Increase in ammonia volatilization from soil in response to N deposition in Inner Mongolia grasslands

Ammonia (NH3) is an important component of the nitrogen (N) cycle in terrestrial ecosystems and is a significant atmospheric pollutant. Results from studies of ammonia emissions from semi-arid grasslands have been infrequently reported although these emissions may be increasing rapidly with a subsequent increase in atmospheric N deposition. Moreover, whether N deposition regimes (fertilization vs. atmospheric N deposition) influence NH3 volatilization from soil is unclear. We conducted an N deposition experiment in an Inner Mongolian grassland with nine levels of N addition (0, 1, 2, 3, 5, 10, 15, 20, and 50 g N m−2 yr−1) and two additional N regimes: a two-pulse addition (N2) simulating fertilization and monthly addition (N12) simulating atmospheric N deposition. We investigated NH3 volatilization from the soil in 2010 and 2011 after the addition of N. NH3 volatilization rates significantly increased with increasing intensities of N addition for both the two-pulse (R2 = 0.993, P < 0.001) and monthly addition (R2 = 0.992, P < 0.001). Cumulative NH3 emissions during the growing season were higher for the monthly addition regime (1.28-3.54 g N m−2) than the two-pulse addition regime (1.12-2.71 g N m−2) (P < 0.001), and there was a significant inter-annual variation in both of the N addition regimes. NH3 volatilization rates and cumulative emissions were significantly positively correlated with soil ammonium concentration in both N addition regimes. Overall, a higher level of N application at a higher frequency resulted in higher NH3 emissions indicating that the assessments made in most N fertilization experiments will underestimate the effects of N deposition on NH3 emissions from soils as well as on N cycles.