Sheep grazing impacts on soil methanotrophs and their activity in typical steppe in the Loess Plateau China

Abstract

Soil methanotrophs are the only biological sink for atmospheric methane (CH4). The activity and the diversity of methanotrophic communities in grassland soils can be impacted by animal grazing. In this study, soils were collected from a 17-year running grazing intensity trial in a typical steppe in the Loess Plateau, China to examine the responses of CH4 oxidation rate and active methanotroph composition and abundance to different levels of sheep grazing. Soils were collected from summer grazed pastures at no (0 sheep ha−1), low (2.67 sheep ha−1), moderate (5.33 sheep ha−1) and high (8.67 sheep ha−1) levels of grazing. We found that all soils were able to oxidize atmospheric and high concentration (10%) CH4 through soil methanotrophs. Compared to ungrazed soils, the atmospheric CH4 oxidation rate and methanotroph abundance were higher in low and high grazed soils but lower in moderately grazed soils. Across all soils, the in-situ methanotroph community was dominated by upland soil cluster gamma methanotrophs and potentially contributed to the atmospheric concentration CH4 oxidation. Soil methanotroph community responses to the high 13CH4 concentration revealed an increase in methanotroph abundance and a transition of the active methanotroph community to traditional Type I (Methylocaldum and Methylobacter) and Type II (Methylocystis) methanotrophs. Methylocaldum responses to the high CH4 concentration were greater in highly grazed soil than in ungrazed soil. The activity of Methylobacter and Methylocystis under high CH4 concentrations was reduced by high grazing intensity. This study provides evidence that recent intensifications of animal grazing can result in particular community assembly of microbial methane oxidizers responsible for CH4 sink capabilities of grassland soils.