Effects of Nitrogen Application on N2O Flux from Fluvo-Aquic Soil Subject to Freezing-Thawing Process

A lab-incubation experiment was conducted to investigate the effects of different forms of nitrogen application (ammonium, NH4+-N; nitrate, NO3−-N; and amide-N, NH2-N) and different concentrations (40, 200 and 800 mg L−1) on N2O emission from the fluvo-aquic soil subjected to a freezing-thawing cycling. N2O emission sharply decreased at the start of soil freezing, and then showed a smooth line with soil freezing. In subject to soil thawing, N2O emission increased and reached a peak at the initial thawing stage. The average N2O emissions with addition of NH4+-N, NO3−-N and NH2-N are 119.01, 611.61 and 148. 22 μg m−2 h−1, respectively, at the concentration of 40 mg L−1; 205.28, 1 084.40 and 106.13 μg m−2 h−1 at the concentration of 200 mg L−1; and 693.95, 1820.02 and 49.74 μg m−2 h−1 at the concentration of 800 mg L−1. The control is only 100.35 μg m−2 h−1. N2O emissions with addition of NH4+-N and NO3−-N increased with increasing concentration, by ranging from 17.49 to 425.67% for NH4+-N, and from 563.38 to 1 458.6% for NO3−-N compared with control. There was a timelag for N2O emission to reach a steady state with an increase of concentration. In contrast, by adding NH2-N to soil, N2O emission decreased with increasing concentration. In sum, NH4+-N or NO3−-N fertilizer incorporated in soil enhanced the cumulative N2O emission from the fluvo-aquic soil relative to amide-N. This study suggested that ammonium and nitrate concentration in overwintering water should be less than 200 and 40 mg L−1 in order to reduce N2O emissions from soil, regardless of amide-N.