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.

Automated summary

Nitrogen removal rates in karst environments are influenced by various factors and processes, such as mineralization and sorption of nitrates. The study highlights the complexities of nitrogen dynamics within karst conduits, with removal processes mainly occurring near conduit entrances and showing higher efficiency during the dry season.