Microbial community responses to changes in rainfall: moving from pattern to process
Rainfall patterns are expected to intensify in the future. Microbial responses to the resulting fluctuations in soil moisture could influence pulses of carbon dioxide that have been observed after extreme rainfall events, which many ecosystem models currently underestimate. So far, it has been difficult to determine the role of microbial communities in these biogeochemical responses because these moisture interactions are on such a small scale, and communities are so diverse. We use empirical and modeling approaches to ask how shifts in microbial community composition (and other mechanisms) influence CO2 flux with soil drying and rewetting. Specifically, we classify microbial taxa into life strategies based on their response to moisture pulses. Then, we test the relative effects of the microbial community and physical mechanisms on CO2 pulses by simulating moisture pulses in a theoretical model that includes different microbial life strategies.
Based on their relative abundance after drying and rewetting pulses, we found bacteria taxa strongly clustered into three distinct ecological strategies: those tolerant, opportunistic, and sensitive to moisture fluctuation, and that the number of species employing each strategy depended on rainfall history. This suggests that moisture pattern is a primary selective pressure for microbes in this environment, and that the functional traits associated with each of these ecological strategies have the potential to influence functions like soil carbon flux. When we examined the influence of microbial life strategy and other processes on respiration-moisture relationships using a theoretical model, we found that cumulative respiration under fluctuating moisture was closer to empirical estimates when multiple life strategies were included. Diffusion limitation and overall physiological stress also allowed model simulations to better approximate high CO2pulses after rainfall events, but it is like that other mechanisms are also involved. We conclude that microbes exhibit coherent ecological strategies in response to moisture, and these strategies may be used to explain community responses to altered rainfall patterns. However, knowledge about the functional traits of these strategies, as well as micro-scale physical responses to moisture fluctuation, will be important for predicting variation in CO2flux with shifts in rainfall patterns.