A field experiment was designed to investigate changes in soil physicochemical properties and microbial communities after six years of biochar amendment. Biochar was applied to a calcareous soil of karst regions in southwest China at four levels: 0%, 1.0%, 5.0%, and 10% (w/w).

Biochar (BC) addition increased soil pH, total carbon (C), total nitrogen (N), total hydrogen (H), and C/N ratio in the rhizosphere and bulk soils. The BC addition changed the composition and a-diversity of soil bacterial communities, especially at higher dosages. As BC additions increased, the relative abundances of Gemmatimonadetes increased in the rhizosphere soil. However, the relative abundances of Bacteroidetes, Firmicutes, and Cyanobacteria decreased. In the bulk soil, the relative abundances of Proteobacteria and Chloroflexi increased with BC application, but the relative abundances of Bacteroidetes and Verrucomicrobia decreased. In addition, canonical correspondence analysis showed that bacterial community composition was related to soil characteristics such as pH, total C, total N, and total H in both the rhizosphere and bulk soils. The changes in these soil properties were closely associated with the rates of BC application, indicating that long-term BC application significantly impact soil bacterial community by changing soil physiochemical properties in karst regions.