UC Davis

The soil microbiome is recognized as a key component of soil health; however, microbial community responses to agricultural management can be inconsistent and context dependent. Here, we investigated the connections between soil microbial communities, soil properties, and agricultural practices in the Driftless Region of Wisconsin and Illinois, spanning 128 fields that have been under organic management for varying durations (from 3 to 51 years). Using structural equation modeling (SEM) we uncovered the effects of tillage, time under organic management, alfalfa use, and cover crop use on total carbon, nitrogen, and pH while quantifying their direct and indirect effects on bacterial biomass, bacterial diversity, relative abundance of arbuscular mycorrhizal fungi (AMF), and fungal diversity. We found that bacterial and fungal community compositions significantly varied among fields that recently transitioned to organic management but converged to a more consistent composition under prolonged organic management (>20 years). SEM suggested that continuous alfalfa use increased bacterial biomass, likely via improved nitrogen availability. Frequent tillage reduced the proportion of AMF, limiting proliferation due to disturbance. This study demonstrates how organic management in dairy forage systems likely shapes microbial community composition, especially through tillage and alfalfa use. These findings confirm the previous understanding of soil disturbance and soil chemical effects on microbial communities and caution that time under organic management per se should not be expected to lead to predictable changes in soil conditions. Rather, combinations of direct and indirect consequences of specific management practices impact soil health over time.