Seasonality of Recharge Drives Spatial and Temporal Nitrate Removal in a Karst Conduit as Evidenced by Nitrogen Isotope Modeling

Nitrogen removal rates can vary with time, space, and external environmental drivers, but are underreported for karst environments. We carried out a multi-year study of a karst conduit where we: (a) measured inputs and outputs of sediment nitrogen (SN and delta N-15(Sed)) and nitrate (NO3- and delta N-15(NO3)); (b) developed, calibrated, and applied a numerical model of nitrogen physics and biogeochemistry; and (c) forecasted the impacts of climate and land use changes on nitrate removal and export. Data results from conduit inputs (SN = 0.43% +/- 0.07%, delta N-15(Sed) = 5.07 parts per thousand +/- 1.01 parts per thousand) and outputs (SN = 0.36% +/- 0.09%, delta N-15(Sed) = 6.45 parts per thousand +/- 0.71 parts per thousand) indicate net-mineralization of SN and increase of delta N-15(Sed) (p < 10(-2)). However, delta N-15(Sed) increase cannot be explained by SN mineralization alone and is instead accompanied by immobilization of isotopically heavier mineral nitrogen (delta N-15(NO3) = 11.25 parts per thousand +/- 6.96 parts per thousand). Modeled SN and delta N-15(Sed) sub-routines provided a boundary condition for DIN simulation and improved NO3- model performance (from NSE = 0.06 to NSE = 0.68). Modeled spatial zones of removal occur in close proximity to conduit entrances, where deposition of labile organic matter promotes a three-fold increase in denitrification (similar to 60 mg N m(-2) d(-1)). Modeled temporal periods of removal occur during the dry-season where longer residence times cause up to 90% removal of NO3- inputs. Projected effects of environmental drivers suggest an increase in denitrification (+14.1%); however, this removal is largely offset by greater nitrate soil leaching (+28.1%) from wetter regional climate. Results suggest that conduits underlying mature karst terrain experience spatiotemporal removal gradients, which are modulated by solute and sediment delivery.