| This dissertation focuses on how moisture variability (soil drying and rewetting) and carbon availability (as a function of soil depth) controls the dynamics of microbial communities and processes in soils of coastal southern California. Short-term effects of a single drying and rewetting cycle included: a significant increase in soil CO2 production, due to mineralization of labile cytoplasmic solutes by microbial cells, and an increase in the pool size of soluble organic C in soil. The frequency of drying and rewetting events (independent of average soil moisture levels) influences microbial community composition (assessed using terminal random fragment length polymorphisms, T-RFLP), C cycling rates, C substrate use efficiencies, and nutrient pool sizes. These changes were largest in soils collected from natural environments where rapid fluctuations in soil moisture are uncommon. Two general conclusions arise from this work: (1) rainfall frequency, not just average rainfall alone, can have a strong influence on soil microbial ecology, and (2) there are strong links between community composition and nutrient cycling dynamics in soil. Based on phospholipid fatty acid analysis (PLFA), community composition changed significantly with depth and the specific changes are predictable based on the gradient in C availability. In the soil profile, community composition also has a direct influence on soil C processing. The subsurface soils are distinct from surface soils in terms of the diversity of C substrates that can be mineralized, the characteristics of organic C processing, and the specific environmental controls on C mineralization rates. |
