Microbial N Turnover and N-Oxide (N₂O/NO/NO₂) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil-atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5-4.1 mg N kg-¹ soil dry weight day-¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m-² h-¹, equivalent to 0.03-0.14 kg N₂O-N ha-¹ y-¹, and were considerably lower than previously reported for the same region. NO x flux rates ranged between 0.16 and 0.48 μg NO x -N m-² h-¹, equivalent to 0.01-0.04 kg NO x -N ha-¹ y-¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.