Evaluation of N2O sources after fertilizers application in vegetable soil by dual isotopocule plots approach

Nitrogen (N) fertilizer is the major deriver of nitrous oxide (N2O) emissions in agricultural soil. In the vegetable fields in China both inorganic and organic fertilizers are largely applied as basic sources of nitrogen. Identifying the effects of fertilizer type on soil microbial activities involved in N2O emissions would be of great help for future development of N2O reduction strategies. N2O isotopocule deltas, including δ15Nbulk, δ18O and SP (the 15N site preference in N2O), have been used to analyze microbial pathways of N2O production under different treatments, including bio–organic fertilizer treatment, half bio–organic fertilizer and half urea (mixed fertilizer) treatment, urea treatment and no fertilizer treatment. We measured environmental factors, N2O fluxes and N2O isotopocule deltas to evaluate the dynamics of N2O emissions and constructed the dual isotopocule plots (δ15Nbulk vs. SP and δ18O vs. SP) of the main N2O emission phases to assess contribution of the involved microbial processes (bacterial nitrification, bacterial denitrification, nitrifier denitrification and fungal denitrification). According to the results of the main N2O emission phases, we found that bio–organic fertilizer and mix fertilizer treatments had significantly lower N2O emissions compared to urea treatment, with average N2O fluxes of 1477 ± 204, 1243 ± 187 and 1941 ± 164 μg m−3 h−1, respectively, but there were no significant effects on mineral N and cabbage yield. In addition, the urea treatment and the mixed fertilizer treatment had close and higher nitrogen use efficiency. Furthermore, the δ18O vs. SP plot was useful for providing insight into microbial processes, showing that fungal denitrification/bacterial nitrification was the dominant microbial pathway and bio–organic fertilizer and mix fertilizer treatments had higher denitrification and N2O reduction compared to urea treatment. Those findings demonstrated that the partial replacement of urea with bio–organic fertilizer was a better choice, by means of enhancing denitrification to reduce N2O emissions and also guaranteeing the nitrogen use efficiency and the cabbage yield.