Oxalic acid is a key root exudate released by plants under phosphorus (P) deficiency and plays a direct role in solubilizing fixed soil P. However, its specific effects on soil microbial community assembly and ecological functions remain less clear. In this study, based on an ex planta soil microcosm incubation experiment, the impacts of oxalic acid input on soil bacterial and fungal community assemblage and functional profiles involved in P mobilization were explored. The results showed that oxalic acid input significantly changed soil bacterial and fungal community composition, decreased their diversity, and enriched bacterial taxa involved in P mobilization and fungal taxa associated with plants, showing the selective effects of oxalic acid on soil microorganisms. Further community assembly analyses (βNTI and NST) showed that oxalic acid input promoted a shift in bacterial community from a stochastic-process-dominated community to a deterministic-process-dominated community, while the fungal community exhibited a converse pattern. These findings reveal the important role of oxalic acid in shaping soil microbial community assembly and ecological functions under P deficiency, broadening our understanding of the role of oxalic acid in plant responses to low-P stress.

Crop straw is an abundant agricultural waste resource, but its lignocellulosic structure limits reuse. The fungus Aspergillus niger (ANG) can promote straw degradation and phosphorus (P) release. However, its degradation efficiency across different crop straws remains unclear. This study compared the straw matter loss and P release of banana leaves (BL), banana stems (BS), and maize straw (MS) by ANG via biomechanical and biochemical pathways. After 7 days of incubation, ANG significantly enhanced the straw dry matter loss of banana stems (41.06%) and maize straw (41.79%), achieving 2.6- and 1.1-fold higher rates compared to treatments without ANG. BL showed a lower straw dry matter loss (20.54%) but had the highest P content of ~3 mg/L among BS and MS. Oxalic acid was the primary organic acid secreted by ANG, achieving the highest value of ~1078 mg/L in the ANG+BS treatment. Meanwhile, these three straws also induced different degrading enzyme activities in ANG. BL stimulated β-glucosidase and lignin peroxidase, whereas BS promoted β-1,4-glucanase in ANG. Overall, ANG is more efficient in BS and MS degradation. Our findings indicate that ANG holds significant potential for straw degradation and the development of specialized fungal inoculants.