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The sediment–water interfaces (SWI) of streams serve as important biogeochemical hotspots in watersheds and contribute to whole-catchment reactive nitrogen budgets and water-quality conditions. Recently, the SWI has been identified as an important source of nitrous oxide (N2O) produced in streams, with SWI residence time among the principal controls on its production. Here, we conducted a series of controlled manipulations of SWI exchange in an urban stream that has high dissolved N2O concentrations and where we concurrently evaluated less-mobile porosity dynamics. Our experiments took place within isolated portions of two sediment types: a coarse sandy stream bed resulting from excess road-sand application in the watershed, and a coarse sand mixed with clay and organic particles. In these manipulation experiments we systematically varied SWI vertical-flux rates and residence times to evaluate their effect on the fate of nitrate and production rates of N2O. Our experiments demonstrate that the fate and transport of nitrate and N2O production are influenced by hydrologic flux rates through SWI sediments and associated residence times. Specifically, we show that manipulations of hydrologic flux systematically shifted the depth of the bulk oxic–anoxic interface in the sediments, and that nitrate removal increased with residence time. Our results also support the emerging hypothesis of a ‘Goldilocks’ timescale for the production of nitrous oxide, when transport and reaction timescales favor incomplete denitrification. Areal N2O production rates were up to threefold higher during an intermediate residence-time experiment, compared to shorter or longer residence times. In our companion study we documented that the studied sediments were dominated by a long-residence-time less-mobile porosity domain, which could explain why we observed N2O production even in bulk-oxic sediments. Overall, we have experimentally demonstrated that changes to SWI hydrologic residence times and SWI substrate associated with urbanization can change the biogeochemical function of the river corridor.

Manganese (Mn) plays a critical role in river-water quality because Mn-oxides serve as sorption sites for contaminant metals. The aim of this study is to understand the seasonal cycling of Mn in an alpine streambed that experiences large spring snowmelt events and the potential responses to changes in snowmelt timing and magnitude. To address this goal, annual variations in river-water/groundwater interaction and Mn(aq) transport were measured and modeled in the bed of East River, Colorado, USA. In observations and numerical models, oxygenated river water containing dissolved organic carbon (DOC) mixes with groundwater rich in Mn(aq) in the streambed. The mixing depth increases during spring snowmelt when river discharge increases, leading to a greater DOC supply to the hyporheic zone and net respiration of Mn-oxides, despite an enhanced supply of oxygen. As groundwater upwelling resumes during the subsequent baseflow period, Mn(aq)-rich groundwater mixes with oxygenated river water, resulting in net accumulation of Mn-oxides until the bed freezes in winter. To explore potential responses of Mn transport to different climate-induced hydrological regimes, three hydrograph scenarios were numerically modeled (historic, low-snow, and storm) for the Rocky Mountain region. In a warming climate, Mn(aq) export to the river decreases, and Mn(aq) oxidation is favored in the upper streambed sediments over more of the year. One important implication is that the streambed may have an increased sorption capacity for metals over more of the year, leading to potential changes in river-water quality.

River systems are important regulators of anthropogenic nitrogen flux between land and ocean. Nitrogen dynamics in small headwater streams have been extensively measured, whereas less is known about contributions of other components of stream networks to nitrogen removal, including larger streams or fluvial wetlands. Here, we quantified nitrate reaction rates in higher-order stream channels and in surface transient storage (STS) zones (sub-systems with greater water residence time than the main channel) of the Ipswich River watershed, a temperate basin characterized by suburban development. We characterized uptake in STS both within higher-order stream channels and in fluvial wetlands that remain connected to advective fluxes but not constrained within channels. We compare reaction rates in these systems to those previously measured in headwater streams in the same basin. We found that (1) nitrate reaction rates (as uptake velocity, υf) in higher-order streams (n = 2) differed from each other but were consistent with previous estimates from headwater streams, (2) nitrate reaction rates in STS zones within higher-order stream channels (n = 2) were higher than rates estimated at the whole-stream scale, (3) ambient nitrate reaction rates in fluvial wetland STS (n = 7) were high but comparable to headwater streams with low nitrate concentration, (4) nitrate reaction rates were higher in fluvial wetland STS compared to headwater stream channels at elevated nitrate concentration, and (5) efficiency loss (EL) similar to that found in headwater streams was also apparent in fluvial wetlands. These results indicate that STS are potential hotspots of biogeochemical activity and should be explicitly integrated into network scale biogeochemical models. Further, experimental evidence of EL in fluvial wetlands suggests that the effectiveness of STS to retain N may decline if N loading increases.