Stream NO3 Research Articles

Land‐use controls on sources and processing of nitrate in small watersheds: insights from dual isotopic analysis

Studies have repeatedly shown that agricultural and urban areas export considerably more nitrogen to streams than forested counterparts, yet it is difficult to identify and quantify nitrogen sources to streams due to complications associated with terrestrial and in-stream biogeochemical processes. In this study, we used the isotopic composition of nitrate (delta15N-NO3- and delta18O-NO3-) in conjunction with a simple numerical model to examine the spatial and temporal variability of nitrate (NO3-) export across a land-use gradient and how agricultural and urban development affects net removal mechanisms. In an effort to isolate the effects of land use, we chose small headwater systems in close proximity to each other, limiting the variation in geology, surficial materials, and climate between sites. The delta15N and delta18O of stream NO3- varied significantly between urban, agricultural, and forested watersheds, indicating that nitrogen sources are the primary determinant of the delta15N-NO3-, while the delta18O-NO3- was found to reflect biogeochemical processes. The greatest NO3- concentrations corresponded with the highest stream delta15N-NO3- values due to the enriched nature of two dominant anthropogenic sources, septic and manure, within the urban and agricultural watersheds, respectively. On average, net removal of the available NO3- pool within urban and agricultural catchments was estimated at 45%. The variation in the estimated net removal of NO3- from developed watersheds was related to both drainage area and the availability of organic carbon. The determination of differentiated isotopic land-use signatures and dominant seasonal mechanisms illustrates the usefulness of this approach in examining the sources and processing of excess nitrogen within headwater catchments.

Seasonal Patterns of Nitrate Discharge from Forested Catchments: Information Derived from Japanese Case Studies

AbstractIn recent decades, the seasonal variation of NO3− discharge from forest ecosystems has been increasingly used by ecologists and hydrologists as a diagnostic indicator of the nutrient status of ecosystems. Major factors controlling the seasonal patterns of stream NO3− concentrations include seasonal variations in (i) nutrient demands of the ecosystem, (ii) solute transport capability of the hydrological condition, and (iii) in‐stream nutrient usage and supply. In this article, we show how case studies have helped elucidate the dominant controlling factors by comparing data from Japanese catchments with previously compiled data from studies in North America and Europe, and explain the different influences hydrological and biogeochemical controls exert in rainy summer regions ( Japanese catchments) and dry summer regions (sites in the northeast United States and Europe). The seasonal variation of hydrological conditions is a predominant controlling factor in Japanese forests, whereas nutrient demand may predominate in the northeast United States and Europe. We emphasize that it is important to compare seasonal patterns among different climate regions to obtain more universal explanations of the seasonal variation in stream NO3− concentration. In addition, multi‐scale investigations will be needed to provide insight into the relative contribution of hillslope biogeochemical effects and the influences of in‐stream biological activities.

Terrestrial and in‐stream influences on the spatial variability of nitrate in a forested headwater catchment

A vast majority of monitoring programs designed to assess nutrient fluxes from headwater systems rely upon temporally intensive sampling at a single position within the stream network, essentially measuring the integrated response of the catchment. Missing from such an approach is spatial information related to how nutrient availability varies throughout the network, where freshwater biota live and where biogeochemical processes ultimately shape the downstream water chemistry. Here, we examine the spatial distribution of nitrate (NO3−) concentrations within the Paine Run catchment, a forested headwater catchment in Shenandoah National Park, Virginia. Nitrate concentrations throughout the stream network were measured as part of synoptic surveys conducted in 1992–1994, in the aftermath of region‐wide gypsy moth defoliation that caused dramatic increases in stream water NO3− concentrations. A follow‐up synoptic survey was conducted in 2007, when the stream water NO3− concentrations had returned to predefoliation levels. Common to each of the eight synoptic surveys were observations of multiple‐fold declines in NO3− concentration along the main stem of the stream network from the headwaters to the catchment outlet. A portion of this decline was caused by dilution, as water input by tributaries at the lower elevations of the catchment tended to have lower NO3− concentrations. A stream network model was applied to determine the relative contributions of terrestrial versus in‐stream processes to the spatial variability of the NO3− concentrations. Model results suggest that even though nitrate removal within the stream network can be substantial, terrestrial factors that determine the NO3− inputs to streams account for the vast majority of the spatial variability in stream water NO3− concentrations.

Land use controls on the delivery, processing, and removal of nitrogen from small watersheds: Insights from the dual isotopic composition of stream nitrate

Studies have repeatedly shown that agricultural and urban areas export considerably more nitrogen to streams than forested counterparts, yet it is difficult to identify and quantify nitrogen sources to streams due to complications associated with terrestrial and in-stream biogeochemical processes. In this study, we used the isotopic composition of nitrate (δ15N-NO3− and δ18O-NO3−) in conjunction with a simple numerical model to examine the spatial and temporal variability of nitrate (NO3−) export across a land-use gradient and how agricultural and urban development affects net removal mechanisms. In an effort to isolate the effects of land use, we chose small headwater systems in close proximity to each other, limiting the variation in geology, surficial materials, and climate between sites. The δ15N and δ18O of stream NO3− varied significantly between urban, agricultural, and forested watersheds, indicating that nitrogen sources are the primary determinant of the δ15N-NO3−, while the δ18O-NO3− was found to reflect biogeochemical processes. The greatest NO3− concentrations corresponded with the highest stream δ15N-NO3− values due to the enriched nature of two dominant anthropogenic sources, septic and manure, within the urban and agricultural watersheds, respectively. On average, net removal of the available NO3− pool within urban and agricultural catchments was estimated at 45%. The variation in the estimated net removal of NO3− from developed watersheds was related to both drainage area and the availability of organic carbon. The determination of differentiated isotopic land-use signatures and dominant seasonal mechanisms illustrates the usefulness of this approach in examining the sources and processing of excess nitrogen within headwater catchments.

The biogeochemical influences of NO3−, dissolved O2, and dissolved organic C on stream NO3− uptake

AbstractStreams are potential hotspots for retention and removal of NO3−, and understanding the mechanisms that enhance NO3− reactivity in stream systems is critical for predicting and preventing eutrophication. Both dissolved organic C (DOC) and dissolved O2 (DO) influence NO3− removal processes. Assessing the individual impacts of NO3−, DO, and DOC concentrations on stream NO3− removal is difficult because these factors covary and are coupled through the C and N cycles. We used an experimental approach to quantify the influences of NO3−, DOC, and DO on NO3− transport in headwater streams of the Ipswich and Parker River watersheds (Massachusetts, USA) with contrasting levels of DOC and DO. In a 1st set of experiments, we added NO3− to address how uptake kinetics differed between a low-DO/high-DOC stream (Cedar Swamp Creek) and a high-DO/low-DOC stream (Cart Creek). In a 2nd set of experiments, we manipulated, for the first time at the reach scale, both DO and DOC in a factorial experiment. DO was added t...

Soil and stream water acidification in a forested catchment in central Japan

Rapid industrialization in East Asia is causing adverse effects due to atmospheric deposition in terrestrial and freshwater ecosystems. Decreasing stream pH and alkalinity and increasing NO3 − concentrations were observed throughout the 1990s in the forested Lake Ijira catchment in central Japan. We investigated these changes using data on atmospheric deposition, soil chemistry, stream water chemistry, and forest growth. Average atmospheric depositions (wet + dry) of 0.83, 0.57, and 1.37 kmol ha−1 year−1 for hydrogen, sulfur, and nitrogen, respectively, were among the highest levels in Japan. Atmospheric deposition generally decreased before 1994 and increased thereafter. The catchment was acid-sensitive; stream alkalinity was low (134 μmolc l−1) and pH in surface mineral soils decreased from 4.5 in 1990 to 3.9 in 2003. Stream NO3 − concentration nearly doubled (from 22 to 42 μmolc l−1) from the late 1980s to the early 2000s. Stream NO3 − concentration was controlled primarily by water temperature before 1996/1997 and by stream discharge thereafter. Stream NO3 − concentrations decreased during the growing season before 1996/1997, but this seasonality was lost thereafter. The catchment became nitrogen-saturated (changing from stage 1 to 2) in 1996/1997, possibly because of declining forest growth rates due to the 1994 summer drought, defoliation of Japanese red pine by pine wilt disease, maturation of Japanese cedar stands, and stimulation of nitrogen mineralization and nitrification due to alkalinization of soils (increased exchangeable Ca2+ and soil pH) after the summer drought. Stream pH and alkalinity began decreasing in 1996/1997. The enhanced growing-season NO3 − discharge since 1996/1997 appeared to be the major cause of stream acidification. Increased atmospheric deposition since 1994 may have contributed to this change.

Impact of an Invasive Exotic Species on Stream Nitrogen Levels in Southern Illinois1

Abstract: Autumn‐olive (Elaeagnus umbellataThunb.) is an invasive, exotic shrub that has become naturalized in the eastern United States. Autumn‐olive fixes nitrogen (N) via a symbiotic relationship with the actinomyceteFrankia. At the plot scale, the presence of autumn‐olive has been related to elevated soil water nitrate‐N (NO3−‐N) concentrations. This study examined the relationship between autumn‐olive cover in a watershed and stream water quality. Stream water nitrate‐N (NO3−‐N) and ammonium‐N (NH4+‐N) concentrations were measured in 12 first order ephemeral streams draining watersheds with mixed forest cover and a range of 0‐35% autumn‐olive cover. Percent autumn‐olive cover was positively correlated with mean stream NO3−‐N concentrations, but was not correlated with mean stream NH4+‐N concentrations. While other studies have demonstrated a significant relationship between native N‐fixers and stream NO3−‐N, this is the first study to document a relationship for an invasive, exotic N‐fixing species. Results suggest that this exotic species can be an additional source of NO3−in local and regional water bodies and demonstrates an additional negative ecosystem consequence of invasion beyond losses in biodiversity.

The Effects of Season and Agriculture on Nitrous Oxide Production in Headwater Streams

Streams and rivers are a globally significant source of nitrous oxide (N(2)O), a potent greenhouse gas. However, there remains much uncertainty in the magnitude of N(2)O emissions from these sources, partly due to an incomplete understanding of the factors that control microbial N(2)O production in lotic sediments. During 2004-2005 we measured sediment N(2)O production in 12 headwater streams across an agricultural land use gradient. Stream water nitrate (NO(3)(-)) concentrations were positively related to the proportion of agricultural land use in the basin and frequently exceeded 20 mg N L(-1) in the stream draining the most agricultural basin. Stream sediments were nearly always a net source of N(2)O, and production rates were positively related to stream water NO(3)(-) concentrations and sediment carbon content. There were no seasonal patterns in N(2)O production rates during 2004, but stream water NO(3)(-) and N(2)O production both peaked during the winter of 2005. The spike in NO(3)(-) concentrations likely resulted from winter rain and snowmelt that flushed NO(3)(-) from the soils following a dry summer and fall. In turn, the elevated stream water NO(3)(-) concentrations stimulated in-stream N(2)O production rates. Overall, we were only able to explain 29% of the variation in N(2)O production rates on a log scale. The unexplained variation may be due to differences in the fraction of denitrified NO(3)(-) that is converted to N(2)O among the study sites, or that our measures of substrate availability in the water column were not reflective of substrate availability in the porewater used by denitrifiers.

Multi-scale measurements and modeling of denitrification in streams with varying flow and nitrate concentration in the upper Mississippi River basin, USA

Denitrification is an important net sink for NO3 − in streams, but direct measurements are limited and in situ controlling factors are not well known. We measured denitrification at multiple scales over a range of flow conditions and NO3 − concentrations in streams draining agricultural land in the upper Mississippi River basin. Comparisons of reach-scale measurements (in-stream mass transport and tracer tests) with local-scale in situ measurements (pore-water profiles, benthic chambers) and laboratory data (sediment core microcosms) gave evidence for heterogeneity in factors affecting benthic denitrification both temporally (e.g., seasonal variation in NO3 − concentrations and loads, flood-related disruption and re-growth of benthic communities and organic deposits) and spatially (e.g., local stream morphology and sediment characteristics). When expressed as vertical denitrification flux per unit area of streambed (U denit, in μmol N m−2 h−1), results of different methods for a given set of conditions commonly were in agreement within a factor of 2–3. At approximately constant temperature (~20 ± 4°C) and with minimal benthic disturbance, our aggregated data indicated an overall positive relation between U denit (~0–4,000 μmol N m−2 h−1) and stream NO3 − concentration (~20–1,100 μmol L−1) representing seasonal variation from spring high flow (high NO3 −) to late summer low flow (low NO3 −). The temporal dependence of U denit on NO3 − was less than first-order and could be described about equally well with power-law or saturation equations (e.g., for the unweighted dataset, U denit ≈26 * [NO3 −]0.44 or U denit ≈640 * [NO3 −]/[180 + NO3 −]; for a partially weighted dataset, U denit ≈14 * [NO3 −]0.54 or U denit ≈700 * [NO3 −]/[320 + NO3 −]). Similar parameters were derived from a recent spatial comparison of stream denitrification extending to lower NO3 − concentrations (LINX2), and from the combined dataset from both studies over 3 orders of magnitude in NO3 − concentration. Hypothetical models based on our results illustrate: (1) U denit was inversely related to denitrification rate constant (k1denit, in day−1) and vertical transfer velocity (v f,denit, in m day−1) at seasonal and possibly event time scales; (2) although k1denit was relatively large at low flow (low NO3 −), its impact on annual loads was relatively small because higher concentrations and loads at high flow were not fully compensated by increases in U denit; and (3) although NO3 − assimilation and denitrification were linked through production of organic reactants, rates of NO3 − loss by these processes may have been partially decoupled by changes in flow and sediment transport. Whereas k1denit and v f,denit are linked implicitly with stream depth, NO3 − concentration, and(or) NO3 − load, estimates of U denit may be related more directly to field factors (including NO3 − concentration) affecting denitrification rates in benthic sediments. Regional regressions and simulations of benthic denitrification in stream networks might be improved by including a non-linear relation between U denit and stream NO3 − concentration and accounting for temporal variation.

The influence of topography on the stream N concentration in the Tanzawa Mountains, Southern Kanto District, Japan

The water chemistry of 51 headwater streams was studied in the Tanzawa Mountains, western fringe of Southern Kanto Plain, Japan. The relationships to soil N processes and catchment topography were also evaluated using a geographic information system with fine-scale map data. The average concentration of total dissolved N was 0.74 mg-N L−1, of which 95% consisted of NO3−-N. Stream N concentrations were not different among bedrock geologies and among vegetations of the catchments. Stream NO3−-N marginally correlated to soil nitrification. Stream NO3−-N also tended to be high in areas with steep and south-facing slopes. These results imply that N transport from Tanzawa forest ecosystems is related to hydrological and biological processes associated with catchment topography.

A modified nitrogen budget for temperate deciduous forests in an advanced stage of nitrogen saturation

Recent advances in ecosystem descriptions associated with nitrogen (N) saturated forests allowed us to revise and improve a model of ecosystem productivity to reflect advanced N saturation. We introduce a new description of nitrogen cycling for a forest that is in stage 2 of N saturation on the basis of recent literature. We compared the accuracy of this new description to the conventional N cycling construct by assessing the degree to which each N cycle description predicted observations in a 100‐year‐old N‐saturated watershed in the Fernow Experimental Forest. The new literature‐based model of saturation outperformed the original model by more accurately predicting aboveground net primary productivity, foliar N, and stream NO3−. Regional estimates of forest productivity and terrestrial carbon sequestration have generally overlooked variable N availability as a vector for forest growth changes. New findings about N‐saturated ecosystem processes refine productivity models and improve estimates of regional ecosystem changes in response to N saturation.

Twenty years apart: Comparisons of DOM uptake during leaf leachate releases to Hubbard Brook Valley streams in 1979 versus 2000

In seven experiments conducted in late summer 1979 and 2000 extracts of autumn‐shed sugar maple leaves and spruce needles were added to two stream reaches of the Hubbard Brook Experimental Forest at concentrations similar to those found during peak leaf fall. Uptake rates for dissolved organic carbon (DOC) ranged from 7 to 408 mg m−2 h−1. In Bear Brook uptake rates for spruce leachate were lower than for sugar maple leachate in both 1979 and 2000. Uptake velocities tended to be higher for more bioavailable DOC fractions: in 1979, monomeric carbohydrates in DOM were removed from the water column more rapidly than the bulk DOC pool in 4 of 4 cases; in 2000, uptake velocities for N‐containing DOC (DON) were higher than those measured for bulk DOC in 2 of 3 cases. Both results suggest that biological assimilation is an important component of DOM uptake, in contrast to earlier reports that DOM uptake in HBEF streams was primarily through abiotic sorption. In both years, uptake velocities for the same forms of DOM were lower in the high DOC stream Cascade Brook than for the low DOC stream Bear Brook. Experiments in 1979 found no change in inorganic nitrogen concentrations during leachate additions, while all leachate releases in 2000 led to reductions in stream NO3− concentrations.

Seasonal dynamics of dissolved nitrogen exports from two High Arctic watersheds, Melville Island, Canada

This study examines the magnitude and seasonal patterns of dissolved N export during the 2006 melt season from two small watersheds on Melville Island, in the Canadian High Arctic. The dominant N species was dissolved organic nitrogen (DON), comprising >80% of the seasonal nitrogen flux from both rivers. The total DON and dissolved inorganic nitrogen (DIN, NH4++NO3−) yields from the catchments were similar (183 and 204 kg DON, and 46–42 kg DIN): however, on a per unit area basis the West catchment had greater yields of both DON and DIN. There were also differences in the temporal patterns and concentrations of the N species between the catchments, which may be a function of inter-catchment differences in vegetation cover. Low end-of-season DIN concentrations in the West river suggest there is strong biogeochemical retention of inorganic N in this catchment during the growing season relative to the East catchment, where stream NO3− concentrations increase through July. A decrease in DOC:DON ratios in the East river at the end of season also indicates a change in the composition of dissolved organic matter in this river that is not evident in the West river. The results illustrate that there can be large differences in the processes controlling nitrogen between two adjacent and similar catchments.

Riparian control on NO3 −, DOC, and dissolved Fe concentrations in mountainous streams, northern Japan

We evaluated (1) the longitudinal pattern of stream chemistry and (2) the effects of the riparian zone on this longitudinal pattern for nitrate (NO3−), dissolved organic carbon (DOC), and total dissolved iron (Fe). We selected two small watersheds; the “southern watershed” had an extending riparian wetland and the “northern watershed” had a narrow riparian area. Stream NO3− concentrations decreased from the spring to outlet of both watersheds. In the southern watershed, stream DOC concentration decreased from the spring to midstream and then increased to the outlet. Stream Fe concentration in the southern watershed longitudinally increased. On the other hand, the northern watershed exhibited no longitudinal pattern for DOC and Fe concentrations. In both watersheds, while NO3− concentrations in the soil and ground water were lower than those in the stream waters, DOC and Fe concentrations exhibited the opposite patterns. The longitudinal decreases of NO3− concentrations in both streams and increase of stream Fe in the southern watershed mainly resulted from the inflow of the soil and ground water to the stream. The decrease in stream DOC from the spring to midstream in the southern watershed was due to the deep groundwater having low DOC, while the subsequent increase to the surrounding soil and ground water. Moreover, considerations of stream solute flow with soil and ground water chemistry suggested other mechanisms adding NO3− and removing/diluting DOC and Fe, especially for the northern watershed; coexistence of oxidizing and reducing conditions in the riparian zone might control the longitudinal concentration change in the stream water chemistry.

Winter climate affects long-term trends in stream water nitrate in acid-sensitive catchments in southern Norway

Abstract. Controls of stream water NO3 in mountainous and forested catchments are not thoroughly understood. Long-term trends in stream water NO3 are positive, neutral and negative, often apparently independent of trends in N deposition. Here, time series of NO3 in four small acid-sensitive catchments in southern Norway were analysed in order to identify likely drivers of long-term changes in NO3. In two sites, stream water NO3 export declined ca 50% over a period of 25 years while in the other sites NO3 export increased with roughly 20%. Discharge and N deposition alone were poor predictors of these trends. The most distinct trends in NO3 were found in winter and spring. Empirical models explained between 45% and 61% of the variation in weekly concentrations of NO3, and described both upward and downward seasonal trends tolerably well. Key explaining variables were snow depth, discharge, temperature and N deposition. All catchments showed reductions in snow depth and increases in winter discharge. In two inland catchments, located in moderate N deposition areas, these climatic changes appeared to drive the distinct decreases in winter and spring concentrations and fluxes of NO3. In a coast-near mountainous catchment in a low N deposition area, these climatic changes appeared to have the opposite effect, i.e. lead to increases in especially winter NO3. This suggests that the effect of a reduced snow pack may result in both decreased and increased catchment N leaching depending on interactions with N deposition, soil temperature regime and winter discharge.

Land-Use Change and Stream Water Fluxes: Decadal Dynamics in Watershed Nitrate Exports

Stream water exports of nutrients and pollutants to water bodies integrate internal and external watershed processes that vary in both space and time. In this paper, we explore nitrate (NO3) fluxes for the 326 km2 mixed-land use Fall Creek watershed in central New York for 1972–2005, and consider internal factors such as changes in land use/land cover, dynamics in agricultural production and fertilizer use, and external factors such as atmospheric deposition. Segmented regression analysis was applied independently to dormant and growing seasons for three portions of the period of record, which indicated that stream water NO3 concentrations increased in both dormant and growing seasons from the 1970s to the early 1990s at all volumes of streamflow discharge. Dormant season NO3 concentrations then decreased at all flow conditions between the periods 1987–1993 and 1994–2005. Results from a regression-based stream water loading model (LOADEST) normalized to mean annual concentrations showed annual modeled NO3 concentration in stream water increased by 34% during the 1970s and 1980s (from 1.15 to 1.54 mg l−1), peaked in about 1989, and then decreased by 29% through 2005 (to 1.09 mg l−1). Annual precipitation had the strongest correlation with stream water NO3 concentrations (r = −0.62, P = 0.01). Among land use factors, corn production for grain was the variable most highly correlated to stream water NO3 concentrations (r = 0.53, P = 0.01). The strongest associative trend determined using Chi-squared Automatic Interaction Detection (CHAID) was found between stream water NO3 concentrations and N-equivalence of dairy production (Bonferroni adjusted P value = 0.0003). Large increases in dairy production were coincident with declining nitrate concentrations over the past decade, which suggest that dairy management practices may have improved in the watershed. However, because dairy production in the Fall Creek watershed has been fueled by large increases in feed imports, the environmental costs of feed production have likely been externalized to other watersheds.

Flow path influence on an N:P ratio in two headwater streams: A paired watershed study

Hydrologic flow paths were assessed and stream water quality was monitored in two adjacent watersheds to improve the understanding of how flow paths impact stream water nitrate nitrogen to total phosphorus ratios (NO3:TP). We hypothesized that a watershed with mostly subsurface flow paths will have high stream water NO3:TP ratios relative to a watershed with a greater tendency to have overland flow paths. Flow paths were assessed with end member mixing analysis (EMMA), stream water temperature, and stream discharge flashiness. The EMMA suggested that groundwater comprised a larger percentage of stream discharge in the west stream compared to the east stream. The analysis of stream water temperature and stream flashiness corroborated the results from the EMMA. Nitrate and TP concentrations were similar in both streams during low discharge. However, during storm flows nitrate decreased and TP increased in the east stream, while nitrate and TP concentrations remained similar during all discharge rates in the west stream. Thus, NO3:TP ratios in the east stream dramatically decreased during peak storm flows, while NO3:TP ratios remained relatively stable during all discharge rates in the west stream. Exploring hydrological processes helped explain the temporal variation within and spatial variation between the two streams, with regard to NO3:TP ratios. Further attention is necessary to understand whether hydrologic flow paths act as a control of other stream water N:P ratios and quantify flow path control relative to in‐stream processes.

Isotopic Apportionment of Atmospheric and Sewage Nitrogen Sources in Two Connecticut Rivers

We used the dual isotope approach to identify sources of nitrate (NO3-) to two mixed land-use watersheds draining to Long Island Sound. In contrastto previous work, we found that sewage effluent NO3- was not consistently enriched in 15N. However, these effluents followed a characteristic denitrification line in delta15N-delta18O space, which could be used as a source signature. We used this signature, together with those of atmospheric deposition and microbial nitrification, to calculate ranges of possible contributions from each of these sources. These estimates are unaffected by any denitrification that may have taken place in soils or streams. Our estimates for atmospheric nitrogen only include unprocessed atmospheric deposition, i.e., NO3-that is not taken up in watershed soils before being delivered to rivers. Using this method, the contribution of atmospheric NO3- could be assessed with good precision and was found to be very low at all our sampling sites during baseflow. During a moderate storm event, atmospheric deposition contributed up to approximately 50% of stream NO3-, depending on the site, with the sites that experienced more stormflow showing a greater contribution of atmospheric NO3-. Our estimates of sewage contribution generally had too large a range to be useful.

DENITRIFICATION IN NITRATE-RICH STREAMS: APPLICATION OF N2:Ar AND15N-TRACER METHODS IN INTACT CORES

Rates of benthic denitrification were measured using two techniques, membrane inlet mass spectrometry (MIMS) and isotope ratio mass spectrometry (IRMS), applied to sediment cores from two NO3(-)-rich streams draining agricultural land in the upper Mississippi River Basin. Denitrification was estimated simultaneously from measurements of N2:Ar (MIMS) and 15N[N2] (IRMS) after the addition of low-level 15NO3- tracer (15N:N = 0.03-0.08) in stream water overlying intact sediment cores. Denitrification rates ranged from about 0 to 4400 micromol N x m(-2) x h(-1) in Sugar Creek and from 0 to 1300 micromol N x m(-2) x h(-1) in Iroquois River, the latter of which possesses greater streamflow discharge and a more homogeneous streambed and water column. Within the uncertainties of the two techniques, there is good agreement between the MIMS and IRMS results, which indicates that the production of N2 by the coupled process of nitrification/denitrification was relatively unimportant and surface-water NO3- was the dominant source of NO3- for benthic denitrification in these streams. Variation in stream NO3- concentration (from about 20 micromol/L during low discharge to 1000 micromol/L during high discharge) was a significant control of benthic denitrification rates, judging from the more abundant MIMS data. The interpretation that NO3- concentration directly affects denitrification rate was corroborated by increased rates of denitrification in cores amended with NO3-. Denitrification in Sugar Creek removed < or = 11% per day of the instream NO3- in late spring and removed roughly 15-20% in late summer. The fraction of NO3- removed in Iroquois River was less than that of Sugar Creek. Although benthic denitrification rates were relatively high during periods of high stream flow, when NO3 concentrations were also high, the increase in benthic denitrification could not compensate for the much larger increase in stream NO3- fluxes during high flow. Consequently, fractional NO3- losses were relatively low during high flow.

Contribution of dissolved organic nitrogen deposition to nitrogen saturation in a forested mountainous watershed in Tsukui, Central Japan

Nitrogen (N) budget was estimated with dissolved inorganic N (DIN) and dissolved organic N (DON) in a forested mountainous watershed in Tsukui, Kanagawa Prefecture, about 50 km west of Tokyo in Central Japan. The forest vegetation in the watershed was dominant by Konara oak (Quercus serrata) and Korean hornbeam (Carpinus tschonoskii), and Japanese cedar (Cryptomeria japonica). Nitrate (NO3−) concentration in the watershed streamwater was averagely high (98.0 ±± 19 (±± SD, n = 36) μmol L−1) during 2001–2003. There was no seasonal and annual changes in the stream NO−3 concentration even though the highest N uptake rate presumably occurred during the spring of plant growing season, a fact indicating that N availability was in excess of biotic demands. The DON deposition rates (DON input rates) in open area and forest area were estimated as one of the main N sources, accounting for about 32% of total dissolved N (TDN). It was estimated that a part of the DON input rate contributed to the excessive stream NO−3 output rate under the condition of the rapid mineralization and nitrification rates, which annual DON deposition rates were positively correlated with the stream NO3− output rates. The DON retention rate in the DON budget had a potential capacity, which contributed to the excessive stream NO−3 output rate without other N contributions (e.g. forest floor N or soil N).