Abstract: Gaseous N losses via ammonia (NH3) volatilization were measured during the 2005 and 2006 growing seasons in a long-term (17 years) experiment with five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha–1). We aim to understand the seasonal variations in ammonia production, the potential carry-over effects of previous grazing and the underlying regulating processes involved. It was found that rates of NH3 volatilization varied seasonally, ranging from 0.88 to 3.52 g N ha–1d–1 during the measurement period, with higher values in spring and early summer and lower values in late summer and autumn. Soil pH value, NH4+-N concentration, moisture and bulk density exerted controls on NH3 volatilization over the two growing seasons. However,?the constraining effect of bulk density on NH3 flux, driven by grazing, muted the supporting effect of above other three environmental factors. Even though no statistically significant effects of different grazing intensities on rates of NH3 volatilization were generally found, grazed treatments tended to result in relatively higher cumulative NH3 volatilization, and relatively lower soil N content, plant growth and productivity. It indicates that restoration of the degraded grazed grassland may be slower in comparison with the ungrazed grassland. The study on NH3 volatilization in grassland ecosystem is meaningful in terms of its contribution to advancing our understanding of the role of N loss in limiting grassland processes and to drawing up grazing guideline and fertilizing regime for utilizing the resources rationally.

Key words: NH₃ volatilization, regulators, N cycle, rangeland, restoration