Background Forest fires are a driver of forest soil and biodiversity, especially in fire-prone ecosystems. The effects of fire on soil microbial communities as well as enzyme activity are complex and largely unknown. Therefore, analyzing the dynamics of soil bacteria and enzyme activities in the short term after fire is of great biological significance for understanding the reconstruction process of forest communities.

Methods The soil of conifer and broad-leaved mixed forest and bamboo forest after the fire in Jinyun Mountain, Chongqing was used as the research object. The activities of sucrase, cellulase and urease as well as the changes of soil nutrients and pH were measured in the soil layers of 0-10 cm and 10-20 cm of conifer and broad-leaved mixed forest and bamboo forests under different fire intensities within one month of the fire, and the changes of soil bacterial community structure were analyzed by high-throughput sequencing technology.

Results 1) The soil bacterial diversity, richness, soil organic matter and soil enzyme activities decreased in the short term after the occurrence of forest fires, and soil pH, available potassium, available nitrogen and available phosphorus increased to varying degrees. Especially in the 0-10 cm soil layer, it was more affected by forest fire. 2) The relative abundance of Proteomycetes, Actinomycetes and phylum Aerugula in the soil was the highest after moderate and severe forest fires. The relative abundance of Acidobacterium phylum was the highest when not exposed to forest fires. The relative abundance of Firmicutes was the highest after mild fire. 3) In the redundant analysis of soil bacterial structure and soil chemical factors, the changes of soil chemical properties after fire also affected soil bacterial communities and soil enzyme activities.

Conclusions Forest fires can lead to a decrease in soil bacterial community diversity and a significant decrease in soil enzyme activity. The results indicate that moderate and severe forest fires have the greatest impact on soil bacterial diversity and enzyme activity, with a positive correlation between Proteobacteria and soil nutrients. Acidobacteriota demonstrates negative correlation with soil nutrients and pH value.