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The compounding effects of anthropogenic legacies for environmental pollution are significant, but not well understood. Here, we show that centennial‐scale legacies of milldams and decadal‐scale legacies of road salt salinization interact in unexpected ways to produce hot spots of nitrogen (N) in riparian zones. Riparian groundwater and stream water concentrations upstream of two mid‐Atlantic (Pennsylvania and Delaware) milldams, 2.4 and 4 m tall, were sampled over a 2 year period. Clay and silt‐rich legacy sediments with low hydraulic conductivity, stagnant and poorly mixed hydrologic conditions, and persistent hypoxia in riparian sediments upstream of milldams produced a unique biogeochemical gradient with nitrate removal via denitrification at the upland riparian edge and ammonium‐N accumulation in near‐stream sediments and groundwaters. Riparian groundwater ammonium‐N concentrations upstream of the milldams ranged from 0.006 to 30.6 mgN L−1while soil‐bound values were 0.11–456 mg kg−1. We attribute the elevated ammonium concentrations to ammonification with suppression of nitrification and/or dissimilatory nitrate reduction to ammonium (DNRA). Sodium inputs to riparian groundwater (25–1,504 mg L−1) from road salts may further enhance DNRA and ammonium production and displace sorbed soil ammonium‐N into groundwaters. This study suggests that legacies of milldams and road salts may undercut the N buffering capacity of riparian zones and need to be considered in riparian buffer assessments, watershed management plans, and dam removal decisions. Given the widespread existence of dams and other barriers and the ubiquitous use of road salt, the potential for this synergistic N pollution is significant. Human activities can combine to exacerbate environmental pollution. We studied the effects of milldams and road salt runoff on nitrogen (N) pollution in streamside/riparian soil and groundwaters in Pennsylvania (Chiques Creek) and Delaware (Christina River). While nitrate‐N concentrations in groundwaters and soils were low, ammonium‐N concentrations for both sites were unexpectedly high. We attributed the high groundwater ammonium concentrations to processes of ammonification and/or dissimilatory nitrate reduction to ammonium that occurred under stagnant and persistently reducing riparian groundwater conditions. Road salt runoff inputs from an interstate highway above the Christina River site likely exacerbated the groundwater ammonium concentrations because of sodium displacement of ammonium‐N from sediment surfaces into solution. We suggest that dam removals could enhance the natural variability in groundwater, induce nitrification‐denitrification removal of N, and thus mitigate N pollution in riparian zones. Greater consideration needs to be given to environmental impacts of human legacies in watershed management. The coupled effects of anthropogenic legacies for nitrogen dynamics are not well understoodAmmonium‐N may accumulate in riparian groundwater and sediments upstream of milldams due to stagnant, poorly mixed, and reducing conditionsRoad salt salinization may further enhance the concentrations of ammonium in riparian groundwaters The coupled effects of anthropogenic legacies for nitrogen dynamics are not well understood Ammonium‐N may accumulate in riparian groundwater and sediments upstream of milldams due to stagnant, poorly mixed, and reducing conditions Road salt salinization may further enhance the concentrations of ammonium in riparian groundwaters

Riparian zones are key ecotones that buffer aquatic ecosystems through removal of nitrogen (N) via processes such as denitrification. However, how dams alter riparian N cycling and buffering capacity is poorly understood. Here, we hypothesized that elevated groundwater and anoxia due to the backup of stream water above milldams may enhance denitrification. We assessed denitrification rates (using denitrification enzyme assays) and potential controlling factors in riparian sediments at various depths upstream and downstream of two relict U.S. mid‐Atlantic milldams. Denitrification was not significantly different between upstream and downstream, although was greater per river km upstream considering deeper and wider geometries. Further, denitrification typically occurred in hydrologically variable shallow sediments where nitrate‐N and organic matter were most concentrated. At depths below 1 m, both denitrification and nitrate‐N decreased while ammonium‐N concentrations substantially increased, indicating suppression of ammonium consumption or dissimilatory nitrate reduction to ammonium. These results suggest that denitrification occurs where dynamic groundwater levels result in higher rates of nitrification and mineralization, while another N process that produces ammonium‐N competes with denitrification for limited nitrate‐N at deeper, more stagnant/poorly mixed depths. Ultimately, while it is unclear whether relict milldams are sources of N, limited denitrification rates indicate that they are not always effective sinks; thus, milldam removal—especially accompanied by removal of ammonium‐N rich legacy sediments—may improve riparian N buffering. Floodplains adjacent to rivers are important ecosystems that provide valuable services including nutrient removal, especially nitrogen, from stream water. Because nitrogen is a major polluter of coastal waters, river floodplains are increasingly being restored as part of watershed best management practices. For example, millions of dollars are being spent annually in the Chesapeake Bay to install 900 miles of riparian buffers and on other watershed practices to mitigate nutrient pollution. However, the impact of small, colonial‐era milldams on floodplain nitrogen mitigation is poorly understood, despite >14,000 such structures still present across streams of the eastern United States. We studied the impact of two small milldams (Roller mill on Chiques Creek, Lancaster, Pennsylvania, and Cooch mill on Christiana River, Newark, Delaware) on the ability of floodplains to remove or store nitrogen. We found that the stagnant water that accumulates behind milldams restricts floodplains from effectively removing nitrogen and may actually cause the accumulation of nitrogen. Whether accumulated nitrogen is released back into streams is unknown but concerning. Removal of dams would likely improve many ecosystem services of both streams and floodplains, with minimal consequences for the nitrogen mitigation abilities of these ecosystems. Riparian denitrification rates are similar above and below milldams but deeper, wider upstream zones result in more nitrogen removalDenitrification rates peak in shallow sediments of riparian areas above milldams with higher hydrologic variabilityStagnant hydrologic conditions upstream of milldams promote nitrogen processes that result in ammonium accumulation at deep sediment depths Riparian denitrification rates are similar above and below milldams but deeper, wider upstream zones result in more nitrogen removal Denitrification rates peak in shallow sediments of riparian areas above milldams with higher hydrologic variability Stagnant hydrologic conditions upstream of milldams promote nitrogen processes that result in ammonium accumulation at deep sediment depths

High‐latitude, coastal wetland biogeochemistry is dynamic in response to climate change, and yet we do not understand, and thus cannot fully predict, how crucial aspects of these systems will change in the future. Temperatures in the Northern Hemisphere have disproportionately increased 4°C in 30 years causing the rate of deglaciation to increase significantly in global high‐latitude river deltas. This will have a prolonged effect on local microbiome metabolism and biodiversity of the subsurface, influencing solid, liquid, and gaseous compounds in the system. Using sediment geochemical analyses, autonomous sampling techniques, and 16S rRNA gene sequencing, we have identified key processes that occur in the Copper River Delta, AK, a model system to study high‐latitude watersheds during rapid climate change. We calculated carbon accumulation rates upwards of 520 ± 60 g C m−2yr−1in outwash pond sediments nearest to the glaciers, which co‐occurred with pronounced suboxic peaks in Fe (III) and Mn (II). Sediment microbial communities across the outwash ponds are structured on the basis of total iron and manganese concentrations, proximity to glaciers, and organic matter content. Additionally, we revealed no methane accumulation in the ponds during ice‐cover, despite high organic matter content. High‐latitude wetland ecosystems are not only influenced by the changing climate, but also have the potential to impact carbon cycling considering high carbon burial rates. These findings show the importance of understanding changing biogeochemical processes in high‐latitude wetlands, as they have the potential to influence carbon cycling. Climate change is reshaping how ecosystems function. Average temperatures are rising most rapidly near Earth's poles, causing these cold regions to lose snow and ice. Water from melting glaciers carries minerals to wetlands where the activities of naturally occurring microorganisms may be altered. Our study focused on describing microbial communities and the chemistry in ponds on the Copper River Delta in Alaska, a glacially influenced coastal wetland, along a gradient between the glaciers and ocean. We sampled water and sediments from wetland ponds over several years and identified that ponds near the glaciers bury carbon in sediments faster than other recorded coastal wetlands. Our findings suggest that iron minerals from the glaciers and the microbes in the system may be associated with this carbon burial and how greenhouse gases are produced and consumed in this watershed. Our findings advance the understanding of the role of microbial communities and the associated biogeochemical processes that store or release carbon in a dynamic, high‐latitude setting. Freshwater glacial outwash ponds in southcentral AK exhibit carbon accumulation rates up to 520 g C m−2yr−1Microbial communities in glacial outwash sediments are shaped by iron, manganese, organic matter, and sediment load from melting glaciersMethane did not buildup during ice cover despite high organic matter content and anoxic conditions Freshwater glacial outwash ponds in southcentral AK exhibit carbon accumulation rates up to 520 g C m−2yr−1 Microbial communities in glacial outwash sediments are shaped by iron, manganese, organic matter, and sediment load from melting glaciers Methane did not buildup during ice cover despite high organic matter content and anoxic conditions

Oregon estuaries provide important opportunities to assess controls on tidal saline wetland carbon burial and sediment accretion as both rates of relative sea level rise (RSLR; −1.4 ± 0.9 to 2.8 ± 0.8 mm yr−1) and fluvial suspended sediment load relative to estuary area (0.23 to 17 × 103t km−2yr−1) vary along the coast. We hypothesized that vertical accretion, measured using excess 210Pb in least‐disturbed wetlands within seven Oregon estuaries, would vary with either RSLR or sediment load relative to estuary area, and carbon burial would correlate strongly to sediment accretion. Mean rates of high marsh accretion (0.8 ± 0.2 to 4.1 ± 0.2 mm yr−1) indicate that Oregon tidal wetlands have mainly kept pace with twentieth‐century RSLR with the exception that the accretionary balance in the central coast is negative, suggesting drowning. Experiencing the fastest rates of RSLR, central‐coast estuaries may foreshadow the fates of other Oregon estuaries under future accelerated sea level rise. Comparison of mass accumulation rates with sediment loads, however, indicates low trapping efficiency and therefore no fluvial sediment limitation. Thus, nonlinear feedback between RSLR and sediment accretion may enhance wetland resistance to drowning. Among wetlands keeping pace with or exceeding RSLR, sediment accretion displays no significant relationship with elevation but rather appears controlled by both the rate of RSLR and relative sediment load, highlighting the importance of incorporating both factors into future studies of tidal saline wetlands. Carbon burial rates, controlled by sediment accretion, will likely increase with future accelerated sea level rise. Salt marshes and brackish scrub‐shrub and forested tidal wetlands, collectively known as tidal saline wetlands, provide valuable services such as habitat for wildlife, protection from coastal flooding, and carbon sequestration. Despite their importance, tidal saline wetlands are threatened by sea level rise and scientists must better understand what controls their growth so as to make predictions of their future survival. Additionally, sediment accumulation within tidal saline wetlands is an important control on the amount of carbon sequestered in coastal areas; thus, understanding how these systems will evolve in the coming decades may shed light on the global carbon cycle and climate change. New results from the Oregon coast indicate that both the local pace of sea level rise and the amount of sediment delivered by rivers control tidal saline wetland growth. Because sea level rise is relatively slow in Oregon and Pacific Northwest rivers deliver relatively high amounts of sediment to the coast, Oregon tidal saline wetlands may be resistant to future drowning. However, of the seven estuaries studied, two showed evidence of drowning over the last century; the reasons for this are unclear but may relate to the local pace of sea level rise or other processes not accounted for in this study. Oregon tidal saline wetlands kept pace with twentieth‐century sea level rise, except in the central coast, the reasons for which are unclearIn estuaries with a positive accretionary balance, both sea level rise and sediment load relative to estuary area control accretionBlue carbon burial rates, provided herein, are the first such measurements for Oregon tidal saline wetlands