Objective Soil bacteria play an important role in maintaining the ecological function and health of the soil ecosystem. Based on understanding the variation of soil bacterial community with the stand ages of walnut forests, combined with soil factors, this paper aims to further explore the relationship between soil microbial community and soil properties. At the same time, the driving factors of soil microbial community composition changes were elucidated.

Method In this study, we sampled soils in walnut consisting of stands of 8, 50 and over 100 years of average age to measure soil physical and chemical properties, enzyme activities, and to describe the composition and diversity of soil microbial community using high-throughput sequencing technology.

Result (1) There was no significant difference in soil bacterial richness under different stand ages, but ancient walnut forests (more than 100 years) significantly reduced soil bacterial Shannon index, Simpson index and Pielou index. (2) Amplicon sequencing revealed Proteobacteria, Acidobacteria, Firmicutes, Gemmatimonadetes, Myxococcota, Actinobacteria, Chloroflexi, and Bacteroidetes were the dominant phylum of soil bacteria under different stand ages. There were significant differences in the relative abundance of some microbial groups under different walnut ages. The relative abundance of Actinobacteria was significantly increased, but Chloroflexi was significantly decreased, and Bacteroidetes first increased and then decreased. (3) Non-metric multi-dimensional scaling showed that there were significant differences between soil bacterial communities under different stand ages. In addition, the composition of bacterial communities was significantly affected by soil nitrate nitrogen content. (4) Correlation analysis also showed a significant or very significant correlation between soil enzymes, soil physicochemical properties and diversity, dominant phylum of soil bacteria.

Conclusion In conclusion, the changes in composition and diversity of soil microbial communities in walnut forests occurring over time can largely be attributed to changes in soil physicochemical properties and enzyme activities. Soil nitrate-nitrogen content is the main driving factor in microbial community formation. Importantly, the bacterial community diversity of ancient walnut forests is significantly reduced, which aggravates the microecology imbalance in the rhizospheric soil of ancient walnut forests. The study is an essential supplement to the analysis of bacterial communities in the forest. In addition to renewing walnut trees in actual production, inorganic and organic fertilizers or microbial fertilizers can be rationally used to maintain the stability of the soil ecosystem of walnut forests.