Aspergillus niger as a key to unlock fixed phosphorus in highly weathered soils

Phosphorus (P) can undergo adsorption and precipitation reactions in the soil. When fixed to iron and aluminum oxyhydroxides in highly weathered soils, P is covalently linked to the soil solid phase and becomes unavailable to crops. Plants and microorganisms produce organic acids as a strategy to acquire P for growth. These compounds have been used to extract P from sparingly soluble phosphate minerals and may play a role in reverting P adsorption and fixation in the soil. The objective of this work was to study the ability of Aspergillus niger FS1 and organic acids to desorb P after 40 d of contact of the nutrient with 75-mu m fractions of a typical highly weathered Oxisol. Samples of the soil fractions from the A and B horizons were incubated for 40 d with P at concentrations corresponding to 90% of the soil maximum P adsorption capacity (MPAC). The ability of A. niger FS1 to desorb P from the soil was performed in a culture medium optimized for phosphate-solubilizing microorganisms. The desorption kinetics of the P bound to the soil fractions was evaluated for oxalic acid at 10 mmol L-1, and for a mixture of oxalic + citric acids, both at 5 mmol L-1. Water at pH 7.0 and at pH 2.0 was used as controls. The fungus released 18.0% of the P from the A and B horizon fractions after 7 days of growth. Oxalic acid was the most efficient compound at desorbing P from the soil, releasing 32.7% of the P from the A horizon fraction, and 18.5%, for the B horizon fraction. These findings contribute to a better understanding of the mechanistic basis of microbial P release from highly weathered soils, opening prospects for the development of microbial techniques allowing plant access to P fixed in the soils and the improvement of P fertilizer use efficiency.