An effective biochar-based slow-release fertilizer for reducing nitrogen loss in paddy fields

As a carbon sequestration material, biochar has attracted much attention due to its potential to enhance rice productivity and nitrogen retention in paddy fields. However, little information is available about the impacts of rice straw-derived biochar on coating materials of slow-release fertilizers especially with bentonite, starch, and humic acid. In this study, a biochar-based slow-release fertilizer was developed and evaluated at field scale. An orthogonal experimental design was applied to investigate the blending ratios of biochar, humic acid, and bentonite with three adhesives, and how these influenced N release. The optimum coating combination was 25% biochar, 4% bentonite, and 10% humic acid with modified cornstarch as the adhesive (herein referred to as CF10). The product not only decreased N leaching and runoff losses at the seeding and tillering stages but also supplied more nutrients to the rice at the heading and maturing stages. The SEM and FT-IR observations revealed that an effective dense layer was formed that slowed N release from the granule. Laboratory- and field-scale studies showed that biochar has played a crucial role in developing a slow-release coating for the compound fertilizer based on its structural properties, porosity, and chemical interaction with other coating ingredients. We conclude that biochar-based slow-release fertilizer is a promising alternative N fertilizer for rice production.