Nitrate enhances N2O emission more than ammonium in a highly acidic soil

PURPOSE: Nitrous oxide (N₂O) is produced naturally in soils through microbial processes of nitrification and denitrification. In recent years, the long-term application of nitrogen-heavy fertilizers has led to the acidification of tea orchard soils with high N₂O emission. The present research aimed at finding out which process (nitrification or denitrification) dominates in N₂O production, whether certain fertilizer managements could reduce N₂O emission, and the effects of fertilizer management on the abundance of functional genes. MATERIALS AND METHODS: Two nitrification inhibitors, 3, 4-dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD), combined with different N fertilizers (ammonium sulfate and potassium nitrate) were applied to highly acidic tea orchard soil in an aerobic incubation experiment. Both amoA and nosZ gene abundances from different treatments were determined by quantitative PCR. An anaerobic nitrate effect test was carried out using C₂H₂ inhibition method. RESULTS AND DISCUSSION: The application of nitrate fertilizers significantly (P < 0.05) enhanced total N₂O emission. A linear regression analysis between total N₂O emission and average nitrate contents indicated that denitrification is the dominant source of N₂O in this tea orchard soil. In the anaerobic incubation, no significant difference of N₂O emission was observed between KNO₃ and no KNO₃ treatments before 96 h. Quantitative PCR revealed lower copy numbers of nosZ in nitrate-associated fertilizer-treated soils than the soils from other treatments. Compared with the control, ammonium fertilizers with DCD or DMPP significantly (P < 0.05) inhibited nitrate production as well as N₂O. CONCLUSIONS: These results showed that denitrification is the dominant source of N₂O in this highly acidic soil. Nitrate addition could significantly inhibit the abundance of nitrous oxide reductase, therefore causing high N₂O emission. The application of ammonium fertilizers with DCD or DMPP could significantly reduce N₂O emission, possibly due to the effective inhibition of nitrate production.