Abstract: Exploring the changes in soil nutrient contents and microbial communities of forest plantation under different combined applications of biochar with fertilizers is of great significance for understanding the mechanism underlying soil carbon cycling. A field experiment in a Cunninghamia lanceolata plantation was conducted with five treatments, including control (CK, no materials applied), biochar (B), biochar combined with nitrogen fertilizer (BN), biochar combined with phosphorus fertilizer (BP), biochar combined with both nitrogen and phosphorus fertilizers (BNP). Soil nutrient contents, microbial biomass, and microbial community structure were measured in different soil layers (0-10, 10-20, 20-40, and 40-60 cm) in a whole growing season, while the main environmental factors influencing soil microbial communities were determined. The results showed that different treatments significantly affected soil nutrient contents and microbial biomass. In the 0-10 cm soil layer, the contents of soil organic carbon (SOC) and total nitrogen (TN) were significantly reduced by 25.3% and 14.9% in the B treatment, 20.5% and 13.5% in the BP treatment, and 23.8% and 14.9% in the BNP treatment, respectively. The microbial biomass nitrogen (MBN) in the BP treatment was significantly higher than that in other treatments. In the 10-20 cm layer, SOC contents and microbial biomass carbon (MBC) were significantly lower in BP and BN treatments than CK, respectively. In the 20-40 cm layer, the MBN was significantly lower in B, BP, BP, and BNP treatments than in CK, and the microbial biomass phosphorus (MBP) in the BNP treatment was significantly higher than that in the B treatment. In the 40-60 cm layer, the available phosphorus (AP) content was significantly lower in B, BP, BP, and BNP treatments than in CK, while the MBP in the BP treatment was significantly higher than that in other treatments. Additionally, the relative abundance of actinomycetes in 20-40 cm and 40-60 cm soil layers significantly increased in BNP treatment. Our results suggested that SOC played a key role in shaping microbial communities. Biochar application provided carbon sources, enhanced microbial biomass, and regulated microbial community structure. This study provides valuable insights for soil carbon management and sustainable development of C. lanceolata plantations.

Key words: combined application of biochar with fertilizer, soil nutrient, microbial biomass, microbial community structure, stoichiometric characteristics