Synergistic effects of biochar and phosphate fertilizer on fungal communities and soybean productivity in microplastic-contaminated alkaline soils

Alkaline soils suffer from phosphorus (P) bioavailability limitations due to fixation and precipitation, which is further exacerbated by microplastics (MPs) disrupting plant-microbe-soil P dynamics and threatening crop productivity. This study investigated the synergistic effects of applying 2 % (w/w) maize-derived biochar (a soil amendment that enhances nutrient retention and microbial activity) and varying phosphate fertilizers (0-180 kg P ha^-1) in MPs-contaminated alkaline soils. Comprehensive investigations were conducted on the effects of the combined addition of phosphate fertilizer and biochar on P dynamics, fungal communities, and soybean growth. Results showed that biochar significantly reshaped soil fungal community structure and composition by modulating pH, nutrient availability, and organic matter dynamics, while increasing Aspergillus and Fusarium abundance, which play a key role in phosphorus solubilization and pollutant degradation. Additionally, co-occurrence network analysis revealed that applying biochar enhanced network complexity, promoted negative interactions, and intensified interspecific competition within the rhizosphere fungal community. Notably, the combined application of biochar with moderate-level P fertilization (60 kg P ha^-1, 90 kg P ha^-1) increased soybean total biomass (40.73 %, 42.58 %), total P uptake (58.19 %, 34.44 %), and yield (74.38 %, 84.16 %), indicating improved P-use efficiency. Additionally, Partial Least Squares Path Modeling (PLS-PM) revealed that the rhizosphere effect promoted soybean P uptake by regulating nutrient cycling, thereby enhancing yield. These findings provide critical insights into sustainable agriculture under dual challenges of MPs pollution and limited P resources, contributing scientific and efficient strategies for nutrient management and rational P fertilizer use in resource-constrained agroecosystems.