Stream NO3 Research Articles

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).

Evaluating Stream Water Quality through Land Use Analysis in Two Grassland Catchments

We evaluated the impacts of natural wetlands and various land uses on stream nitrogen concentration in two grassland-dominated catchments in eastern Hokkaido, Japan. Analyzing land use types in drainage basins, measuring denitrification potential of its soil, and water sampling in all seasons of 2003 were performed. Results showed a highly significant positive correlation between the concentration of stream NO3-N and the proportion of upland area in drainage basins in both catchments. The regression slope, which we assumed to reflect the impact on water quality, was 24% lower for the Akkeshi catchment (0.012 +/- 0.001) than for the Shibetsu catchment (0.016 +/- 0.001). In the Akkeshi catchment, there was a significant negative correlation between the proportion of wetlands in the drainage basins and stream NO3-N concentration. Stream dissolved organic nitrogen (DON) and carbon (DOC) concentrations were significantly higher in the Akkeshi catchment. Upland and urban land uses were strongly linked to increases in in-stream N concentrations in both catchments, whereas wetlands and forests tended to mitigate water quality degradation. The denitrification potential of the soils was highest in wetlands, medium in riparian forests, and lowest in grasslands; and was significant in wetlands and riparian forests in the Akkeshi catchment. The solubility of soil organic carbon (SOC) and soil moisture tended to determine the denitrification potential. These results indicate that the water environment within the catchments, which influences denitrification potential and soil organic matter content, could have caused the difference in stream water quality between the two catchments.

Effect of Reduced Winter Precipitation and Increased Temperature on Watershed Solute Flux, 1988–2002, Northern Michigan

Since 1987 we have studied weekly change in winter (December–April) precipitation, snowpack, snowmelt, soil water, and stream water solute flux in a small (176-ha) Northern Michigan watershed vegetated by 65–85 year-old northern hardwoods. Our primary study objective was to quantify the effect of change in winter temperature and precipitation on watershed hydrology and solute flux. During the study winter runoff was correlated with precipitation, and forest soils beneath the snowpack remained unfrozen. Winter air temperature and soil temperature beneath the snowpack increased while precipitation and snowmelt declined. Atmospheric inputs declined for H+, NO3−, NH4+, dissolved inorganic nitrogen (DIN), and SO42−. Replicated plot-level results, which could not be directly extrapolated to the watershed scale, showed 90% of atmospheric DIN input was retained in surface shallow (<15 cm deep) soils while SO42− flux increased 70% and dissolved organic carbon (DOC) 30-fold. Most stream water base cation (CB), HCO3−, and Cl− concentrations declined with increased stream water discharge, K+, NO3−, and SO42− remained unchanged, and DOC and dissolved organic nitrogen (DON) increased. Winter stream water solute outputs declined or were unchanged with time except for NO3− and DOC which increased. DOC and DIN outputs were correlated with the percentage of winter runoff and stream discharge that occurred when subsurface flow at the plot-level was shallow (<25 cm beneath Oi). Study results suggest that the percentage of annual runoff occurring as shallow lateral subsurface flow may be a major factor regulating solute outputs and concentrations in snowmelt-dominated ecosystems.

Simulating Spatial Nitrogen Dynamics in a Forested Reference Watershed, Hubbard Brook Watershed 6, New Hampshire, USA

We demonstrate that available information on spatial heterogeneity in biotic, topographic, and climatic variables within a forested watershed, Hubbard Brook Experimental Forest (HBEF) Watershed 6, New Hampshire, USA, was sufficient to reproduce the observed elevational pattern in stream NO3 concentration during the 1982–1992 period. Five gridded maps (N mineralization factor, N uptake factor, precipitation, elevation, and soil depth factor) were created from spatial datasets and successively added to the spatially explicit model SINIC-S as spatially varying input parameters. Adding more spatial information generally improved model predictions, with the exception of the soil depth factor. Ninety percent of the variation in the observed stream NO3 concentration was explained by the combination of the spatial variation of the N mineralization and N uptake factors. Simulated streamflow NO3 flux at the outlet point was improved slightly by introducing spatial variability in the model parameters. The model exhibited substantial cell-to-cell variation in soil N dynamics and NO3 loss within the watershed during the simulation period. The simulation results suggest that the spatial distributions of forest floor organic matter and standing biomass are most responsible for creating the elevational pattern in stream NO3 concentration within this watershed.

Decadal-scale Dynamics of Water, Carbon and Nitrogen in a California Chaparral Ecosystem: DAYCENT Modeling Results

The Mediterranean climate, with its characteristic of dry summers and wet winters, influences the hydrologic and microbial processes that control carbon (C) and nitrogen (N) biogeochemical processes in chaparral ecosystems. These biogeochemical processes in turn determine N cycling under chronic N deposition. In order to examine connections between climate and N dynamics, we quantified decadal-scale water, C and N states and fluxes at annual, monthly and daily time steps for a California chaparral ecosystem in the Sierra Nevada using the DAYCENT model. The daily output simulations of net mineralization, stream flow and stream nitrate (NO3−) export were developed for DAYCENT in order to simulate the N dynamics most appropriate for the abrupt rewetting events characteristic of Mediterranean chaparral ecosystems. Overall, the magnitude of annual modeled net N mineralization, soil and plant biomass C and N, nitrate export and gaseous N emission agreed with those of observations. Gaseous N emission was a major N loss pathway in chaparral ecosystems, in which nitric oxide (NO) is the dominant species. The modeled C and N fluxes of net primary production (NPP), N uptake and N mineralization, NO3− export and gaseous N emission showed both high inter-annual and intra-annual variability. Our simulations also showed dramatic fire effects on NPP, N uptake, N mineralization and gaseous N emission for three years of postfire. The decease in simulated soil organic C and N storages was not dramatic, but lasted a longer time. For the seasonal pattern, the predicted C and N fluxes were greatest during December to March, and lowest in the summer. The model predictions suggested that an increase in the N deposition rate would increase N losses through gaseous N emission and stream N export in the chaparral ecosystems of the Sierra Nevada due to changes in N saturation status. The model predictions could not capture stream NO3− export during most rewetting events suggesting that a dry-rewetting mechanism representing the increase in N mineralization following soil wetting needs to be incorporated into biogeochemical models of semi-arid ecosystems.

Long-term Decreases in Stream Nitrate: Successional Causes Unlikely; Possible Links to DOC?

Huntington (2005) poses the hypothesis that ‘‘immobilization of inorganic N, in combination with region-wide recovery from past disturbances may explain the decrease in stream water nitrate in recent decades’’ that we observed in 28 White Mountain, New Hampshire streams (Goodale and others 2003). We focus our response on Huntington s suggestions, and note that a range of additional factors not discussed here can have marked impacts on long-term patterns of NO3 ) loss (for example, N deposition, insect outbreaks, climate variation and extreme events such as soil frost). Below, we address Huntington s suggestion in two parts, first considering the role of region-wide recovery from historical disturbances and then considering recent changes in environmental factors that might have increased soil N immobilization through increased belowground productivity. We then introduce a hypothesis that suggests that changes in stream NO3 ) might be partly linked to observed changes in dissolved organic carbon (DOC). We agree with Huntington that historical disturbances have had substantial impacts on forest carbon and nitrogen dynamics in the White Mountains of New Hampshire (see for example, Bormann and Likens 1979; Covington 1981; Thorne and Hamburg 1985). We have previously considered in some depth the role of the region s disturbance history as a major driver of patterns of organic matter accumulation and watershed N retention (see for example, Vitousek and Reiners 1975; Aber and Driscoll 1997; Goodale and others 2000; Goodale and Aber 2001). We agree that re-accumulation of soil organic matter in re-growing forests provides important sinks for atmospheric CO2 and N deposition across large parts of the eastern U.S. (see for example, Huntington 1995; Hooker and Compton 2003). However, we do not believe that forest re-growth and soil re-accumulation explain the approximately 70% decrease in stream NO3 ) concentration (mean decrease of 25 lmol/L) that we observed between 1973)4 and 1996–97 (Goodale and others 2003). First, we emphasize that we observed large decreases in stream NO3 ) concentration in both aggrading successional systems and in several oldgrowth watersheds that had no evidence of standlevel disturbance by historical logging or fire (see Goodale and others 2003 Figure 6; see also Martin and others 2000). Second, the observed decreases in stream NO3 ) between the 1970s and the 1990s would require a synchronous increase in the rate of soil re-accumulation during this time period across all watersheds. Few successional factors could cause an increase in soil re-accumulation rate, and such factors are unlikely to occur simultaneously in both old-growth and successional watersheds.

Effects of a clearcut on the net rates of nitrification and N mineralization in a northern hardwood forest, Catskill Mountains, New York, USA

The Catskill Mountains of southeastern New York receive among the highest rates of atmospheric nitrogen (N) deposition in eastern North America, and ecosystems in the region may be sensitive to human disturbances that affect the N cycle. We studied the effects of a clearcut in a northern hardwood forest within a 24-ha Catskill watershed on the net rates of N mineralization and nitrification in soil plots during 6 years (1994–1999) that encompassed 3-year pre- and post-harvesting periods. Despite stream NO3− concentrations that increased by more than 1400 μmol l−1 within 5 months after the clearcut, and three measures of NO3− availability in soil that increased 6- to 8-fold during the 1 st year after harvest, the net rates of N mineralization and nitrification as measured by in situ incubation in the soil remained unchanged. The net N-mineralization rate in O-horizon soil was 1– 2 mg N kg−1 day−1 and the net nitrification rate was about 1 mg N kg−1 day−1, and rates in B-horizon soil were only one-fifth to one-tenth those of the O-horizon. These rates were obtained in single 625 m2 plots in the clearcut watershed and reference area, and were confirmed by rate measurements at 6 plots in 1999 that showed little difference in N-mineralization and nitrification rates between the treatment and reference areas. Soil temperature increased 1 ± 0.8 °C in a clearcut study plot relative to a reference plot during the post-harvest period, and soil moisture in the clearcut plot was indistinguishable from that in the reference plot. These results are contrary to the initial hypothesis that the clearcut would cause net rates of these N-cycling processes to increase sharply. The in situ incubation method used in this study isolated the samples from ambient roots and thereby prevented plant N uptake; therefore, the increases in stream NO3− concentrations and export following harvest largely reflect diminished uptake. Changes in temperature and moisture after the clearcut were insufficient to measurably affect the net rates of N mineralization and nitrification in the absence of plant uptake. Soil acidification resulting from the harvest may have acted in part to inhibit the rates of these processes.

Tracing sources of nitrate in snowmelt runoff using a high‐resolution isotopic technique

The denitrifier method to determine the dual isotopic composition (δ15N and δ18O) of nitrate is well suited for studies of nitrogen contributions to streams during runoff events. This method requires only 70 nmol of NO3− and enables high throughput of samples. We studied nitrate sources to a headwater stream during snowmelt by generating a high‐temporal resolution dataset at the Sleepers River Research Watershed in Vermont, USA. In the earliest phase of runoff, stream NO3− concentrations were highest and stream discharge, NO3− concentrations, and δ18O of NO3− generally tracked one another during diurnal melting. The isotopic composition of stream NO3− varied in‐between atmospheric and groundwater NO3− end members indicating a direct contribution of atmospherically‐derived NO3− from the snow pack to the stream. During the middle to late phases of snowmelt, the source shifted toward soil NO3− entering the stream via shallow subsurface flow paths.

Changes in biogeochemical cycling following forest defoliation by pine wilt disease in Kiryu experimental catchment in Japan

AbstractChanges in nutrient budgets and hydrological processes due to the natural disturbance of pine wilt disease (PWD) were monitored in a small, forested watershed in Japan. The disturbance caused changes in soil nitrogen transformations. Pre‐disturbance, mineralized nitrogen remained in the form of NH4+, whereas in disturbed areas most mineralized nitrogen was nitrified. Stream NO3− concentrations increased following PWD. There was a delay between time of disturbance and the increase of NO3− in ground and stream waters. Stream concentrations of NO3− and cations (Ca2+ + Mg2+) were significantly correlated from 1994 to 1996, whereas the correlation among NO3−, H+, and SO42− was significant only in 1995. Although both cation exchange and SO42− adsorption buffered protons, cation exchange was the dominant and continuous mechanism for acid buffering. SO42− adsorption was variable and highly pH dependent. The disturbance also resulted in slight delayed changes of input–output nutrient balances. The nitrogen contribution to PWD litter inputs was 7·39 kmol ha−1, and nitrogen loss from streamwater was less than 0·5 kmol ha−1 year−1 throughout the observation period. This large discrepancy suggested substantial nitrogen immobilization. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Export mechanisms for dissolved organic carbon and nitrate during summer storm events in a glaciated forested catchment in New York, USA

AbstractNitrate and dissolved organic carbon (DOC) concentrations during a summer storm for a forested catchment in the Adirondack Mountains displayed a clear separation in trajectories and timing of maximum values. Nitrate concentrations peaked early on the rising limb of the hydrograph, whereas DOC concentrations gradually increased through the rising limb with maximum concentrations following the discharge peak. Solute data from precipitation, throughfall, soilwater, and ground/till water indicated till water and near‐surface soil waters as the controlling end members for stream NO3− and DOC concentrations respectively. Streamflow concentrations of major base cations (Ca2+ and Mg2+), which were assumed to represent water originating from deep flow paths, matched the NO3− trajectory. These data suggest that streamflow NO3− concentrations are derived from till groundwater and that DOC is derived from near‐surface soil waters. We attributed the early expression of NO3− to the displacement of till waters by infiltrating precipitation. In contrast, we hypothesized that the delayed DOC concentrations occurred with surface and near‐surface runoff from near‐stream wetlands/peatlands and isolated saturated areas that became connected only under conditions of maximum water content in the catchment. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO3−) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO3− concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO3- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO3-. At the regional scale, we observed high variability in the magnitude of NO3- losses: while six of the surveyed watersheds showed accelerated rates of NO3− loss, three did not. Moreover, in contrast to the strong linear relationship between NO3− loss and crown damage within HBEF watersheds [r2: (W1 = 0.91, W6 = 0.85)], stream water NO3− concentrations were weakly related to crown damage (r2 = 0.17) across our regional sites. The efflux of NO3− associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO3− fluxes associated with forest harvesting. Because over one half of the entire year’s worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems.

Effect of a whole-catchment N addition on stream detritus processing

The Bear Brook Watershed in Maine (BBWM) is a paired catchment study investigating ecosystem effects of N and S deposition. Because of the decade long (NH4)2SO4 addition, the treatment catchment has higher stream NO3− and enriched foliar N concentrations compared to the reference catchment. We investigated how both stream N and foliar N affect stream detritus processing. Differences in litter processing were assessed by measuring mass loss, tissue softness, and shredder biomass. To examine both stream effects and leaf source, N-enriched and reference litter bags were prepared for 3 leaf species and placed in each stream. Red maple leaves were examined in 1997, 1998, and 1999. Sugar maple and American beech were examined in 1999. In all years, the only stream effects were increased mass loss for sugar maple and higher shredder biomass for red maple in 1998 in the treatment stream. Several leaf source effects were observed. N-enriched leaves of sugar maple, American beech, and red maple in 1998 had significantly higher microbial activity, as indicated by softer tissue, and had higher total mass loss. Further, shredder biomass tended to be highest in N-enriched litter bags. Although significant effects were detected, our results suggest that elevated dissolved N concentrations as a result of N deposition play a minimal role in regulating stream detritus processing at BBWM. Increased foliar N, however, did influence rates of stream detritus processing by increasing microbial activity, and possibly increasing shredder biomass. Our study shows how N deposited on entire catchments can affect litter processing in stream ecosystems.

Episodic increases in nitrate concentrations in streamwater due to the partial dieback of a pine forest in Japan: runoff generation processes control seasonality

AbstractNitrogen dynamics were investigated in tandem with groundwater monitoring in a temperate forest catchment in Japan, to determine the effect of the partial dieback of dominant tree stands on nutrient cycling and streamwater chemistry, especially temporal and spatial changes in the NO3− concentrations in soil and ground water. Decreased nitrogen uptake by roots and increased nitrogen supply from the litter fall that occurred with the 1992–94 pine dieback caused by pine‐wilt disease brought on a threefold increase in nitrate concentrations in streamwater and subsurface groundwater. It was found that, from 1992 to 1996, seasonal peaks occurred in the stream NO3− concentrations during the rainy season (July to August). This seasonal variation corresponded directly to groundwater levels in the riparian zone near the catchment outlet, suggesting that seasonal changes in groundwater levels were the dominant factor controlling temporal variation in stream NO3− concentrations. The mechanism of the seasonal pattern found in this watershed emphasizes the importance of hydrological seasonality with the high precipitation, groundwater levels, and runoff rates that are seen in summer in Japan, whereas in forested watersheds in Europe and the eastern USA the smaller inorganic nitrogen pools and low transportation forces simultaneously affect NO3− leaching during summer. Copyright © 2003 John Wiley &amp; Sons, Ltd.

An Unexpected Nitrate Decline in New Hampshire Streams

Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO3 concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376 –381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973–74 and 1996 –97, mean NO3 concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 mol/L), Ca 2 decreased by 24% (8 mol/L), and Mg 2 decreased by 22% (5 mol/L).

Analysis of δ15N and δ18O to differentiate NO3− sources in runoff at two watersheds in the Catskill Mountains of New York

To quantify the movement of atmospheric nitrogen deposition through two forested watersheds in the Catskill Mountains of New York, dual‐isotope analysis (δ15N and δ18O) was used to differentiate NO3− derived from precipitation from NO3− derived by microbial nitrification and to quantify the contributions of these sources to NO3− in drainage waters. Samples of stream water, soil water, precipitation, snowmelt, and O‐horizon soil were collected during the March and April snowmelt period of 1994 and throughout an 18‐month period from August 1995 through February 1997. The mean δ18O‐NO3− value of precipitation was +50.5‰, whereas the mean values for stream water and soil water were +17.7‰ and +23.6‰, respectively. The mean δ15N‐NO3− of precipitation was −0.2‰, that of soil water was +1.4‰, and that of stream water was +2.3‰; these values showed greater overlap among the three different waters than did the δ18O‐NO3− values, indicating that δ15N‐NO3− was not as useful for source separation. Soil water δ18O‐NO3− values decreased, and δ15N‐NO3− values increased, from the O to the B and C horizons, but most of the differences among horizons were not statistically significant. Nitrate derived by nitrification in incubated soil samples had a wide range of δ15N‐NO3− values, from +1.5‰ to +16.1‰, whereas δ18O‐NO3− values ranged more narrowly, from +13.2‰ to +16.0‰. Values of δ18O‐NO3− indicated that NO3− in stream water is mainly derived from nitrification. Only during a high‐flow event that exceeded the annual flood was precipitation a major contributor to stream water NO3−. Values of δ18O‐NO3− and δ15N‐NO3− changed at differing rates as NO3− cycled through these watersheds because δ18O‐NO3− values change sharply through the incorporation of oxygen from ambient water and gas during nitrification, whereas δ15N‐NO3− values change only incrementally through fractionation during biocycling processes. The results of this study show that most NO3− is first cycled through the biota and nitrified before entering the stream.

A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (&lt;0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.

Regional Distribution of Natural Stream Nitrate in Central Japan

Over 60 natural streams with forested catchments were surveyed for nitrate (NO3−) concentration in central Japan. Stream NO3− concentrations showed a clear zonal gradient with a range nearly as large as 100-fold, where their small seasonal variations (av. CV = 27%) justified respective statuses. Elevated NO3− concentrations (more than 70 µM) described a distribution of N saturation along a course of polluted air advection in the Kanto Plain. The extent of NO3− leaching from forests was not proportional to atmospheric N deposition probably because of ecosystem N retention. A dependency of NO3− level on the watershed altitude and quite constant δ15N of stream NO3− over a wide NO3− range were observed, which could be also related to this cause.

Soil C/N Ratio as Affected by Climate: An Ecological Factor of Forest NO3− Leaching

The soil nitrogen cycle was investigated for several watershed forests of which stream waters have distinctively different nitrate (NO3−) concentrations. In a watershed with stream water NO3− of more than 100 µM, soil NO3− content increased even beneath the rooting zone, revealing "nitrogen saturation" status. A laboratory soil experiment demonstrated that the proportion of net NO3− production to CO2 production was largely regulated by a soil C/N ratio, suggesting a key parameter for NO3− abundance. In the respective watershed soils, little nitrogen was actually present as NO3− above a soil C/N ratio of 20. The annual mean soil temperature recorded at the sites was correlated with a shift in the C/N ratio in watershed soils (a soil C/N ratio increase of 0.5 per 1°C decrease) along the stream NO3− gradient of 30 fold. The results suggest that soil microbial metabolisms affected by C/N ratio may be a direct agent regulating NO3− leaching from watersheds under the influence of an atmospheric nitrogen load and climate.

Low stream nitrate concentrations associated with oak forests on the Allegheny high plateau of Pennsylvania

Stream nitrate (NO3−) concentrations were 2.6–7.0 times lower in watersheds in which red oak (Quercus rubra) stands were present than in other forested watersheds in northwestern Pennsylvania. Also, seasonal variations in NO3− concentrations were smaller in watersheds with oak stands than in watersheds without oak stands. Stream water concentrations of calcium, magnesium, potassium, sodium, chloride, and sulfate did not differ significantly between the two groups of watersheds. The variability in NO3− concentrations among streams did not appear to be due to differences in atmospheric nitrogen deposition among watersheds. Low stream NO3− concentrations occurred even if only 10–20% of a watershed was covered by oak stands. We suggest that oaks, rather than directly controlling NO3− losses, instead merely occur on soils which have low NO3− losses.

FOG AND ACIDIFICATION IMPACTS ON ION BUDGETS OF BASINS IN NOVA SCOTIA, CANADA

ABSTRACT: We examined hydrogeochemical records for a dozen watersheds in and near Kejimkujik National Park in southwestern Nova Scotia by relating stream ion concentrations and fluxes to atmospheric deposition, stream type (lake inlet versus outlet; brown versus clear water), and watershed type (catchment area, topography, soils, and dominant forest cover type). We found that fog and dry deposition make important contributions to S, N, Cl, H, Ca, Mg, K, and Na inputs into these watersheds. Seasalt chloride deposition from rain, snow, fog, and dry deposition equal total stream outputs on a region-wide basis. Chloride outputs, however, differ among watersheds by a factor of about two, likely due to local differences in air flow and vegetational fog interception. We found that most of the incoming N is absorbed by the vegetation, as stream water NO3- and NH4+ are very low. Our results also show that the vegetation and the soils absorb about half of the incoming SO42. In comparison with other North American watersheds with similar forest vegetation, Ca outputs are low, while Mg and K outputs are similar to other regions. Soil exchangeable Ca and soil cation exchange capacity are also very low. We found that first-order forest streams with no upstream lakes have a distinct seasonal pattern that neither corresponds with the seasonal pattern of atmospheric deposition, nor with the seasonal pattern of downstream lake outlets.