Changes of δ15N values during the volatilization process after applying urea on soil.

Ammonia (NH3) 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 NH3 volatilization. The measurements of 15N natural abundance (δ15N) could be used as a complementary tool for apportioning emissions sources to resolve the contribution of multiple NH3 emission sources to air NH3 pollution. However, information of the changes of δ15N-NH3 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 δ15N values of NH3 during volatilization process. Results showed that volatilization process depleted 15N in NH3. The average δ15N value of NH3 volatilized from the 0, 20, 180, and 360 kg N ha-1 treatment was -16.2 ± 7.3‰, -26.0 ± 5.4‰, -34.8 ± 4.8‰, and -40.6 ± 5.7‰. Overall, δ15N-NH3 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. δ15N-NH3 values during the NH3 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 NH4+-N and NH3 volatilization rate and δ15N-NH3 values were observed in this study. Our results could be used as evidences of NH3 source apportionments and N cycle.