Evaluating microbial role in reducing N2O emission by dual isotopocule mapping following substitution of inorganic fertilizer for organic fertilizer

Organic and inorganic fertilizers are major sources of N2O emission in vegetable field and their combined applications are recommended in China. However, the microbial processes for N2O production and consumption after combined applications of organic and inorganic fertilizers to vegetable field remain unclear making the optimal fertilization scheme less accurate. To address this issue, five substitution rates of organic fertilizer for inorganic fertilizer including 0 (0M), 20 (20M), 50 (50M), 80 (80M), and 100% (100M) and a no fertilizer control (NF) were evaluated on Chinese cabbage (Brassica pekinensis L. var). N2O fluxes, N2O isotopocule ratios, Chinese cabbage yield and environmental parameters were measured. According to the revised dual isotopocule mapping of δ18O vs. δ15NSP (difference value of 15N in Nα–Nβ–O molecule site preference), the nitrification contribution for the 100M, 80M, 50M, 20M, 0M and NF treatments were 41, 28, 33, 34, 39, and 37%, respectively, and the extent of N2O reduction were 74, 67, 48, 35, 11, and 78%, respectively. Such results suggest that denitrification was the major microbial process and was promoted by adding organic fertilizer. Increasing the substitution rates of organic fertilizer improved the extent of N2O reduction and thus mitigated N2O emission. In addition, increasing the substitution rates of organic fertilizer improved C/N ratio and N2O isotopocule deltas, yet reduced N2O emission, Chinese cabbage yield, and soil mineral nitrogen content. In all the evaluated substitution rates, the 20M treatment had the best performance in maintaining the Chinese cabbage yield and reducing the N2O emission. These findings reveal that the dual double isotopocule mapping is useful for partitioning N2O source and sink on a field scale. Adding organic fertilizer could reduce N2O emission by enhancing N2O reduction to N2 and the 20% substitution rate of organic fertilizer is the appropriate substitution rate. Furthermore, an optimal substitution rate of organic fertilizer should be explored between 20% and 50% in later studies.