Winter-grazing reduces methane uptake by soils of a typical semi-arid steppe in Inner Mongolia, China

Steppe ecosystems are regarded as an important sink of atmospheric methane (CH4) and grazing is hypothesized to reduce CH4 uptake. However, firm experimental evidence is required to prove this hypothesis. Using a fully automated, chamber-based measuring system, we conducted continuous high-frequency (at a 3-h interval) measurements of CH4 uptake in a Leymus chinensis steppe, which is a typical grassland ecosystem in Inner Mongolia, China. Two management regimes were investigated: ungrazed since 1999 (UG99) and winter-grazed since 2001 (WG01). Measurements were carried out continuously during the periods of June–September 2004, May–September 2005 and March–June 2006. During all of these periods, significantly lower mean CH4 uptake (±S.E.) at WG01 (28±0.7 μg C m−2 h−1) as compared to UG99 (56±1.0 μg C m−2 h−1) (p<0.01) was found. Total CH4 uptake during the growing seasons (May–September) 2004 and 2005 at WG01 and UG99 was quantified as 1.15 and 2.15 kg C ha−1, respectively. Annual rates of CH4 uptake were approximately 1.91 (WG01) and 3.58 kg C ha−1 (UG99), respectively. These results indicate that winter-grazing of steppe significantly reduced atmospheric CH4 uptake by ca. 47%. The winter-grazing practice may have inhibited CH4 uptake by (a) increasing the likelihood of physiological water stress for CH4-consuming bacteria during dry periods, (b) decreasing gas diffusion into the soil and, (c) reducing the populations of CH4 oxidizing bacteria. These three mechanisms could have collectively or independently facilitated the observed inhibitory effects. Our results suggest that grazing exerts a considerable negative impact on CH4 uptake in semi-arid steppes at regional scales. Notwithstanding, further studies involving year-round, intensive measurements of CH4 uptake are needed.