Budget of methane emissions from soils, livestock and the river network at the regional scale of the Seine basin (France)

We used various approaches to establish a comprehensive budget of methane (CH₄) emissions from the Seine basin, including direct emissions from livestock and soils as well as emissions from the drainage network. For the direct emissions from livestock, we used official livestock census numbers and emission factors (CH₄emitted by each animal species per head per year) available in the literature. For the emissions from soils, we based our estimates on experimental measurements in closed chambers installed on different agricultural plots, forest, and grasslands in 2008 and 2009. The results were extrapolated to the whole Seine basin, including grassland, cropland, and forest soil distributions in the Seine basin. The CH₄emissions from the Seine drainage network were also based on measurements of sampled waters in various rivers and streams (from headwaters to estuary) during different seasons in 2007, 2008, and 2010. After chemical analysis of CH₄concentrations in the water samples using a gas chromatographic technique and calculation of the CH₄supersaturation by stream order in rivers of the Seine basin (from 1 to 8) and by season we could estimate the CH₄emissions for the whole water surface area of the Seine drainage network. The livestock of the Seine basin produce CH₄emissions amounting to 166 × 10⁶ kg C year⁻¹, among which cattle are responsible for 85 %. The total CH₄emission from the Seine drainage network was estimated at 0.3 × 10⁶ kg C year⁻¹, large rivers being responsible for the largest proportion. Ebullition could account for an additional 0.2 × 10⁶ kg C year⁻¹. Soils of the Seine basin are a net sink for CH₄(9.4 × 10⁶ kg C year⁻¹). The water and soils fluxes are low with regard to emissions by livestock, but domestic waste, through landfills, could contribute an additional 40 × 10⁶ kg C year⁻¹.