**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 OXYGEN (O2) AVAILABILITY IS A CRITICAL TRIGGER OF MICROBIAL PRODUCTION AND CONSUMPTION OF NITROUS OXIDE (N2O), A POTENT GREENHOUSE GAS THAT IS RAPIDLY ACCUMULATING IN THE ATMOSPHERE. IN PARTICULAR, DENITRIFICATION (NITRATE REDUCTION UNDER ANAEROBIC CONDITIONS) IS HIGHLY SENSITIVE TO O2 CONCENTRATION AND OFTEN A MAJOR CONTRIBUTOR OF N2O. DESPITE ITS IMPORTANCE, KNOWLEDGE ON O2 REGULATION OF N2O IS LIMITED AS O2 AVAILABILITY IS OFTEN SUBSTITUTED BY SOIL MOISTURE STATUS ASSUMING THAT VARIABILITY IN SOIL O2 CONCENTRATION IS PRIMARILY REGULATED BY WATER REDISTRIBUTION IN THE SOIL PROFILE. HOWEVER, ACCELERATED SUBSTRATE-INDUCED HETEROTROPHIC RESPIRATION IN PRESENCE OF AMPLE DECOMPOSING ORGANIC RESIDUES CAN ALSO CONSUME O2TO PROMOTE ANOXIC CONDITIONS FOR N2O PRODUCTION. THIS IS RELEVANT BECAUSE INCREASING ADOPTION OF COVER CROP-BASED SOIL HEALTH PRACTICES THAT INTRODUCE A LARGE AMOUNT OF RESIDUE WHICH COULD REGULATE N2O FLUXES DIFFERENTLY IN RELATION TO O2 AND SOIL MOISTURE CONDITIONS. WE PROPOSE TO UNDERSTAND AND MODEL SOIL O2 CONTROLS ON N2O FLUXES FROM LONG-TERM CONSERVATION AGRICULTURAL PRACTICES. THIS GOAL WILL BE ACHIEVED THROUGH THREE MAIN AIMS: 1) FIELD-SCALE MEASUREMENTS OF O2 CONCENTRATION, N2O FLUXES, AND BIOGENIC SOURCES OF N2O IN RESPONSE TO LONG-TERM TILLAGE (CONVENTIONAL VS NOT-TILL), COVER CROP (NO COVER, GRASS, AND LEGUME SPECIES), AND NITROGEN FERTILIZATION PRACTICES; 2) MESOCOSM-SCALE INVESTIGATION OF COVER CROP RESIDUE-TRIGGERED O2 CONSUMPTION BY HETEROTROPHIC RESPIRATION TO DRIVE N2O LOSSES FROM DENITRIFICATION EVEN UNDER SUB-OPTIMAL MOISTURE CONDITIONS; AND 3) DEVELOPING A MACHINE LEARNING MODELING APPROACH FOR N2O PREDICTION USING SOIL O2 AND OTHER BIOGEOCHEMICAL VARIABLES AS PREDICTORS. THE FUNDAMENTAL KNOWLEDGE GAINED FROM THIS PROJECT WOULD CONTRIBUTE TO OUR MECHANISTIC UNDERSTANDING OF SOIL HEALTH-CLIMATE CHANGE MITIGATION NEXUS AND IMPROVED PREDICTABILITY OF SOIL N2O FLUXES - A HIGHLY VARIABLE SOIL N CYCLING PROCESS.