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The Yangtze River, which is the largest in Euro-Asian, receives tremendous anthropogenic nitrogen input and is typically characterized by severe eutrophication and hypoxia. Two major processes, denitrification and anaerobic ammonium oxidation (anammox), play vital roles for removing nitrogen global in nitrogen cycling. In the current study, sediment samples were collected from both latitudinal and longitudinal transects along the coastal Yangtze River and the East China Sea (ECS). We investigated community composition and distributions of nosZ gene-encoded denitrifiers by high throughput sequencing, and also quantified the relative abundances of both denitrifying and anammox bacteria by q-PCR analysis. Denitrifying communities showed distinct spatial distribution patterns that were impacted by physical (water current and river runoffs) and chemical (nutrient availability and organic content) processes. Both denitrifying and anammox bacteria contributed to the nitrogen removal in Yangtze Estuary and the adjacent ECS, and these two processes shifted from coastal to open ocean with reverse trends: the abundance of nosZ gene decreased from coastal to open ocean while anammox exhibited an increasing trend based on quantifications of hzsB and 16S rRNA genes. Further correspondence correlation analysis revealed that salinity and nutrients were the main factors in structuring composition and distribution of denitrifying and anammox bacteria. This study improved our understanding of dynamic processes in nitrogen removal from estuarine to open ocean. We hypothesize that denitrification is the major nitrogen removal pathway in estuaries, but in open oceans, low nutrient and organic matter concentrations restrict denitrification, thus increasing the importance of anammox as a nitrogen removal process., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Seasonal shifts of dissolved inorganic nitrogen (DIN) and the dynamics of microbial communities for nitrogen transformation were investigated in the water column of Chesapeake Bay. The relative abundance of nitrogen over phosphorus (N) showed a strong seasonal and spatial pattern: gradually decreased from upstream to downstream; high in winter and low in summer. Because the phosphorus concentration remained relatively stable, the spatiotemporal pattern of N implied that a substantial fraction of DIN was removed in the bay, especially in summer. Correlation analyses indicated the functional microbial communities and environmental variables, such as temperature, dissolved oxygen, salinity, played important roles for connecting the seasonal variation of N. Among them, temperature was the trigger factor. High temperature in the summer induced the growth of functional microbes, which subsequently consumed a large portion of DIN inputted from the tributaries and reduced the N. The current study provided the relative importance of microbial communities and environmental variables in driving the DIN loss in the bay.

Groundwater nitrate‐N isotopes (δ15N‐NO3− ${{\text{NO}}_{3}}^{-}$) 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‐NO3− ${{\text{NO}}_{3}}^{-}$ for riparian zones affected by relict milldams and road salt salinization. Contrary to natural, undammed conditions, groundwater δ15N‐NO3− ${{\text{NO}}_{3}}^{-}$ values declined from the upland edge through the riparian zone and were lowest near the stream. Groundwater δ15N‐NO3− ${{\text{NO}}_{3}}^{-}$ values increased for low electron donor (dissolved organic carbon) to acceptor NO3− $\left({{\text{NO}}_{3}}^{-}\right)$ ratios but decreased beyond a change point in ratios. Groundwater δ15N‐NO3− ${{\text{NO}}_{3}}^{-}$ 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‐NO3− ${{\text{NO}}_{3}}^{-}$ can provide valuable insights into process mechanisms and can serve as "imprints" of anthropogenic activities and legacies. Plain Language Summary: Human activities and their legacies can alter our environment and the imprints can last for a long time. Here, we show the groundwater stable nitrogen isotopes can provide new and unexpected insights into nitrogen processing and cycling in riparian zones affected by dams and road salt salinization. Using such metrics/indicators we can investigate the changes in process mechanisms and the thresholds at which they change. This knowledge will help us better manage environments impacted by human landuse. Key Points: Groundwater N isotopes revealed altered riparian N processing due to milldams and salinizationGroundwater δ15N‐NO3− ${{\text{NO}}_{3}}^{-}$ values changed along the riparian transect and in response to electron donor to acceptor (DOC:NO3− ${{\text{NO}}_{3}}^{-}$) ratiosLow groundwater δ15N‐NO3− ${{\text{NO}}_{3}}^{-}$ values were attributed to suppression of denitrification due to persistent anoxic conditions and salinization [ABSTRACT FROM AUTHOR]

Stream, floodplain, and wetland restorations enhance water quality and ecological function; however, soil health is prioritized infrequently in restoration planning and monitoring. Buried, historic, hydric soils—common across U.S. mid‐Atlantic valley bottoms beneath legacy sediments—are not included in most floodplain restoration designs, though they may retain favorable biogeochemical characteristics and host legacy microbial communities that could support ecosystem recovery if exhumed and preserved. To assess the efficacy of including historic hydric soils in floodplain restoration for nitrogen (N) removal, we characterized pre‐Euro‐American settlement wetland soils buried below legacy sediments and now exposed along incised streambanks across the mid‐Atlantic. We compared carbon (C) and N contents; C:N ratios; nitrate‐N and ammonium‐N concentrations; denitrification rates; functional genes for denitrification (nosZ) and nitrification (amoA for ammonia oxidizing archaea [AoA] + ammonia oxidizing bacteria [AoB]); and phospholipid fatty acid biomasses of historic wetland soils with contemporary wetland soils before and after an 1‐year incubation in a recently restored floodplain. Compared to modern wetland soils, historic hydric soils buried by legacy sediment are less nutrient‐rich, have fewer functional genes for and lower rates of denitrification, and possess significantly less microbial biomass. Following the 1‐year incubation, many of these concentrations, rates, and gene counts increased in historic soils, though not substantially. Ultimately, our results suggest that while inclusion of historic, hydric soils and their legacy microbiomes is valuable for N‐removal in floodplain restoration, the recovery of historic, hydric soils is predictably slow, and attainment of restoration goals, such as increased denitrification, may require multiple years. [ABSTRACT FROM AUTHOR]

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−1 while 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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Dam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over 2 years. Soil N concentrations and process rates and δ15N values were also determined. Denitrification rates and soil δ15N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate‐N concentrations were high (median 4.5 mg N L−1), riparian groundwater nitrate‐N values were low (median 0.09 mg N L−1), indicating that nitrate‐N leakage was minimal. We attribute the low groundwater nitrate‐N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate‐N concentrations were high (median 7.6 mg N L−1) and contrary to our dam‐removal hypothesis displayed a watershed‐wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management. Plain Language Summary: Dams are being removed to allow fish passage and improve safety for water users. Dam removal results in a drop of stream water level and a drying‐out of the streamside (riparian) zones. We investigated if these changes would undermine the N‐filtering service of riparian zones and increase N concentrations in groundwater and stream waters. We monitored soil and water N concentrations for 2 years following the removal of a 1.5 m milldam on Chiques Creek in Pennsylvania. Our data showed that while denitrification in soils did decrease, the N concentrations in riparian groundwaters and stream waters did not increase over the study period. Key Points: The effect of dam removal on riparian zone nitrogen (N) cycle processes and groundwater N concentrations is unknownWe studied changes in riparian N cycle processes associated with the removal of a 1.5 m milldam for 2 yearsWhile soil denitrification and δ15N decreased following dam removal, an increase in groundwater nitrate‐N concentrations was not observed [ABSTRACT FROM AUTHOR]

While streambank legacy sediment has been increasingly recognized as an important contributor of fine sediment to watersheds, little is known about its contributions to suspended sediment‐bound nutrients. This uncertainty is exacerbated by the ubiquitous presence of legacy sediments in valley‐bottoms of the Mid‐Atlantic region of the United States. Using a sediment fingerprinting model, we quantified suspended sediment and sediment‐bound nutrient sources for a 114 km2 agricultural, Piedmont watershed. Sediment sources included agricultural, developed, and forested soils as well as streambanks legacy sediments. Suspended sediment and sediment‐bound nitrogen (N) and phosphorus (P) concentrations were determined for 10 storms over the study period of August 1, 2017 to July 31, 2018. Streambanks contributed 50% of the annual suspended sediment yield of 1,348 kg/ha followed by agricultural (49%) and forested (1%) soils. Streambanks amounted to 26% and 32% of the suspended sediment‐bound N and P exports, respectively, whereas the corresponding loads from agricultural sources were 73 and 67%, respectively. Bank contributions varied with storm magnitude and seasonality, with the highest exports associated with the largest peak flows. These results highlight that streambank sediments constitute a significant fraction of suspended sediment‐bound nutrient exports and must be considered in watershed models, regulatory assessments, and management practices. [ABSTRACT FROM AUTHOR]

Abstract: Large storm events possess significant erosive energy capable of mobilizing large amounts of sediment and particulate organic matter (POM) into fluvial systems. This study investigated how stream POM composition varied as a function of the watershed POM source, particle size, storm event magnitude, and seasonal timing. POM composition was characterized for multiple watershed sources and for stream POM following storms in a second‐order forested stream. Carbon (C) and nitrogen (N) amount, C:N ratio and isotopic content (13C and 15N) were determined for solid phase POM, whereas dissolved organic C, total N concentrations, and fluorescence characteristics were determined for solution/extracted POM. Key findings from this study were the following: (1) Composition of POM varied greatly with watershed sources with forest floor litter being C and N rich and labile, while stream banks and bed were C and N poor and recalcitrant. (2) Summer storms mobilized more carbon and nitrogen‐rich labile sources, while winter events mobilized more carbon‐ and nitrogen‐poor refractory material from near‐stream sources. (3) POM composition varied by size class, with the coarse POM showing more C and N rich and labile properties, while the fine POM displayed more degraded and refractory properties. If climate variability increases the magnitude and intensity of large storm events, our observations suggest that this will not only increase the inputs of POM to aquatic systems but also result in the delivery of coarser, C and N rich, and more bioavailable POM to the stream drainage network. [ABSTRACT FROM AUTHOR]

Large storms can erode, transport, and deposit substantial amounts of particulate nitrogen (PN) in the fluvial network. The fate of this input and its consequence for water quality is poorly understood. This study investigated the transformation and leaching of PN using a 56-day incubation experiment with five PN sources: forest floor humus, upland mineral A horizon, stream bank, storm deposits, and stream bed. Experiments were subjected to two moisture regimes: continuously moist and dry–wet cycles. Sediment and porewater samples were collected through the incubation and analyzed for N and C species, as well as the quantification of nitrifying and denitrifying genes (amoA, nirS, nirK). C- and N-rich watershed sources experienced decomposition, mineralization, and nitrification and released large amounts of dissolved N, but the amount of N released varied depending on the PN source and moisture regime. Drying and rewetting stimulated nitrification and suppressed denitrification in most PN sources. Storm deposits released large amounts of porewater N regardless of the moisture conditions, indicating that they could readily act as N sources under a variety of conditions. The inputs, processing, and leaching of large, storm-driven PN inputs become increasingly important as the frequency and intensity of large storms is predicted to increase with global climate change. [ABSTRACT FROM AUTHOR]

While stream restorations are increasingly being adopted to mitigate sediment and nutrient inputs and to meet water quality regulatory targets, less information is available on the drivers behind the design, implementation, effectiveness, and cost of restorations. We address these issues for a $4.2 million stream restoration for legacy sediments implemented for a rural Piedmont stream in Maryland, USA. A total of 1668 m of stream was restored in three phases, which included the partial removal of legacy sediments, the grading of streambanks, floodplain creation, channel reshaping with meanders and pool-riffle forms, the raising of the stream bed, and the planting of riparian vegetation. The sediment, nitrogen, and phosphorus concentrations and fluxes were monitored before- and during the restoration phases. The sites selected for restoration had legacy sediments vulnerable to erosion and were on state-owned land. The restoration design was based on the need to maintain mature riparian trees and preserve existing sensitive wetland habitats. Water quality monitoring indicated that the sediment and nutrient fluxes increased during the restoration phase and were attributed to disturbance associated with construction activities and increased runoff. We also recommend that soil health needs to be included as an integral component to enhance the effectiveness and resilience of stream restorations. [ABSTRACT FROM AUTHOR]