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

Abstract

AbstractNitrogen 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 δ15NSed) and nitrate (NO3− and δ15NNO3); (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%, δ15NSed = 5.07‰ ± 1.01‰) and outputs (SN = 0.36% ± 0.09%, δ15NSed = 6.45‰ ± 0.71‰) indicate net‐mineralization of SN and increase of δ15NSed (p < 10−2). However, δ15NSed increase cannot be explained by SN mineralization alone and is instead accompanied by immobilization of isotopically heavier mineral nitrogen (δ15NNO3 = 11.25‰ ± 6.96‰). Modeled SN and δ15NSed 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 (∼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.