Biochar Application Improves Eggplant Yield in Association with Changes in Soil Microbial Communities and Soil Multifunctionality

Biochar is widely used to improve soil quality and crop productivity; however, the relationships among biochar addition, soil multifunctionality (SMF), microbial communities, and crop yield remain unclear. A field experiment was conducted from May to September 2025 in a continuous eggplant (Solanum melongena L.) cropping system established on Mollisols. Four biochar application rates (0, 5.40, 6.75, and 8.10 Mg ha−1) were evaluated using a randomized complete block design with three replicates. Biochar addition significantly affected eggplant growth, selected photosynthetic traits, root development, and yield, with the most consistent positive responses observed at 6.75 Mg ha−1. Biochar also altered soil physicochemical and biological properties, with these effects being more pronounced in the topsoil and at the fruiting stage. SMF was positively correlated with yield, and this relationship was stronger at the fruiting stage than at the flowering stage. Random forest analysis showed that yield was strongly associated with soil biological properties, particularly extracellular enzyme activities involved in carbon, nitrogen, and phosphorus acquisition. Structural equation modeling further examined depth-specific statistical associations among biochar addition, microbial communities, SMF, and yield; topsoil bacterial communities and subsoil fungal communities showed the strongest associations with SMF. Overall, the application of 6.75 Mg ha−1 biochar produced the most consistent improvements in SMF and eggplant productivity.