Crop straw degradation and phosphorus release by Aspergillus niger under humic acid and fulvic acid supply

Humic substances can enhance fungal physiological functions by promoting microbial growth and stimulating the production of metabolites. However, the effects of different humic substances on crop straw degradation by fungi remain unclear. This study investigated the impact of humic acid (HA) and fulvic acid (FA) on maize ( Zea mays L.) and wheat ( Triticum aestivum L.) straw degradation and P release by Aspergillus niger Tiegh. ( A. niger ) under laboratory and field conditions. Straw was incubated with A. niger and 0.5%, 1%, or 2% HA or FA for 7 days, followed by 90-day field validation using 2% treatments. After 7 days, maize straw degradation percentage was 1.5~3.9 times that of wheat straw. Notably, HA outperformed FA at equivalent concentrations across all treatments. The supply of 2% HA produced the highest degradation percentages and soluble P concentrations, reaching 43.2% and 0.60 mg/L for maize straw and 21.9% and 0.21 mg/L for wheat straw. In maize straw, 2% HA also resulted in the highest fungal biomass of 1.86 g and enhanced pyruvate dehydrogenase, lignin peroxidase, cellulase, β-1,4-glucanase, and acidic xylanase activities to 34.4, 518.9, 270.4, 322.9, and 332.0 nmol/min/g, respectively. The fungal biomass, lignin peroxidase activities, pyruvate dehydrogenase activities, and oxalic acid produced by A. niger dominated the degradation of maize straw under HA supplementation. Meanwhile, 2% HA also showed the highest degradation percentages under field conditions, with 48.0% and 38.2% degradation observed for maize and wheat straw respectively after 90 days of incubation. Overall, HA was more effective than FA in enhancing straw degradation by A. niger , and the positive effect of 2% HA was maintained under field conditions, suggesting its potential for improving the efficiency of straw bioconversion and nutrient cycling.