Annual methane uptake by temperate semiarid steppes as regulated by stocking rates, aboveground plant biomass and topsoil air permeability

Overgrazing-induced degradation of temperate semiarid steppes may affect the soil sink for atmospheric methane (CH4). However, previous studies have primarily focused on the growing season and on single grazing patterns. Thus, the response of annual CH4 uptake by steppes compared with various grazing practices is uncertain. In this study, we investigated the effects of grazing on the annual CH4 uptake by two typical Eurasian semiarid steppes (the Stipa grandis steppe and the Leymus chinensis steppe) located in Inner Mongolia, China. The CH4 fluxes were measured yearround using static chambers and gas chromatography at 12 field sites that differed primarily in grazing intensities. Our results indicated that steppe soils were CH4 sinks throughout the year. The annual CH4 uptake correlated with stocking rates, whereas the seasonality of CH4 uptake was primarily dominated by temperature. The annual CH4 uptake at all sites averaged 3.7 +/- 0.7 kgCha(-1) yr(-1) (range: 2.3-4.5), where approximately 35% (range: 23-40%) occurred during the nongrowing season. Light-to-moderate grazing (stocking rate <= 1 sheep ha(-1) yr(-1)) did not significantly change the annual CH4 uptake compared with ungrazed steppes, but heavy grazing reduced annual CH4 uptake significantly (by 24-31%, P<0.05). These findings imply that easing the pressure of heavily grazed steppes (e. g. moving to light or moderate stocking rates) would help restore steppe soil sinks for atmospheric CH4. The empirical equations based on the significant relationships between annual CH4 uptake and stocking rates, aboveground plant biomass and topsoil air permeability (P<0.01) could provide simple approaches for the estimation of regional CH4 uptake by temperate semiarid steppes.