Grazing effects on the greenhouse gas balance of a temperate steppe ecosystem

Abstract

Although a significant fraction of the global soil–atmosphere exchange of greenhouse gases (GHGs) occurs in semi-arid zones little is known about the magnitude of fluxes in grazed steppe ecosystems and the interference with grazing intensity. In order to assess GHG burdens and to identify options of climate-optimized livestock farming, GHG emissions of sheep grazing in Inner Mongolia steppe were analyzed. Carbon sequestration and field-fluxes of methane (CH4) and nitrous oxide (N2O) were measured at a range of steppe sites differing in grazing intensity and management, i.e. ungrazed (UG), ungrazed with hay cutting (HC), lightly grazed (LG), moderately grazed (MG), and heavily grazed (HG). In addition, GHG emissions from enteric fermentation, manure management, and farming inputs (i.e. fossil fuels) were quantified for LG, MG, and HG. Monte Carlo simulation was used to estimate uncertainty. Sheep grazing changed the net GHG balance of the steppe from a significant sink at UG (−1476 ± 2481 kg CO2eq ha−1 year−1) to a significant source at MG (2350 ± 1723 kg CO2eq ha−1 year−1) and HG (3115 ± 2327 kg CO2eq ha−1 year−1). In a similar way, the GHG intensity increased from 8.6 ± 79.2 kg CO2eq kg−1 liveweight gain at LG up to 62.2 ± 45.8 and 62.6 ± 46.7 kg CO2eq kg−1 liveweight gain at MG and HG, respectively. GHG balances were predominantly determined by CO2 from changes in topsoil organic carbon. In grazing systems, CH4 from enteric fermentation was the second most important component. The results suggest that sheep grazing under the current management changes this steppe ecosystem from a sink to a source of GHGs and that grazing exclusion holds large potential to restore soil organic carbon stocks and thus to sequester atmospheric CO2. The balance between grazing intensity and grazing exclusion predominantly determines GHG balances of grass-based sheep farming in this region. Therefore, a high proportion of ungrazed land is most important for reducing GHG balances of sheep farms. This can be either achieved by high grazing intensity on the remaining grazed land or by confined hay feeding of sheep.