Exchanges Of Greenhouse Gases From Steppe Ecosystems In An Agro-pastoral Econe Of Northern China

Grassland plays a key role for the climate system due to the about 40% covering of the land area, and the importance of grasslands as source or sink of greenhouse gases. However, there are still high uncertainties in estimates of annual exchange of greenhouse gases in grassland ecosystem, because most studies on gas fluxes in grassland ecosystem focused on the growing season, little is known about the magnitude of fluxes during the no-growing season. In this study, using static chamber technique, we quantify annual greenhouse gas exchanges from four land uses in the agro-pastoral ecotone in northern China: a summer-grazed steppe (SG), a winter-grazed steppe (WG), an ungrazed steppe (UG) since 1997 and a cropland (OC) originated from the cultivation of grassland. In the contrasting grassland and farmland, we estimated soil CO2 flux and subsurface CO2 production dynamics based on continuous CO2 measurement using solid sensor, and quantifed the vertical distribution of CO2 production in the soil profile. Finally, we estimated net greenhouse gas emissions under different land uses. The main results are as follows.All land uses always functioned as net sinks for atmospheric CH4 throughout the study period. The CH4 consumption was still observed during snow-cover period. The CH4 uptake fluxes showed a significant increase in SG, WG and OC following spring thaw and soil warming in late April, whereas such increase in CH4 uptake flux was not observed in the UG site. The annual CH4 uptake from SG, WG, UG and OC was 1.36、2.17、1.01 and 2.50 kg C ha-1 yr-1, respectively. The CH4 uptake in the growing season dominated the annual CH4 uptake, and the no-growing season contributed approximately 29-39% of total annual uptake. The results suggested the conversion from the steppe to cropland and grazing intended to enhance CH4 uptake.N2O fluxes at all land uses were low most of the time. N2O emissons occurred mainly after farmyard manure fertilization and during spring thaw periods. The annual N2O emissions from SG, WG, UG and OC was 0.19、0.15、0.43 and 0.98 kg N ha-1 yr-1, respectively. The spring-thaw N2O emissions of UG and OC dominated the total annual N2O 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 were only 31% and 33% for SG and WQ respectively. Thus, the importance of the no-growing season, particularly spring thaw season for the assessment of annual N2O budget. N2O fluxes during spring thaw season were positively related to soil mineral N. These results suggested that the conversion from the native grassland to cropland increased N2O flux. Instead, grazing has the potential to decrease annual N2O losses mainly through reducing spring-thaw N2O emissions.The ecosystem respiration from SG, WG, UG and OC was 722、657、817 and 901 g C m-2 yr-1, respectively. The CO2 emissions during the growing season accounted for 92%-95%。 The CO2 fluxes during the no-growing season (NEE) were ranged from 0 to 72 mg C m-2 h-1. NEE during the no-growing season could not be ignored when net CO2 exchange was estimated.Soil CO2 concentration generally increased with depth. For certain depth, soil CO2 concentration measured in the grassland was higher than farmland. Soil CO2 production at 0-20 cm、20-40 cm and 40-60 cm depth was 360,123,28 g C.m-2 for the grassland, and 267,45,15 g C.m-2 for the farmland. The relative contribution to total CO2 production at 0-20 cm,20-40 cm,40-60 cm depth were averaged 69%, 22%,9% in the grassland, and 81%,14%,5% in the farmland. Soil CO2 production in the grassland was greater compared to farmland due to perennial fibrous root biomass and higher soil organic carbon.The net greenhouse gas exchange from SG、WG、UG and OC was 2249、665、-1826、-376 kg CO2 eq yr-1, respectively. In the Sulutan administration region, the regional greenhouse gas budget was 789 t CO2 eq yr-1. The area ratio of OC, WG, SG site and UG, which is 0/0.2/0.5/0.3 (OC/WG/SG/UG), the regional greenhouse gas emission was lower 36% compared to current situation.