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 δ¹⁵N^bulk, 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 (δ¹⁵N^bulk 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.