** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** INCREASED AGRICULTURAL INTENSIFICATION HAS LED TO EXCESS NITROGEN INPUTS ON SOIL AND DECLINED WATER QUALITY IN MANY REGIONS OF THE UNITED STATES (US). THE US CORN-BELT STATES USE LARGE QUANTITIES OF NITROGEN FERTILIZER, AND MOST OF THESE REGIONS CONTRIBUTE TO CONTINUED POLLUTION OF LOCAL GROUNDWATER. UNDER TYPICAL CONDITIONS, CROPS TAKE UP ~47% OF APPLIED NITROGEN, AND THE REST IS LOST THROUGH VOLATILIZATION, RUN-OFF, AND LEACHING. A SUBSTANTIAL AMOUNT IS LEACHED TO THE VADOSE ZONE, WHICH IS THE SOIL LAYER BETWEEN THE LAND SURFACE AND THE WATER TABLE. THE VADOSE ZONE ACTS AS A TEMPORARY NITROGEN RESERVOIR, STORING THE LEACHED NITROGEN. GLOBALLY, THE VADOSE ZONE IS ESTIMATED TO STORE 605-1814 MILLION TONS OF NITROGEN AS NITRATE, PRIMARILY COMING FROM APPLIED FERTILIZERS. WHILE MOST VADOSE ZONE STUDIES ONLY MEASURE NITRATE, OUR PAST STUDIES SHOW THE WIDESPREAD OCCURRENCE OF BOTH NITRATE AND AMMONIUM IN DEEP VADOSE ZONE SEDIMENTS. MOREOVER, THE IRRIGATION METHOD (FLOOD VERSUS PIVOT) CORRELATES WITH AMMONIUM OCCURRENCE, SUGGESTING THAT WATER INPUT PLAYS A CRITICAL UNDERAPPRECIATED ROLE IN THE FATE OF NITROGEN BENEATH THE LAND SURFACE.A WIDE VARIETY OF BIOGEOCHEMICAL REACTIONS CONTROL THE OCCURRENCE OF SPECIFIC NITROGEN SPECIES IN THE VADOSE ZONE. HOWEVER, INACCESSIBILITY OF THE VADOSE ZONE MAKES IT CHALLENGING TO STUDY AND IDENTIFY THESE BIOGEOCHEMICAL REACTIONS, WHICH CAN CONTROL THE NITROGEN MOVEMENT TO THE GROUNDWATER RESOURCES. FURTHER, THESE NITROGEN SPECIES TRANSFORMATION REACTIONS ARE LINKED WITH SURFACE PROCESSES OCCURRING IN THE AGROECOSYSTEMS SUCH AS IRRIGATION PRACTICE AND FERTILIZER TYPES. THIS PROJECT WILL MEASURE THE OCCURRENCE OF MULTIPLE NITROGEN SPECIES, WHICH WILL INCLUDE INORGANIC AND ORGANIC NITROGEN SPECIES BENEATH GRAVITY, PIVOT IRRIGATED AND DRYLAND CORN USING DEEP CORING, ELUCIDATE TRANSFORMATION PATHWAYS BETWEEN DIFFERENT NITROGEN SPECIES BY COMPREHENSIVE CHEMICAL ANALYSIS, AND COLUMN EXPERIMENTS WITH ISOTOPE-LABELED FERTILIZERS AND SIMULATE REACTIVE TRANSPORT OF NITROGEN WITH APPROPRIATE MODIFICATION OF A WELL-DEVELOPED USDA-ARS MANAGEMENT MODEL - ROOT ZONE WATER QUALITY MODEL (RZWQM2). THE NEW KNOWLEDGE FROM FIELD AND COLUMN EXPERIMENTS AND THE MODIFIED MODEL CAN BE USED AS A DECISION-SUPPORT TOOL BY REGULATORY AGENCIES TO SHOW HOW PROPOSED MANAGEMENT PRACTICES MITIGATE GROUNDWATER POLLUTION. ULTIMATELY THIS WORK WILL SUPPORT THE DEVELOPMENT OF PROACTIVE MANAGEMENT PRACTICES TO PROTECT GROUNDWATER QUALITY.