Stream Nitrate Concentrations Research Articles

Nitrate attenuation in a small temperate wetland following forest harvest

Elevated nitrate concentrations in streams and groundwater are frequently observed following forest harvest. In addition to depleting nutrients available for forest regeneration, elevated nitrate export following harvest can have deleterious effects on downstream aquatic ecosystems. As part of a forest harvest experiment conducted at the Turkey Lakes Watershed, Ontario, Canada, stable isotope techniques were employed to investigate nitrate attenuation in a natural wetland receiving high concentrations of nitrate as a result of clear-cutting in the catchment. Isotopic analysis of nitrate (δ 18O, δ 15N) and vegetation (δ 15N) demonstrated that both denitrification and plant uptake of nitrate resulted in significantly lower nitrate concentrations in wetland outflow compared to incoming stream water. Although the 0.2-ha forested swamp (4% of catchment by area) was too small to be featured on standard topographic maps, the wetland remove 65–100% of surface water nitrate inputs, thereby protecting downstream aquatic habitats from the full effect of N release from forest harvest. The δ 15N enrichment factor associated with nitrate attenuation in wetland surface water was lower than typically observed during denitrification in groundwaters, suggesting that nitrate removal is complete in some areas of the wetland. Plant assimilation of nitrate was also partially responsible for the low observed enrichment factor. Wetland plants recorded the high δ 15N associated with denitrification activity in portions of the wetland. Apportionment of nitrate sources using δ 18O–NO 3 − at the outlet weir was unaffected by the wetland nitrate attenuation under pre- and post-harvest conditions due to the mid-catchment position of the wetland. Future forest management practices designed to recognize and preserve small wetlands could reduce the potentially detrimental effects of forest harvest on aquatic systems.

Watershed-Scale Impacts of Nitrogen from On-Site Wastewater Systems: Parameter Sensitivity and Model Calibration

A numerical watershed model was used to evaluate the potential influence of various point and nonpoint sources including on-site wastewater systems (OWS) on stream nitrate concentration in Turkey Creek Watershed, Colorado. A watershed analysis risk management framework model was used for this study, and was calibrated to observed stream nitrate concentrations using an automatic calibration tool. Parameter sensitivity analysis was done to select critical parameters for calibration and to reduce uncertainty in the simulated results. Sensitivity analysis of nitrate transport and transformation parameters showed that stream nitrate concentration is highly sensitive to cation exchange capacity, nitrification rate, base saturation of ammonium, initial concentration of ammonium in the soil, and some of the crop growth related parameters. The calibrated model was used to evaluate scenarios related to OWS including the impacts of population growth and new development and impacts of conversion of OWS to conventional sewers. The results showed that there would be a significant increase in stream nitrate concentration with increasing population. Conversion of OWS to sewers increased stream nitrate concentration but decreased nitrate concentration in the bottom soil layer indicating that OWS are beneficial with respect to stream nitrate concentration but may increase nitrate concentrations in groundwater.

Seeps Regulate Stream Nitrate Concentration in a Forested Appalachian Catchment

Surface seeps can be defined as locations where upwelling ground water saturates the surface for most of the year and excess ground water can be delivered to the stream channel via surface flowpaths. If a stream is predominantly fed by seeps, then ground water added to the stream via these surface flowpaths may result in reduced interactions with the subsurface riparian zone. It is generally believed that seep ground water that upwells and then flows along surface flowpaths can be subject to diminished denitrification and biologic uptake processes. Seep effects on stream nitrate (NO(3)) concentration were studied in Baldwin Creek (5.35 km(2)), southwestern Pennsylvania. Nitrate retention within seep zones was evaluated over a 1-yr period (May 2002-2003) using a monthly, nested (top and bottom of seep) sampling approach along 15 individual seeps. Seep samples were analyzed for NO(3)-N, NH(3)-N, and dissolved organic carbon, along with stream waters and streamflow measurements at seven stream stations. Seeps were generally NO(3) sinks with concentrations decreasing downseep: 31% median annual reduction and 73% maximum monthly reduction. During cold and wet periods, seeps frequently behaved as NO(3) sources to the stream (NO(3) concentrations increased or remained constant downseep). Seep temperature and discharge were related to seasonal variability in seep NO(3) retention. Seasonal variations in stream NO(3) concentration have been attributed to upland soil and vegetation processes in numerous watersheds. At Baldwin Creek, seep NO(3) processing regulated the seasonal variability of stream NO(3) concentrations. These results suggest that seeps provide important water quality functions and can modulate the effects of elevated regional N deposition in Appalachian catchments.

Impact of Wildfire on Stream Nutrient Chemistry and Ecosystem Metabolism in Boreal Forest Catchments of Interior Alaska

With climatic warming, wildfire occurrence is increasing in the boreal forest of interior Alaska. Loss of catchment vegetation during fire can impact streams directly through altered solute and debris inputs and changed light and temperature regimes. Over longer time scales, fire can accelerate permafrost degradation, altering catchment hydrology and stream nutrient dynamics. In 2004, the 217,000 ha Boundary Fire burned 65% of an established study site in the Caribou-Poker Creeks Research Watershed. We used this opportunity to investigate the impact of wildfire on stream chemistry and metabolism in boreal forest catchments. Wildfire impacts on chemistry were evaluated by examining solute chemistry in four catchments from 2002 to 2007. Ecosystem metabolism was measured over the summer of 2005 in one burned and two unburned catchments. Wildfire led to stream nitrate concentration increasing up to threefold, whereas dissolved organic carbon (DOC) and dissolved organic nitrogen concentrations decreased post-fire. Average stream gross primary production in the burned catchment was double that of the unburned sites (2.4 and 1.2 g O2 m−2 day−1, respectively). Respiration rate was also elevated in the burned stream (6.6 g O2 m−2 day−1) compared with the control streams (1.2 and 4.5 g O2 m−2 day−1). Climatic warming has the potential to impact boreal forest streams through permafrost thaw and increased fire frequency, leading to altered solute inputs and production and respiration rates.

Climate‐induced changes in high elevation stream nitrate dynamics

AbstractMountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). We explored the consequences of a temperature‐warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991–1999. Mean annual N export increased by 28% from 2.03 kg N ha−1 yr−1 before 2000 to 2.84 kg N ha−1 yr−1 in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha−1 yr−1) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.12 °C yr−1 in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for the period 2000–2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Vale are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.

The role of climate on inter-annual variation in stream nitrate fluxes and concentrations

In recent decades, temporal variations in nitrate fluxes and concentrations in temperate rivers have resulted from the interaction of anthropogenic and climatic factors. The effect of climatic drivers remains unclear, while the relative importance of the drivers seems to be highly site dependent. This paper focuses on 2–6 year variations called meso-scale variations, and analyses the climatic drivers of these variations in a study site characterized by high N inputs from intensive animal farming systems and shallow aquifers with impervious bedrock in a temperate climate. Three approaches are developed: 1) an analysis of long-term records of nitrate fluxes and nitrate concentrations in 30 coastal rivers of Western France, which were well-marked by meso-scale cycles in the fluxes and concentration with a slight hysteresis; 2) a test of the climatic control using a lumped two-box model, which demonstrates that hydrological assumptions are sufficient to explain these meso-scale cycles; and 3) a model of nitrate fluxes and concentrations in two contrasted catchments subjected to recent mitigation measures, which analyses nitrate fluxes and concentrations in relation to N stored in groundwater. In coastal rivers, hydrological drivers (i.e., effective rainfall), and particularly the dynamics of the water table and rather stable nitrate concentration, explain the meso-scale cyclic patterns. In the headwater catchment, agricultural and hydrological drivers can interact according to their settings. The requirements to better distinguish the effect of climate and human changes in integrated water management are addressed: long-term monitoring, coupling the analysis and the modelling of large sets of catchments incorporating different sizes, land uses and environmental factors.

Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 54:233-241 (2009) - DOI: https://doi.org/10.3354/ame01272 Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters E. K. Payne1,2, A. J. Burgin2,3,*, S. K. Hamilton2 1Life Sciences and Society Program, University of Michigan School of Public Health, 109 South Observatory Road, Ann Arbor, Michigan 48105, USA 2W.K. Kellogg Biological Station and Department of Zoology, Michigan State University, 3700 Gull Lake Drive, Hickory Corners, Michigan 49060, USA 3Cary Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545, USA *Corresponding author. Email: burginam@gmail.com ABSTRACT: Anthropogenic nitrogen (N) loading to agricultural and populated landscapes has resulted in elevated nitrate (NO3–) concentrations in ground water, streams and rivers, ultimately causing problems in coastal marine environments such as eutrophication, hypoxia and harmful algal blooms. Nitrate removal along hydrologic flow paths through landscapes intercepts much of the N before it reaches coastal zones. We used traditional porewater equilibrators in a novel way to add nitrate to the sediment porewater of 8 wetlands in southwestern Michigan. Nitrate losses and changes in porewater chemistry were examined to elucidate N removal processes, with particular focus on the potential coupling of bacterial sulfur (S) oxidation to (1) dissimilatory nitrate reduction to ammonium (DNRA) and (2) denitrification. We hypothesized that, if S oxidizers utilized the added nitrate, porewater sulfide concentrations should decrease and sulfate concentrations should increase. Additionally, if the nitrate is used in DNRA, ammonium concentrations should increase as well. Nitrate additions caused decreases in dissolved hydrogen sulfide and increases in sulfate relative to controls at all sites. Ammonium also tended to increase, though the response was less consistent due to a high background ammonium pool. These results provide evidence that microbial S transformations may play an important role in nitrate removal in these freshwater wetland sediments. KEY WORDS: Denitrification · Nitrate removal · Dissimilatory nitrate reduction to ammonium · DNRA · Sulfur oxidation · Porewater equilibrators · Freshwater sediments · Sulfate Full text in pdf format PreviousNextCite this article as: Payne EK, Burgin AJ, Hamilton SK (2009) Sediment nitrate manipulation using porewater equilibrators reveals potential for N and S coupling in freshwaters. Aquat Microb Ecol 54:233-241. https://doi.org/10.3354/ame01272 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 54, No. 3. Online publication date: February 24, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.

Modelling denitrification at the catchment scale

The objective of this work was to evaluate the importance of heterotrophic denitrification in the fate of nitrogen surpluses at the catchment scale. For that purpose we modified the denitrification module of TNT2 model and calibrated the model on a small catchment where denitrification measurements had been performed in different locations. The main interest of the TNT2 model is its ability to simulate the dynamics of the zones where soil and shallow water table interact, making it possible to spatialize the denitrification process. Daily water and nitrogen flux at the outlet were relatively well simulated (Nash of 0.85 and 0.77). In average, the model correctly simulates the denitrification measurements ( R = 0.68). Nitrogen flux towards the atmosphere, at the catchment scale (4.70 g N m − 2 year − 1 ), is of the same order of magnitude as the soluble N flux in the stream. The model was able to reproduce the distribution of denitrification in the riparian (mean of 9.26 g N m − 2 year − 1 ) and hillslope (mean of 3.45 g N m − 2 year − 1 ) domains of the catchment. The results confirm the importance of riparian denitrification, but show also that hillslope soils contribute significantly (60%) to the whole catchment denitrification. The variations of denitrification rates, and also of nitrate concentrations in stream were not very well simulated by the model, highlighting the complexity of the spatial and temporal controls of nitrogen dynamics in areas with high inputs of nitrogen fertilizers, especially under organic forms.

Short-term Effects of Wildfire on Montane Stream Ecosystems in the Southern Rocky Mountains: One and Two Years Post-burn

The frequency and intensity of wildfire in the western United States has increased over the last century, creating a heterogeneous mosaic of landscapes in various stages of recovery. The 2002 Hayman Fire was one of the largest wildfires in Colorado history and was unprecedented for its speed and intensity, with over 50%–70% of the burn area classified as moderate to high severity where much of the canopy crown was consumed. We evaluated the short-term impact of the Hayman Fire on ecological properties in montane stream ecosystems in the summers of 2003 and 2004, one and two years post-fire. Fire significantly altered surface temperature, dissolved oxygen concentration, and the chemical composition of stream water, including concentrations of nitrate, phosphate, and mineral salts. However, fire did not have significant impacts on stream conductivity, pH, or total concentrations of cations or anions during our study period. Streams in the burn area contained fewer benthic macroinvertebrate taxa compared to unburned streams during the year after fire and contained lower invertebrate densities and biomass compared to reference streams 2 years post-fire. Average C:N ratio of the benthic macroinvertebrate community was significantly and negatively related to stream nitrate concentration, possibly due to a shift in community composition or invertebrate nitrogen acquisition in fire-affected streams.

Stream nitrate and DOC dynamics during three spring storms across land uses in glaciated landscapes of the Midwest

Many studies have investigated nitrate and dissolved organic carbon (DOC) export mechanisms during storms in forested mountainous to hilly catchments. However, the number of studies across land uses in artificially drained landscapes of the Midwest remains relatively small despite the importance of nitrate and carbon losses to streams on water quality in this region. This study investigates water, nitrate, and DOC delivery (timing) to streams during storms, and the mechanisms (flowpaths) affecting nitrate and DOC flushing trajectories during three spring storms in an agricultural watershed and a mixed land use watershed in glaciated landscape of the Midwest. Hydrograph separation techniques using oxygen-18 isotopes of water and the analysis of major cation flushing trajectories were used to determine the flowpath associated with the export of nitrate and DOC in each watershed. Higher anthropogenic N inputs in the agricultural watershed are associated with higher stream nitrate concentration during storms, while DOC concentration in streams across land uses is mainly influenced by storm characteristics/discharge. In both watersheds, DOC concentration quickly increases and decreases with discharge. In the mixed land use watershed, the peak in nitrate is consistently delayed relative to the peak in DOC; however, nitrate peaks vary in relation to discharge in the agricultural watershed. The comparison of the nitrate/DOC concentration patterns to the concentration patterns of major cations during the storms studied suggests that DOC is likely exported via a combination of overland flow and preferential flow through soil macropores in both watersheds. Data suggest nitrate is exported with groundwater, either as tile-drain flow or subsurface flow to the streams. Results also indicate that the connectivity between nitrate/DOC reservoirs and the stream is an important control mechanism on the timing of delivery of nitrate and DOC to the stream in the watersheds studied. Finally, results indicate that even in agricultural watersheds where tile-drain flow is an important contributor to stream flow during storms, there is not necessarily a consistent export mechanism for all solutes. These results underscore the need for further studies investigating the relative importance of matrix flow, overland flow and macropore flow at the watershed scale in order to fully understand event scale processes controlling the transport of solutes to streams in glaciated landscapes of the Midwest under various land use conditions.

Denitrification in the mainstem Neuse River and tributaries, USA

We conducted quarterly (spring, summer, fall and winter) determinations of denitrification at representative sites in the mainstem Neuse River and tributaries, USA. Denitrification measurements were made on intact sediment cores using the N 2 /Ar and isotope pairing techniques. We measured nitrate (NO 3 - -N) and ammonium (NH 4 + -N) concentrations in the water column at the time of core collection and evaluated rates of sediment-water exchange of dissolved oxygen (O 2 ) in conjunction with denitrification rate determinations. Nitrate concentrations in the tributary streams and river water varied over an order or magnitude, from about 10 to 110 λmol/L, while NH 4 + -N concentrations were lower, varying from 0.6 to 10.8 μmol/L. Sediments consistently consumed dissolved O 2 , with rates varying from 256 to 1418 pmol m -2 h -1 . Most cores actively denitrified with rates ranging to 222 μmol N 2 -N m -2 h -1 . On average, coupled nitrification-denitrification accounted for 66 % of total denitrification. Lowest rates of denitrification were generally observed during December, when the water temperature was lowest at 4 °C. No other relationship was identified between rates of denitrification and measured physicochemical variables within or among sites or dates as sampling frequency and replication were low. Our study suggests that denitrification represents a sink of about 5 % for the NO 3 - -N load to the mainstem Neuse River and tributary streams.

Long-Term Trends in Stream Nitrate Concentrations and Losses Across Watersheds Undergoing Recovery from Acidification in the Czech Republic

Stream nitrogen (N) export and nitrate \((\text {NO}^-_3) \) concentration were measured at 14 forested watersheds (GEOMON network) in the Czech Republic between 1994 and 2005. In the last several decades, emissions of sulfur (S) and N compounds have decreased throughout much of Europe. In the Czech Republic, atmospheric deposition of S has decreased substantially since the beginning of 1990s, whereas N deposition remains largely unchanged at most sites. The mean dissolved inorganic nitrogen (DIN) streamwater export ranged from 0.2 to 12.2 kg ha−1 y−1 at the GEOMON sites. Despite decades of elevated N deposition, 44–98% of DIN inputs to these watersheds were retained or denitrified, and many watersheds showed seasonal variation in nitrate concentrations. Dissolved organic N export was quantified in 1 year only and ranged from 0.05 to 3.5 kg ha−1 y−1. Spatial variability in DIN export among watersheds was best explained by spatial variability in average acidic deposition, particularly S deposition (R 2 = 0.81, P < 0.001); DIN input and forest floor carbon:nitrogen (C/N) also provided significant explanatory power. DIN export was strongly influenced by the forest floor C/N ratio and depth of the forest floor soils (R 2 = 0.72, P < 0.001). The only variable that predicted variations in forest floor C/N (R 2 = 0.32, P < 0.05) among watersheds was S deposition. Forest floor depth was also related to deposition variables, with S deposition providing the most explanatory power (R 2 = 0.50, P < 0.01). Variation in forest floor depth was also associated with climatic factors (precipitation and temperature). Temporal variability in DIN export was primarily associated with changes in acidic deposition over time; S deposition explained 41% of variability in DIN exports among all watersheds and years. Extensive acidification of forested watersheds was associated with the extraordinarily high S inputs to much of the Czech Republic during earlier decades. We hypothesize that recovery from acidification has led to improved tree health as well as enhanced microbial activity in the forest floor. As these watersheds move into a new regime with dramatically lower sulfur inputs, we expect continued declines in nitrate output.

Climate control on sulphate and nitrate concentrations in alpine streams of Northern Italy along a nitrogen saturation gradient

Abstract. The role of meteorology, hydrology and atmospheric deposition on the temporal pattern of SO4 and NO3 concentrations was investigated for three streams draining alpine catchments in Northern Italy. The study sites lie on a gradient of atmospheric fluxes of SO4 and NO3 (from about 50 to 80 meq m−2 y−1, and from 40 to 90 meq m−2 y−1, respectively). As a consequence of the increasing N input, the three catchments are also representative of aggrading levels of N saturation. Different methods of statistical analysis were applied to monthly data for the period 1997–2005 to identify which variables (temperature, precipitation, hydrology, SO4 and NO3 deposition) were the main predictors of water chemistry and its change in time. Hydrological changes and snow cover proved to be the main confounding factors in the response to atmospheric deposition in the River Masino catchment. Its particular characteristics (small catchment area, rapid flushing during runoff and thin soil cover) meant that this site responded without a significant delay to SO4 deposition decrease. It also showed a clear seasonal pattern of NO3 concentration, in response to hydrology and biological uptake in the growing season. The selected driving variables failed to model the water chemistry at the other study sites. Nevertheless, temperature, especially extreme values, turned out to be important in both SO4 and NO3 export from the catchments. This result might be largely explained by the effect of warm periods on temperature-dependent processes such as mineralization, nitrification and S desorption. Our findings suggest that surface waters in the alpine area will be extremely sensitive to a climate warming scenario: higher temperatures and increasing frequency of drought could exacerbate the effects of high chronic N deposition.

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20-25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

Role of water table dynamics on stream nitrate export and concentration in agricultural headwater catchment (France)

The objective of this work is to identify hydrological processes controlling nitrate export and base flow concentration at the year scale in agricultural headwater catchment streams fed by an unconfined aquifer. The study is based on the hydrological and hydrochemical monitoring of the stream and shallow groundwater of three headwater catchments (0.1­5 km2) in Western France over three to five water years. Results show that at the year scale nitrate export from the three catchments is a transport-limited process. The stream nitrate flux depends on how much water flows in the stream and not on the distribution of the flow over the year. Seasonal variations of concentration over the water year were complex. Variations were different for a given year from one catchment to another, and also different for a given catchment from one year to another. We show that the seasonal variations are controlled by water table depth dynamics along hillslope associated with spatially distributed nitrate concentration in the groundwater. The groundwater displays high nitrate concentrations, from 6 to 22 mg, in upland and in the deeper zones of bottom lands. Persistence of such high concentrations results from the geochemical and mineralogical properties of the aquifers that consist of old, very oxidised and strongly weathered material, such that any primary electron donors have been leached. In riparian zones, concentrations are close to zero due to denitrification with oxidation of organic matter. In winter, stream nitrate concentration is controlled by the nitrate rich groundwater flow from upland. In summer, groundwater flow from upland decreases and stream concentration is controlled by bottom land hydrological and biogeochemical processes. The shift between winter and summer control depends on the water table dynamics along hillslopes. As long as the water table remains deep in upland, summer controls prevail. As soon as water table rises in upland, hydraulic gradient and groundwater flow from this zone increase, leading to an increase in the stream nitrate concentration. The shift from summer to winter control can be considered as the result of a connection between the upland nitrate rich groundwater and the stream, connection triggered by upland water table rise.

Evaluation and indirect estimation of nitrate losses from the agricultural microbasin Rybárik

The long-term trends of mean monthly nitrate concentrations in stream and drainage runoff were evaluated in the experimental microbasin Rybarik (0.119 km2) at the Institute of Hydrology, Slovak Academy of Sciences, during the period 1987–2005. The results of analyses indicate a decreasing trend of nitrate concentration after the year 1989, but with relatively high losses in some years and relatively low losses in other years. This decreasing trend is mainly caused by a decrease in the use of nitrogen fertilizers. The nitrate concentration in surface runoff strongly correlates with runoff and fertilization. Based on measured data, an empirical relation was found describing the dependence of annual nitrate transport in the stream on annual runoff depth and on the annual amount of applied nitrogen fertilizers.

Influence of shifting flow paths on nitrogen concentrations during monsoon floods, San Pedro River, Arizona

Hydrologic flow paths control transport, and therefore are a major constraint on the cycling and availability of nutrients within stream ecosystems. This control is particularly evident in semiarid streams, where hydrologic connectivity between stream, riparian, and upland systems increases greatly during storms in the rainy season. We measured chloride concentrations in base flow, precipitation, soil water, and stream water to quantify the hydrologic connectivity and solute flux between soil water, groundwater, and the stream channel during six summer floods in 2001 (a wet year; 25 cm winter rain) and 2002 (a dry year; 5 cm winter rain) in the San Pedro River, southeastern Arizona. This hydrologic information was used to evaluate observed patterns in nitrate, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations in floods. The first floods of each year showed increased stream nitrate concentration that was approximately two orders of magnitude higher than base flow concentration. DOC consistently doubled to tripled during storm events, while DON in 2001 showed no response and showed a marked increase in 2002. A chloride mixing model indicated that soil and groundwater contributions to storm water discharge were related to antecedent conditions and to flood magnitude. Soil and groundwater contributions were the highest early in the 2001 monsoon season following the wet winter, much lower early in 2002 following a dry winter, and lowest during the largest floods of the 2002 monsoon season when flows were derived primarily from precipitation and overland flow. Stream water nitrate‐N concentrations during floods were consistently 0.2–0.5 mg/L higher in 2002 than during 2001, suggesting greater over‐winter accumulation of soil nitrate during the drier year. This result is consistent with higher mean nitrate‐N concentrations in soil water of the riparian zone in 2002 (3.1 mg/L) than in 2001 (0.56 mg/L). These data highlight the importance of seasonal and interannual variability of hydrology in semiarid regions, and the role of water availability in driving patterns of soil nutrient accumulation and their transport to the stream.

Quantifying nitrogen cycling beneath a meander of a low gradient, N‐impacted, agricultural stream using tracers and numerical modelling

AbstractIn watersheds impacted by nitrate from agricultural fertilizers, nitrification and denitrification may be decoupled as denitrification in the hyporheic zone is not limited to naturally produced nitrate. While most hyporheic research focuses on the 1–2 m of sediment beneath the stream bed, there are a limited number of studies that quantify nitrogen (N) cycling at larger hyporheic scales (10s of metres to kms). We conducted an investigation to quantify N cycling through a single meander of a low gradient, meandering stream, draining an agricultural watershed. Chemistry (major ions and N species) and hydrologic data were collected from the stream and groundwater beneath the meander. Evidence indicates that nearly all the shallow groundwater flowing beneath the meander originates as stream water on the upgradient side of the meander, and returns to the stream on the downgradient side. We quantified the flux of water beneath the meander using a numerical model. The flux of N into and out of the meander was quantified by multiplying the concentration of the important N species (nitrate, ammonium, dissolved organic nitrogen (DON)) by the modelled water fluxes. The flux of N into the meander is dominated by nitrate, and the flux of N out of the meander is dominated by ammonium and DON. While stream nitrate varied seasonally, ammonium and DON beneath the meander were relatively constant throughout the year. When stream nitrate concentrations are high (&gt;2 mg litre−1), flow beneath the meander is a net sink for N as more N from nitrate in stream water is consumed than is produced as ammonium and DON. When stream nitrate concentrations are low (&lt;2 mg litre−1), the flux of N entering is less than exiting the meander. On an annual basis, the meander hyporheic flow serves as a net sink for N. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Factors controlling nitrogen release from two forested catchments with contrasting hydrochemical responses

AbstractQuantifying biogeochemical cycles of nitrogen (N) and the associated fluxes to surface waters remains challenging, given the need to deal with spatial and temporal variability and to characterize complex and heterogeneous landscapes. We focused our study on catchments S14 and S15 located in the Adirondack Mountains of New York, USA, which have similar topographic and hydrologic characteristics but contrasting stream nitrate ($\hbox{NO}_{3}^{-}$) concentrations. We characterized the mechanisms by which $\hbox{NO}_{3}^{-}$ reaches the streams during hydrological events in these catchments, aiming to reconcile our field data with our conceptual model of factors that regulate nutrient exports from forested catchments. Combined hydrometric, chemical and isotopic (δ$\hbox{NO}_{3}^{-}$) data showed that the relative contributions of both soil and ground water sources were similar between the two catchments. Temporal patterns of stream chemistry were markedly different between S14 and S15, however, because the water sources in the two catchments have different solute concentrations. During late summer/fall, the largest source of $\hbox{NO}_{3}^{-}$ in S14 was till groundwater, whereas shallow soil was the largest $\hbox{NO}_{3}^{-}$ source in S15. $\hbox{NO}_{3}^{-}$ concentrations in surface water decreased in S14, whereas they increased in S15 because an increasing proportion of stream flow was derived from shallow soil sources. During snowmelt, the largest sources of $\hbox{NO}_{3}^{-}$ were in the near‐surface soil in both catchments. Concentrations of $\hbox{NO}_{3}^{-}$ increased as stream discharge increased and usually peaked before peak discharge, when shallow soil water sources made the largest contribution to stream discharge. The timing of peaks in stream $\hbox{NO}_{3}^{-}$concentrations was affected by antecedent moisture conditions. By elucidating the factors that affect sources and transport of N, including differences in the soil nutrient cycling and hydrological characteristics of S14 and S15, this study contributes to the overall conceptualization of $\hbox{NO}_{3}^{-}$ release from temperate forested catchments. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Evaluating the implementation of the Nitrates Directive in Denmark and England using an actor‐orientated approach

AbstractThe Nitrates Directive (91/676/EEC) was adopted to address the issue of water pollution caused by nitrate derived from agricultural practices across the European Union (EU). This study evaluates the implementation of the Nitrates Directive by considering the perspectives of respondents from significant actor groups involved in the policy implementation process in Denmark and England. Structured field interviews were conducted and discovered that Nitrate Vulnerable Zone (NVZ) designation in England was a lengthy and inefficient process compared with that in Denmark. Generally, respondents from both countries considered that whole country designation was a more effective policy instrument than designation of discrete zones. Denmark's Action Programme measures reflected the severity of its water quality problems and go beyond the demands of the Nitrates Directive. The respondents recognized that Denmark's strict measures have led to a reduction in nitrate leaching and a decline in nitrate concentrations in streams from cultivated catchments. England's current Action Programme measures do not demand any substantial changes to farming practice and so have limited potential to reduce nitrate losses. This study concludes that a number of vague and ill prepared guidelines within the Nitrates Directive undermine its ability to protect water quality adequately, and hence current implementation of the Water Framework Directive (WFD) is an opportunity to re‐shape water policy in relation to nitrate. Copyright © 2007 John Wiley &amp; Sons, Ltd and ERP Environment.