Background/Question/Methods   In recent decades, human activities have vastly increased the amount of biologically available nitrogen (N) in the biosphere. The resulting increase in N availability has broadly affected ecosystems, through increased productivity, changes in species composition, altered nutrient cycles, and increases in invasion by exotic plant species, especially in systems that were historically low in N. California serpentine grasslands are a rare ecosystem type dominated by endemic species, including several threatened and endangered plants and animals. However, in recent decades invasion by exotic grasses has increased, presumably due to increased N deposition. While the exotic species have been effectively managed with grazing by cattle thus far, the effects of future increases in N deposition and cattle grazing are unclear. We examined the interactive effects of cattle grazing and N deposition on plant community composition, invasion resistance, and microbial processes in serpentine grasslands to determine the efficacy of current management strategies as well as the biogeochemical consequences of exotic species invasion.

Results/Conclusions   In the first two years of the study, aboveground net primary productivity decreased in response to grazing and increased in response to nitrogen addition. However, contrary to our hypotheses the change in productivity was not due to an increase in exotic species cover as there was no effect of grazing or N addition on species composition. Microbial activity was more responsive to grazing and N. Potential net N mineralization rates increased with N addition, but were not affected by grazing. In contrast, soil respiration rates were inhibited by grazing, but were not affected by N addition; suggesting strong carbon-limitation of soil microbial activity, particularly under grazing. We expect that the unusually dry conditions in the first two growing seasons inhibited the growth of exotic species and minimized the effects of excluding grazers and N addition on species composition.