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 (CO₂, CH₄ and N₂O); 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, NH₃ 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 N₂O 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 CO₂, CH₄ and NH₃ (-78.38%; -83.33%; -27.46%), while temperate climate reduced N₂O emissions by -70.36%. For different crops, only rice demonstrated reduction in CO₂, CH₄, N₂O and NH₃ losses. On the other hand, our findings revealed a mitigating trade-off between CO₂ and CH₄ emissions on medium-textured soils and N₂O emissions on fine-textured soils. A significant reduction in N₂O and NH₃ 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 NH₃ losses but showed a noticeable N₂O reduction. We found that polymer-coated urea reduced CH₄ and N₂O emissions and NH₃ losses at the expense of higher CO₂ emissions. Moreover, application of a lower dose of coated urea (0-100 kg N ha^-1) enhanced CO₂ and CH₄ mitigation, while N₂O 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.