Nickel titanate (NiTiO₃) semiconductors and graphitic carbon nitride (g-C₃N₄) have attracted great attention as photocatalysts in the degradation of environmental pollutants because of their visible-light-driven activity. But the utilizations of both semiconductors are limited by their low specific surface area. In this study, a nano-NiTiO₃/g-C₃N₄ photocatalyst was successfully synthesized by optimizing the preparation method of photocatalyst precursors. Compared with the bulk g-C₃N₄ and bulk NiTiO₃/g-C₃N₄ composite photocatalysts, the nano-NiTiO₃/g-C₃N₄ composite catalyst displayed a larger specific surface area, a more abundant pore size structure, and superior carrier separation capabilities. According to the pseudo-first-order, the degradation rate of MB was more than 2.5–19.7 times higher than that of previous studies. The superoxide radicals (·O₂⁻) and holes (h⁺) played significant roles in the photocatalytic reaction of MB. This study provides a new idea for the synthesis of photocatalysts and the improvement in photocatalytic performance.

Phosphorus (P) leaching from dryland from purple soils poses a significant risk to water quality, yet effective mitigation strategies and their underlying microbial mechanisms remain poorly understood. This study aimed to evaluate the efficacy of biochar (B), a silicon-based conditioner (Si), and their combination (BSi) in controlling P leaching, hypothesizing that B would immobilize P while Si would mobilize it. The indoor soil column leaching experiments were conducted with four treatments (CK, B, Si, BSi), measuring leachate P fractions and soil P forms, and employed metagenomic sequencing combined with partial least-squares path modeling (PLS-PM) and Bayesian structural equation modeling (BSEM) to explore microbial functional mechanisms. Results showed that B alone reduced cumulative leaching of inorganic P (IP), organic P (OP), and total P (TP) by a range of 5.4–6.3%, while increasing available phosphorus (Olsen-P) by 39.4% in the surface layer. Si and BSi promoted leaching, with BSi reducing available P sharply, despite raising TP. Metagenomic analysis revealed that B suppressed subsurface IP solubilization genes (e.g., gcd, ppx) and optimized OP mineralization, whereas Si inhibited mineralization via reducing key microbial taxa. BSEM further identified water-soluble P (Water-P) and total nitrogen (TN) as direct positive drivers of inorganic P dissolution. Collectively, the key biological mechanisms for leaching reduction involve inhibiting subsurface IP solubilization, optimizing surface OP mineralization, and enhancing P transport/starvation responses. Collectively, biochar applied alone offers the optimal balance between P retention and crop-available P supply in dryland purple soils, and provides mechanistic insights—through functional gene profiling—that can inform the design of more sustainable P fertilization and leaching control practices.