A COUPLED BIOGEOCHEMICAL/HYDROLOGICAL APPROACH FOR ELUCIDATING TRANSPORT AND CYCLING OF NITROGEN WITHIN MANTLED KARST WATERSHEDS

Animal production and associated pasture application of animal manures in vulnerable karst regions poses a significant threat to water quality. Balanced nutrient application presupposes an understanding of biogeochemical processes and controls on nitrogen transport, transformation, and sequestration. Concentration and bioavailability of dissolved organic carbon were investigated as important factors controlling nutrient cycling and transport. Additionally, concentration and isotopic composition of NO₃^- was used to determine the extent of denitrification and immobilization of nitrate. The study incorporates two synoptic sampling events, storm and base flow, to characterize three components of a shallow system of a karst watershed: (1) surface-water/ground-water interface (soil/regolith) zone, (2) interflow (low permeability) zone, and (3) focused flow zone. The first two are presumably the zones with increased biogeochemical processing of nutrients due to longer retention time and greater matrix/water ratios. Initial results (storm flow) indicated decreasing values of DOC concentration along the flow-path from the interface zone to the interflow zone down to the focused flow zone. Once the biogeochemical mechanisms proposed are completely elucidated, the impact of agricultural practices on the integrity of these zones, and the way the processes occurring within these zones can be capitalized upon for nutrient management can be tested.