Annual methane uptake by typical semiarid steppe in Inner Mongolia

Abstract

Steppe ecosystems cover approximately 10% of the global land surface. Recent measurements have shown that steppe soils function as a significant sink for atmospheric methane (CH4). However, precise quantification of the annual CH4 uptake by steppe is challenged by infrequent measurements of exchange rates, which often only cover the growing season. In order to understand the annual dynamics and magnitude of CH4 exchange, especially contribution of nongrowing season to the cumulative annual CH4 exchange, we conducted year‐round CH4 flux measurements at high temporal resolution at two adjacent steppe sites. One was ungrazed and fenced since 1999 (UG99) and the other was grazed during the winter (WG01). The measurements were supplemented with observations of CH4 concentrations in the soil profile. Sites were located in typical Leymus chinensis steppe in Inner Mongolia, China. The results show that the typical semiarid steppe functioned exclusively as a sink for atmospheric CH4 throughout the entire year. Even during the spring soil thawing, a period with high water content in the top soil, CH4 uptake was dominant. The seasonality of CH4 uptake displayed a strong dependency on the seasonal variation in soil temperature. Soil moisture increased in importance when temperature was not the limiting factor. For example, CH4 rates decreased sharply following summer rainfall events. The annual CH4 uptake by the ungrazed UG99 and the winter‐grazed WG01 sites was 3.7 and 2.1 kg C ha−1, respectively. The contribution of the nongrowing season (October–April) to the cumulative annual CH4 uptake was approximately 30% (25%–36%). Additionally, our data suggest that winter grazing significantly alters the capacity of steppe soils for CH4 uptake. However, more measurements at paired ungrazed/grazed sites are needed to assess how grazing might affect the CH4 uptake capacity of steppe soils at a larger regional or global scale.