Moderate grazing increases the abundance of soil methane-oxidizing bacteria and CH4 up-take rate in a typical steppe of Inner Mongolia, China.

Microbial oxidation is the only biological sink of atmospheric methane (CH₄ ). It is essential to understand the variation of CH₄ fluxes among different grassland use types for developing low-emission management system. Here, we measured the CH₄ flux and the soil methane-oxidizing bacteria abundance in a typical steppe under grazing, mowing and fencing management in central Inner Mongolia, with the aims to determine the effects of these grassland use types on CH₄ flux, and to test the hypothesis that pmoA functional gene abundance regulates CH₄ fluxes. The measurements were conducted on the experimental grassland that had experienced four grassland use treatments over five years. The treatments were whole growing season grazing from May to September (T, ), spring and summer grazing (twice in May and July) (T₂), autumn mowing (T₃) and enclosure (To). We measured CH₄ flux using static chamber method, and quantified the abundance of pmoA functional genes using molecular techniques. Moreover, we measured plant biomass and soil physi- cochemical properties. The results showed that moderate grazing significantly enhanced CH₄uptake rate and the methane-oxidizing bacteria abundance (i.e., the pmoA gene copy number per gram of dry soil). The pmoA gene copy number ranged from 6.9x10⁴ to 3.9x10⁵ per gram of dry soil in growing season. The CH₄ uptake rate was (68.21 ±3.01) (µg ⋅ m^-2 ⋅ h^-1 under T₁, which was 22.1%, 37.5% and 30.9% higher than that under T₂, T₃ or T₀, respectively. The CH₄ uptake rate was positively correlated with abundance of CH₄ oxidizing bacteria and soil sand content, but negatively correlated with soil silt content, soil moisture, NH₄⁺-N and NO₃^--N content, and plant biomass. These results suggested that the steppe ecosystem is a CH₄ sink under all land-use types in central Inner Mongolia, and that moderate grazing would enhance methane-oxidizing bacteria abundance and CH₄uptake by improving soil sand content, reducing soil mineral nitrogen content and plant production in the typical steppe ecosystem. These results were of significance for the development of low-emission grassland management system. [ABSTRACT FROM AUTHOR]