Seasonality of Recharge Drives Spatial and Temporal Nitrate Removal in a Karst Conduit as Evidenced by Nitrogen Isotope Modeling
Admin Husic, James F. Fox, Nabil Al Aamery, William I. Ford, Erik D. Pollock, Jason Backus
Abstract
Abstract 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 δ 15 N Sed ) and nitrate (NO 3 − and δ 15 N 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%, δ 15 N Sed = 5.07‰ ± 1.01‰) and outputs (SN = 0.36% ± 0.09%, δ 15 N Sed = 6.45‰ ± 0.71‰) indicate net‐mineralization of SN and increase of δ 15 N Sed ( p < 10 −2 ). However, δ 15 N Sed increase cannot be explained by SN mineralization alone and is instead accompanied by immobilization of isotopically heavier mineral nitrogen (δ 15 N NO3 = 11.25‰ ± 6.96‰). Modeled SN and δ 15 N Sed sub‐routines provided a boundary condition for DIN simulation and improved NO 3 − 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 NO 3 − 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.