Coupling Hyporheic Nitrification-Denitrification: Evaluating Net Nitrate Source-Sink Dynamics as a Function of Transport and Reaction Kinetics
Abstract
The fate of biologically-available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. Hyporheic and riparian zones, where ground waters and stream waters mix, can be important locations controlling N and C transformations because they create strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O2), ammonium (NH4), nitrate (NO3), and dissolved organic carbon (DOC). The model successfully simulated the O2, NH4, NO3 and DOC concentration profiles observed in the hyporheic zone at our study site. We then used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification-denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in hyporheic systems and the uptake rate of O2 from either respiration and/or nitrification determined whether a hyporheic system was a source or a sink of NO3 to the stream. We further show that the net NO3 source or sink function of a hyporheic system is determined by the ratio of characteristic transport time to the characteristic reaction time of O2 (i.e., the Damköhler number, DaO2), where hyporheic systems with DaO2 < 1 will be net nitrification environments and hyporheic systems with DaO2 >> 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within hyporheic zones allows us to explain the widely contrasting results of previous investigations of hyporheic N dynamics which variously identify the hyporheic zone as either a net source or sink of NO3. Our model results also suggest that only estimates of residence times and O2 uptake rates are necessary to predict whether a hyporheic system will be either a net source or sink of NO3.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B12D..05Z
- Keywords:
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- 0412 BIOGEOSCIENCES / Biogeochemical kinetics and reaction modeling;
- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling;
- 0469 BIOGEOSCIENCES / Nitrogen cycling;
- 1830 HYDROLOGY / Groundwater/surface water interaction