Fifteen-year no tillage of a Mollisol with residue retention indirectly affects topsoil bacterial community by altering soil properties

Soil microbes play an important role in nutrient cycling in agricultural soils and can be influenced by tillage. Conservation tillage aims to reduce energy inputs and conserve soil and water by decreasing disturbance of soil and returning a portion of crop residue. The response of the soil microbial community to conservation tillage is complex and quantitative analysis is largely absent regarding how tillage and depth, combined with soil properties, affect soil microbial diversity and community composition. Here, the diversity and composition of the soil bacterial community and its relationship with soil properties were explored by high-throughput sequencing technology and Structural Equation Modeling (SEM) at two soil depths (0–5 cm and 15–20 cm) under conventional tillage and no tillage with residue retention. No tillage significantly increased alpha diversity (Chao 1 and Shannon) at 0-5 cm and altered the composition of the bacterial community, coinciding with changes in physicalchemical properties. Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, and Gemmatimonadetes were the most abundant phyla across all samples. Alpha diversity was significantly correlated with soil bulk density (BD) and pH. The SEM showed that tillage and depth explained 86% of the bacterial diversity and 84% of the composition. In addition, tillage and depth had indirect effects on bacterial diversity and composition by affecting soil BD, pH and soil organic carbon. Results indicate that the soil bacterial community is altered by conservation tillage, especially in the topsoil, and highlight the importance of soil physical-chemical properties in shaping the diversity and composition of the soil bacterial community. Our findings contribute to a broad understanding of tillage disturbance and differentiated effects in the soil profile for the bacterial community.