Underlying Ecosystem Emissions Exceed Cattle-Emitted Methane from Subtropical Lowland Pastures

Cattle are a major methane (CH4) source from pasture ecosystems; however, the underlying landscape can be a significant and unaccounted source of CH4. In general, landscape CH4 emissions are poorly quantified, vary widely across time and space, and are easily underestimated if emission hotspots or episodic fluxes are overlooked. In this study, CH4 emissions from subtropical lowland pastures were quantified using static chambers, eddy covariance, and mobile spectrometer surveys. Landscape emissions were the dominant CH4 source, and cattle were responsible for 19-30% of annual emissions. The entire ecosystem emitted 33.84 +/- A 2.25 g CH4 m(-2) y(-1) as estimated by eddy covariance-measured fluxes. Landscape emissions were highly variable, and seasonal flooding drove high magnitude emissions from the underlying landscape. Large CH4 emissions were observed from wetlands and, to a lesser extent, from the entire landscape during the wet season. In contrast, during the dry season, there were no appreciable landscape CH4 emissions, although canals, which cover only 1.7% of the total land area, were responsible for 97.7% of dry-season emissions. Ecosystem CH4 fluxes, measured by eddy covariance, varied seasonally and positively correlated to water table depth, soil and air temperatures, and topsoil water content. The results presented here are the first to use mobile spectrometers to map biogenic CH4 emissions at the landscape scale, and strongly suggest that the underlying landscape is a strong CH4 source that must be considered in addition to cattle emissions.