Biogeochemistry of unpolluted forested watersheds in the Oregon Cascades: temporal patterns of precipitation and stream nitrogen fluxes

We analyzed long-term organic and inorganic nitrogen inputs and outputs in precipitation and streamwater in six watersheds at the H.J. Andrews Experimental Forest in the central Cascade Mountains of Oregon. Total bulk N deposition, averaging 1.6 to 2.0 kg N ha^–1 yr^–1, is low compared to other sites in the United States and little influenced by anthropogenic N sources. Streamwater N export is also low, averaging <1 kg ha^–1 yr^–1. DON is the predominant form of N exported from all watersheds, followed by PON, NH₄-N, and NO₃-N. Total annual stream discharge was a positive predictor of annual DON output in all six watersheds, suggesting that DON export is related to regional precipitation. In contrast, annual discharge was a positive predictor of annual NO₃-N output in one watershed, annual NH₄-N output in three watersheds, and annual PON output in three watersheds. Of the four forms of N, only DON had consistent seasonal concentration patterns in all watersheds. Peak streamwater DON concentrations occurred in November-December after the onset of fall rains but before the peak in the hydrograph, probably due to flushing of products of decomposition that had built up during the dry summer. Multiple biotic controls on the more labile nitrate and ammonium concentrations in streams may obscure temporal DIN flux patterns from the terrestrial environment. Results from this study underscore the value of using several watersheds from a single climatic zone to make inferences about controls on stream N chemistry; analysis of a single watershed may preclude identification of geographically extensive mechanisms controlling N dynamics.     

Forest nitrogen sinks in large eastern U.S. watersheds: estimates from forest inventory and an ecosystem model

The eastern U.S. receives elevated rates of Ndeposition compared to preindustrial times, yetrelatively little of this N is exported indrainage waters. Net uptake of N into forestbiomass and soils could account for asubstantial portion of the difference between Ndeposition and solution exports. We quantifiedforest N sinks in biomass accumulation andharvest export for 16 large river basins in theeastern U.S. with two separate approaches: (1)using growth data from the USDA ForestService's Forest Inventory and Analysis (FIA)program, and (2) using a model of forestnitrogen cycling (PnET-CN) linked to FIAinformation on forest age-class structure. Themodel was also used to quantify N sinks in soiland dead wood, and nitrate losses below therooting zone. Both methods agreed that netgrowth rates were highest in the relativelyyoung forests on the Schuylkill watershed, andlowest in the cool forests of northern Maine. Across the 16 watersheds, wood export removedan average of 2.7 kg N ha^–1 yr^–1(range: 1–5 kg N ha^–1 yr^–1), andstanding stocks increased by 4.0 kg N ha^–1yr^–1 (–3 to 8 kg N ha^–1 yr^–1). Together, these sinks for N in woody biomassamounted to a mean of 6.7 kg N ha^–1yr^–1 (2–9 kg N ha^–1 yr^–1), or73% (15–115%) of atmospheric N deposition. Modeled rates of net N sinks in dead wood andsoil were small; soils were only a significantnet sink for N during simulations ofreforestation of degraded agricultural sites. Predicted losses of nitrate depended on thecombined effects of N deposition, and bothshort- and long-term effects of disturbance. Linking the model with forest inventoryinformation on age-class structure provided auseful step toward incorporating realisticpatterns of forest disturbance status acrossthe landscape.     

Residual phosphorus in runoff from successional forest on abandoned agricultural land: 2. Hydrological and soluble reactive P Budgets

Residual P from historical farm practices hasbeen linked to elevated soluble reactivephosphorus (SRP) transport in runoff from afield study site in the Catskills Mountains,New York, U.S.A., with a P source assay indicatingthat successional forest floor biomass was themajor contributor to runoff SRP. In thispaper, we assemble hydrological and SRP budgetsthat indicate net SRP loss of 0.123 kgha^–1 yr^–1 occurs from the site(composed of 0.044 kg ha^–1 yr^–1precipitation input, with 0.143 kg ha^–1yr^–1 and 0.024 kg ha^–1 yr^–1losses in runoff and groundwater,respectively). These findings contrast withconservative P cycling reported for undisturbedforests. Coupled hydrological and SRP data areanalyzed suggesting that catchment ambient andequilibrium SRP concentrations corresponding togroundwater and long-term average runoffconcentrations are in the range capable ofcontributing to eutrophication of receivingwaters. A physically based variable sourcearea hydrological model is tested to simulateSRP export using deterministic concentrations. The three-layer model (surface runoff, shallowlateral flow, and groundwater) is parameterizedusing spatially distributed data fromadditional P source assays and fieldhydrological monitoring for the site. Differences in simulated and observed outflowand SRP export are partially explained byforest evapotranspiration and frozen soilprocesses. The field data, SRP budgets andsimulations show that sufficient residual Ppools exist to prolong net SRP loss rates untilecosystem processes re-establish moreconservative P cycling.     

Nitrogen Fluxes and Retention in Urban Watershed Ecosystems

Although the watershed approach has long been used to study whole-ecosystem function, it has seldom been applied to study human-dominated systems, especially those dominated by urban and suburban land uses. Here we present 3 years of data on nitrogen (N) losses from one completely forested, one agricultural, and six urban/suburban watersheds, and input–output N budgets for suburban, forested, and agricultural watersheds. The work is a product of the Baltimore Ecosystem Study, a long-term study of urban and suburban ecosystems, and a component of the US National Science Foundations long-term ecological research (LTER) network. As expected, urban and suburban watersheds had much higher N losses than did the completely forested watershed, with N yields ranging from 2.9 to 7.9 kg N ha^–1 y^–1 in the urban and suburban watersheds compared with less than 1 kg N ha^–1 y^–1 in the completely forested watershed. Yields from urban and suburban watersheds were lower than those from an agricultural watershed (13–19.8 kg N ha^–1 y^–1). Retention of N in the suburban watershed was surprisingly high, 75% of inputs, which were dominated by home lawn fertilizer (14.4 kg N ha^–1 y^–1) and atmospheric deposition (11.2 kg N ha^–1 y^–1). Detailed analysis of mechanisms of N retention, which must occur in the significant amounts of pervious surface present in urban and suburban watersheds, and which include storage in soils and vegetation and gaseous loss, is clearly warranted.     

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N<Subscript>2</Subscript>-fixing Red Alder

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N₂-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r ² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha^–1 y^–1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.     

Nitrogen export by surface runoff from a small agricultural watershed in southeast China: seasonal pattern and primary mechanism

The seasonal pattern and primary mechanism of nitrogen (N) export by surface runoff from the Wuchuan subwatershed (WCW), an agricultural upper watershed (1.88 km²) located in southeast China, were investigated based on extensive streamwater measurements in 2004–2005 under subtropical climatic conditions. The results disclosed a highly variable but strong linkage between hydrological and anthropogenic controls and N export. N export via surface runoff presented a significant seasonal pattern caused by changes in rainfall and watershed N input. Approximately 75% of the annual N export (67 kg ha⁻¹) was flushed by those storm runoff mainly occurred during the wet season (March through September). The WCW dataset of N concentrations and loads during both baseflow and stormflow implied an interactive effects of anthropogenetic N input and hydrology conditions: N export was flush-driven in late spring, summer and autumn (wet season), but highly related with soil N in winter and early spring. Compared to undisturbed watersheds under similar rainfall conditions, WCW exported a considerable amount of N due to intensive fertilizer application (a mean of 690 kg N ha⁻¹ year⁻¹, commonly as surface applications). This work provides a first characterization of a small agricultural Chinese catchment under subtropical climates and its associated N export behavior.     

Retention of nutrients in river basins

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha^–1 and 0.63 kg P ha^–1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export.Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m^–2 yr^–1 and 3.7–8.3 g P m^–2 yr–1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m^–2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr^–1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr^–1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m^–2 yr^–1 and 0.30 g P m^–2 yr^–1, respectively, during the period 1989–1995.The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr^–1 and that P-retention increased from –0.80 to 0.90 tonnes yr^–1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.     

Yield of nitrogen from minimally disturbed watersheds of the United States

Watersheds of the US Geological Survey's Hydrologic Benchmark Network program were used in estimating annual yield of total nitrogen and nitrogen fractions (ammonium, nitrate, dissolved organic N, particulate N) in relation to amount of runoff, elevation, and watershed area. Only watersheds minimally disturbed with respect to the nitrogen cycle were used in the analysis (mostly natural vegetation cover, no point sources of N, atmospheric deposition of inorganic N < 10 kg ha^–1 y^–1). Statistical analysis of the yields of total nitrogen and nitrogen fractions showed that elevation and watershed area bear no significant relationship to nitrogen yield for these watersheds. The yields of total nitrogen and nitrogen fractions are, however, strongly related to runoff (r ² = 0.91 for total N). Annual yield increases as runoff increases, but at a rate lower than runoff; annual discharge-weighted mean concentrations decline as annual runoff increases. Yields of total nitrogen and most nitrogen fractions bear a relationship to runoff that is nearly indistinguishable from a relationship that was documented previously for minimally disturbed watersheds of the American tropics. Overall, the results suggest strong interlatitudinal convergence of yields and percent fractionation for nitrogen in relation to runoff.     

The nitrogen cycle in lodgepole pine forests,southeastern Wyoming

Storage and flux of nitrogen were studied in several contrasting lodgepole pine (Pinus contorta spp.latifolia) forests in southeastern Wyoming. The mineral soil contained most of the N in these ecosystems (range of 315–860 g · m^–2), with aboveground detritus (37.5–48.8g · m^–2) and living biomass (19.5–24.0 g · m^–2) storing much smaller amounts. About 60–70% of the total N in vegetation was aboveground, and N concentrations in plant tissues were unusually low (foliage = 0.7% N), as were N input via wet precipitation (0.25 g · m^–2 · yr^–1), and biological fixation of atmospheric N (<0.03 g · m^–2 · yr^–1, except locally in some stands at low elevations where symbiotic fixation by the leguminous herbLupinus argenteus probably exceeded 0.1 g · m^–2 · yr^–1).Because of low concentrations in litterfall and limited opportunity for leaching, N accumulated in decaying leaves for 6–7 yr following leaf fall. This process represented an annual flux of about 0.5g · m^–2 to the 01 horizon. Only 20% of this flux was provided by throughfall, with the remaining 0.4g · m^–2 · yr^–1 apparently added from layers below. Low mineralization and small amounts of N uptake from the 02 are likely because of minimal rooting in the forest floor (as defined herein) and negligible mineral N (< 0.05 mg · L^–1) in 02 leachate. A critical transport process was solubilization of organic N, mostly fulvic acids. Most of the organic N from the forest floor was retained within the major tree rooting zone (0–40 cm), and mineralization of soil organic N provided NH₄ for tree uptake. Nitrate was at trace levels in soil solutions, and a long lag in nitrification was always observed under disturbed conditions. Total root nitrogen uptake was calculated to be 1.25 gN · m^–2 · yr^–1 with estimated root turnover of 0.37-gN · m^–2 · yr^–1, and the soil horizons appeared to be nearly in balance with respect to N. The high demand for mineralized N and the precipitation of fulvic acid in the mineral soil resulted in minimal deep leaching in most stands (< 0.02 g · m^–2 · yr^–1). These forests provide an extreme example of nitrogen behavior in dry, infertile forests.     

N deposition, N transformation and N leaching in acid forest soils

Nitrogen deposition, mineralisation, uptake and leaching were measured on a monthly basis in the field during 2 years in six forested stands on acidic soils under mountainous climate. Studies were conducted in three Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantations (D20: 20 year; D40: 40 yr; D60: 60 yr) on abandoned croplands in the Beaujolais Mounts; and two spruce (Picea abies Karst.) plantations (S45: 45 yr; S90: 90 yr) and an old beech (Fagus sylvatica L.) stand (B150: 150 yr) on ancient forest soils in a small catchment in the Vosges Mountains. N deposition in throughfall varied between 7–8 kg ha^–1 year^–1 (D20, B150, S45) and 15–21 kg ha^–1 yr^–1 (S90, D40, D60). N in annual litterfall varied between 20–29 kg ha^–1 (D40, D60, S90), and 36–43 kg ha^–1 (D20, S45, B150). N leaching below root depth varied among stands within a much larger range, between 1–9 kg ha^–1 yr^–1 (B150, S45, D60) and 28–66 kg ha^–1 yr^–1 (D40, S90, D20), with no simple relationship with N deposition, or N deposition minus N storage in stand biomass. N mineralisation was between 57–121 kg ha^–1 yr^–1 (S45, D40, S90) and between 176–209 kg ha^–1 yr^–1 in (B150, D60 and D20). The amounts of nitrogen annually mineralised and nitrified were positively related. Neither general soil parameters, such as pH, soil type, base saturation and C:N ratio, nor deposition in throughfall or litterfall were simply related to the intensity of mineralisation and/or nitrification. When root uptake was not allowed, nitrate leaching increased by 11 kg ha^–1 yr^–1 at S45, 36 kg ha^–1 yr^–1 at S90 and between 69 and 91 kg ha^–1 yr^–1 at D20, D40, B150 and D60, in relation to the nitrification rates of each plot. From this data set and recent data from the literature, we suggest that: high nitrification and nitrate leaching in Douglas-fir soils was likely related to the former agricultural land use. High nitrification rate but very low nitrate leaching in the old beech soil was related to intense recycling of mineralised N by beech roots. Medium nitrification and nitrate leaching in the old spruce stand was related to the average level of N deposition and to the deposition and declining health of the stand. Very low nitrification and N leaching in the young spruce stand were considered representative of fast growing spruce plantations receiving low N deposition on acidic soils of ancient coniferous forests. Consequently, we suggest that past land use and fine root cycling (which is dependent on to tree species and health) should be taken into account to explain the variability in the relation between N deposition and leaching in forests.     

Nitrogen input–output budgets for lake-containing watersheds in the Adirondack region of New York

The Adirondack region of New York is characterized by soils and surface waters that are sensitive to inputs of strong acids, receiving among the highest rates of atmospheric nitrogen (N) deposition in the United States. Atmospheric N deposition to Adirondack ecosystems may contribute to the acidification of soils through losses of exchangeable basic cations and the acidification of surface waters in part due to increased mobility of nitrate (NO₃^–). This response is particularly evident in watersheds that exhibit nitrogen saturation. To evaluate the contribution of atmospheric N deposition to the N export and the capacity of lake-containing watersheds to remove, store, or release N, annual N input–output budgets were estimated for 52 lake-containing watersheds in the Adirondack region from 1998 to 2000. Wet N deposition was used as the N input and the lake N discharge loss was used as the N output based on modeled hydrology and measured monthly solute concentrations. Annual outputs were also estimated for dissolved organic carbon (DOC). Wet N deposition increased from the northeast to the southwest across the region. Lake N drainage losses, which exhibited a wider range of values than wet N deposition, did not show any distinctive spatial pattern, although there was some evidence of a relationship between wet N deposition and the lake N drainage loss. Wet N deposition was also related to the fraction of N removed or retained within the watersheds (i.e., the fraction of net N hydrologic flux relative to wet N deposition, calculated as [(wet N deposition minus lake N drainage loss)/wet N deposition]). In addition to wet N deposition, watershed attributes also had effects on the exports of NO₃^–, ammonium (NH₄⁺), dissolved organic nitrogen (DON), and DOC, the DOC/DON export ratio, and the N flux removed or retained within the watersheds (i.e., net N hydrologic flux, calculated as [wet N deposition less lake N drainage loss]). Elevation was strongly related with the lake drainage losses of NO₃^–, NH₄⁺, and DON, net NO₃^– hydrologic flux (i.e., NO₃^– deposition less NO₃^– drainage loss), and the fraction of net NO₃^– hydrologic flux, but not with the DOC drainage loss. Both DON and DOC drainage losses from the lakes increased with the proportion of watershed area occupied by wetlands, with a stronger relationship for DOC. The effects of wetlands and forest type on NO₃^– flux were evident for the estimated NO₃^– fluxes flowing from the watershed drainage area into the lakes, but were masked in the drainage losses flowing out of the lakes. The DOC/DON export ratios from the lake-containing watersheds were in general lower than those from forest floor leachates or streams in New England and were intermediate between the values of autochthonous and allochthonous dissolved organic matter (DOM) reported for various lakes. The DOC/DON ratios for seepage lakes were lower than those for drainage lakes. In-lake processes regulating N exports may include denitrification, planktonic depletion, degradation of DOM, and the contribution of autochthonous DOM and the influences of in-lake processes were also reflected in the relationships with hydraulic retention time. The N fluxes removed or stored within the lakes substantially varied among the lakes. Our analysis demonstrates that for these northern temperate lake-containing watershed ecosystems, many factors, including atmospheric N deposition, landscape features, hydrologic flowpaths, and retention in ponded waters, regulated the spatial patterns of net N hydrologic flux within the lake-containing watersheds and the loss of N solutes through drainage waters.     

Regional analysis of inorganic nitrogen yield and retention in high-elevation ecosystems of the Sierra Nevada and Rocky Mountains

Yields and retention of dissolved inorganic nitrogen (DIN: NO₃ ^– + NH₄ ⁺) and nitrate concentrations in surface runoff are summarized for 28 high elevation watersheds in the Sierra Nevada of California and Rocky Mountains of Wyoming and Colorado. Catchments ranged in elevation from 2475 to 3603 m and from 15 to 1908 ha in area. Soil cover varied from 5% to nearly 97% of total catchment area. Runoff from these snow-dominated catchments ranged from 315 to 1265 mm per year. In the Sierra Nevada, annual volume-weighted mean (AVWM) nitrate concentrations ranged from 0.5 to 13 M (overall average 5.4 M), and peak concentrations measured during snowmelt ranged from 1.0 to 38 M. Nitrate levels in the Rocky Mountain watersheds were about twice those in the Sierra Nevada; average AVWM NO₃ ^– was 9.4 M and snowmelt peaks ranged from 15 to 50 M. Mean DIN loading to Rocky Mountain watersheds, 3.6 kg ha^–1 yr^–1, was double the average measured for Sierra Nevada watersheds, 1.8 kg ha^–1 yr^–1. DIN yield in the Sierra Nevada, 0.69 kg ha^–1 yr^–1, was about 60% that measured in the Rocky Mountains, 1.1 kg ha^–1 yr^–1. Net inorganic N retention in Sierra Nevada catchments was 1.2 kg ha^–1 yr^–1 and represented about 55% of annual DIN loading. DIN retention in the Rocky Mountain catchments was greater in absolute terms, 2.5 kg ha^–1 yr^–1, and as a percentage of DIN loading, 72%.A correlation analysis using DIN yield, DIN retention and surface water nitrate concentrations as dependent variables and eight environmental features (catchment elevation, slope, aspect, roughness, area, runoff, soil cover and DIN loading) as independent variables was conducted. For the Sierra Nevada, elevation and soil cover had significant (p > 0.1) Pearson product moment correlations with catchment DIN yield, AVWM and peak snowmelt nitrate concentrations and DIN retention rates. Log-linear regression models using soil cover as the independent variable explained 82% of the variation in catchment DIN retention, 92% of the variability in AVWM nitrate and 85% of snowmelt peak NO₃ ^–. In the Rocky Mountains, soil cover was significantly (p < 0.05) correlated with DIN yield, AVWM NO₃ ^– and DIN retention expressed as a percentage of DIN loading (%DIN retention). Catchment mean slope and terrain roughness were positively correlated with steam nitrate concentrations and negatively related to %DIN retention. About 91% of the variation in DIN yield and 79% of the variability in AVWM NO₃ ^– were explained by log-linear models based on soil cover. A log-linear regression based on soil cover explained 90% of the variation of %DIN retention in the Rocky Mountains.     

Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem

Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO₃ ^–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH₄ ⁺) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha^–1yr^–1, significant in comparison to wet Ndeposition (530 mol ha^–1 yr^–1) andstream NO₃ ^– export (25 mol ha^–1yr^–1) in this northern forest ecosystem. Soil solution fluxes of P_i from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha^–1 yr^–1;this elevated mobilization of P_i coincidedwith heightened NO₃ ^– leaching. Elevated leaching of P_i from the forestfloor was coupled with enhanced retention ofP_i in the mineral soil Bs horizon. Thequantities of P_i mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha^–1) as well asnet P mineralization rates in the forest floor(180 mol ha^–1 yr^–1). Increased fineroot mortality was likely an important sourceof mobile N and P_i from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.     

Nitrogen supply rate in Scots pine (Pinus sylvestris L.) forests of contrasting slope aspect

We studied Nitrogen (N) transformations in Pinus sylvestris forest stands in the foothills of the SE Pre-Pyrenees (NE Spain). Plots were selected in two contrasting aspects (two plots per aspect) and N supply rate was measured by the resin-core incubation technique once every three months. N leaching through litter layers (L and F horizons) was evaluated by 5 zero-tension lysimeters in each plot. NH₄ ⁺-N, NO₃ ^--N and soluble organic-N were determined in all solutions. N supply rate showed a clear seasonal pattern. Ammonification and nitrification were segregated in space and in time. While ammonification showed a peak in spring, nitrification was higher in summer. There was evidence suggesting that nitrification occurs mostly in A1 horizon. Nitrification rates differed significantly among plots. N supply rate was 12.7–23.5 kg N·ha^-1·yr^-1 but it did not differ between aspects or plots. Inorganic-N leached through litter layers was 14–17 kg N·ha^-1·yr^-1, and represented a high proportion of N supply rate. Organic-N leached through litter layers (27.8–37.0 kg N·ha^-1·yr^-1) was higher than leached inorganic-N. However, in most cases organic-N did not represent a high proportion of changes in soluble organic-N pools in H and A1 horizons (about 240 kg N·ha^-1·yr^-1). This large decrease in soluble organic-N was much greater than the increase in inorganic-N. The possible fate of these large amounts of organic-N is discussed.     

Nitrogen solutes in an Adirondack forested watershed: Importance of dissolved organic nitrogen

Nitrogen (N) dynamics were evaluated from 1 June 1995 through 31 May 1996 within the Arbutus Lake watershed in the Adirondack Mountains of New York State, USA. At the Arbutus Lake outlet dissolved organic nitrogen (DON), NO₃ ^- and NH₄ ⁺ contributed 61%, 33%, and 6% respectively, to the total dissolved nitrogen (TDN) flux (259 mol ha^-1 yr^-1). At the lake inlet DON, NO₃ ^-, and NH₄ ^- constituted 36%, 61%, and 3% respectively, of TDN flux (349 mol ha^-1 yr^-1). Differences between the factors that control DON, NO₃ ⁺, and NH₄ ⁺ stream water concentrations were evaluated using two methods for estimating annual N flux at the lake inlet. Using biweekly sampling NO₃ ^- and NH₄ ⁺ flux was 10 and 4 mol ha^-1 yr^-1 respectively, less than flux estimates using biweekly plus storm and snowmelt sampling. DON flux was 18 mol ha^-1 yr^-1 greater using only biweekly sampling. These differences are probably not of ecological significance relative to the total flux of N from the watershed (349 mol ha^-1 yr^-1). Dissolved organic N concentrations were positively related to discharge during both the dormant (R² = 0.31; P < 0.01) and growing season (R² = 0.09; P < 0.01). There was no significant relationship between NO₃ ^- concentration and discharge during the dormant season, but a significant negative relationship was found during the growing season (R² = 0.29; P < 0.01). Biotic controls in the growing season appeared to have had a larger impact on stream water NO₃ ^- concentrations than on DON concentrations. Arbutus Lake had a major impact on stream water N concentrations of the four landscape positions sampled, suggesting the need to quantify within lake processes to interpret N solute losses and patterns in watershed-lake systems.     

Modeling the hydrochemistry of the Choptank River basin using GWLF and Arc/Info: 2. Model Validation and Application

We used the hydrochemical model GWLF to estimate terrestrial diffuse fluxes from ungauged areas of a coastal plain catchment, the Choptank River basin. The gauged area of the basin is 17% of the land surface, and we divided the remaining ungauged area into 21 subbasins. Three comparative approaches were used: (1) application of area yield coefficients based on 11 years of observations from the gauged area to extrapolate over ungauged subbasins without modeling, (2) application of GWLF to estimate export from all subbasins using model parameters calibrated in the gauged subbasin, and (3) application of GWLF with parameter adjustments based on the local characteristics in each subbasin. Comparison of the predicted export from 6 selected subbasins with observed export data showed that application of GWLF with local adjustments reduced model errors of N export from 43% to 27%. With only one adjustment for sediment P, application of GWLF alone reduced errors of P export from 92%to 40–45%, with or without local adjustments for flow and sediment retention. The data supported the hypothesis that significant spatial variations in N and P yields introduce large errors when extrapolating from gauged to ungauged subbasins, and estimated TN and TP yield coefficients varied over 1–21 kg N and 0.1–0.5 kg P ha^–1 y^–1 in ungauged areas due to varying human population densities, soil drainage characteristics, and amounts of agriculture. The most accurate estimates of terrestrial diffuse sources were combined with point source discharges and wet atmospheric inputs to estimate annual average inputs of 2.5 × 10⁶ kg N and 5.8 × 10⁴ kg P y^–1 to the Choptank estuary during 1980–1996. These results illustrate the problems of spatial extrapolation from gauged to ungauged areas and emphasize the need for application of local characteristics for accurate assessment of watershed export.     

Retention of soluble organic nutrients by a forested ecosystem

We document an example of a forested watershed at the Coweeta HydrologicLaboratory with an extraordinary tendency to retain dissolved organic matter(DOM) generated in large quantities within the ecosystem. Our objectives weretodetermine fluxes of dissolved organic C, N, and P (DOC, DON, DOP,respectively),in water draining through each stratum of the ecosystem and synthesizeinformation on the physicochemical, biological and hydrologic factors leadingtoretention of dissolved organic nutrients in this ecosystem. The ecosystemretained 99.3, 97.3, and 99.0% of water soluble organic C, N and P,respectively, produced in litterfall, throughfall, and root exudates. Exportsinstreamwater were 4.1 kg ha^–1yr^–1of DOC, 0.191 kg ha^–1 yr^–1 ofDON, and 0.011 kg ha^–1 yr^–1 ofDOP. Fluxes of DON were greater than those of inorganic N in all strata. MostDOC, DON, and DOP was removed from solution in the A and B horizons, with DOCbeing rapidly adsorbed to Fe and Al oxyhydroxides, most likely by ligandexchange. DON and DOC were released gradually from the forest floor over theyear. Water soluble organic C produced in litterfall and throughfall had adisjoint distribution of half-decay times with very labile and veryrefractory fractions so that most labile DOC was decomposed before beingleachedinto the mineral soil and refractory fractions dominated the DOC transportedthrough the ecosystem. We hypothesize that this watershed retained solubleorganic nutrients to an extraordinary degree because the soils have very highcontents of Fe and Al oxyhydroxides with high adsorption capacities and becausethe predominant hydrologic pathway is downwards as unsaturated flow through astrongly adsorbing A and B horizon. The well recognized retention mechanismsforinorganic nutrients combine with adsorption of DOM and hydrologic pathway toefficiently prevent leaching of both soluble inorganic andorganic nutrients in this watershed.     

Biogeochemical storm response in agricultural watersheds of the Choptank River Basin,Delmarva Peninsula,USA

Stream discharge and chemistry (total suspended solids TSS, nitrogen N, and phosphorus P) were monitored for 15 months in six agricultural watersheds on the U.S. Mid-Atlantic coastal plain. Watersheds with similar land uses and a range of hydric soils were used to test the hypothesis that hydric soils generate large storm discharges due to low permeability, resulting in watershed areas with high loss rates of N, P, and TSS. To test the hypothesis, discharge was monitored continuously, and a flow separation method quantified the base and stormflow contributions. Another primary goal was to measure base and stormflow chemistry to quantify N, P, and TSS export. Baseflow chemistry was monitored monthly, and 31 storm events were sampled. Baseflow chemistry varied little over the 15 months, but stormflow chemistry was dynamic, with three major patterns: (1) TSS and particulate N and P had large, brief peaks during the rising limb of storm hydrographs; (2) phosphate and ammonium had broader peaks close to maximum discharges; and (3) nitrate concentrations decreased during the rising limb, slowly returning to pre-storm levels. Event water yields were correlated with volume-weighted mean concentrations (VWMs) of N, P, and TSS, providing a basis for estimating VWMs of unsampled events. Export coefficients (kg ha^?1 year^?1) ranged over 22–33 for TN, 0.9–1.4 for TP, and 240–1140 for TSS. Most P and TSS export occurred during storms (71–99%), while most N export occurred during baseflow (52–84%). The discharge data did not support the hypothesis, and watershed slope, not hydric soils, was the major control on storm discharge. Surface ponding of water on hydric soils intercepted runoff, reducing the impacts of the low infiltration rates.     

Cation distribution,cycling, and removal from mineral soil in Douglas-fir and red alder forests

Overstory species influence the distribution and dynamics of nutrients in forest ecosystems. Ecosystem-level estimates of Ca, Mg, and K pools and cycles in 50-year old Douglas-fir and red alder stands were used to determine the effect of overstory composition on net cation removal from the mineral soil, i.e. cation export from the soil in excess of additions. Net cation removal from Douglas-fir soil was 8 kg Ca ha^–1 yr^–1, 1 kg Mg ha^–1 yr^–1, and 0.3 kg K ha^–1 yr^–1. Annual cation export from soil by uptake and accumulation in live woody tissue and O horizon was of similar magnitude to leaching in soil solution. Atmospheric deposition partially off-set export by adding cations equivalent to 28–88% of cation export. Net cation removal from red alder soil was 58 kg Ca ha^–1 yr^–1, 9 kg Mg ha^–1 yr^–1, and 11 kg K ha^–1 yr^–1. Annual cation accumulation in live woody tissue and O horizon was three times greater than in Douglas-fir, while cation leaching in soil solution was five to eight times greater. The lack of excessive depletion of exchangeable cations in the red alder soil suggests that mineral weathering, rather than exchangeable cations, was the source of most of the removed cations. Nitric acid generated during nitrification in red alder soil led to high rates of weathering and NO₃-driven cation leaching.