The impacts of different nitrogen forms on fungal weathering of carbonate and insoluble phosphate in the karst red soil

Karst soils contain abundant carbonates but commonly exhibit phosphorus (P) deficiency. Fungal weathering plays a central role in P and carbon (C) cycling in these soils. However, this process is often shaped by environmental factors such as soil pH, minerals composition, and nitrate or C availability. This study investigated how different nitrogen (N) forms (nitrate, ammonium, and urea) influence fungal weathering of phosphate and carbonate in karst soil. Ammonium and urea were more effective than nitrate in promoting fungal carbonate weathering, significantly enhancing the conversion of carbonate to calcium oxalate in karst soil. Meanwhile, Ammonium additions significantly increased available P concentrations from 3.9 to 9.3 mg/kg after 8 weeks of incubation. In contrast, nitrate is more suitable than ammonium and urea for fungal growth and hyphal development. The formed fungal hyphae facilitated the biomechanical weathering of carbonate. According to the loss mass ratio at 450-900 o C, the theoretical fungal carbonate weathering rate driven by ammonium was 93% higher than that without N input. This research clarifies how different N shapes carbonate fungal weathering in karst soil and highlights links between P and C utilization that may help enhance P availability in karst regions. • N supply can promote the fungal weathering of insoluble phosphate and carbonate in karst soil. • Both biochemical and biomechanical pathways contribute to fungal weathering of insoluble phosphate and carbonate under different N forms. • The formation of calcium oxalate promotes the weathering of carbonate by fungi in karst soil. • Ammonium and urea are more efficient than nitrate in fungal carbonate weathering. • Under NH 4 + and urea supply, the ideal fungal carbonate weathering rate increased 80% and 93% compared to no N input.