Nitrogen cycling dynamics over space and time in annual and perennial grasslands in California

California grasslands were originally perennial-dominated, but experienced an extreme shift in plant community composition with the invasion of annual grasses from the Mediterranean Basin. Given that annuals differ from perennials in ways that may influence nitrogen (N) use, it is likely that annual invasion resulted in shifts N cycling dynamics in California grasslands. Phenological differences between the grasses, combined with seasonal variation in N availability, may be important in mediating competitive interactions between individual plants, leading to long-term consequences for community structure and function. Annual grasses also differ from perennials in stature, growth form, and rooting depth, suggesting that the shift from perennial to annual-dominance in California grasslands may have led to changes in the spatial structure of soil properties and processes in these systems. Because N often limits plant growth in grassland ecosystems, our understanding of how these systems function is constrained by our knowledge of the temporal and spatial dynamics of N availability and transformation in soils. Therefore, I used a multi-pronged approach to study N dynamics in California grasslands. I tracked seasonal changes in N pool sizes, processing rates, and microbial biomass in grassland soils. Using a 15N tracer experiment, I evaluated how N partitioning between plant, soil, and microbial pools differed between stands of perennial and annual grasses. I addressed the problem of how different grasses influence soil N dynamics by measuring net and gross rates of N cycling in experimentally-planted annual and perennial-dominated grasslands. Using geostatistical analysis and mapping techniques to quantify spatial heterogeneity in mixed grasslands, I related aboveground patterns in grass cover to the spatial structure of belowground resources and processes. I found that season and grass type both influence N cycling dynamics in grassland soils. While perennials take up less N, their soils leach more N than annual soils. With more annual cover, N pool sizes and cycling rates increase, and soils become more homogenous across space with regard to soil N, suggesting that the invasion of California grasslands resulted in large shifts in N cycling in these systems.