A BIOGEOCHEMICAL/HYDROLOGICAL APPROACH TO CHARACTERIZE TRANSPORT AND CYCLING OF NITROGEN IN MANTLED KARST WATERSHEDS

Animal production and associated pasture application of animal manures in karst poses a significant threat to water quality due to thin soils, rapid infiltration, a predominance of conduit flow, and minimal opportunity for processing of nutrients. Balanced nutrient application presupposes an understanding of biogeochemical processes and controls on nitrogen transport and cycling, an understanding that is not well evolved for karst. Nitrate processing was characterized during storm and base flow conditions in karst hydrogeological zones, particularly in the interflow zone – the intermediate zone between focused-flow and diffuse-flow (soil) zones where a strong horizontal component of flow occurs over permeability contrasts such as at the epikarst. Nutrient processing that may occur in the interflow zone, which has increased ground-water retention time and water-matrix interaction is important because of the lack of processing in the focused-flow and thin, karst soil zones.

Nitrate processing was characterized using dissolved organic carbon concentration and bioavailability, and concentration of reactive (nitrate) versus conservative (chloride) species. Additionally, concentration and isotopic composition of NO3- was used to determine the extent of denitrification and immobilization of nitrate in the interflow zone. Study results show that as much as 30% of nitrate moving through the interflow zone can be microbially processed within this zone, and the level of processing is highly dependent upon flow-path and hydrologic conditions. Bioavailability of DOC in the interflow is increased relative to the focused-flow zone under high flow conditions. Nitrogen and oxygen stable isotope data for nitrate suggest denitrification is occurring in the interflow. The interflow zone appears to be a potentially important zone for nitrate attenuation in karst settings.