Spring-water Nitrate Increased with Removal of Livestock Grazing in a California Oak Savanna

Abstract

We characterized spatial and temporal changes in nitrate concentrations of the leachate from annual grasslands and subsequently emergent spring-waters and tested the effect of livestock grazing removal on them. Nitrate patterns indicated that annual grassland soils are a likely N source to spring-fed wetlands, which appear to intercept and transform N along its hydrologic path from upland soils to spring-fed, headwater streams. Aboveground biomass and soil N extractions suggested that removal of livestock grazing from these wetlands impaired this function by allowing dead plant material to accumulate inhibiting plant production (hence, plant N demand), resulting in elevated stream-water nitrate (NO3−) concentrations. Nitrous oxide (N2O) fluxes indicated that grazing removal may increase the relative importance of this N-loss pathway. Microbial biomass varied with season but was not affected by grazing treatments suggesting that N2O losses were related to differences in NO3− availability rather than grazing effects on microbial community composition or their activity. Spring-fed wetlands provide important ecosystem services such as plant uptake and denitrification at transition zones between terrestrial and aquatic ecosystems. These N-retention and transformation functions may be enhanced through biomass harvesting by livestock.