Enhancing Phosphorus Availability Through Bagasse Biochar Addition and Changes in phoD Bacterial Communities of Karst and Non-Karst Forest Soils

Biochar can enhance microbial-mediated organic phosphorus mineralization, but the underlying mechanisms remain unknown in forest soils with varying pH values. An incubation experiment was conducted using karst (alkaline) and non-karst (acidic) forest soils. Four amounts of bagasse biochar were applied (0, 5, 10, and 15 t·hm−2) to assess their effects on soil phosphorus availability and microbial community structure. Olsen-P content of alkaline and acidic forest soils increased with increasing bagasse biochar addition and incubation time, especially in non-karst forest soil. The structure and diversity of phoD-harboring bacterial community of acidic forest soil were significantly altered by the amount of bagasse biochar added and the incubation time, whereas those in alkaline karst forest soil were not significantly affected. The relative abundance of the dominant order Burkholderiales reached (43%) in acidic forest soil, significantly exceeding the (9%) recorded in alkaline karst forest soil. The phoD bacteria in acidic forest soil had more complex microbial networks and were more closely related to phosphorus fractions than those in alkaline forest soil. Structural equation modeling indicated that soil phosphorus availability was directly controlled by bagasse biochar input in acidic forest soil, with an indirect pathway linked to phoD bacterial community structure. The contribution of phoD bacteria to the variation in phosphorus availability was higher in acidic forest soil than in alkaline forest soil based on variance partitioning, indicating that enhancing soil phosphorus availability with bagasse biochar depends on the amount added, soil type, and its regulation of phoD bacterial communities.