Underlying Ecosystem Methane Emissions Exceed Cattle-Derived Methane from Subtropical Lowland Pastures.

Abstract:

Grazing cattle are a major methane (CH₄) source from pasture ecosystems, however the underlying landscape is a potentially significant CH₄ source that has received far less attention. Ecosystem surface emissions of CH₄ are poorly quantified, vary widely across time and space, and are easily underestimated if emission hotspots or episodic fluxes are overlooked. We used static chambers, eddy covariance, and mobile cavity-ringdown spectrometry surveys to quantify spatially and temporally variable CH₄ emissions from subtropical lowland pastures. We conclude emissions from soil and standing water are the dominant CH₄ source, and cattle were responsible for only 13% of annual CH₄emissions. The ecosystem emit 33.8 ± 2.2 g CH₄ m^-2 yr^-1, however surface CH₄ emissions were highly variable in both time and space. Seasonal flooding of pastures and low-lying landforms (canals, ditches, wetlands) drove high magnitude CH₄ emissions. We observed large CH₄ emissions from wetlands and, to a lesser extent, the entire landscape during the wet season. In contrast, during the dry season there was no appreciable CH₄ accumulation in pastures when cattle were not present, and canals, which comprise 1.7% of the total land area, were responsible 97.7 % of dry season emissions. Ecosystem CH₄ fluxes, measured by eddy covariance, varied seasonally and positively correlated to soil and air temperature, topsoil water content, and water table depth. Our work is the first to use mobile spectrometers to map biogenic CH₄ emissions at the landscape scale, and demonstrates that soils and water are a strong pasture CH₄ source that must be considered in addition to cattle emissions.