**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** PLANTING DIVERSE COVER CROP MIXTURES HAS A VARIETY OF AGRICULTURAL AND ECOLOGICAL BENEFITS, BUT IT REMAINS DIFFICULT TO RELIABLY PREDICT WHICH BENEFITS WILL BE REALIZED FROM PARTICULAR COMBINATIONS OF COVER CROP SPECIES. AT THE SAME TIME, THE SOIL MICROBIOME (I.E., THE COMPLETE COMMUNITY OF BACTERIA, FUNGI, AND OTHER MICROORGANISMS LIVING IN SOIL) IS KNOWN TO MEDIATE CROP GROWTH AND ECOLOGICAL BENEFITS, BUT ITS COMPLEXITY MAKES IT DIFFICULT TO MANAGE DIRECTLY. WE HYPOTHESIZE THAT THE SOIL MICROBIOME IS A KEY LINK BETWEEN COVER CROP DIVERSIFICATION AND AGRO-ECOLOGICAL BENEFITS, BUT AN APPLIED UNDERSTANDING OF RELATIONSHIPS BETWEEN PLANT DIVERSITY AND SOIL MICROBIAL DIVERSITY IS STILL LACKING. WE PROPOSE TO DIRECTLY MEASURE THE DIVERSITY OF SOIL MICROBIAL COMMUNITIES SELECTED FOR BY INDIVIDUAL COVER CROP SPECIES. WE WILL THEN DESIGN COVER CROP MIXTURES THAT VARY IN THREE DISTINCT WAYS: THE NUMBER OF CROP SPECIES; THE DIVERSITY OF DIFFERENT ORGANISM TYPES (I.E., PLANT FUNCTIONAL TRAITS VS. SOIL MICROBIOME); AND THE DEGREE OF HOMOGENEITY OR HETEROGENEITY IN THE DIVERSITY. THE AIM IS TO GENERATE NEW KNOWLEDGE ABOUT THE RELATIONSHIPS AMONG COVER CROP DIVERSITY, THE SOIL MICROBIOME, AND ECOSYSTEM FUNCTIONS THAT TRANSLATE TO MORE RELIABLE IMPROVEMENTS IN PLANT PRODUCTION SYSTEMS. THIS WORK IS DIRECTLY RELEVANT TO THE FOUNDATIONAL KNOWLEDGE OF AGRICULTURAL PRODUCTION SYSTEMS PROGRAM AREA PRIORITY BY EXPERIMENTALLY MANIPULATING CROP DIVERSITY IN AN AGRO-ECOLOGICAL CONTEXT TO DETERMINE HOW PRODUCTION SYSTEMS CAN IMPACT SOIL MICROBIAL COMMUNITIES, PRODUCTIVITY, AND NATURAL RESOURCE STEWARDSHIP. THE EXPECTED BENEFITS WILL RESULT IN A SIGNIFICANT STEP TOWARDS USING CROP DIVERSIFICATION MORE PURPOSEFULLY TO MANAGE THE SOIL MICROBIOME, AND BY EXTENSION, IMPROVING YIELD, SOIL HEALTH, AND AGRICULTURAL SUSTAINABILITY.