Modeling the Competition and Controls for Denitrification and Dissimilatory Nitrate Reduction to Ammonium (DNRA) in Riparian Sediments

Abstract Understanding the balance between denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in riparian systems is essential for managing watershed nitrogen (N) budgets and evaluating restoration practices. This balance is influenced by several factors including concentrations and ratio of various organic and inorganic electron donors (dissolved organic carbon [DOC], Fe 2+ ) to acceptor (NO 3 − ). In riparian sediments, these factors can change rapidly over space and time, complicating measurement and quantification. We used a PFLOTRAN batch reactor model calibrated to laboratory microcosm experiments where the denitrification and DNRA rates in riparian sediments were measured using 15 N stable isotopes. Although DOC/NO 3 − ratios influenced the relative proportions of denitrification and DNRA, the processes were also affected by elemental concentrations. For a starting DOC concentration of 0.12 mgL −1 , DNRA exceeded denitrification at DOC/NO 3 − = 6; however, this shift was not observed within a range of DOC/NO 3 − = 30 at higher DOC concentration of 12 mgL −1 . Heterotrophic pathways dominated NO 3 ‐N reduction with smaller contribution from autotrophic pathways. These findings suggest that although heterotrophic pathways are important in carbon‐rich sediments, autotrophic pathways can be significant in carbon‐depleted conditions in the presence of inorganic electron donors such as Fe 2+ . Our simulations also highlighted key challenges with constraining model rate constants and parameters and the need for site specific calibrations. This work highlights the value of process‐based modeling in quantifying denitrification‐DNRA partitioning and the variable controls of electron donors and acceptors. Such simulations could be extended to riparian buffers to determine if they are effective management sinks for N mitigation.