Nitrogen-fixing Trees Increase Weathering Indices and Soil Nutrients in coral sand substrates

Aims Nitrogen-fixing (NF) trees are widely used pioneer species of afforestation to increase ecosystem nitrogen (N) inputs and accelerate weathering for nutrient releases, but their effects on highly alkaline coral sands remains unclear. Methods We conducted a greenhouse experiment using a coral sand-lateritic red soil mixture, comparing three NF and three non-nitrogen-fixing (non-NF) tree species with an unplanted control. We quantified plant effects on calcium (Ca) and sodium (Na) depletion using aluminum (Al)-normalized mass transfer indices (W _Ca and W _Na ) and assessed concomitant changes in soil and plant properties, fine root traits, soil enzyme activities and microbial community. Results NF trees triggered higher soil Ca and Na depletion (W _Ca and W _Na increased by 116% and 145%) than non-NF trees, accompanied by lower soil pH and higher soil organic carbon (SOC), total N (TN), and total phosphorus (TP). NF trees also exhibited higher fine root N and P concentrations, with higher specific root length (SRL) and specific surface area (SSA) but lower root tissue density (RTD). Soil Ca and Na depletion were most strongly linked to lower pH and these acquisitive root traits, whereas enzyme activities and microbial community closely mirrored rhizosphere conditions. Conclusions Our results suggest that NF trees can enhance early-stage carbonate weathering and promote nutrient accumulation, with implications for accelerating soil development during coral island restoration.