干旱区生态环境知识资源中心

Changes in precipitation and temperature in Asian continental steppelands may affect soil physical, chemical and biological processes that control the biosphere-atmosphere exchange of N-trace gases. The changes include regional desertification, global warming and strong El Nino events that impact the large steppe land area in China and Mongolia. The area is so large that feedbacks to the global greenhouse gas balance may occur. In this study we investigated how changes in soil moisture and temperature, and especially drying-rewetting and freeze-thaw events, affect nitric oxide (NO) and nitrous oxide (N2O) fluxes from large intact soil cores taken from representative land use/cover types in the region of the Xilin River catchment, Inner Mongolia. These soil cores were incubated under varying conditions with respect to temperature (ranging from -10 to 15 degrees C) and simulated rainfall (25, 45 and 65 mm). Following drying-rewetting and freeze-thaw transitions, we observed pulses of NO and N2O emissions from the soils of typical steppe, mountain meadow, sand dune and marshland. A comparable trend in soil CO2 emissions and soil air N2O concentrations indicated that the high substrate availability and rapid recovery of microbial activity after soil wetting and thawing resulted in high gas fluxes. Across the whole temperature range, NO and N2O fluxes from all soils, except for N2O emissions from marshland soils, showed a positive exponential relationship with soil temperature. A combination of soil temperature and soil moisture explained most of the observed variations in NO (up to 74-90%) and N2O (up to 67-89%) fluxes for individual soils. Spatial differences in NO emissions between land use/cover types could be explained by differences in soil organic carbon and pH, whereas spatial variations of N2O fluxes were primarily correlated with differences in soil microbial biomass. On the basis of the incubation under controlled conditions, the average annual flux, weighted by the areal extent of the different investigated land use/cover types in the region, was estimated at similar to 3.9 +/- 1.1 kg N ha(-1) yr(-1) for NO and 0.53 +/- 0.20 kg N ha(-1) yr(-1) for N2O, respectively. It is noteworthy that our measurements were conducted using soil cores without a vegetation cover, which probably resulted in an overestimation of N-trace gas fluxes. However, our results indicate that the rarely determined NO formation appears to be a significant pathway in the N cycle of semiarid steppe, which is highly sensitive to the climatic change taking place in these regions, especially an increase in intensity and frequency of drying-wetting and freeze-thaw cycles.