Serpentine grasslands harbor some of the most diverse plant communities in California, but the spread of invasive grasses is increasingly threatening these centers of native biodiversity. One factor that is potentially facilitating these invasions is increased rates of nitrogen (N) deposition. Cattle grazing is commonly used to combat the spread of invasive grasses in these grasslands, but increasing N deposition may compromise the effectiveness of grazing. Soil microbial activity strongly influences the cycling of carbon (C) and N in soil, and ultimately affects N availability and plant species composition. Microbial responses to the combined effects of N deposition and grazing will likely mediate future changes in serpentine plant communities. This research addresses three main questions: 1) How do N addition and grazing (alone and in combination) affect microbial biomass and activity? 2) How do microbial biomass and activity vary on intra and interannual timescales, and how do N addition and grazing modify these temporal patterns? 3) Which biotic and abiotic factors exert the strongest controls on microbial biomass and activity? To answer these questions, grazing and N addition treatments were applied in a factorial design at a serpentine grassland in California. Soil microbial biomass and extracellular enzyme activities were measured for six dates from October 2008 through May 2009.

Results/Conclusions

Results from October - December indicate a significant stimulatory effect of N addition on most hydrolytic enzyme activities (p < 0.05), and a trend towards higher activities in grazed plots. Grazing significantly suppressed oxidative enzyme activities, while N additions had no effect. Although there were no significant interactions between the treatments, grazing reduced the magnitude of the effect of N addition for most enzymes. Both oxidative and hydrolytic enzyme activities show significant positive correlations with soil moisture. Enzyme activities measured in October prior to the first rainfall event were generally higher and more sensitive to soil moisture than during subsequent measurements. Overall, these results indicate that increasing N deposition in serpentine grasslands may stimulate microbial C and nutrient cycling, creating a positive feedback that could lead to higher soil nutrient availability, and facilitate the spread of invasive grasses. However, grazing may continue to be an effective management strategy for combating these invasions by mitigating the stimulatory effects of N deposition on microbial nutrient cycling.