Metagenomic insight into microbial regulation of nutrient cycling in amended bauxite residue under various planting strategies: Implications for soil formation

Sustainable rehabilitation of bauxite residue relies on a consistent supply of plant-available nutrients, with functional microbial communities playing a pivotal role in nutrient cycling. However, the mechanisms by which microbial functional genes regulate nutrient cycling during early plant growth in amended bauxite residue remain poorly understood. In this study, we conducted a greenhouse pot experiment to examine shifts in microbial functional genes related to carbon (C), nitrogen (N), and phosphorus (P) cycling under various planting strategies, including single and mixed plantings of Lolium perenne, Cynodon dactylon, and Trifolium repens, using metagenomic sequencing. Results showed that planting significantly reduced alkalinity (pH decreased by 1.58–1.67 units) and enhanced nutrient availability, with available N and P increasing by 12.47–18.00-fold and 4.07–8.56-fold, respectively, while enzyme activities increased 4.18–179-fold, from catalase to urease. Dominant C cycling pathways were the reductive tricarboxylic acid (rTCA) cycle (28.1–29.2 %) and the dicarboxylate-4-hydroxybutyrate (DC4HB) cycle (29.1–30.4 %). N cycling was primarily mediated through organic nitrogen metabolism (47.6–49.0 %), whereas P cycling was driven by translocation-related processes (35.0–36.9 %). Planting with Lolium perenne enhanced C sequestration, whereas Cynodon dactylon promoted N and P cycling. Notably, mixed planting with Trifolium repens weakened certain metabolic pathways related to C, N, and P cycling. Co-occurrence network analysis indicated that most nutrient cycling genes were positively correlated, with glpx-SEBP, gltB, and phoU identified as key regulatory genes. Partial least squares path modeling (PLS-PM) revealed that planting directly enhanced enzyme activity by improving residue physicochemical properties and indirectly regulated C, N, and P cycling genes through shifts in bacterial community composition. Proteobacteria and Actinobacteria consistently dominated C, N, and P cycling gene groups. These findings provide mechanistic insights into microbial regulation of nutrient cycling and inform planting strategies for sustainable bauxite residue rehabilitation.