Improving soil health and tomato production in a reduced nitrogen system: The role of biochar and organic fertilizer coupling

Conventional nitrogen fertilization frequently results in soil degradation and environmental pollution. While the isolated effects of nitrogen reduction, organic amendments, or biochar have been explored, their synergistic interactions remain inadequately understood. This two-year field study in Yunnan, China, addressed this gap by evaluating the coupling effects of biochar (applied at 10, 15, and 20 t ha⁻¹) with three organic fertilizers (chicken manure, cattle manure, and earthworm castings) under a consistent 40% nitrogen reduction. The primary objective was to establish a technical pathway for synergistic fertilizer reduction, waste utilization, and sustainable soil management. Results demonstrated that the combination of earthworm castings and biochar (15 t ha⁻¹), designated as treatment F3B2, most significantly enhanced soil nutrient supply and microbial diversity. This treatment yielded the highest relative abundance of the top 10 bacterial genera and maximum bacterial Chao1 indices of 3028.06 and 3087.18 in the first and second years, representing increases of 10.13% and 18.50% compared to the control (CK), respectively. Agronomically, the F3B2 treatment increased tomato yield by 23.75% and vitamin C content by 14.65% in the second year. Furthermore, TOPSIS analysis consistently ranked F3B2 as the optimal integrated management strategy. This study provides a replicable paradigm for green and efficient production in protected vegetable cultivation systems. In conclusion, the integrated application of organic fertilizer and biochar under reduced nitrogen conditions synergistically optimizes soil health—particularly by enhancing the microbial community—and significantly promotes tomato growth, yield, and quality.