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Abstract The seasonal controls of hydrology, temperature, hypoxia, and biogeochemical conditions for groundwater ammonium–N (NH₄⁺) concentrations are not well understood. Here we investigated these controls for riparian groundwaters located upstream of two milldams over a period of 4 years. Groundwater chemistry was sampled monthly while groundwater elevations, hydraulic gradients, and temperatures were recorded sub‐hourly. Distinct seasonal patterns for NH₄⁺were observed which differed among the wells. For wells that displayed a strong seasonal pattern, NH₄⁺concentrations increased through the summer and peaked in October–November. These elevated concentrations were attributed to ammonification, suppression of nitrification, and/or dissimilatory nitrate reduction to ammonium (DNRA). These processes were driven by high groundwater temperatures, low hydraulic gradients (or long residence times), hypoxic/anoxic groundwater conditions, and increased availability of dissolved organic carbon as an electron donor. In contrast, NH₄⁺concentrations decreased in the riparian groundwater from January to April during cool and wet conditions. A groundwater well with elevated total dissolved iron (TdFe) concentrations had elevated NH₄⁺concentrations but displayed a muted seasonal response. In addition to hydrologic controls, we attributed this response to additional NH₄⁺contribution from Fe‐driven autotrophic DNRA and/or ammonification linked to dissimilatory Fe reduction. Understanding the temporal patterns and factors controlling NH₄⁺in riparian groundwaters is important for making appropriate watershed management decisions and implementing appropriate best management practices. 

Abstract Groundwater nitrate‐N isotopes (δ¹⁵N‐) have been used to infer the effects of natural and anthropogenic change on N cycle processes in the environment. Here we report unexpected changes in groundwater δ¹⁵N‐ for riparian zones affected by relict milldams and road salt salinization. Contrary to natural, undammed conditions, groundwater δ¹⁵N‐ values declined from the upland edge through the riparian zone and were lowest near the stream. Groundwater δ¹⁵N‐ values increased for low electron donor (dissolved organic carbon) to acceptor ratios but decreased beyond a change point in ratios. Groundwater δ¹⁵N‐ values were particularly low for the riparian milldam site subjected to road‐salt salinization. We attributed these N isotopic trends to suppression of denitrification, occurrence of dissimilatory nitrate reduction to ammonium (DNRA), and/or effects of road salt salinization. Groundwater δ¹⁵N‐ can provide valuable insights into process mechanisms and can serve as “imprints” of anthropogenic activities and legacies. 

ABSTRACT Controls of hydrology, landscape and soil characteristics on redox potentials, particularly at fine spatial and temporal scales, are poorly quantified and understood. Using high‐frequency sensors (30 min), the variability of redox potentials (Eh) in riparian soils was investigated for multiple depths and across diel, event and seasonal time scales over a period of 2 years. Sampling was performed for two riparian sites upstream of dams with contrasting soil textures. Incremental changes in soil moisture triggered abrupt and disproportionately large shifts in soil Eh values, a threshold‐like behaviour that has not been reported before. Contrary to expectations, Eh values initially decreased as the soil dried and then abruptly increased with further drying and oxidation of soil. Diel Eh patterns differed between sites, with the sand‐rich site oxidising by day and reducing at night, while the clay‐ and silt‐rich site showed the opposite pattern. These differences were attributed to differential oxygen diffusion and consumption by processes such as photosynthesis, evapotranspiration and microbial respiration. Sharp Eh shifts also occurred at soil depths with textural discontinuities. These redox hot spots and hot moments were attributed to differences in site topography, hydrology and soil characteristics such as texture, moisture, organic matter content and microbial biomass. Identifying and understanding these fine‐scale spatial and temporal patterns of Eh is critical for improved characterisation and quantification of biogeochemical processes in redox‐sensitive environments. This study underscores the value of high‐frequency redox sensors in making such assessments. 

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], Fe²⁺) to acceptor (NO₃⁻). 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 using¹⁵N stable isotopes. Although DOC/NO₃⁻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/NO₃⁻ = 6; however, this shift was not observed within a range of DOC/NO₃⁻ = 30 at higher DOC concentration of 12 mgL⁻¹. Heterotrophic pathways dominated NO₃‐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 Fe²⁺. 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.