Technologies and Implementation Pathways for Improving Nitrogen Use Efficiency in Rice in Highly Leached Tropical Region

Tropical regions, particularly Hainan Island in southern China, provide abundant natural resources for crop growth because of their unique climatic conditions. The highly weathered, strongly leached acidic soils in these regions are prone to substantial nutrient losses, creating a vicious cycle in which excessive fertilization is applied to increase yield, while fertilizer use efficiency remains low. Under such production conditions, however, a practical pathway for reducing fertilizer inputs while improving fertilizer use efficiency under these specific tropical soil and climatic conditions remains limited. To identify such a pathway, we conducted a three-year field experiment across eight experimental environments to evaluate three approaches for improving fertilizer use efficiency: slow-release fertilizer (SRF), side-deep fertilization (SBF), and organic substitution (SRF+20% OF), with conventional farmer fertilization as the control and optimized formula fertilization (OPT) as a comparison treatment. The combination of slow-release fertilizer and organic substitution reduced nitrogen input from 220 kg ha−1 to 150 kg ha−1, while conventional farmer fertilization supplied 220 kg N ha−1, 180 kg P2O5 ha−1, and 180 kg K2O ha−1. Meanwhile, the N rate under OPT was progressively reduced from 180 kg N ha−1 in 2023 to 160 kg N ha−1 in 2024 and 150 kg N ha−1 in 2025. Compared with OPT under the same 27.3% reduction in fertilizer input, slow-release fertilizer increased grain yield by 6.5–12.2% and nitrogen use efficiency by 1.4–26.3%, whereas side-deep fertilization increased grain yield by 2.8–13.6% and nitrogen use efficiency by 3.6–20.5%. Compared with OPT under the same 31.8% reduction in fertilizer input, SRF+20% OF resulted in grain yield changes ranging from −7.5% to 1.3%, while NUE changes ranged from −10.9% to 12.5%. This study provides a practical pathway for improving nitrogen use efficiency in tropical regions and contributes to the sustainable development of agriculture.