NITROGEN (N) IS AN ESSENTIAL ELEMENT, OFTEN LIMITING PLANT PRODUCTIVITY IN MANY ECOSYSTEMS. AS A RESULT, N FERTILIZERS ARE ADDED TO ENHANCE CROP YIELDS WHICH HAS ALSO INTRODUCED REACTIVE N INTO THE ENVIRONMENT. DESPITE RECOGNITION OF THE IMPORTANCE OF N AS A NUTRIENT AND POTENTIAL CONTAMINANT, UNCERTAINTY REMAINS REGARDING THE FATE OF ADDED AMMONIUM INPUTS TO SOIL. AMMONIUM CAN BE BOUND TO CLAY MINERALS IN AN ABIOTIC PROCESS. IN ADDITION, IT CAN BE CONVERTED TO ORGANIC N FORMS BY BIOLOGICAL IMMOBILIZATION PROCESSES MEDIATED BY MICROORGANISMS. HISTORICALLY, THESE TWO FATES OF AMMONIUM HAVE BEEN STUDIED IN ISOLATION. ACCORDINGLY, THE OVERALL GOAL OF THIS PROPOSED PROJECT IS TO INVESTIGATE THE CONTRIBUTION OF ABIOTIC AND BIOLOGICAL IMMOBILIZATION PROCESSES AND NITRIFICATION PROCESSES IN NATURAL CLAY MINERAL ASSEMBLAGES AND BULK SOILS FROM UNDISTURBED SOD AND LONG TERM NO-TILLAGE SOIL SAMPLES FROM FOUR LOCATIONS SPANNING A CLIMATE GRADIENT (MAP 400-1300 MM). WE WILL COMBINE WET CHEMICAL WITH CUTTING EDGE SPECTROSCOPIC EXPERIMENTS TO UNDERSTAND THE FATE OF AMMONIUM IN SOIL UNDER THESE DIFFERENT LAND USES TAKEN OVER THE CLIMATE GRADIENT. IT IS EXPECTED THAT THE CHANGES IN MINERALOGY OBSERVED OVER THE CLIMATE GRADIENT WILL BE REFLECTED IN THE CONTRIBUTION OF ABIOTIC VERSUS BIOLOGICAL IMMOBILIZATION. THESE RESULTS WILL HELP TO OPTIMIZE NITROGEN FERTILIZER ADDITIONS IN ORDER TO MINIMIZE ADVERSE ENVIRONMENTAL CONSEQUENCES OF REACTIVE N ADDITIONS INTO ECOSYSTEMS.