Influence of Drying–Rewetting Frequency on Soil Bacterial Community Structure

Soil drying and rewetting represents a common physiological stress for the microbial communities residing in surface soils. A drying–rewetting cycle may induce lysis in a significant proportion of the microbial biomass and, for a number of reasons, may directly or indirectly influence microbial community composition. Few studies have explicitly examined the role of drying–rewetting frequency in shaping soil microbial community structure. In this experiment, we manipulated soil water stress in the laboratory by exposing two different soil types to 0, 1, 2, 4, 6, 9, or 15 drying–rewetting cycles over a 2-month period. The two soils used for the experiment were both collected from the Sedgwick Ranch Natural Reserve in Santa Ynez, CA, one from an annual grassland, the other from underneath an oak canopy. The average soil moisture content over the course of the incubation was the same for all samples, compensating for the number of drying–rewetting cycles. At the end of the 2-month incubation we extracted DNA from soil samples and characterized the soil bacterial communities using the terminal restriction fragment length polymorphism (T-RFLP) method. We found that drying–rewetting regimes can influence bacterial community composition in oak but not in grass soils. The two soils have inherently different bacterial communities; only the bacteria residing in the oak soil, which are less frequently exposed to moisture stress in their natural environment, were significantly affected by drying–rewetting cycles. The community indices of taxonomic diversity and richness were relatively insensitive to drying–rewetting frequency. We hypothesize that drying–rewetting induced shifts in bacterial community composition may partly explain the changes in C mineralization rates that are commonly observed following exposure to numerous drying–rewetting cycles. Microbial community composition may influence soil processes, particularly in soils exposed to a significant level of environmental stress.