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

Esch, D.L. Hernandez, J.R. Pasari, R.S.G. Kantor, and P.C. Selmants. 2012. Plant and Soil 366:671-682

Abstract

Background and aims: Exotic species, nitrogen (N) deposition, and grazing are major drivers of change in grasslands. However little is known about the interactive effects of these factors on below-ground microbial communities. Methods: We simulated realistic N deposition increases with low-level fertilization and manipulated grazing with fencing in a split-plot experiment in California's largest serpentine grassland. We also monitored grazing intensity using camera traps and measured total available N to assess grazing and nutrient enrichment effects on microbial extracellular enzyme activity (EEA), microbial N mineralization, and respiration rates in soil. Results: Continuous measures of grazing intensity and N availability showed that increased grazing and N were correlated with increased microbial activity and were stronger predictors than the categorical grazing and fertilization measures. Exotic cover was also generally correlated with increased microbial activity resulting from exotic-driven nutrient cycling alterations. Seasonal effects, on abiotic factors and plant phenology, were also an important factor in EEA with lower activity occurring at peak plant biomass. Conclusions: In combination with previous studies from this serpentine grassland, our results suggest that grazing intensity and soil N availability may affect the soil microbial community indirectly via effects on exotic cover and associated changes in nutrient cycling while grazing directly impacts soil community function.

Key Words

California; Cattle; Extracellular enzyme activity; Fertilization; Festuca perennis; Invasive species