Tillage and pesticide seed treatments have distinct effects on soil microbial diversity and function

Conservation tillage practices can benefit soil health relative to conventional tillage; however, conservation tillage systems are often coupled with the use of pesticide seed treatments (PST; fungicides and insecticides, including neonicotinoids), which may undermine these benefits by disrupting microbial communities and processes. We examined the effects of tillage system (conventional moldboard plowing versus two conservation tillage practices: no tillage and strip tillage) and PST on microbial diversity and microbial function in a field experiment with a soybean-corn rotation. Enzyme activities and the microbial functional capacity to degrade a variety of substrates was intermediate in strip tillage compared with conventional and no tillage. Microbial community composition under strip tillage was similar to those under no tillage but distinct from conventional tillage, and community differences were associated with soil C/N and pH. However, tillage system had no effect on alpha diversity. Communities under strip tillage and no tillage had different substrate preferences to those under conventional tillage, which could be explained by the major C sources under each management practice. Pesticide seed treatment affected microbial substrate preference and altered the relative abundance of various fungal taxa, indicating a direct (i.e., toxic) effect. Despite this, PST had no effect on enzyme activities or alpha diversity of the microbial community. There were limited interactions between tillage and PST on microbial properties. Our results support that microbial communities are valuable indicators of soil health and show that strip tillage maintains soil microbial health better than conventional tillage but not as well as no tillage, and that PST may negatively affect soil microbial health.