[Effects of Controlled-Release Urea Application on N₂O Emission in Maize-Cultivated Sandy Loam Soil]

A field experiment was conducted in maize-cultivated sandy loam soil in the old flooded area of the Yellow River to evaluate the responses of N₂O emissions to application of different type of controlled-release urea. An inorganic N fertilizer was applied at 270 kg·hm^-2 during the maize season. Urea was applied alone and in combination with sulfur-coated urea (SCU) or polyurethane-coated urea (PCU) at N ratios of 30%:70%, 50%:50%, and 70%:30%, respectively. Cumulative N₂O emission under urea treatment alone (CN) was 1.78 kg·hm^-2 with a N₂O emission factor of 0.38%. In comparison to CN, 70% urea+30% SCU, 50% urea+50% SCU, and 30% urea+70% SCU treatments reduced N₂O emission by 1.12%, 22.5%, and 11.2%, respectively. In contrast, application of urea in combination with PCU (with the proportion varied from 30%-70%) increased N₂O emission by 0.02-0.41 kg·hm^-2 compared with the CN, while 30% urea+70% PCU treatment showed a 23.0% increase. Regression analysis showed that N₂O flux was significantly (P<0.01) correlated with soil temperature at 10 cm depth and concentrations of soil NH₄⁺-N and NO₃^--N in all the treatments, but not with soil moisture or dissolved organic carbon concentration. Compared with the CN, the 50% urea+50% SCU and 50% urea+50% PCU treatments slightly, but not significantly, increased the maize yield, whereas the 30% urea+70% SCU treatment showed a reduction effect. Overall, the mitigation effect of controlled-release urea on N₂O emission may primarily depend on its coating material and application rate.