Core-shell biochar-zeolite fertilizer with starch coating: Boosting nutrient use in saline-alkaline rice soils via smart drone delivery & economic assessment.

Saline-alkaline soils severely constrain global rice production by impairing nutrient use efficiency (NUE) and soil health. Conventional fertilizers exacerbate these issues through rapid nutrient loss. This study developed a new core-shell slow-release fertilizer (SC-ZEO/BCF) composed of a NPK-loaded biochar-zeolite core encapsulated within a starch coating, designed for enhanced nutrient retention and slow-release in challenging environments. Comprehensive characterization confirmed successful synthesis with a porous core for adsorption and a continuous starch shell as a physical barrier. Soil column leaching tests demonstrated SC-ZEO/BCF's superior slow-release performance, significantly retarding nitrogen and phosphorus release compared to uncoated formulations. A field trial in a saline-alkali paddy showed that SC-ZEO/BCF application resulted in the highest agronomic traits (grains per panicle, 1000-grain weight), yield (7.35 t/ha, 22.14% higher than the compound fertilizer), agronomic nitrogen use efficiency (14.08 kg/kg), and nitrogen recovery efficiency (25.32%). It also improved soil health by increasing organic matter and available nutrients while reducing salinity. The systematic innovation of combining SC-ZEO/BCF with a drone-based variable-rate system (as a hypothetical scenario) was then assessed: the drone would use YOLOv12 to recognize rice density and growth stage in real time, adjusting fertilization rates accordingly. This integration would synergize the fertilizer's slow-release properties with precision delivery, potentially lowering costs and increasing net profit to $597.02/ha compared to conventional practices. This study validates the core-shell SC-ZEO/BCF combined with precision agriculture as a highly effective and economically viable strategy for sustainable rice production in saline-alkaline soils.