Nitrogen Retention, Removal, and Saturation in Lotic Ecosystems

Diel variation of hydrochemistry in streams in protected areas in northeastern Brazil

This research investigates the diel variations in nutrient concentrations in preserved streams located in the Cerrado and Atlantic Forest biomes of Bahia, Brazil. By examining both organic and inorganic forms of nitrogen and phosphorus, the study aims to understand how these nutrients fluctuate in response to environmental factors such as precipitation and landscape features. This study contributes to a deeper understanding of nutrient cycling in tropical stream ecosystems, providing insights that could aid in conservation strategies. Water samples were collected every hour using an automatic sampler over a 24-h period, covering both dry and rainy seasons to reflect seasonal variability. The abiotic variables pH, DO, conductivity, and temperature were measured during sampling, and subsequent nutrient analyses were conducted (including nitrate, nitrite, ammonium, and both dissolved and particulate organic nitrogen and phosphorus) along with Chlorophyll-a. This system-wide analysis provides a more comprehensive understanding, enabling us to effectively link nutrient dynamics with environmental conditions. Results indicate that abiotic variables were the only parameters showing diel variation, with higher values during the daytime. A comparison of parameters between Cerrado (M1) and Atlantic Forest (M2) revealed that almost all values were higher in the Atlantic Forest during the rainy season. Our findings suggest that vegetation cover, soil characteristics, and biogeochemical processes in soil and water were more influential in the variations between areas than diel fluctuations.

Study on the purification mechanism for ammonia nitrogen in micro-polluted rivers by herbaceous plant - Rumex japonicus Houtt

Water eutrophication caused by nitrogen pollution is an urgent global issue that requires attention. The Qingyi River is a typical micro-polluted river in China. In this study, we took this river as the research object to investigate the nitrogen pollution purification capacity of a herbaceous plant, Rumex japonicus Houtt. (RJH). Compared to nitrate nitrogen (NO3--N) and nitrite nitrogen (NO2--N), RJH showed better purification performance on total nitrogen (TN), total phosphorus (TP) and ammonia nitrogen (NH4+-N), with a highest removal rate of 37.22%, 52.13%, and 100%, respectively. RJH could completely remove ammonia nitrogen and exhibit excellent resistance to pollutant interference when the initial concentration of ammonia nitrogen in the cultivation devices increased from 1 mg/L to 10 mg/L or in the actual river. This indicated the great application potential of RJH in ammonia nitrogen removal from natural micro-polluted rivers. In addition, combined effects of nitrification of roots, absorption of self-growth, stripping, and others contributed to nitrogen removal by RJH. Particularly, the nitrification of roots played a dominant role, accounting for 73.85% ± 8.79%. High-throughput sequencing results indicate that nitrifying bacteria accounted for over 75% of all bacterial species in RJH. Furthermore, RJH showed good growth status and strong adaptability. The correlation coefficients of its relative growth rate with chlorophyll A and the degradation rate of absorption were 0.9677 and 0.9594, respectively. Our research demonstrates that RJH is one of the excellent varieties for ammonia removal. This provides a very promising and sustainable method for purifying micro-polluted rivers.

Spatiotemporal patterns and main driving factors of drainage water quality of an arid irrigation district with shallow groundwater table

Drainage water quality is a crucial factor reflecting the regime of agricultural non-point source pollution in irrigation districts and is closely related to land use, soil texture, cropping pattern, fertilization, and irrigation and drainage conditions. However, the response of drainage water quality to various natural and anthropogenic factors needs further exploration in irrigation districts affected by shallow groundwater table. Spatiotemporal patterns of chemical oxygen demand (COD), total phosphorus (TP), total nitrogen (TN), and ammonium nitrogen (NH4-N) were monitored and analyzed in ten agricultural drainage ditches in the arid region of China from 2011 to 2019. Spatially, water pollution in agricultural drainage ditches with small water quantity can be significantly exacerbated by urban sewage, whereas a large amount of agricultural drainage can effectively dilute the pollution of urban sewage. Severe soil salinization in the cropland increases the risk of water pollution due to easier losses of soil nutrient and organic matter. Soil salinization is a key factor in the crop distribution pattern based on the crop salt tolerance, and the maize/wheat field with a higher fertilizer application rate generally results in poorer drainage water quality. Temporally, for the agricultural drainage ditches, the monthly and annual COD, TP, TN, and NH4-N concentrations fluctuate inversely with drainage water quantity and are positively correlated with fertilizer application, among which the monthly COD concentration in drainage water has larger variation in severe salinized areas. There exist critical annual and monthly drainage amounts, above which the probabilities of higher concentrations of COD, TP, TN, and NH4-N reduce greatly.

Processes of nitrogen removal from rainwater runoff in bioretention filters modified with ceramsite and activated carbon

Conventional bioretention filters lack satisfactory performance in nitrogen removal. In this study, we used a mixture of cultivated soil and river sand as the bioretention filter to remove nitrogen pollutants from simulated rainwater runoff. To improve its permeability and nitrogen removal performance, both activated carbon and ceramsite were used as additives. The nitrogen removal processes and its mass accumulation in the modified bioretention filters were studied. The contribution of adsorption and biotransformation processes, together with the effects of percolate rate on nitrogen removal performance was explored. The results showed that an activated carbon layer in the bioretention filters could obviously improve nitrogen removal efficiencies, but its location made no significant difference in nitrogen removal performance. Bioretention filters modified with 20% of ceramsite could achieve the optimal percolate rate and nitrogen removal efficiencies. At given conditions, the average removal efficiencies of ammonium nitrogen (NH3-N), nitrate-nitrogen (NO3-N), and total nitrogen (TN) by the modified bioretention filter reached 80.27%, 41.48%, and 59.45%, respectively. During the leaching processes, organic nitrogen originated in the filter materials can be mineralised into NH3-N, then be denitrified and completely removed in the anaerobic environment under flooding conditions. Biotransformation in the modified bioretention filters caused a reduction of NH3-N removal efficiency by 15.41% and an increase of NO3-N removal efficiency by 31.03%. The modified bioretention filter can withstand a long-term operation. Compared with NO3-N and TN, the pollutant of NH3-N in rainwater runoff is not easy to form a mass accumulation in the modified bioretention filter.Highlights The modified bioretention filter showed high percolation rate and nitrogen removal.Hydraulic residence time is a critical design parameter to achieve nitrogen removal.NH3-N is not easy to form a mass accumulation in the filler media as NO3-N.Biodegradation increased NO3-N removal efficiency by 31.03% at given conditions.

Suburban agriculture increased N levels but decreased indirect N2O emissions in an agricultural-urban gradient river

The effects of land use on riverine N₂O emissions are not well understood, especially in suburban zones between urban and rural with distinct anthropogenic perturbations. Here, we investigated in situ riverine N₂O emissions among suburban, urban, and rural sections of a typical agricultural-urban gradient river, the Qinhuai River of Southeastern China from June 2010 to September 2012. Our results showed that suburban agriculture greatly increased riverine N concentration compared to traditional agricultural rivers (TAR). The mean total dissolved nitrogen (TDN) concentration was 8.18 mg N L^-1 in the suburban agricultural rivers (SUAR), which was almost the same as that in the urban rivers (UR, of 8.50 mg N L^-1), compared to that in TAR (0.92 mg N L^-1). However, the annual average indirect N₂O flux from the SUAR was only 27.15 μg N₂O-N m^-2 h^-1, which was slightly higher than that from the TAR (13.14 μg N₂O-N m^-2 h^-1) but much lower than that from the UR (131.10 μg N₂O-N m^-2 h^-1). Moreover, the average N₂O emission factor (EF_5r, N₂O-N/DIN-N) in the SUAR (0.0002) was significantly lower than those in the TAR (0.0028) and UR (0.0004). The limited indirect N₂O fluxes from the SUAR are best explained by the high riverine dissolved organic carbon (DOC) and low dissolved oxygen, which probably reduced the denitrification source N₂O by favoring complete denitrification to produce N₂ and inhibited the nitrification source N₂O, respectively. An exponential decrease model incorporating dissolved inorganic nitrogen and DOC could greatly improve our EF_5r predictions in the agricultural-urban gradient river. Given the unprecedented suburban agriculture in the world, more studies in suburban agricultural rivers are needed to further refine the EF_5r and better reveal the mechanisms behind indirect N₂O emissions as influenced by suburban agriculture.

Wastewater treatment plant effluent inputs influence the temporal variability of nutrient uptake in an intermittent stream

Wastewater treatment plant (WWTP) effluents alter water chemistry and in-stream nutrient uptake rates of receiving freshwaters, thus changing the magnitude and fate of the nutrients exported. In Mediterranean regions, the dilution capacity of receiving streams can vary strongly over time due to the seasonal occurrence of floods and droughts, causing temporal variability of nutrient uptake. We assessed the temporal patterns and the controlling factors of net nutrient uptake in an intermittent Mediterranean stream receiving WWTP effluent inputs. We compiled the longitudinal concentration profiles of ambient dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) along a 800 m reach on 47 sampling dates between 2001 and 2017, encompassing a wide range of hydrological conditions. We estimated net nutrient uptake in the receiving stream. In 72% of the dates, high rates of net ammonium uptake co-occurred with net releases of either nitrate or nitrite. This pattern suggests that the receiving stream has a high nitrification capacity. Conversely, 75% of the dates did not show any longitudinal pattern in SRP concentration, suggesting that uptake and release processes for this element were either counterbalanced or both occurred at very low rates. Finally, net ammonium uptake was low when the stream had a low dilution capacity (< 40%) and ammonium concentration was high. Overall, we demonstrate that consideration of the receiving stream’s dilution capacity is imperative to the management of freshwaters to guarantee an adequate dilution of WWTP effluent inputs and avoid saturation of in-stream nutrient uptake capacity under low flow conditions in urban landscapes.

Classification of Geomorphic Units and Their Relevance for Nutrient Retention or Export of a Large Lowland Padma River, Bangladesh: A NDVI Based Approach

Geomorphic classification of large rivers identifies morphological patterns, as a foundation for estimating biogeochemical and ecological processes. In order to support the modelling of in-channel nutrient retention or export, the classification of geomorphic units (GUs) was done in the Padma River, Bangladesh, a large and geomorphically-complex lowland river. GUs were classified using the normalized difference vegetation index (NDVI) four times over a year, so as to cover the seasonal variation of water flows. GUs were categorized as primary and secondary channels (C & S); longitudinal bar (L); transverse bar (T); side bar (SB); unvegetated bank (EK); dry channel (ED); island (VI); and water depression (WD). All types of GUs were observed over the four distinct annual seasons, except ED, which was absent during the high flow, monsoon season. Seasonal variation of the surface area of GUs and discharge showed an inverse relation between discharge and exposed surface areas of VI, L, T, and SB. Nutrients mainly enter the river system through water and sediments, and during monsoon, the maximum portion of emergent GUs were submerged. Based on the assumption that nutrient retention is enhanced in the seasonally inundated portions of GUs, nutrient retention-/export-relevant geomorphic units (NREGUs) were identified. Seasonal variation in the area of NREGUs was similar to that of GUs. The mean NDVI values of the main identified NREGUs were different. The variation of NDVI values among seasons in these NREGUs resulted from changes of vegetation cover and type. The variation also occurred due to alteration of the surface area of GUs in different seasons. The changes of vegetation cover indicated by NDVI values across seasons are likely important drivers for biogeochemical and ecological processes.

Context is Everything: Interacting Inputs and Landscape Characteristics Control Stream Nitrogen

To understand the environmental and anthropogenic drivers of stream nitrogen (N) concentrations across the conterminous US, we combined summer low-flow data from 4997 streams with watershed information across three survey periods (2000-2014) of the US EPA's National Rivers and Streams Assessment. Watershed N inputs explained 51% of the variation in log-transformed stream total N (TN) concentrations. Both N source and input rates influenced stream NO3/TN ratios and N concentrations. Streams dominated by oxidized N forms (NO3/TN ratio > 0.50) were more strongly responsive to the N input rate compared to streams dominated by other N forms. NO3 proportional contribution increased with N inputs, supporting N saturation-enhanced NO3 export to aquatic ecosystems. By combining information about N inputs with climatic and landscape factors, random forest models of stream N concentrations explained 70, 58, and 60% of the spatial variation in stream concentrations of TN, dissolved inorganic N, and total organic N, respectively. The strength and direction of relationships between watershed drivers and stream N concentrations and forms varied with N input intensity. Model results for high N input watersheds not only indicated potential contributions from contaminated groundwater to high stream N concentrations but also the mitigating role of wetlands.

Sediment Nutrient Flux Rates in a Shallow, Turbid Lake Are More Dependent on Water Quality Than Lake Depth

The bottom sediments of shallow lakes are an important nutrient sink; however, turbidity may alter the influence of water depth on sediment nutrient uptake by reducing light and associated oxic processes, or altering nutrient availability. This study assessed the relative influence of water quality vs. water depth on sediment nutrient uptake rates in a shallow agricultural lake during spring, when sediment and nutrient loading are highest. Nitrate and soluble reactive phosphorus (SRP) flux rates were measured from sediment cores collected across a depth and spatial gradient, and correlated to water quality. Overlying water depth and distance to shore did not influence rates. Both nitrate and SRP sediment uptake rates increased with greater Secchi depth and higher water temperature, and nitrate and SRP rates increased with lower water total N and total P, respectively. The importance of water temperature on N and P cycling was confirmed in an additional experiment; however, different patterns of nitrate reduction and denitrification suggest that alternative N2 production pathways may be important. These results suggest that water quality and temperature can be key drivers of sediment nutrient flux in a shallow, eutrophic, turbid lake, and water depth manipulation may be less important for maximizing spring runoff nutrient retention than altering water quality entering the lake.

Fate of bioavailable nutrients released to a stream during episodic effluent releases from a municipal wastewater treatment lagoon

Municipal wastewater lagoons are common across North America and, unlike larger mechanical wastewater treatment plants, typically release nutrient-rich effluent directly to rivers in intermittent pulses. However, little is known about the fate of nutrients from these episodic events, which may happen under varying hydrologic or thermal conditions. We assessed fate of nitrogen (N) and phosphorus (P) from lagoon effluent during three releases to Deadhorse Creek, Manitoba, Canada. Using net nutrient uptake lengths and natural abundance stable isotope ratios of dissolved inorganic nitrogen (DIN) and primary producers, we found that DIN was processed during the summer releases though the dominant mechanism was unclear. However, nitrate was largely exported in autumn. Primary producers assimilated lagoon N but did not appear to reduce DIN concentrations. The longitudinal pattern of soluble reactive phosphorus (SRP) varied between releases and in summer 2019 the stream became a net source of SRP despite concomitant processing of DIN. We hypothesize that low demand for P in Deadhorse Creek, as suggested by upstream SRP > 0.05 mg P L-1, and nutrient ratios indicative of N limitation, reduced instream processing of P. Furthermore, our results indicated that cool or high flow conditions may result in the export of much of the lagoon nutrient load downstream. Our findings suggest the processes that transform wastewater nutrients are overwhelmed during effluent releases. Managers should consider increasing effluent dilution via continuous release of effluent rather than pulsed delivery. However, management of upstream nutrient supply may also be needed when relying upon the self-purifying capacity of rivers.

Nutrient and sediment fluxes in microbasins with different conservation states in the northeastern Brazil

The implications of land use change in small watersheds through the conversion of forests to agropastoral areas have altered the natural nutrient cycle, intensifying exports under freshwater ecosystems. This study aimed to investigate the land use effects on nutrient and sediment exports in two small watersheds in northeastern Brazil to understand if anthropogenic disturbance alters the structure end functioning of these systems. Thus, land use mapping and hydrological treatment of a digital elevation model were made to characterize the basins. Water samples were collected monthly from Aug. 2016 to Jan. 2017 to evaluate suspended sediments and dissolved nutrient fluxes ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and dissolved organic nitrogen and phosphorus). The results indicated that land use change had a greater influence on exports from the most disturbed basin, where the nutrient and sediment increments were respectively an average 6.61 and 5.81 times higher than the most preserved basin. Thus, the conservation status of the forest cover has influenced the assimilation capacity of diffuse loads, highlighting the differences between the microbasins of this study.

Reactive Silica Traces Manure Spreading in Alluvial Aquifers Affected by Nitrate Contamination: A Case Study in a High Plain of Northern Italy

In the northern sector of the Po River Plain (Italy), widespread intensive agriculture and animal farming are supported by large amounts of water from Alpine lakes and their emissaries. Flood irrigation and excess fertilization with manure affect both the hydrology and the chemical quality of surface and groundwater, resulting in diffuse nitrogen pollution. However, studies analyzing the mechanisms linking agricultural practices with vertical and horizontal nitrogen paths are scarce in this area. We investigated groundwater quality and quantity in an unconfined, coarse-grained alluvial aquifer adjacent to the Mincio River (a tributary of the Po River), where steep summer gradients of nitrate (NO3−) concentrations are reported. The effects of manure on solutes’ vertical transport during precipitation events in fertilized and in control soils were simulated under laboratory conditions. The results show high SiO2 and NO3− leaching in fertilized soils. Similarly, field data are characterized by high SiO2 and NO3− concentrations, with a comparable spatial distribution but a different temporal evolution, suggesting their common origin but different processes affecting their concentrations in the study area. Our results show that SiO2 can be used as a conservative tracer of manure spreading, as it does not undergo biogeochemical processes that significantly alter its concentrations. On the contrary, nitrate displays large short-term variations related to aquifer recharge (i.e., flood irrigation and precipitation). In fact, aquifer recharge may promote immediate solubilization and stimulate nitrification, resulting in high NO3− concentrations up to 95.9 mg/L, exceeding the Water Framework Directive (WFD) thresholds. When recharge ends, anoxic conditions likely establish in the saturated zone, favoring denitrification and resulting in a steep decrease in NO3− concentrations.

Power law scaling model predicts N2O emissions along the Upper Mississippi River basin

Nitrous oxide (N₂O) is widely recognized as one of the most important greenhouse gases, and responsible for stratospheric ozone destruction. A significant fraction of N₂O emissions to the atmosphere is from rivers. Reliable catchment-scale estimates of these emissions require both high-resolution field data and suitable models able to capture the main processes controlling nitrogen transformation within surface and subsurface riverine environments. Thus, this investigation tests and validates a recently proposed parsimonious and effective model to predict riverine N₂O fluxes with measurements taken along the main stem of the Upper Mississippi River (UMR). The model parameterizes N₂O emissions by means of two denitrification Damköhler numbers; one accounting for processes occurring within the hyporheic and benthic zones, and the other one within the water column, as a function of river size. Its performance was assessed with several statistical quantitative indexes such as: Absolute Error (AE), Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR). Comparison of predicted N₂O gradients between water and air (ΔN₂O) with those quantified from field measurements validates the predictive performance of the model and allow extending previous findings to large river networks including highly regulated rivers with cascade reservoirs and locks. Results show the major role played by the water column processes in contributing to N₂O emissions in large rivers. Consequently, N₂O productions along the UMR, characterized by regulated flows and large channel size, occur chiefly within this surficial riverine compartment, where the suspended particles may create anoxic microsites, which favor denitrification.

Effects of macrophytes on ecosystem metabolism and net nutrient uptake in a groundwater fed lowland river

Transport and transformation of inorganic nutrients are influenced by abiotic-biotic interactions and determine downstream water quality. Macrophytes play an important role in these complex ecological interactions. The role of macrophytes was studied in three reaches of the groundwater-fed, oligotrophic River Fischa with different macrophyte coverage and biomass. This was done by measuring metabolism and calculating changes in nutrient loading and concentrations, which were determined via an upstream-downstream mass balance approach. As the dominant autotrophs, we expected macrophytes (i) to have a direct effect by uptake and release, and (ii) an indirect effect by slowing down flow, which results in changed sedimentation patterns and altered conditions for heterotrophic microbial organisms implicating higher turnover and uptake rates. The seasonal development of macrophytes in 2017 had a strong impact on gross primary production, but not on ecosystem respiration. Increase in macrophyte biomass led to higher GPP (max. 5.4 g O₂m^-2d^-1). ER was highest in autumn in the reach with intermediate macrophyte biomass (max. 10.1 g O₂m^-2d^-1). We observed that the autotrophic uptake of phosphorus accounted for 80-145% of the P-PO₄-flux and concluded that P-uptake by macrophytes from the sediment is an important source of phosphate for macrophytes in the river. By accumulating fine sediment, macrophytes are improving the availability of phosphate for their own long-term development. N-NO₃, represented >99% of the nitrogen flux. N-NO₃ net uptake was higher in the reaches with more macrophytes (0.84 vs. 0.12 g m^-2d^-1), but in average only 21% of the net uptake could be related to autotrophic nitrogen uptake in the reach with high macrophyte biomass. Dissimilatory uptake by heterotrophic organisms, most probably denitrification, were of high relevance. Macrophytes supported microbial uptake and release by improving conditions and slowing down flow. In the River Fischa, an oligotrophic river with low variability of environmental parameters, macrophytes greatly affected nutrient uptake by direct and indirect pathways.

Bulk sedimentary phosphorus in relation to organic carbon, sediment textural properties and hydrodynamics in the northern Beibu Gulf, South China Sea

Bulk sedimentary phosphorus (P) is studied to evaluate its source, distribution, preservation and enrichment in relation with organic carbon (OC), sediment textures and moisture contents in the northern Beibu Gulf. Approximately 80% of surface sediments in the investigated sites were composed of coarse sandy texture (>63 μm). Total P (TP), inorganic P (IP) and organic P (OP) contents were lower to medium range compared to the levels reported for other marginal seas. Sedimentary OC and P were derived from mixed sources, with high terrestrial influence in the coastal areas (molar OC/OP ratios >250:1). The distribution of P corroborated with the variation tendency of fine-grained sediments, moisture contents and OC. Both IP and OP may significantly influence the trophic state of seawater if released from surface sediments. Influenced by hydrodynamics, frequent resuspension and high abundance of sand, TP is less preserved, and shows low to moderate enrichment in surface sediments.

Cross-scale controls on the in-stream dynamics of nitrate and turbidity in semiarid agricultural waterway networks

Stream and riparian zone networks embedded in agricultural landscapes provide a potential intervention point to ameliorate the negative effects of agricultural runoff by reducing transport of nitrate (NO₃^-) and suspended sediments (SS) downstream. However, our ability to support and promote NO₃^- and SS attenuation is limited by our understanding of vegetative and hydrogeomorphic controls in realistic management contexts. In addition, agricultural landscapes are heterogenous on multiple management scales, from farm field to regional water management scales, and the effect of these heterogeneities and how they interact across scales to affect vegetative and hydrogeomorphic controls is poorly explored in many settings. This is especially true in irrigated agricultural settings, where stream and riparian networks are entwined with and sensitive to water management systems. To fill these gaps, we related the vegetative and hydrogeomorphic features of 67 waterway reaches across two water management districts in the California Central Valley to reach-scale NO₃^- and turbidity attenuation and district-scale water quality patterns. We found that in-stream NO₃^- attenuation was rare, but, when it did occur, it was promoted by shallow and wide riparian banks, low flows, and high channel-edge denitrification potential. Nitrate concentrations were consistently higher in upstream reaches compared to water district outlets, suggesting that while exports from the district were low, agricultural runoff may impair within-district water resources. Turbidity attenuation was highly variable and unrelated to vegetative or hydrogeomorphic features, suggesting that onfield controls are crucial to managing suspended sediments. We conclude that waterway networks have the potential to mitigate the effects of agricultural NO₃^- runoff in this setting, but that more effective monitoring and adoption of NO₃^- attenuating features is needed. Using our findings, we make specific management and monitoring recommendations at both reach and water district scales.

Relative influence of watershed and geomorphic features on nutrient and carbon fluxes in a pristine and moderately urbanized stream

Streams are important sites of elemental transformations due to the relatively high contact rates between flowing water and biogeochemically reactive sediments. Increased urbanization typically results in higher nutrient and carbon (C) inputs to streams from their watersheds and increased flow rates due to modification in channel form, reducing within stream net retention and increasing downstream exports. However, less is known on how moderate urbanization might influence the joint processing of C, nitrogen (N), and phosphorus (P) in streams or the relative influence of changes in watershed and stream features on their fluxes. In this study, we performed mass-balances of different C, N, and P species in multiple reaches with contrasting land use land cover and geomorphic features (pools, riffles, runs) to determine the effects of geomorphology versus human influence on elemental fluxes in a pristine and a semi-urban stream. N was the most responsive of all elements, where nitrate concentrations were 3.5-fold higher in the peri-urban stream. Dissolved organic carbon was only slightly higher in the peri-urban site whereas total P not significantly different between streams. In terms of fluxes, nitrate behaved differently between the streams with net retention occurring in the majority of the reaches of the pristine site, whereas net export was observed in all of the reaches of the semi-urban one. We found a decrease in nitrate concentrations with an increase in excess deuterium of the water (d-excess), an indicator of how overall water retention capacity of the watershed favored N loss. Within the stream, the presence of pools, and reduced channel slope, which also increase water retention time, again favored N loss. Overall, nitrate was the most sensitive nutrient to slight urbanization, where higher export to stream was influenced by land use, but where geomorphic features were more important in driving retention capacity.

Evaluating transient storage and associated nutrient retention in a nutrient-rich headwater stream: a case study in Lake Chaohu Basin, China

Transient storage has been studied intensively in small streams, but some processes and mechanisms are not yet entirely understood regarding this issue, especially in chronically nutrient-enriched streams. The exploration of transient storage dynamics in nutrient-rich headwater streams has great significance for stream nutrient management in China and other developing countries, which are suffering from eutrophication. In the present study, we conducted five instantaneous slug additions composed of a conservative tracer dissolved with two nonconservative nutrients injections in a suburban small stream (Guanzhen Creek), Lake Chaohu Basin, China. Transient storage metrics were estimated using the model-fitted hydrologic parameters from the one-dimensional transport with inflow and storage (OTIS) model. Regression analyses were performed to examine the relationship between hydraulic parameters and transient storage metrics. Moreover, nitrogen and phosphorus retention efficiency was qualitatively evaluated based on the OTIS model-fitted nutrient parameters. Our results showed that the OTIS model-fitted hydrologic parameters in Guanzhen Creek were within the range of previously published literature. The transient storage metrics of Guanzhen Creek were generally comparable to those in streams with low-to-moderate nutrient levels in other catchments. Moreover, most of the transient storage metrics showed a strong relationship with stream discharge, while only hydrological retention factor showed a markedly negative correlation with flow rate. Given the negative uptake rates for NH₄-N and SRP in half cases, we reasonably concluded that Guanzhen Creek was hardly incapable of retaining nitrogen and phosphorus.

Nitrate availability affects denitrification in Phragmites australis sediments

Understanding relationships between an increase in nitrate (NO₃ ^- ) loading and the corresponding effects of wetland vegetation on denitrification is essential to designing, restoring, and managing wetlands and canals to maximize their effectiveness as buffers against eutrophication. Although Phragmites australis (Cav.) Trin. ex Steud. is frequently used to remediate nitrogen (N) pollution, no information is available on how NO₃ ^- concentration may affect plant-mediated denitrification. In the present study, denitrification was measured in outdoor vegetated and unvegetated mesocosms incubated in both summer and winter. After spiking the mesocosms with NO₃ ^- concentrations typical of agricultural drainage water (0.7-11.2 mg N L^-1 ), denitrification was quantified by the simultaneous measurement of NO₃ ^- consumption and dinitrogen gas (N₂ ) production. Although denitrification rates varied with vegetation presence and season, NO₃ ^- availability exerted a significant positive effect on the process. Vegetated sediments were more efficient than bare sediments in adapting their mitigation potential to an increase in NO₃ ^- , by yielding a one-order-of-magnitude increase in NO₃ ^- removal rates, under both summer (743-6007 mg N m^-2 d^-1 ) and winter (43-302 mg N m^-2 d^-1 ) conditions along the NO₃ ^- gradient. Denitrification was the dominant sink for water NO₃ ^- in winter and only for vegetated sediments in summer. Nitrification likely contributed to fuel denitrification in summer unvegetated sediments. Since denitrification rates followed Michaelis-Menten kinetics, P. australis-mediated depuration may be considered optimal up to 5.0 mg N L^-1 . The present outcomes provide experimentally supported evidence that restoration with P. australis can work as a cost-effective means of improving water quality in agricultural watersheds.

A Mass Balance of Nitrogen in a Large Lowland River (Elbe, Germany)

Nitrogen (N) delivered by rivers causes severe eutrophication in many coastal waters, and its turnover and retention are therefore of major interest. We set up a mass balance along a 582 km river section of a large, N-rich lowland river to quantify N retention along this river segment and to identify the underlying processes. Our assessments are based on four Lagrangian sampling campaigns performed between 2011 and 2013. Water quality data served as a basis for calculations of N retention, while chlorophyll-a and zooplankton counts were used to quantify the respective primary and secondary transformations of dissolved inorganic N into biomass. The mass balance revealed an average N retention of 17 mg N m−2 h−1 for both nitrate N (NO3–N) and total N (TN). Stoichiometric estimates of the assimilative N uptake revealed that, although NO3–N retention was associated with high phytoplankton assimilation, only a maximum of 53% of NO3–N retention could be attributed to net algal assimilation. The high TN retention rates in turn were most probably caused by a combination of seston deposition and denitrification. The studied river segment acts as a TN sink by retaining almost 30% of the TN inputs, which shows that large rivers can contribute considerably to N retention during downstream transport.

Limited nitrogen retention in an urban river receiving raw sewage and wastewater treatment plant effluent

Excessive dissolved inorganic nitrogen (DIN) added to urban river systems by point-source (PS) inputs, including raw sewage and wastewater treatment plant (WWTP) effluent, constitutes a water-quality problem of growing concern worldwide. However, the quantification of their impacts on DIN retention capacity and pathways in receiving water still remains partial. In this study, a spatially intensive water quality monitoring campaign was conducted to support the application of a water quality model to a PS-impacted urban river in Hefei City, China. The DIN retention capacities and pathway of a reference upstream Reach A, a raw-sewage-impacted Reach B and a WWTP-effluent-dominated Reach C were quantified using the model results after a Bayesian approach for parameter estimation and uncertainty analysis. The results showed that the raw sewage discharge elevated the assimilatory uptake rate but lowered its efficiency in Reach B, while the WWTP effluent discharge elevated both the denitrification rate and efficiency and made Reach C a denitrification hotspot with an increased nitrate concentration and hypoxic environment. The effects of the PS inputs on the DIN retention pathways (assimilatory uptake vs. denitrification) were regulated by their impacts on river metabolism. Despite different pathways, the total DIN retention ratios of Reaches A, B and C under low-flow conditions were 30.3% km-1, 14.3% km-1 and 6.5% km-1, respectively, which indicated that the instream DIN retention capacities were significantly impaired by the PS inputs. This result suggests that the DIN discharged from PS inputs to urban rivers will be transported with the potential to create long-term ecological implications not only locally but also more distant downstream.

Nutrient attenuation dynamics in effluent dominated watercourses

In-stream attenuation of dissolved and particulate forms of carbon, nitrogen and phosphorus are a crucial ecosystem service, especially in watercourses downstream of chemical pollution point-sources (i.e. wastewater treatment plants). Most chemical-fate models assume that attenuation is directly proportional to the concentration of available dissolved organic carbon, and inorganic nitrogen and phosphorus compounds in watercourses, but there are multiple evidences of saturation and even inhibition of attenuation at higher concentrations. Our current comprehension of nutrient attenuation kinetics in streams remains a limiting factor for the development and calibration of predictive models of the chemical fate of these compounds in rivers, thus hindering the development and implementation of more effective regulatory strategies. Here, we assessed the in-stream attenuation of dissolved organic carbon, inorganic nitrogen (NH₄⁺, NO₂^-, NO₃^-) and phosphorus (PO₄^3-) compounds at increasing concentrations of these compounds, and analyzed the interaction between attenuation kinetics and biofilm structure and function. Specifically, the net balances of these compounds were assessed in artificial streams exposed to eight treatments following the gradient of WWTP contribution to the river flow (0, 14, 29, 43, 58, 72, 86, and 100% of WWTP effluent water). Results indicate that biological in-stream attenuation by a given biofilm of an effluent dominated watercourse might be saturated if exposed for short periods to high nutrient concentrations such as during combined sewer overflow events, but that communities can adapt if exposed long enough to high concentrations, therefore avoiding or at least minimizing saturation. More attention should be therefore given to the management of effluent-dominated watercourses, as reductions in the temporal variability of the discharged wastewater by WWTP might enhance attenuation and thus reduce water quality issues downstream.

Leachates from Helophyte Leaf-Litter Enhance Nitrogen Removal from Wastewater Treatment Plant Effluents

Bioengineering techniques are currently used  in a wide variety of wastewater treatment systems. Aquatic plants (i.e., helophytes) used in these techniques reduce excess nitrogen (N) from water column via assimilation. Moreover, leachates from plant leaf-litter can serve as an additional source of labile dissolved organic matter (DOM), which can promote aerobic respiration and N removal via denitrification. We tested the influence of leaf-litter leachates from  Iris pseudacorus and Phragmites australis on the structure and activity of freshwater biofilms grown in flumes fed by effluent from a wastewater treatment plant (WWTP). The responses of the epilithic biofilm to the inputs of leaf-litter leachates were compared to those measured using a brewery byproduct rich in sugars and to the WWTP effluent water (i.e., control). All DOM sources significantly enhanced aerobic respiration and denitrification of the biofilm when compared to the controls, with increases in total microbial abundance but not in denitrifier abundance. The results suggest that metabolic activity of biofilms may be limited by bioavailability of DOM in WWTP effluent; and leaf-litter leachates of helophytes used in bioengineering techniques could alleviate this limitation by enhancing microbial N and C uptake.

Influence of urban river restoration on nitrogen dynamics at the sediment-water interface

River restoration projects focused on altering flow regimes through use of in-channel structures can facilitate ecosystem services, such as promoting nitrogen (N) storage to reduce eutrophication. In this study we use small flux chambers to examine ammonium (NH₄⁺) and nitrate (NO₃^-) cycling across the sediment-water interface. Paired restored and unrestored study sites in 5 urban tributaries of the River Thames in Greater London were used to examine N dynamics following physical disturbances (0–3 min exposures) and subsequent biogeochemical activity (3–10 min exposures). Average ambient NH₄⁺ concentrations were significantly different amongst all sites and ranged from 28.0 to 731.7 μg L^-1, with the highest concentrations measured at restored sites. Average NO₃^- concentrations ranged from 9.6 to 26.4 mg L^-1, but did not significantly differ between restored and unrestored sites. Average NH₄⁺ fluxes at restored sites ranged from -8.9 to 5.0 μg N m^-2 sec^-1, however restoration did not significantly influence NH₄⁺ uptake or regeneration (i.e., a measure of release to surface water) between 0–3 minutes and 3–10 minutes. Further, average NO₃^- fluxes amongst sites responded significantly between 0–3 minutes ranging from -33.6 to 97.7 μg N m^-2 sec^-1. Neither NH₄⁺ nor NO₃^- fluxes correlated to sediment chlorophyll-a, total organic matter, or grain size. We attributed variations in overall N fluxes to N-specific sediment storage capacity, biogeochemical transformations, potential legacy effects associated with urban pollution, and variations in river-specific restoration actions.

Channelizing Streams for Agricultural Drainage Impairs their Nutrient Removal Capacity

In agricultural basins, fluvial ecosystems can work as filters when retaining the nutrient excess from agricultural activities, mitigating the impacts downstream. In frequently flooded areas, like the Pampas Region of Argentina, natural streams are being channelized to reduce flood frequency and intensity, thus increasing land suitability for crop production, but the impact of these interventions on nutrient removal capacity by streams is unknown. To evaluate the effects of channelizing streams on the assimilation rate of nitrate, ammonia, and phosphorus, nutrient addition experiments were performed in streams of the southern Pampas under three different conditions: (i) channelized reaches without (C.A. Mey.) Palla (reeds), (ii) unchannelized reaches without reeds, and (iii) unchannelized reaches with reeds. Assimilation rates were estimated by applying the one-dimensional transport with inflow and storage (OTIS) model, which considers the solute transport with lateral flow and storage. Nitrate and ammonia uptake rates were higher in unchannelized than in channelized stream reaches, and a higher nitrate assimilation rate was found in the presence of reeds, indicating an important role of this macrophyte in the nitrate uptake. In the case of phosphorous, uptake rates were higher in unchannelized reaches with reeds than in the channelized reaches. These results suggest that channelizing first-order streams in agricultural landscapes of the Argentine Pampas may significantly reduce the ability of streams to mitigate nutrients loss to continental and marine water sinks.

Public Water Supply Is Responsible for Significant Fluxes of Inorganic Nitrogen in the Environment

Understanding anthropogenic disturbance of macronutrient cycles is essential for assessing the risks facing ecosystems. For the first time, we quantified inorganic nitrogen (N) fluxes associated with abstraction, mains water leakage, and transfers of treated water related to public water supply. In England, the mass of nitrate-N removed from aquatic environments by abstraction (ABS-NO₃-N) was estimated to be 24.2 kt N/year. This is equal to six times the estimates of organic N removal by abstraction, 15 times in-channel storage of organic N, and 30 times floodplain storage of organic N. ABS-NO₃-N is also between 3 and 39% of N removal by denitrification in the hydrosphere. Mains water leakage of nitrate-N (MWL-NO₃-N) returns 3.62 kt N/year to the environment, equating to approximately 15% of ABS-NO₃-N. In urban areas, MWL-NO₃-N can represent up to 20% of the total N inputs. MWL-NO₃-N is predicted to increase by up to 66% by 2020 following implementation of treated water transfers. ABS-NO₃-N and MWL-NO₃-N should be considered in future assessments of N fluxes, in order to accurately quantify anthropogenic disturbances to N cycles. The methodology we developed is transferable, uses widely available datasets, and could be used to quantify N fluxes associated with public water supply across the world.

Give and Take: A Watershed Acid Rain Mitigation Experiment Increases Baseflow Nitrogen Retention but Increases Stormflow Nitrogen Export

In many temperate forested watersheds, hydrologic nitrogen export has declined substantially in recent decades, and many of these watersheds show enduring effects from historic acid deposition. A watershed acid remediation experiment in New Hampshire reversed many of these legacy effects of acid deposition and also increased watershed nitrogen export, suggesting that these two phenomena may be coupled. Here we examine stream nitrate dynamics in this watershed acid remediation experiment for indicators of nitrogen saturation in the terrestrial and aquatic ecosystems. Post-treatment, the (positive) slope of the relationship between nitrate concentration and discharge increased by a median of 82% ( p = 0.004). This resulted in greater flushing of nitrate during storm events, a key indicator of early stage nitrogen saturation. Hysteretic behavior of the concentration-discharge relationship indicated that the mobilization of soil nitrate pools was responsible for this increased flushing. In contrast to this evidence for nitrogen saturation in the terrestrial ecosystem, we found that nitrogen uptake increased, post-treatment, in the aquatic ecosystem, substantially attenuating growing-season nitrate flux by up to 71.1% ( p = 0.025). These results suggest that, as forests slowly recover from acid precipitation, terrestrial, and aquatic ecosystem nitrogen balance may be substantially altered.

Quantifying Urban Bioswale Nitrogen Cycling in the Soil, Gas, and Plant Phases

Bioswales are a common feature of urban green infrastructure plans for stormwater management. Despite this fact, the nitrogen (N) cycle in bioswales remains poorly quantified, especially during dry weather in the soil, gas, and plant phases. To quantify the nitrogen cycle across seven bioswale sites located in the Bronx, New York City, we measured rates of ammonium and nitrate production in bioswale soils. We also measured soil nitrous oxide gas emissions and plant foliar nitrogen. We found that all mineralized nitrogen underwent nitrification, indicating that the soils were nitrogen-rich, particularly during summer months when nitrogen cycling rates increase, as indicated by higher levels of ammonium in the soil. In comparison to mineralization (0 to 110 g N m−2 y−1), the amounts of nitrogen uptake by the plants (0 to 5 g N m−2 y−1) and of nitrogen in gas emissions from the soils (1 to 10 g N m−2 y−1) were low, although nitrous oxide gas emissions increased in the summer. The bioswales’ greatest influx of nitrogen was via stormwater (84 to 591 g N m−2 y−1). These findings indicate that bioswale plants receive overabundant nitrogen from stormwater runoff. However, soils currently used for bioswales contain organic matter contributing to the urban nitrogen load. Thus, bioswale designs should use less nitrogen rich soils and minimize fertilization for lower nitrogen runoff.

The response of chironomid taxonomy- and functional trait-based metrics to fish farm effluent pollution in lotic systems

The lotic habitats affected by trout farm waste are colonized with a particular invertebrate community of which chironomids are the most abundant group. However, there is little information available regarding how chironomid community structures respond to this type of pollution at the highest taxonomic resolution. Eight fish farms, together with their lotic systems as recipients, were used to test the variability of the chironomid community and its surrogates (taxonomic and functional metrics) across spatially arranged sampling sites to form a gradual decrease in the trout farm influence. The self organizing map (SOM) classified six different types of chironomid communities which were characteristic for both the control and affected habitats. The species indicator analyses listed 32 taxa as positive indicators of water pollution. The SOM and Kruskal-Wallis test revealed that the pattern of chironomid community structure obtained was mainly driven by six environmental parameters (Altitude, conductivity, distance from the outlet, hardness, HN₄-N, NO₃-N). Categorical principal components analysis (CATPCA) derived three models for each type of biotic metric, in which for diversity-, taxonomy- and functional feeding group-based metrics, the first two dimensions explained 55.2%, 58.3% and 55.4%, of the total variance respectively for 315 sampling sites. According to this analysis, the total number of taxa (S), abundance and the Shannon-Wiener index (H') (as a diversity metric), as well as the proportion of Tanypodinae (as taxonomic group) and grazers/scraper (GRA) and gatherer collector (GAT)(as FFG metrics), were related to the outlet distance gradient, thus showing great potential to be used in the multimetric approach in bioassessment.

The Functioning of a Water Body Within a Fluvio-Lacustrine System as an Effect of Excessive Nitrogen Loading—The Case of Lake Symsar and its Drainage Area (Northeastern Poland)

Generally, in water ecosystems, it is assumed that rivers play a transport role. In turn, lakes have accumulation properties. However, in fluvio-lacustrine systems, each water body located on a river track can disrupt naturally occurring processes. One such process is the nitrogen cycle. An analysis of the nitrogen cycle, at both the global and local levels, is of extreme significance in view of the progressive degradation of aquatic ecosystems. In this study, we attempted to show that the specific properties of reservoirs located in river–lake systems contribute to an adequate reaction of these reservoirs to situations involving an excessive pollution load. Despite the intensive exchange of water in lakes, they were mainly shown to have an accumulation function. In particular, in those located in the lower part of the system, the total nitrogen load transported outside the example reservoir decreased by 4.3%. The role of these reservoirs depends on the morphometric, hydrologic, and meteorological conditions. The actual loading of the water body was shown to be more than double the permitted critical loading. The creation of conditions similar to those occurring in river–lake systems by, for example, delaying the outflow of water, may favor the protection of surface water from the last element of the system, because this limits the transport of pollutants. This study of the functioning and evolution of lakes’ fluvio-lacustrine systems, including the balance of the nutrient load, enables the prediction of the aquatic ecosystem’s responses in the future and their changes.

Nitrogen-fixation activity and the abundance and taxonomy of nifH genes in agricultural, pristine, and urban prairie stream sediments chronically exposed to different levels of nitrogen loading

Small streams exert great influences on the retention and attenuation of nitrogen (N) within stream networks. Human land use can lead to increased transport of dissolved inorganic N compounds and downstream eutrophication. Microbial activity in streams is important for maintaining an actively functioning N cycle. Chronically high N loading in streams affects the rates of the central processes of the N cycle by increasing rates of nitrification and denitrification, with biota exhibiting decreased efficiency of N use. The LINXII project measured N-cycle parameters in small streams using 15NO3- tracer release experiments. We concurrently measured N2 fixation rates in six streams of three types (agricultural, pristine, and urban prairie streams) as part of this broader study of major N-cycle processes. Nitrogen fixation in streams was significantly negatively correlated with nitrate levels, dissolved inorganic N levels, and denitrification rates. Algal mat and leaf litter samples generally exhibited the highest rates of N2 fixation. The abundance of nifH genes, as measured by real-time PCR, was marginally correlated with N2-fixation rates, but not to other N-cycle processes or stream characteristics. The nifH sequences observed were assigned to cyanobacteria, Deltaproteobacteria, Methylococcus, and Rhizobia. Seasonal changes, disturbances, and varying inputs may encourage a diverse, flexible, stable N2-fixing guild. Patchiness in the streams should be considered when assessing the overall impact of N2 fixation, since algal biomass exhibited high rates of N2 fixation.

Wastewater influences nitrogen dynamics in a coastal catchment during a prolonged drought

Ecosystem function measurements can enhance our understanding of nitrogen (N) delivery in coastal catchments across river and estuary ecosystems. Here, we contrast patterns of N cycling and export in two rivers, one heavily influenced by wastewater treatment plants (WWTP), in a coastal catchment of south Texas. We measured N export from both rivers to the estuary over 2 yr that encompass a severe drought, along with detailed mechanisms of N cycling in river, tidal river, and two estuary sites during prolonged drought. WWTP nutrient inputs stimulated uptake of N, but denitrification resulting in permanent N removal accounted for only a small proportion of total uptake. During drought periods, WWTP N was the primary source of exported N to the estuary, minimizing the influence of episodic storm-derived nutrients from the WWTP-influenced river to the estuary. In the site without WWTP influence, the river exported very little N during drought, so storm-derived nutrient pulses were important for delivering N loads to the estuary. Overall, N is processed from river to estuary, but sustained WWTP-N loads and periodic floods alter the timing of N delivery and N processing. Research that incorporates empirical measurements of N fluxes from river to estuary can inform management needs in the face of multiple anthropogenic stressors such as demand for freshwater and eutrophication.

Seasonal Differences in Relationships between Nitrate Concentration and Denitrification Rates in Ditch Sediments Vegetated with Rice Cutgrass

Increased application of nitrogen (N) fertilizers in agricultural systems contributes to significant environmental impacts, including eutrophication of fresh and coastal waters. Rice cutgrass [ (L.) Sw.] can significantly enhance denitrification potential in agricultural ditch sediments and potentially reduce N export from agricultural watersheds, but relationships with known drivers are not well understood. To address this, we examined effects of nitrate (NO) availability on dinitrogen gas (N) and NO fluxes seasonally. Net denitrification rates were measured as positive N fluxes from vegetated intact sediment cores using membrane inlet mass spectrometry (MIMS). We developed Michaelis-Menten models for N fluxes across NO gradients in the spring, summer, and fall seasons. Summer N models exhibited the highest (maximum amount of net N flux) and (concentration of NO in the overlying water at which the net N flux is half of ), with a maximum production of N of ∼20 mg N m h. Maximum percentage NO retention occurred at 1 mg NO L in the overlying water in all seasons, except summer where maximum retention persisted from 1 to 5 mg NO L. Denitrification rates were strongly correlated with NO uptake by vegetated sediments in spring ( = 0.94, < 0.0001) and summer ( = 0.97, < 0.0001), but low NO uptake in fall and winter resulted in virtually no net denitrification during these seasons. Our results indicate that vegetated ditch sediments may act as effective NO sinks during the growing season. Ditch sediments vegetated with cutgrass not only immobilized a significant fraction of NO entering them but also permanently removed as much as 30 to 40% of the immobilized NO through microbial denitrification.

Pesticide and nitrate transport in an agriculturally influenced stream in Indiana

Agrochemicals can be transported from agricultural fields into streams where they might have adverse effects on water quality and ecosystems. Three enrichment experiments were conducted in a central Indiana stream to quantify pesticide and nitrogen transport dynamics. In an enrichment experiment, a compound solution is added at a constant rate into a stream to increase compound background concentration. A conservative tracer (e.g., bromide) is added to determine discharge. Water and sediment samples are taken at several locations downstream to measure uptake metrics. We assessed transport of nitrate, atrazine, metolachlor, and carbaryl through direct measurement of uptake length (S _w ), uptake velocity (V _f ), and areal uptake (U). S _w measures the distance traveled by a nutrient along the stream reach. V _f measures the velocity a nutrient moves from the water column to immobilization sites. U represents the amount of nutrient immobilized in an area of streambed per unit of time. S _w varied less than one order of magnitude across pesticides. The highest S _w for atrazine suggests greater transport to downstream ecosystems. Across compounds, pesticide S _w was longest in August relative to October and July. V _f varied less than one order of magnitude across pesticides with the highest V _f for metolachlor. U varied three orders of magnitude across pesticides with the highest U associate with sediment-bound carbaryl. Increasing nitrate S _w suggests a lower nitrate demand of biota in this stream. Overall, pesticide transport was best predicted by compound solubility which can complement and improve models of pesticide abundance used by water quality programs and risk assessments.

Nitrogen and phosphorus in sediments in China: A national-scale assessment and review

A national-scale investigation of total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC), and pH in sediments was performed. The sediment samples investigated in this study were collected from 10 major basins in China (Songhua River Basin (SRB), Liao River Basin (LRB), Hai River Basin (HRB), Yellow River Basin (YRB), Huai River Basin (HuRB), Yangtze River Basin (YtRB), Southeastern River Basin (SeRB), Pearl River Basin (PRB), Southwestern River Basin (SwRB), and Northwestern River Basin (NwRB)). And then, a pollution assessment was performed by comparing the data with established sediment quality guidelines (SQGs) and organic nitrogen index values. Results demonstrated that the mean TN content in the sediments of the 10 basins was 1.070g/kg, while the mean TP content was 0.733g/kg. The TN contents displayed significantly positive correlations with the TP contents in the sediments of SRB, LRB, YtRB, SeRB, PRB, and NwRB. Moreover, the concentrations of TN in the sediments of nine basins (SRB, LRB, HRB, YRB, HuRB, YtRB, SeRB, PRB, and NwRB) and TP concentrations of four basins (LRB, YtRB, SeRB, and PRB) were possibly related to the TOC contents, and the distributions of TN concentrations in eight basins (SRB, LRB, HRB, YRB, YtRB, SeRB, PRB, and NwRB) as well as the TP concentrations in LRB might be affected by the pH of sediments. By comparing the data in our study with those obtained in other periods (1990-2013), we found that the TN contamination situation in HuRB and the TP contamination situation in PRB have potentially worsened over time, which deserves more attention. According to the results of SQGs and organic nitrogen index assessment, among the 10 basins, SeRB was the worst watershed polluted by N and HRB was the worst watershed polluted by P.

Small Reservoirs as a Beneficial Management Practice for Nitrogen Removal

There are few beneficial management practices (BMPs) with demonstrated efficacy in snowmelt-dominated regions. Small reservoirs are a BMP that can help mitigate flooding and reduce sediment transport, while reducing export of dissolved nutrients. To understand controls on nitrate removal and assess how this ecosystem service can be optimized, denitrification activity was measured in reservoirs and stream pools of the Tobacco Creek Model Watershed (Manitoba, Canada) via the chloramphenicol-amended acetylene block technique. Denitrification activity was positively correlated with nitrate and sediment organic carbon (SOC), and negatively correlated with sediment particle size and pH. Reservoirs exhibited higher denitrification activity than stream pools and were associated with higher levels of SOC, higher nitrate in early summer, and lower concentrations of dissolved oxygen. Nitrate was added to a set of samples to test for nitrate saturation, an indicator of poor ecological status, where nitrate concentrations exceed the denitrification capacity of microbes. Forty-nine percent of measurements demonstrated nitrate saturation, indicative of the need for additional remediation activity. Findings from this research suggest this BMP has higher capacity for nitrogen removal than stream pools because of higher denitrification rates and a higher apparent threshold for nitrate saturation, coupled with increased residence times. Results also inform the construction of additional reservoirs, which have been identified as a priority BMP in this region. Siting reservoirs in areas where conditions contribute to buildup of fine sediments and planting riparian vegetation to foster high organic C availability may help optimize denitrification, although tradeoffs in terms of other ecosystem services must be considered.

Modeling nitrous oxide emission from rivers: a global assessment

Estimates of global riverine nitrous oxide (N₂ O) emissions contain great uncertainty. We conducted a meta-analysis incorporating 169 observations from published literature to estimate global riverine N₂ O emission rates and emission factors. Riverine N₂ O flux was significantly correlated with NH₄ , NO₃ and DIN (NH₄  + NO₃ ) concentrations, loads and yields. The emission factors EF(a) (i.e., the ratio of N₂ O emission rate and DIN load) and EF(b) (i.e., the ratio of N₂ O and DIN concentrations) values were comparable and showed negative correlations with nitrogen concentration, load and yield and water discharge, but positive correlations with the dissolved organic carbon : DIN ratio. After individually evaluating 82 potential regression models based on EF(a) or EF(b) for global, temperate zone and subtropical zone datasets, a power function of DIN yield multiplied by watershed area was determined to provide the best fit between modeled and observed riverine N₂ O emission rates (EF(a): R²  = 0.92 for both global and climatic zone models, n = 70; EF(b): R²  = 0.91 for global model and R²  = 0.90 for climatic zone models, n = 70). Using recent estimates of DIN loads for 6400 rivers, models estimated global riverine N₂ O emission rates of 29.6-35.3 (mean = 32.2) Gg N₂ O-N yr^-1 and emission factors of 0.16-0.19% (mean = 0.17%). Global riverine N₂ O emission rates are forecasted to increase by 35%, 25%, 18% and 3% in 2050 compared to the 2000s under the Millennium Ecosystem Assessment's Global Orchestration, Order from Strength, Technogarden, and Adapting Mosaic scenarios, respectively. Previous studies may overestimate global riverine N₂ O emission rates (300-2100 Gg N₂ O-N yr^-1 ) because they ignore declining emission factor values with increasing nitrogen levels and channel size, as well as neglect differences in emission factors corresponding to different nitrogen forms. Riverine N₂ O emission estimates will be further enhanced through refining emission factor estimates, extending measurements longitudinally along entire river networks and improving estimates of global riverine nitrogen loads.

Thinking beyond the Bioreactor Box: Incorporating Stream Ecology into Edge-of-Field Nitrate Management

Around the world, artificially drained agricultural lands are significant sources of reactive nitrogen to stream ecosystems, creating substantial stream health problems. One management strategy is the deployment of denitrification enhancement tools. Here, we evaluate the factors affecting the potential of denitrifying bioreactors to improve stream health and ecosystem services. The performance of bioreactors and the structure and functioning of stream biotic communities are linked by environmental parameters like dissolved oxygen and nitrate-nitrogen concentrations, dissolved organic carbon availability, flow and temperature regimes, and fine sediment accumulations. However, evidence of bioreactors' ability to improve waterway health and ecosystem services is lacking. To improve the potential of bioreactors to enhance desirable stream ecosystem functioning, future assessments of field-scale bioreactors should evaluate the influences of bioreactor performance on ecological indicators such as primary production, organic matter processing, stream metabolism, and invertebrate and fish assemblage structure and function. These stream health impact assessments should be conducted at ecologically relevant spatial and temporal scales. Bioreactors have great potential to make significant contributions to improving water quality, stream health, and ecosystem services if they are tailored to site-specific conditions and implemented strategically with land-based and stream-based mitigation tools within watersheds. This will involve combining economic, logistical, and ecological information in their implementation.

Evaluation of nutrients and major ions in streams-implications of different timescale procedures

Small watersheds are characterized by a high degree of sensitivity to changes observed in their environment, making them important sampling and management units. Due to this high sensitivity, several studies have shown that intensive collecting may be more effective in these systems compared to other timescale procedures. The aim of this study was to evaluate the concentration of organic and inorganic nutrients and major ions dissolved in two small watersheds with different land uses to determine whether there are differences between these watersheds with different levels of impact and to identify the most appropriate timescale procedure for the variables under analysis. Therefore, monthly, daily, and hourly samples were taken in the two streams in the northeast of Brazil. One of the streams is located in an undisturbed area (environmental protected area) (S1) and one in a disturbed area (S2). The results showed significant differences for conductivity, temperature, pH, dissolved oxygen (%), sodium (Na(+)), and chloride (Cl(-)) ions and higher values presented in the anthropogenic stream. Dissolved inorganic nitrogen (DIN) in S2 mainly comprised ammonium (NH4 (+)), while nitrate (NO3 (-)) predominated in S1. The considerable increase in the concentration of NO3 (-) and dilution of Na(+) and Cl(-) after rain in April in S1 shows how precipitation may change the chemical composition of the water in a 1-day period. No changes were observed in the concentrations of major ions and nutrients that could be related to the cyclical variation of the hours during the day in both small watersheds. Daily collections allow better monitoring of the dynamics of streams and greater robustness of the data.

Linking nitrogen management, seep chemistry, and stream water quality in two agricultural headwater watersheds

Riparian seepage zones in headwater agricultural watersheds represent important sources of nitrate-nitrogen (NO-N) to surface waters, often connecting N-rich groundwater systems to streams. In this study, we examined how NO-N concentrations in seep and stream water were affected by NO-N processing along seep surface flow paths and by upslope applications of N from fertilizers and manures. The research was conducted in two headwater agricultural watersheds, FD36 (40 ha) and RS (45 ha), which are fed, in part, by a shallow fractured aquifer system possessing high (3-16 mg L) NO-N concentrations. Data from in-seep monitoring showed that NO-N concentrations generally decreased downseep (top to bottom), indicating that most seeps retained or removed a fraction of delivered NO-N (16% in FD36 and 1% in RS). Annual mean N applications in upslope fields (as determined by yearly farmer surveys) were highly correlated with seep NO-N concentrations in both watersheds (slope: 0.06; = 0.79; < 0.001). Strong positive relationships also existed between seep and stream NO-N concentrations in FD36 (slope: 1.01; = 0.79; < 0.001) and in RS (slope: 0.64; = 0.80; < 0.001), further indicating that N applications control NO-N concentrations at the watershed scale. Our findings clearly point to NO-N leaching from upslope agricultural fields as the primary driver of NO-N losses from seeps to streams in these watersheds and therefore suggest that appropriate management strategies (cover crops, limiting fall/winter nutrient applications, decision support tools) be targeted in these zones.

Effects of atrazine, metolachlor, carbaryl and chlorothalonil on benthic microbes and their nutrient dynamics

Atrazine, metolachlor, carbaryl, and chlorothalonil are detected in streams throughout the U.S. at concentrations that may have adverse effects on benthic microbes. Sediment samples were exposed to these pesticides to quantify responses of ammonium, nitrate, and phosphate uptake by the benthic microbial community. Control uptake rates of sediments had net remineralization of nitrate (−1.58 NO₃ µg gdm⁻¹ h⁻¹), and net assimilation of phosphate (1.34 PO₄ µg gdm⁻¹ h⁻¹) and ammonium (0.03 NH₄ µg gdm⁻¹ h⁻¹). Metolachlor decreased ammonium and phosphate uptake. Chlorothalonil decreased nitrate remineralization and phosphate uptake. Nitrate, ammonium, and phosphate uptake rates are more pronounced in the presence of these pesticides due to microbial adaptations to toxicants. Our interpretation of pesticide availability based on their water/solid affinities supports no effects for atrazine and carbaryl, decreasing nitrate remineralization, and phosphate assimilation in response to chlorothalonil. Further, decreased ammonium and phosphate uptake in response to metolachlor is likely due to affinity. Because atrazine target autotrophs, and carbaryl synaptic activity, effects on benthic microbes were not hypothesized, consistent with results. Metolachlor and chlorothalonil (non-specific modes of action) had significant effects on sediment microbial nutrient dynamics. Thus, pesticides with a higher affinity to sediments and/or broad modes of action are likely to affect sediment microbes' nutrient dynamics than pesticides dissolved in water or specific modes of action. Predicted nutrient uptake rates were calculated at mean and peak concentrations of metolachlor and chlorothalonil in freshwaters using polynomial equations generated in this experiment. We concluded that in natural ecosystems, peak chlorothalonil and metolachlor concentrations could affect phosphate and ammonium by decreasing net assimilation, and nitrate uptake rates by decreasing remineralization, relative to mean concentrations of metolachlor and chlorothalonil. Our regression equations can complement models of nitrogen and phosphorus availability in streams to predict potential changes in nutrient dynamics in response to pesticides in freshwaters.

Characterizing storm-event nitrate fluxes in a fifth order suburbanizing watershed using in situ sensors

Land use influences the distribution of nonpoint nitrogen (N) sources in urbanizing watersheds and storm events interact with these heterogeneous sources to expedite N transport to aquatic systems. In situ sensors provide high frequency and continuous measurements that may reflect storm-event N variability more accurately compared to grab samples. We deployed sensors from April to December 2011 in a suburbanizing watershed (479 km2) to characterize storm-event nitrate-N (NO3-N) and conductivity variability. NO3-N concentrations exhibited complex patterns both within and across storms and shifted from overall dilution (source limitation) before summer baseflows to subsequent periods of flushing (transport limitation). In contrast, conductivity generally diluted with increasing runoff. Despite diluted NO3-N concentrations, NO3-N fluxes consistently increased with flow. Sensor flux estimates for the entire deployment period were similar to estimates derived from weekly and monthly grab samples. However, significant differences in flux occurred at monthly time scales, which may have important implications for understanding impacts to temporally sensitive receiving waters. Evidence of both supply (nutrient-poor) and transport (nutrient-rich) limitation patterns during storms is consistent with watersheds undergoing land use transitions. Tracking shifts in these patterns could indicate N accumulation in developing watersheds and help identify mitigation opportunities prior to N impairment.

Temporary storage or permanent removal? The division of nitrogen between biotic assimilation and denitrification in stormwater biofiltration systems

The long-term efficacy of stormwater treatment systems requires continuous pollutant removal without substantial re-release. Hence, the division of incoming pollutants between temporary and permanent removal pathways is fundamental. This is pertinent to nitrogen, a critical water body pollutant, which on a broad level may be assimilated by plants or microbes and temporarily stored, or transformed by bacteria to gaseous forms and permanently lost via denitrification. Biofiltration systems have demonstrated effective removal of nitrogen from urban stormwater runoff, but to date studies have been limited to a 'black-box' approach. The lack of understanding on internal nitrogen processes constrains future design and threatens the reliability of long-term system performance. While nitrogen processes have been thoroughly studied in other environments, including wastewater treatment wetlands, biofiltration systems differ fundamentally in design and the composition and hydrology of stormwater inflows, with intermittent inundation and prolonged dry periods. Two mesocosm experiments were conducted to investigate biofilter nitrogen processes using the stable isotope tracer 15NO3(-) (nitrate) over the course of one inflow event. The immediate partitioning of 15NO3(-) between biotic assimilation and denitrification were investigated for a range of different inflow concentrations and plant species. Assimilation was the primary fate for NO3(-) under typical stormwater concentrations (∼1-2 mg N/L), contributing an average 89-99% of 15NO3(-) processing in biofilter columns containing the most effective plant species, while only 0-3% was denitrified and 0-8% remained in the pore water. Denitrification played a greater role for columns containing less effective species, processing up to 8% of 15NO3(-), and increased further with nitrate loading. This study uniquely applied isotope tracing to biofiltration systems and revealed the dominance of assimilation in stormwater biofilters. The findings raise important questions about nitrogen release upon plant senescence, seasonally and in the long term, which have implications on the management and design of biofiltration systems.

Effects of hydromorphology and riparian vegetation on the sediment quality of agricultural low-order streams: consequences for stream restoration

Intensive agricultural land use imposes multiple pressures on streams. More specifically, the loading of streams with nutrient-enriched soil from surrounding crop fields may deteriorate the sediment quality. The current study aimed to find out whether stream restoration may be an effective tool to improve the sediment quality of agricultural headwater streams. We compared nine stream reaches representing different morphological types (forested meandering reaches vs. deforested channelized reaches) regarding sediment structure, sedimentary nutrient and organic matter concentrations, and benthic microbial respiration. Main differences among reach types were found in grain sizes. Meandering reaches featured larger mean grain sizes (50-70 μm) and a thicker oxygenated surface layer (8 cm) than channelized reaches (40 μm, 5 cm). Total phosphorous amounted for up to 1,500 μg g(-1) DW at retentive channelized reaches and 850-1,050 μg g(-1) DW at the others. While N-NH(4) accumulated in the sediments (60-180 μg g(-1) DW), N-NO(3) concentrations were generally low (2-5 μg g(-1) DW). Benthic respiration was high at all sites (10-20 g O(2) m(-2) day(-1)). Our study shows that both hydromorphology and bank vegetation may influence the sediment quality of agricultural streams, though effects are often small and spatially restricted. To increase the efficiency of stream restoration in agricultural landscapes, nutrient and sediment delivery to stream channels need to be minimized by mitigating soil erosion in the catchment.