**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** THE BUILDING AND MAINTENANCE OF SOIL ORGANIC MATTER (SOM) IS A FUNDAMENTAL PILLAR OF SOIL HEALTH. SOIL ORGANIC MATTER PROVIDES IMPROVED NUTRIENTS FOR FOOD PRODCUTION, LONG TERM STORAGE OF CARBON REMOVED FROM THE ATMOSPHERE, AND ENHANCED BIODIVERSITY IN AGRICULTURAL SYSTEMS. SOIL MICROORGANISMS SUCH AS BACTERIA AND FUNGI ARETHE MAJOR COMPONENTS OF SOIL ECOSYSTEMS THAT TRANSFORM AND BUILD STABLE SOM. HOWEVER, THE SOIL MICROBIOME IS SO COMPLEXTHAT AFTER MORE THAN A DECADE OF HUNDREDS OF SOIL MICROBIOME STUDIES, WE DO NOTYET HAVE A THOROUGH UNDERSTANDING OF HOW THE SOIL MICROBIOME IMPACTS SOM. METAGENOMIC APPROACHES HAVE GREAT PROMISE BECAUSE THEYFOCUS NOT JUST ON WHICHMICROORGANISMS ARE PRSENT BUT ALSO THE GENES THEY USE TO CARRY OUT DIFFERENT METABOLIC FUNCTIONS. FURTHERMORE, CHANGES IN AMOUTNS OF SOM IN SOIL ARE VERY SLOW TO CHANGE (E.G., DECADES), MAKING IT CHALLENGING TO DETERMINE IF SOIL HEALTH PRACTICES ARE HAVINGDESIRABLE BENEFITS OVER SHORT TIME SCALES. HOWEVER, USING ISOTOPICALLY LABELED CARBON AS A TRACER IN SOIL MAKES IT POSSIBLE TO DETECT SMALL CHANGES IN THE MOST STABLE FRACTIONS OF SOIL CARBON OVER RELATIVELY SHORT TIME SCALES. UNFORTUNATLELY, ALTHOUGH METAGENOMICS AND ISOTOPIC LABELING ARE VERY POWERFUL FOR UNDERSTANDING COMPLEX SYSTEMS,THESE METHODS ARE GENERALLY STILL TOO COSTLY, TECHNICAL,AND TIME-CONSUMING FOR WIDESPREAD MONITORING OF SOIL HEALTH. WE AIM TO COUPLEMETAGENOMIC AND ISOTOPIC APPROACHES IN A SUITE OF CONTROLLED LABORATORY AND FIELD EXPERIMENTS TOIDENTIFY WHICH SOIL MICROBIAL COMMUNITY TRAITS PREDICTTHE ACCUMULATION OF STABLE SOM. OUR ULTIMATE AIM IS THAT THESE TRAITS - ONCE ELUCIDATED - WILL OPEN THE DOOR FOR DEVELOPMENT OF NEXT-GENERATION ASSAYS OF SOIL HEALTH THAT CAN BE SCALED FOR WIDESPREAD USE. WE PROPOSE TO FIRST IDENTIFY THESE TRAITS BY INVESTIGATING HOW SOIL MICROORGRANISMS PROCESSCARBON FROM PLANT MATERIAL IN SMALL EXPERIMENTAL SOIL SYSTEMS IN THE LAB THAT REPRESENT A WIDE RANGE OF SOIL TYPES.WE WILL THEN VALIDATE THESE TRAITS IN A SOIL HEALTH MANAGEMENT CONTEXT BY SAMPLING A VAREITY OF SOIL HEALTH BUILDING COVER CROP SYSTEMS IN THE FIELD. FIRSTLY, WE WILL EVALUATE WHETHER SOIL MICROBIAL TRAITS CAN BE USED TO INDICATE THE TRAJECTORYOF SOIL HEALTH IN SOILS THAT HAVE ONLY RECENLTY RECEIVED NEW COVER CROP TREATMENTS. IN OTHER WORDS, CAN THESE TRAITS BE USED TO EVALUATE WHETHER RECENTLY IMPLEMENTED MANAGEMENT PRACTICES ARE MOVING TORWARDDESIRED LONG-TERM OUTCOMES? SECONDLY, WE WILL EVALUATE WHETHER SOIL MICROBIAL TRAITS CAN INDICATE THE STATUSOF SOIL HEALTH AMONG SITES THAT HAVE BEEN MANGAGED DIFFERENLTY OVER LONG PERIODS (E.G., DECADES). WE WILL DO THIS BY SAMPLING LONG-TERM RESEARCH PLOTS EMPLOYING A VARIETY OF ORGANIC AND CONVENTIONAL CROPPING SYSTEMS CONTINUOUSLY SINCE 1986 AT THE LOWER CHESAPEAKE BAY LONG-TERM AGRICULTURAL RESEARCH SITE IN BELTSVILLE, MD.THIS WORK IS DIRECTLY RELEVANT TO THE SOIL HEALTH PROGRAM AREA 1401 BY FOCUSING ON THE BIOLOGICAL MECHANISMS THAT LEAD TO THE RESTORATION OF SOM, A KEY SOILHEALTH METRIC.