Changes of δN values during the volatilization process after applying urea on soil.

Ammonia (NH) volatilized from soils plays an important role in N cycle and air pollution, thus it is important to trace the emission source and predict source contributions to development strategies mitigating the environmental harmful of soil NH volatilization. The measurements of N natural abundance (δN) could be used as a complementary tool for apportioning emissions sources to resolve the contribution of multiple NH emission sources to air NH pollution. However, information of the changes of δN-NH values during the whole volatilization process under different N application rates are currently lacking. Hence, to fill this gap, we conducted a 15-day incubation experiment included different urea-N application rates to determine δN values of NH during volatilization process. Results showed that volatilization process depleted N in NH. The average δN value of NH volatilized from the 0, 20, 180, and 360 kg N ha treatment was -16.2 ± 7.3‰, -26.0 ± 5.4‰, -34.8 ± 4.8‰, and -40.6 ± 5.7‰. Overall, δN-NH values ranged from -46.0‰ to -4.7‰ during the whole volatilization process, with lower in higher urea-N application treatments than those in control. δN-NH values during the NH volatilization process were much lower than those of the primary sources, soil (-3.4 ± 0.1‰) and urea (-3.6 ± 0.1‰). Therefore, large isotopic fractionation may occur during soil volatilization process. Moreover, negative relationships between soil NH-N and NH volatilization rate and δN-NH values were observed in this study. Our results could be used as evidences of NH source apportionments and N cycle.