Annual N2O emissions from conventionally grazed typical alpine grass meadows in the eastern Qinghai–Tibetan Plateau

Abstract

Annual nitrous oxide (N2O) emissions from high-altitude alpine meadow grasslands have not been effectively characterized because of the scarcity of whole-year measurements. The authors performed a year-round measurement of N2O fluxes from three conventionally grazed alpine meadows that represent the typical meadow landscape in the eastern Qinghai–Tibetan Plateau (QTP). The results showed that annual N2O emissions averaged 0.123±0.053 (2SD, i.e., the double standard deviation indicating the 95% confidence interval) kgNha−1yr−1 across the three meadow sites. N2O flux pulses during the spring freezing-thawing period (FTP) were observed at only one site, indicating a large spatial variability in association with soil moisture differences. Approximately 34–57% (mean: 46%) of the annual N2O emissions occurred in the non-growing season, highlighting the substantial importance of accurate flux observations during this period. The simultaneous observations showed conservative, marginal nitric oxide (NO) fluxes of 0.058±0.032 (2SD) kgNha−1yr−1. The N2O fluxes across the three field sites correlated negatively with the soil nitrate concentrations during the entire year-round period (P<0.05). Furthermore, a significant joint regulatory effect of topsoil temperature and moisture on the N2O and NO fluxes was observed during the relatively warm periods. Based on the results of the present and previous studies, a simple extrapolation roughly estimated the annual total N2O emission from Chinese grasslands to be 73±15 (2SD) GgNyr−1 (1Gg=109g). A linear dependence of the annual N2O fluxes on the aboveground net primary productivity (ANPP) was also found. This result may provide a simple approach for estimating the N2O emission inventories of frigid alpine or temperate grasslands that are ungrazed either in the summer or year round. However, further confirmation of this relationship with a wider ANPP range is still needed in the future studies.