Objective Long-term effects on nitrogen form and bacterial community in the soil at an experimentation station treated with biochar 12 years previously were examined.

Method In 2010, a field study was started at the Tea Research Institute of Fujian Academy of Agricultural Sciences. At the time, 5 levels of biochar applications at 0 (CK), 8 t·hm⁻² (B1), 16 t·hm⁻² (B2), 32 t·hm⁻² (B3), and 64 t·hm⁻² (B4) were implemented. In the past 12 years since, tea was continuously cultivated on the land. After the spring harvest in 2022, 0-20 cm layer soil were sampled for nitrogen form and microbial determinations. The collected data were analyzed using non-metric multidimensional scaling (NMDS), Pearman correlation, and redundancy analysis (RDA).

Result Over the years, the various biochar treatments significantly increased the contents in the soil of organic carbon by 7.95-21.40%, total nitrogen by 11.69-25.54%, nitrate nitrogen by 53.45-115.84%, ammonium nitrogen by 1.13-38.86%, available potassium by 28.49-114.13%, pH by 5.51-12.46%, and sand by 21.11-67.88%. Whereas alkaline nitrogen declined by 11.97-17.25% (except B1), available phosphorus by 15.18-50.36%, clay by 15.64-38.21%, and silt by 8.57-30.63%. Meanwhile, the abundance and diversity of the soil microbiome were significantly enhanced. In comparison to CK, the biochar treatments enriched Proteobacteria, Actinobacteria, Gemmatimonadota, and Myxococcota, while reduced Chloroflexi, Acidobacteria, and Planctomycetes. NMDS and linear discriminant analysis (LDA) showed apparent distinctions between B0 and B4 on the soil microbial community structure, and 19 biomarkers highly sensitive to the changes brought about by B4. The correlation analyses indicated that available phosphorus, available potassium, nitrate nitrogen, and clay in soil were the major factors affecting the microbial community.

Conclusion Long after 12 years of the biochar application, the carbon, nitrogen, available nutrients, and structure of the soil continued to improve which benefited the land fertility as well as the microbiome. Significantly, the application of biochar at 64 t·hm⁻² would effectively increase the nitrogen supply for a tea plantation.