Alkalinity exacerbates phosphorus deficiency in subtropical red soils: Insights from phosphate-solubilizing fungi

Red soils in subtropical regions are often low in available phosphorus (P), a vital plant nutrient. Phosphate-solubilizing microorganisms (PSMs) can release P from phosphate reservoir, making it accessible to plants. However, the complex interactions between PSMs and minerals in red soils are not yet fully understood. In this study, we investigated the effects of Aspergillus niger, a typical phosphate-solubilizing fungus (PSF), on phosphate dissolution in two representative red soils – an acidic soil and an alkaline soil. In the acidic red soil, the fungal abundance reached 3.01 × 10 ⁷ cfu g^?1 after a 28-day incubation period, with respiration of ~2000 mg C kg^?1. The secretion of oxalic acid promoted P release from inorganic phosphate (from ~1 to 187 mg kg^?1). Additionally, the contents of amorphous Fe/Al oxides decreased, which otherwise could have contributed to P sorption in the soil. In contrast, P availability declined in the alkaline red soil after the addition of A. niger, regardless of the P source (inorganic or organic phosphate). Meanwhile, the fungal respiration decreased to ~780 mg C kg^?1. Therefore, alkaline red soils with abundant carbonates are susceptible to P deficiency due to both the diminished function of PSMs and strong soil buffering. These findings have important implications for sustainable agriculture on alkaline red soils, as they suggest that the use of PSMs to improve P availability may be limited.