Slow-release urea fertilizer with polymer and biochar-based organic coatings: Design, field trials and global economic implications

Conventional slow-release urea fertilizers often rely on polymeric coatings, which can be costly, non-biodegradable, and environmentally problematic. To address these limitations, we developed a slow-release urea fertilizer using a combination of biodegradable polymeric (specifically polyvinyl alcohol/starch film) and biochar-based organic coatings, designated as U-PVA/ST/BOF. The slow-release performance of this fertilizer was evaluated through column leaching tests, where U-PVA/ST/BOF exhibited the most effective slow-release characteristics, with the lowest cumulative nitrogen release rate (18.3 %) over 28 days, while U/BOF (lacking the polymeric coating) released 36.4 %, while U-PVA/ST (without the biochar-based organic coating) released 42.7 %. Mechanistic investigations revealed that biochar-based organic coating in U-PVA/ST/BOF acts as a barrier, slowing water penetration and preventing premature swelling of the PVA/ST membrane. As water gradually diffuses through the biochar-based organic layer, the PVA/ST matrix absorbs moisture and swells, thereby releasing nitrogen in a sustained manner. Field trials showed that U-PVA/ST/BOF significantly improved wheat growth, grain-setting rate, and yield, resulting in a 40 %-44 % increase in yield compared to other fertilization treatments. Economic analysis indicated that U-PVA/ST/BOF provides a cost-effective solution for reducing nitrogen inputs, potentially lowering global nitrogen use by up to 29.75 million tons, offering both ecological benefits and economic viability for global agricultural systems. • U-PVA/ST/BOF shows 18.3 % cumulative N release over 28 days, outperforming others. • Biochar coating in U-PVA/ST/BOF slows water penetration, enhancing slow-release. • U-PVA/ST/BOF boosts wheat yield by 40 %-44 %, improving growth and grain-setting. • The application of U-PVA/ST/BOF can reduce global nitrogen use by 29.75 million tons.