**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** WIDESPREAD DECLINES IN SOIL HEALTH ARE A PERSISTENT PROBLEM FOR AGRICULTURE. SOIL ORGANIC MATTER (SOM) IS THE FOUNDATION OF SOIL HEALTH, ITS EFFECTS ON SOIL MOISTURE, STRUCTURE, FERTILITY, AND SOIL BIOTIA ARE THE BASIS FOR SOIL SUSTAINABILITY. SOM RESTORATION AND MAINTENANCE IS ACHIEVED THROUGH JUDICIOUS USE OF COVER CROPS. HOWEVER, THEIR EFFECTS ON SOM DIFFER BETWEEN SPECIES, SOILS, AND SYSTEMS. THOUGH PLANT-MICROBE-SOIL INTERACTIONS GOVERN SOM DYNAMICS, WE LACK KNOWLEDGE OF THESE BELOWGROUND PROCESSES. CURRENT COVER CROP RECOMMENDATIONS FOR BUILDING SOIL HEALTH LACK A SOLID FOUNDATION OF FUNDAMENTAL KNOWLEDGE. OUR GOAL IS TO DEFINE THE BELOWGROUND MECHANISMS THROUGH WHICH PLANT-MICROBE-SOIL INTERACTIONS GOVERN SOM DYNAMICS. WE FOCUS ON FOUR OBJECTIVES: (I) QUANTIFY COVER CROP EFFECTS ON BELOWGROUND PROCESSES THAT REGULATE SOM POOLS, (II) DEFINE COVER CROP EFFECTS ON BELOWGROUND PROCESSES IN DIFFERENT SOIL HEALTH CONTEXTS (DEGRADED VERSUS HEALTHY SOILS), (III) QUANTIFY CONTRIBUTIONS OF COVER CROP ROOTS AND SHOOTS TO SOM POOLS, AND (IV) DESCRIBE THE MICROBIAL MECHANISMS THAT REGULATE SOM POOLS. THESE OBJECTIVES WILL BE ACHIEVED THROUGH GREENHOUSE EXPERIMENTS THAT MINIMIZE VARIATION AND ISOLATE CRITICAL VARIABLES THAT CONTROL COMPLEX PLANT-MICROBE-SOIL INTERACTIONS DIFFICULT TO DISENTANGLE IN THE FIELD. WE WILL MEASURE THE DYNAMICS OF DISCRETE SOM POOLS WITH RESPECT TO PLANT AND MICROBIAL INPUTS, SOIL PROCESSES, AND RHIZOSPHERE MICROBIOME COMPOSITION. THIS FUNDAMENTAL RESEARCH WILL DEFINE THE MECHANISMS WHEREBY PLANT-MICROBE-SOIL INTERACTIONS REGULATE SOM FORMATION. SUCH KNOWLEDGE IS ESSENTIAL FOR IMPROVING THE PRECISION OF COVER CROP RECOMMENDATIONS DESIGNED FOR ENHANCEMENT AND SUSTAINABILITY OF SOIL HEALTH.