Surrounding land use and land cover shape the soil microbiome to a similar extent as local diversification practices across organic farms

Soils harbor a significant proportion of Earth’s biodiversity, especially microorganisms. Yet understanding the drivers of soil microbial diversity, community composition, and interactions remains far from complete, especially in agricultural landscapes where management at multiple scales could strongly influence soil microbiomes, and in turn, where humans depend on multiple soil-based ecosystem services supported by microbes. We studied how variation in local, field-scale diversification practices (i.e. crop diversity, cover cropping, organic amendments, and reduced tillage) and proportions of different land use / land cover classes in the surrounding landscape (i.e. landscape composition) affected soil bacterial and fungal communities across organically managed lettuce fields in an intensively-managed agricultural region. Across 28 organic farms, only crop diversity positively affected metrics of soil microbial diversity, including taxonomic diversity of bacteria and ecological functional diversity of fungi, while cover cropping reduced the diversity of fungal ecological functional diversity and the metabolic richness of bacteria. All diversification practices significantly influenced the community structure of both bacteria and fungi, collectively explaining 5.6 and 8.4% of community structure variation, respectively. Surrounding landscape composition explained 1.8-fold and 1.1-fold more variation in bacterial and fungal community structure as these field-scale diversification practices. Landscape composition also affected five fungal and one bacterial metrics of diversity and richness. Field-scale diversification practices, especially crop diversity, modulated bacterial and fungal network structure, in complex ways. Our results highlight the importance of both field management and surrounding landscapes in shaping multiple aspects of soil microbial communities. Similar to aboveground ecological communities in agricultural landscapes, scales beyond the local must be considered to steward the soil microbiome.