** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** SOIL IS ONE OF OUR MOST VALUABLE RESOURCES, PROVIDING PHYSICAL SUPPORT TO PLANTS, REGULATING DIFFERENT BIOGEOCHEMICAL CYCLES, AND MITIGATING GLOBAL WARMING THROUGH ORGANIC CARBON STOCKS. EMPIRICAL DATA SUGGESTS THAT THE RATE AND MAGNITUDE OF NET C ACCUMULATION IS OFTEN LOWER THAN THE TOTAL AGRICULTURAL INPUT AND WE HAVE YET TO UNDERSTAND HOW ORGANIC C IS STABILIZED IN SOILS. WE PROPOSE, THROUGH A SERIES OF TRANSFORMATIVE EXPERIMENTS, TO QUANTIFY THREE KEY SOIL COMPONENTS: MICROBIOME STRUCTURE (BIOLOGY), ORGANIC MATTER STRUCTURE (CHEMISTRY) AND SOIL AGGREGATE STRUCTURE (PHYSICS), CONSIDERED THE FUNCTIONAL UNIT OF SOIL. THIS RESEARCH WILL BE GUIDED BY THE FOLLOWING SPECIFIC AIMS:T1. SEPARATING AGGREGATES INTO FOUR DIFFERENT CLASSES AND DETERMINE MICROBIAL CARBON USE EFFICIENCY (CUE) IN EACH OF THOSE AGGREGATES USING 18O ISOTOPES.T2. OBTAINING METAGENOMIC SEQUENCING OF MICROBIAL COMMUNITIES AT DIFFERENT AGGREGATE LEVELS AND ESTIMATING LIFE-HISTORY TRAITS ASSOCIATED WITH CUE.T3. CHARACTERIZE C ACCUMULATION AND SOURCES (&DELTA;13C RATIOS) OF SOM FRACTIONS FOR AGGREGATES OF DIFFERENT SIZE CLASSES: DISSOLVED ORGANIC C, MICROBIAL BIOMASS C, MINERAL ASSOCIATED ORGANIC MATTER (OM), OCCLUDED PARTICULATE OM AND FREE PARTICULATE OM.T4. IDENTIFY SPECIFIC DIFFERENCES IN C PROCESS UTILIZATION USING A SUPERVISED MACHINE LEARNING APPROACH.T5. EXPLAIN C STABILIZATION AT THE FIELD SCALE BY INCORPORATING GENOMIC-ENABLED C PROCESSES INTO THE NEXT-GENERATION SOIL C MODEL - THE MILLENNIAL MODEL - TO SIMULATE MICROBIAL GROUP DYNAMICS AND THEIR FUNCTIONAL ROLES.THIS RESEARCH PROPOSAL ADDRESSES THE NIFA-AFRI FOUNDATIONAL PROGRAM: SOIL HEALTH. SPECIFICALLY, THE RESEARCH WILL FUNCTIONALLY CHARACTERIZE SOIL MICROBIOMES, THEIR FUNCTIONAL TRAITS RESPONSIBLE FOR C STABILIZATION AND MODEL THE OUTCOME IN AGRICULTURAL SOILS UNDER DIFFERENT MANAGEMENT PRACTICES.