Response of soil enzyme activity to grazing, nitrogen deposition, and exotic cover in a serpentine grassland

Background/Question/Methods

Exotic species, nitrogen (N) deposition, and grazing are major drivers of change in grasslands. In California serpentine grasslands, there has been an increase in invasion of exotic annual grasses (particularly Festuca perennis), presumably due to increased N deposition. In response to the increase in exotic cover, cattle grazing is used as a management tool to decrease exotic grass cover and maintain native species richness. Previous research has demonstrated that N deposition and grazing have interactive effects on plant community composition. However, their effects on soil microbial activity have not been investigated. Both N deposition and grazing may influence the availability of nutrients directly and indirectly through effects on exotic cover, and thus have significant impacts on microbial processes. We examined the interactive effects of N deposition and grazing in California's largest serpentine grassland. We established a 2x2 factorial experiment with N addition and grazing measured as both categorical and continuous variables. We measured microbial extracellular enzyme activity (EEA) at the beginning of the growing season and a peak biomass. We also measured N mineralization and respiration rates at peak biomass. We measured activities of five enzymes involved in the acquisition of C, N, and phosphorous (P): cellobiohydrolase (CBH), l-leucine aminopeptidase (LAP), N-acetylglucosaminidase (NAG), phosphatase (PHOS), and β-xylosidase (XYL).

Results/Conclusions

Seasonality affected all enzymes except LAP (p<0.001), showing greater activity in January than at peak biomass. Soil C and nutrient availability is likely low when winter rains increase microbial activity, but at peak biomass, increased resource availability decreases microbial EEA. Continuous measures of grazing intensity and N availability were stronger predictors than the categorical grazing and fertilization measures. At peak biomass, higher grazing intensity increased CBH, NAG, PHOS, XYL, and N mineralization rates in a backward stepwise regression model using Akaike's Information Criterion. Greater soil N availability increased CBH, LAP, XYL, and N mineralization activity in grazed sites. At peak biomass, CBH, LAP, and N mineralization activity increased with greater exotic cover while CBH, LAP, NAG, and XYL activities increased with exotic cover early in the growing season (p<0.05). Our results suggest that grazing directly impacts soil community function while N deposition may affect the soil microbial community indirectly via effects on exotic cover and associated changes in nutrient cycling.