Grazing removal decreases the magnitude of methane and the variability of nitrous oxide emissions from spring-fed wetlands of a California oak savanna

Abstract

Spring-fed wetlands are embedded within Californian oak savannas whose understory is dominated by annual grasslands that are grazed by livestock. Because there is mounting pressure to remove livestock from riparian areas in the western U.S., we excluded livestock from one-half of three spring-fed wetlands and monitored greenhouse gas (CH4 and N2O) fluxes in 2000 and 2002. In 2003, we also measured several ecosystem characteristics to help understand treatment differences in gas fluxes. Bootstrapped estimates of mean CH4 and N2O fluxes over the study period showed that these wetlands were sources of CH4 and N2O to the atmosphere; we compare the magnitude of these fluxes to estimates from other wetland studies. Grazing removal decreased the magnitude of CH4 emissions and their variability during our study period. A regression tree analysis showed lower soil temperature and higher soil water content to be the best predictors of lower CH4 emissions, both of which were observed under grazing removal. The magnitude of N2O emissions was not influenced by grazing removal, but fluxes from ungrazed plots were less variable. Grazing exclusion during hot summer months in California should reduce CH4 emissions from spring-fed wetlands, but have little effect on the magnitude of N2O loss to the atmosphere. Implications of climate change for these processes are discussed.