Can urea-coated fertilizers be an effective means of reducing greenhouse gas emissions and improving crop productivity?

Given the significance of nitrogen (N) as the most constraining nutrient in agro-ecosystems, it is crucial to develop an updated model for N fertilizers management to achieve higher crop yields while minimizing the negative impacts on the environment. Coated urea is touted as one of the most important controlled-release N fertilizers used in agriculture to reduce cropland emissions and improve nitrogen use efficiency (NUE) for optimal crop yields. The sustainability of coated urea depends on the trade-offs between crop productivity, NUE and greenhouse gas emissions (CO, CH and NO); however, role of various agro-edaphic factors in influencing these trade-offs remains unclear. To determine the effects of soil properties, climatic conditions, experimental conditions, and type of coated urea on greenhouse gas emissions, NH losses, crop productivity, and NUE, we conducted a meta-analysis using data from 76 peer-reviewed studies. Our results showed that the application of coated urea under field conditions contributed to a greater reduction in NO emissions (-48.67%) and higher NUE (58.72%), but crop yields were not significant. Across different climate regions, subtropical monsoon climate showed a perceptible mitigation for CO, CH and NH (-78.38%; -83.33%; -27.46%), while temperate climate reduced NO emissions by -70.36%. For different crops, only rice demonstrated reduction in CO, CH, NO and NH losses. On the other hand, our findings revealed a mitigating trade-off between CO and CH emissions on medium-textured soils and NO emissions on fine-textured soils. A significant reduction in NO and NH losses was evident when coated urea was applied to soils with a pH > 5.5. Interestingly, application of coated urea to soils with higher C/N ratios increased NH losses but showed a noticeable NO reduction. We found that polymer-coated urea reduced CH and NO emissions and NH losses at the expense of higher CO emissions. Moreover, application of a lower dose of coated urea (0-100 kg N ha) enhanced CO and CH mitigation, while NO mitigation increased linearly with increasing dose of coated urea. Most importantly, our results showed that the application of coated urea leads to a large mismatch between NUE, crop yields and greenhouse gas mitigation. By and large, the application of coated urea did not correspond with higher crop yields despite significant reduction in the emissions and improved NUE. Overall, these results suggest that site-specific agro-edaphic conditions should be considered when applying coated urea to reduce these emissions and N volatilization losses for increasing NUE and crop yields.