation of dormant microbes in soil.