Nitrous oxide emissions from an agro-pastoral ecotone of northern China depending on land uses

Overgrazing and intensive farming have led to severe land degradation in the past half century in the agro-pastoral ecotone of northern China. Currently, complete and periodical exclusions of grazing are commonly adopted for the restoration of these degraded grasslands. However, little is known about the effects of such land uses on nitrous oxide (N2O) emission in this region. Using static chamber technique, we quantified annual N2O emissions (from May 2012 to September 2013) from four land uses: summer-grazed grassland (SG), winter-grazed grassland (WG), ungrazed grassland since 1997 (UG) and oat cropland (OC). N2O emissions occurred mainly after farmyard manure fertilization and during spring thaw periods. Annual N2O fluxes from the SG, WG, UG and OC were 0.19, 0.15, 0.43 and 0.98 kg N ha−1 yr−1, respectively. The spring-thaw N2O emissions from UG and OC dominated the annual emission and accounted for 70% and 65% of the annual fluxes, respectively. In contrast, the contributions of spring thaw fluxes to total annual N2O emissions for SG and WG were only 32%. N2O fluxes during spring thaw season were positively related to soil NH4+ +  NO3− content accounting for 80% of N2O flux variability across all land uses. Land use conversion from the native grassland to cropland increased N2O flux both during growing and spring thaw seasons due to farmyard manure application. Instead, grazing has the potential to decrease annual N2O losses mainly through reducing spring-thaw N2O emissions.