Microbial community composition and soil organic matter dynamics in agroecosystems

The objective of this dissertation was to examine the effects of different agricultural management practices on microbial community composition and the influence that community structure has on the formation and stabilization of soil organic matter (SOM) in agricultural soils. My research included field observations collected from several long-term agricultural experiments located in the wheat and corn-growing regions of the U.S., as well as laboratory and field experiments designed to test specific process-level hypotheses. These studies examined (1) the effects of no-tillage (NT) and conventional tillage (CT) management on microbial community composition, (2) controls on microbial community composition, (3) nitrogen (N) translocation through fungal hyphae in a NT system, and (4) the impacts of protozoan grazing on patterns of carbon utilization by the soil microbial community.; Shifts in the relative abundances of bacteria and fungi were observed and were found to be related to soil water availability which varied locally with tillage treatment and geographically with climate. Fungal biomass and the proportion of the total biomass composed of fungi increased in surface soil in response to reduced tillage and across a gradient of increasing soil moisture, while bacterial biomass was not strongly affected by tillage and remained relatively constant in response to changing soil moisture.; Fungal translocation of N from the soil inorganic N pool into surface residues was a significant exogenous N source for fungi associated with residues decomposing in a NT field and accounted for the observed net N immobilized by those residues. Both residue quality and soil N availability were important controls on fungal biomass associated with surface residues and rates of soil-to-residue N translocation.; Protozoan grazing significantly influenced patterns of glucose utilization by the soil microbial community. The total amount of glucose utilized did not vary with protozoan grazing intensity, but a high level of grazing increased the rate of glucose use and significantly reduced the amount of measurable microbial biomass C. Microbial biomass turnover was significantly faster in high compared to low grazing treatments.