Soil Moisture Fluctuation Rather Than Water Saturation Enhances Carbon Decomposition via Microbial Physiological Responses

Biogeochemical processes in soil regulated by microbes are increasingly disrupted by climate change, particularly extreme rainfall events with varying frequencies and intensities. Understanding how microbial physiological states and activities respond to the disturbance of soil moisture helps to elucidate the key roles of active microbes in regulating soil organic matter decomposition. In the 10-week incubation experiment, we investigated the effects of altered water availability on microbial physiological responses and soil carbon decomposition, including treatments with low and high average moisture contents or low and high moisture fluctuation patterns. Highly fluctuating soil moisture increased microbial mortality by approximately 12.2%, while changes in average water availability had little impact. Active cells exhibited distinct functional responses, with higher relative abundances of anabolic genes under high moisture and genes encoding various dehydrogenases under highly fluctuating conditions. Moreover, active microbial communities exhibited 1.6-fold more associations with soil functions compared to the entire microbial communities, which concurred with a 12.5% reduction in soil respiration rate and a shift in the composition of dissolved organic matter. Collectively, our findings elucidate a mechanistic link between microbial physiological responses of active cells and carbon decomposition under hydrological extremes, offering critical insights into the role of active bacteria in regulating soil carbon dynamics.