Deciphering the microbial and physicochemical pathways of biochar-based fertilizer in driving resource-use...

Global agricultural intensification relies heavily on mineral fertilizers, inadvertently triggering environmental degradation and declining resource-use efficiency (RUE), particularly in acidic agroecosystems. Although biochar-based fertilizer (BBF) offer a promising alternative, their driving pathways across contrasting soil fertility and pH conditions remain poorly understood. This study evaluated BBF responses in two distinct acidic soils: a high-fertility, strongly acidic paddy soil and a low-fertility, weakly acidic lateritic red soil. Six treatments were applied: a no-fertilizer control (CK) and five BBF substitution levels replacing chemical nitrogen (N) fertilizer at N-equivalent ratios of 0% (B0), 25% (B1), 50% (B2), 75% (B3), and 100% (B4). The results revealed that BBF efficacy is highly context-dependent. In the lateritic red soil, the 75% BBF substitution (B3) emerged as the optimal strategy, significantly enhancing water- and N-use efficiency while boosting tomato yield by 191.7%-266.5% relative to B0. These gains were primarily driven by direct nutrient replenishment and acidity alleviation, which concurrently triggered a major rhizospheric microbial structural transition, characterized by an integrated bacterial co-occurrence network and the recruitment of metabolic taxa such as Actinobacteriota. Conversely, in the fertile, strongly acidic paddy soil, BBF performance was constrained by severe acidity and insufficient pH buffering. The bacterial community prioritized structural homeostasis within a fragmented network by recruiting stress-tolerant taxa like Chujaibacter, resulting in minimal RUE improvements. Overall, BBF effectiveness is governed by soil-specific constraints. These findings underscore the necessity of tailoring BBF application ratios to specific fertility-pH contexts, providing a mechanistic framework guiding future biochar-based fertilization strategies in contrasting acidic fields.