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.