**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 NON-GROWING SEASON IS A CRITICAL PERIOD FOR NUTRIENT LOSS IN COLD AGRICULTURAL REGIONS. HOWEVER, SOIL AND WATER NUTRIENT DYNAMICS DURING THE WINTERTIME AND TRANSITION PERIODS BETWEEN FROZEN AND NON-FROZEN CONDITIONS ARE POORLY UNDERSTOOD. THIS STUDY WILL INTEGRATE HYDROLOGY, SOIL PHYSICS, AND NUTRIENT MEASUREMENTS WITH PROCESS-BASED MODELING TO UNDERSTAND FROZEN AND NON-FROZEN HYDROLOGIC PROCESSES WITH THE GOAL TO PROTECT WATER QUALITY AT THE WATERSHED SCALE IN MIDWEST AGROECOSYSTEMS. THIS MULTI-SCALE (LABORATORY, PLOT, FIELD, AND WATERSHED) AND REGIONAL (OHIO, WISCONSIN, AND MINNESOTA) STUDY WILL GENERATE NEW DATA AND LEVERAGE EXISTING COLD SEASON DATA SETS THAT SPAN MULTIPLE STATES AND YEARS, IMPROVE WATERSHED-SCALE SIMULATION OF COLD SEASON HYDROLOGY AND NUTRIENT TRANSPORT, AND PROJECT THE IMPACTS OF AGRICULTURAL MANAGEMENT PRACTICES ON WATER QUALITY.LABORATORY EXPERIMENTS WILL PROVIDE CONTROLLED AND REPLICATED CONDITIONS TO UNDERSTAND HOW COLD TEMPERATURES AFFECT KEY HYDROLOGIC AND NUTRIENT TRANSPORT PROCESSES WITHIN THE SOIL MATRIX. THE PLOT SCALE ADDS COMPLEXITY IN TERMS OF WEATHER AND MANAGEMENT WHILE STILL PROVIDING REPLICATION OF HYDROLOGICALLY ISOLATED PLOTS. KNOWLEDGE GAINED FROM THE PLOT EXPERIMENTS SERVES TO BRIDGE BETWEEN THE LABORATORY AND FIELD SCALES, AS THE PLOT SCALE WILL PROVIDE GREATER CONFIDENCE THAN THE FIELD SCALE IN THE IDENTIFICATION OF FACTORS THAT CONTROL HYDROLOGICAL AND NUTRIENT PROCESSES, AS WELL AS THE OPPORTUNITY TO HONE IN ON ACTUAL FIELD CONDITIONS/PROCESSES. KNOWLEDGE GAINED FROM BOTH THE LABORATORY AND PLOT SCALES WILL INFORM OUR INTERPRETATION OF FIELD-SCALE RESULTS. FINALLY, THE DATA COLLECTED AT FIELD SCALES WILL BE USED TO EVALUATE AND VALIDATE MODEL IMPROVEMENTS. EVALUATING CONSERVATION PRACTICES ACROSS SCALES IS CRITICAL FOR UNDERSTANDING THE ROLE OF COLD SEASON PROCESSES IN WATER QUALITY AT THE POINT OF IMPACT AND GUIDING MANAGEMENT DECISIONS.UNDERSTANDING CHANGES IN SOIL NUTRIENT DYNAMICS IS IMPORTANT FOR FOOD AND ENERGY SECURITY, ENVIRONMENTAL PROTECTION, AND WATER QUALITY SUSTENANCE. PROPER UNDERSTANDING OF CROP PRODUCTION SYSTEMS, INCLUDING NUTRIENT MANAGEMENT AND RECYCLING, IS IMPORTANT NOW AND INTO THE FUTURE TO PROMOTE BOTH ECONOMIC AND ENVIRONMENTAL SUSTAINABILITY. GIVEN POTENTIAL CHANGES TO NUTRIENT AND SOIL DYNAMICS BROUGHT ON BY CLIMATE CHANGE, KNOWLEDGE ON HOW NUTRIENT DYNAMICS ARE AFFECTED BY SOIL TEMPERATURE GRADIENTS AND FREEZE-THAW PROCESSES IS NEEDED SO THAT STAKEHOLDERS ARE AWARE OF ADAPTATION CHALLENGES AND OPTIONS. COLD REGIONS CONTRIBUTE TO PERVASIVE NATIONAL AND REGIONAL WATER QUALITY ISSUES, SUCH AS GULF HYPOXIA AND GREAT LAKES HARMFUL ALGAL BLOOMS. WHILE THE CRITICAL PERIOD FOR THESE IMPAIRMENTS IS GENERALLY SUMMER, WHEN CONDITIONS ARE CONDUCIVE TO EXCESS ALGAL GROWTH, NUTRIENT LOADING TO WATERWAYS THROUGHOUT THE YEAR CONTRIBUTES TO SUMMER EUTROPHICATION. THE PROPOSED WORK WILL FILL IMPORTANT KNOWLEDGE GAPS IN HYDROLOGIC AND NUTRIENT CYCLING PROCESSES OF FROZEN/PARTIALLY FROZEN SOIL TO DEVELOP EVIDENCE-BASED GUIDANCE FOR REDUCING NONPOINT SOURCE NUTRIENT POLLUTION IN COLD REGION AGROECOSYSTEMS.