Abstract: Agricultural and forestry residues, being abundant and easily collectible, serve as ideal feedstocks for biochar production. We established four biochar application rates: 0% (CK), 0.5%, 2%, and 4%, (w/w), to conduct a short-term indoor incubation experiment (lasting 56 days) with red soil and investigated the changes in soil physicochemical properties after biochar addition. Biochar was produced with eucalyptus branches, camphor tree branches, and guava fruit branches. By using high-throughput sequencing, we analyzed the diversity of bacterial communities. The results showed that: 1) Compared with CK, biochar amendment increased soil pH by 0.25-1.69 units and significantly enhanced organic carbon (19.2%-323.5%), available phosphorus (31.8%-69.8%), available potassium (10.3%-448.7%), and nitrate (46.9%-118.2%) contents. 2) Urease activity was increased by 108.5% with 4% guava biochar, while sucrase and cellulase activities was enhanced by 26.8%-43.0% and 8.6%-8.9%, respectively, under 4% eucalyptus or camphor biochar. 3) High-throughput sequencing revealed that 1% guava biochar, 1% camphor biochar, and 4% eucalyptus biochar elevated bacterial diversity, as indicated by the significant increases in Shannon, Simpson, and Chao1 indices. Dominant phyla included Proteobacteria (23.1%-31.4%) and Acidobacteria (11.7%-23.3%). Redundancy analysis identified available potassium, pH, available nitrogen, organic carbon, and sucrase as key environmental drivers, demonstrating that biochar indirectly modulates bacterial composition through physicochemical modifications. In conclusion, all three types of biochar effectively improved soil quality and bacterial diversity of red soil, with the 4% application rate achieved the best effect.

Key words: forest soil, biochar, soil physicochemical properties, bacterial diversity, soil enzyme activity