Synergy of Arbuscular Mycorrhizal Fungi and Biochar-Based Fertilizer Reshapes Soybean Nutrient Acquisition and Drives Yield Enhancement

Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with most crops. They function as promising sustainable agricultural amendments by synergizing with biochar to enhance plant nutrient uptake. However, the effects of AMF and biochar interactions on the yield and nutrient uptake of leguminous crops and the underlying mechanisms remain insufficiently understood. This study employed a two-factor experimental design. Under the baseline conditions of no fertilization (CK), chemical fertilizer application (CF), and biochar-based fertilizer application (BF), treatments with and without AMF inoculation were established, resulting in a total of six experimental treatments. Compared to BF treatment alone, the combined application of AMF and BF (AM + BF) synergistically increased soybean biomass (12.81%) and grain yield (19.45%). This synergistic effect was accompanied by increased plant nitrogen (14.04%) and potassium (21.82%) accumulation. Notably, despite the highest yield, the AM + BF treatment showed a 22.22% reduction in nodule formation rate. This reveals that plant nitrogen acquisition strategies have shifted from relying on biological nitrogen fixation to efficient mycorrhizal pathways, reflecting an inherent optimization of carbon economy. The PLS-SEM model revealed that AMF inoculation altered yield-driving mechanisms: in the absence of AMF, yield could be directly predicted by soil nutrient levels; however, this relationship was disrupted after AMF inoculation. The soil nutrient pathway became non-significant, indicating a transition from a soil chemistry-dependent model to a biologically driven one, where AMF–plant symbiosis became the primary regulator of nutrient uptake. These findings highlight that AMF-BF synergy creates a novel soil–plant feedback mechanism that enhances nutrient acquisition efficiency and optimizes carbon allocation, providing a sustainable approach to boost legume crop yields and reduce environmental footprints.