Nitrification, ammonia-oxidizing communities, and N2O and CH4 fluxes in an imperfectly drained agricultural field fertilized with coated urea with and without dicyandiamide

Agricultural soil is a major source of nitrous oxide (N2O), and the application of nitrogen and soil drainage are important factors affecting N2O emissions. This study tested the use of polymer-coated urea (PCU) and polymer-coated urea with the nitrification inhibitor dicyandiamide (PCUD) as potential mitigation options for N2O emissions in an imperfectly drained, upland converted paddy field. Fluxes of N2O and methane (CH4), ammonia oxidation potential, and ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) abundances were monitored after the application of PCU, PCUD, and urea to upland soil. The results showed that urea application increased the ammonia oxidation potential and AOB and AOA abundances; however, the increase rate of AOB (4.6 times) was much greater than that of AOA (1.8 times). These results suggested that both AOB and AOA contributed to ammonia oxidation after fertilizer application, but the response of AOB was greater than AOA. Although PCU and PCUD had lower ammonia oxidation potential compared to urea treatment, they were not effective in reducing N2O emissions. Large episodic N2O emissions (up to 1.59 kg N ha(-1) day(-1)) were observed following heavy rainfall 2 months after basal fertilizer application. The episodic N2O emissions accounted for 55-80 % of total N2O emissions over the entire monitoring period. The episodic N2O emissions following heavy rainfall would be a major source of N2O in poorly drained agricultural fields. Cumulative CH4 emissions ranged from -0.017 to -0.07 kg CH4 ha(-1), and fertilizer and nitrification inhibitor application did not affect CH4 oxidation.