Annual methane uptake from different land uses in an agro-pastoral ecotone of northern China

The semi-arid grasslands in northern China are subjected to accelerating land use change due to population growth and food demand. Considerable uncertainty exists on annual methane (CH4) fluxes of different land uses because most measurements have only been conducted during the growing season. Using static chamber – gas chromatographic technique, we quantified and characterized annual CH4 fluxes (from 2012 to 2015) from four land uses common in an agro-pastoral ecotone of northern China: summer-grazed grassland (SumGrazed), winter-grazed grassland (WinGrazed), ungrazed grassland since 1997 (Ungrazed) and oat cropland (OatCrop). The soil at all land uses functioned exclusively as a sink for atmospheric CH4 through the entire three years. Annual CH4 uptake rates averaged 1.42, 2.36, 1.12 and 2.57kgCha−1 yr−1 for SumGrazed, WinGrazed, Ungrazed and OatCrop, respectively, during 2012–2015. Compared to Ungrazed, OatCrop and WinGrazed increased annual CH4 uptake by 129 and 111%, respectively. Non-growing season (October–April) contributed 28–43% of the annual CH4 uptake at all land uses. Across all four land uses, annual cumulative CH4 uptake decreased with increasing soil water-filled pore space (WFPS) explaining 81% of the variance in annual CH4 uptake. WFPS negatively correlated to CH4 uptake in the growing season (R2 =0.16-0.35, P<0.001). CH4 uptake increased with soil temperature through the entire observed period (R2 =0.38-0.63, P<0.001) and the non-growing season (R2 =0.51–0.74, P<0.001). We conclude that grazing has the potential to increase CH4 uptake from the atmosphere and consequently, contribute positively to CH4 related part of C budget.