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.     

A ground water nitrogen budget for a headwater swamp in an area of permanent ground water discharge

Ground water inputs and outputs of N were studied for a small ground water discharge swamp situated in a headwater drainage basin in southern Ontario, Canada. Darcy's equation with data for piezometers was used to measure inputs of shallow local ground water at the swamp margin and deep regional ground water beneath the swamp. Ground water flux was also quantified by measuring ground water discharge to the outlet stream draining the swamp in combination with a chemical mixing model to separate shallow and deep ground water components based on chloride differences. Estimates of shallow ground water flux determined by these two approaches agreed closely however, the piezometer data seriously underestimated the deep ground water input to the swamp. An average ground water input-output budget of total N (TN) total organic nitrogen (TON) ammonium (NH₄ ⁺-N) and nitrate (NO₃ ^--N) was estimated for stream base flow periods which occurred on an average of 328 days each year during 1986–1990. Approximately 90% of the annual NO₃ ^--N input was contributed by shallow ground water at the swamp margin. Deep ground water represented about 65% of the total ground water input and a similar proportion of TON and NH₄ ⁺-N inputs. Annual ground water NO₃ ^--N inputs and outputs were similar whereas NH₄ ⁺-N retention was 4 kg ha^-1 representing about 68% of annual ground water input. Annual TON inputs in ground water exceeded outputs by 7.7 kg ha (27%). The capacity of the swamp to regulate ground water N fluxes was influenced by the N chemistry of ground water inputs and the hydrologic pathways of transport within the swamp.     

Dissolved organic nitrogen budgets for upland, forested ecosystems in New England

Relatively high deposition ofnitrogen (N) in the northeastern United States hascaused concern because sites could become N saturated.In the past, mass-balance studies have been used tomonitor the N status of sites and to investigate theimpact of increased N deposition. Typically, theseefforts have focused on dissolved inorganic forms ofN (DIN = NH₄-N + NO₃-N) and have largelyignored dissolved organic nitrogen (DON) due todifficulties in its analysis. Recent advances in themeasurement of total dissolved nitrogen (TDN) havefacilitated measurement of DON as the residual of TDN– DIN. We calculated DON and DIN budgets using data onprecipitation and streamwater chemistry collected from9 forested watersheds at 4 sites in New England. TDNin precipitation was composed primarily of DIN. Netretention of TDN ranged from 62 to 89% (4.7 to 10 kgha^{minus 1} yr^{minus 1}) of annual inputs. DON made up themajority of TDN in stream exports, suggesting thatinclusion of DON is critical to assessing N dynamicseven in areas with large anthropogenic inputs of DIN.Despite the dominance of DON in streamwater,precipitation inputs of DON were approximately equalto outputs. DON concentrations in streamwater did notappear significantly influenced by seasonal biologicalcontrols, but did increase with discharge on somewatersheds. Streamwater NO₃-N was the onlyfraction of N that exhibited a seasonal pattern, withconcentrations increasing during the winter months andpeaking during snowmelt runoff. Concentrations ofNO₃-N varied considerably among watersheds andare related to DOC:DON ratios in streamwater. AnnualDIN exports were negatively correlated withstreamwater DOC:DON ratios, indicating that theseratios might be a useful index of N status of uplandforests.     

The influence of nitrogen concentration and ammonium/nitrate ratio on N-uptake,mineral composition and yield of citrus

In short-term water culture experiments with different ¹⁵N labeled ammonium or nitrate concentrations, citrus seedlings absorbed NH₄ ⁺ at a higher rate than NO₃ ^–. Maximum NO₃ ^– uptake by the whole plant occurred at 120 mg L^–1 NO₃ ^–-N, whereas NH₄ ⁺ absorption was saturated at 240 mg L^–1 NH₄ ⁺-N. ¹⁵NH₄ ⁺ accumulated in roots and to a lesser degree in both leaves and stems. However, ¹⁵NO₃ ^– was mostly partitioned between leaves and roots.Adding increasing amounts of unlabeled NH₄ ⁺ (15–60 mg L^–1 N) to nutrient solutions containing 120 mg L^–1 N as ¹⁵N labeled nitrate reduced ¹⁵NO₃ ^– uptake. Maximum inhibition of ¹⁵NO₃ ^– uptake was about 55% at 2.14 mM NH₄ ⁺ (30 mg L^–1 NH₄ ⁺-N) and it did not increase any further at higher NH₄ ⁺ proportions.In a long-term experiment, the effects of concentration and source of added N (NO₃ ^– or NH₄ ⁺) on nutrient concentrations in leaves from plants grown in sand were evaluated. Leaf concentration of N, P, Mg, Fe and Cu were increased by NH₄ ⁺ versus NO₃ ^– nutrition, whereas the reverse was true for Ca, K, Zn and Mn.The effects of different NO₃ ^–-N:NH₄ ⁺-N ratios (100:0, 75:25, 50:50, 25:75 and 0:100) at 120 mg L^–1 total N on leaf nutrient concentrations, fruit yield and fruit characteristics were investigated in another long-term experiment with plants grown in sand cultures. Nitrogen concentrations in leaves were highest when plants were provided with either NO₃ ^– or NH₄ ⁺ as a sole source of N. Lowest N concentration in leaves was found with a 75:25 NO₃ ^–-N/NH₄ ⁺-N ratio. With increasing proportions of NH₄ ⁺ in the N supply, leaf nutrients such as P, Mg, Fe and Cu increased, whereas Ca, K, Mn and Zn decreased. Yield in number of fruits per tree was increased significantly by supplying all N as NH₄ ⁺, although fruit weight was reduced. The number of fruits per tree was lowest with the 75:25 NO₃ ^–-N:NH₄ ⁺-N ratio, but in this treatment fruits reached their highest weight. Rind thickness, juice acidity, and colour index of fruits decreased with increasing NH₄ ⁺ in the N supply, whereas the % pulp and maturity index increased. Percent of juice in fruits and total soluble solids were only slightly affected by NO₃ ^–:NH₄ ⁺ ratio.     

Seasonal Variations of Dissolved Nitrogen and DOC:DON Ratios in an Intermittent Mediterranean Stream

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃–N and NH₄–N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km²) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000–2001 was 70 kg km⁻¹ year⁻¹, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃–N, DON and NH₄–N, respectively.     

Return of Salmon-Derived Nutrients from the Riparian Zone to the Stream during a Storm in Southeastern Alaska

Spawning salmon deliver nutrients (salmon-derived nutrients, SDN) to natal watersheds that can be incorporated into terrestrial and aquatic food webs, potentially increasing ecosystem productivity. Peterson Creek, a coastal watershed in southeast Alaska that supports several species of anadromous fish, was sampled over the course of a storm during September 2006 to test the hypothesis that stormflows re-introduce stored SDN into the stream. We used stable isotopes and PARAFAC modeling of fluorescence excitation–emission spectroscopy to detect flushing of DOM from salmon carcasses in the riparian zone back into a spawning stream. During the early storm hydrograph, streamwater concentrations of NH₄–N and total dissolved phosphorus (TDP), the fluorescent protein tyrosine and the δ¹⁵N content of DOM peaked, followed by a rapid decrease during maximum stormflow. Although δ¹⁵N has previously been used to track SDN in riparian zones, the use of fluorescence spectroscopy provides an independent indicator that SDN are being returned from the riparian zone to the stream after a period of intermediate storage outside the stream channel. Our findings further demonstrate the utility of using both δ¹⁵N of streamwater DOM and fluorescence spectroscopy with PARAFAC modeling to monitor how the pool of streamwater DOM changes in spawning salmon streams.     

滇西北高原纳帕海湿地土壤氮矿化特征

采用树脂芯原位培育法,研究了纳帕海沼泽、沼泽化草甸和草甸土壤氮的矿化特征。结果表明,铵态氮（NH₄⁺-N）为沼泽、沼泽化草甸土壤中无机氮的主要存在形式,分别占无机氮含量的96.76%和75.24%,而硝态氮（NO₃^--N）为草甸土壤中无机氮的主要存在形式,占无机氮含量的58.77%。植物生长期内,纳帕海湿地土壤的净氮矿化速率表现为沼泽化草甸 > 草甸 > 沼泽,表明干湿交替的土壤环境更利于土壤氮矿化作用的进行,土壤中氮素有效性和维持植物可利用氮素的能力更强。整个生长季,沼泽和草甸土壤氮矿化为硝化作用,而沼泽化草甸土壤氮矿化为氨化作用。土壤硝态氮含量、有机质含量、碳氮比和含水量均对纳帕海沼泽、沼泽化草甸和草甸土壤的氮矿化产生显著影响。     

A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream

Patterns of dissolved organic carbon (DOC) and nitrogen (DON) delivery were compared between times of stormflow and baseflow in Paine Run, an Appalachian stream draining a 12.4 km² forested catchment in the Shenandoah National Park (SNP), Virginia. The potential in-stream ecological impact of altered concentrations and/or chemical composition of DOM during storms also was examined, using standardized bacterial bioassays. DOC and DON concentrations in Paine Run were consistently low during baseflow and did not show a seasonal pattern. During storms however, mean DOC and DON concentrations approximately doubled, with maximum concentrations occurring on the rising limb of storm hydrographs. The rapid response of DOM concentration to changes in flow suggests a near-stream or in-stream source of DOM during storms. Stormflow (4% of the time, 36% of the annual discharge) contributed >50% of DOC, DON and NO₃ ^– flux in Paine Run during 1997. In laboratory bacterial bioassays, growth rate constants were higher on Paine Run stormflow water than on baseflow water, but the fraction of total DOM which was bioavailable was not significantly different. The fraction of the total stream DOC pool taken up by water column bacteria was estimated to increase from 0.03 ± 0.02% h^–1 during baseflow, to 0.15 ± 0.04% h^–1 during storms. This uptake rate would have a minimal effect on bulk DOM concentrations in Paine Run, but storms may still have considerable impact on the bacterial stream communities by mobilizing them into the water column and by supplying a pulse of DOM.     

The effects of forest on stream water quality in two coastal plain watersheds of the Chesapeake Bay

Forest had varying effects on stream nutrients in two coastal plain basins of the Delmarva Peninsula, USA. In the Choptank basin, forest was strongly associated with low stream total nitrogen (TN) and nitrate (NO₃⁻) concentrations (r²0.70), and forest placement along first order streams was important in maintaining low stream nitrogen (N) concentrations (r²0.35). In addition, a multiple regression model explained 40% of the stream total phosphorus (TP) variance and indicated that forest directly adjacent to streams (0–100 m) acted as a TP source and forest further away (100–300 m) from streams acted as a TP sink. In contrast, stream nutrients in the nearby Chester basin demonstrated a strong relationship with soil hydrologic properties. Forest had no significant effect on stream N and P because the finer-textured soils, higher stream slopes, and higher runoff potential of the Chester basin appeared to result in less baseflow compared to that in the Choptank basin. This reduced the opportunity for forest to intercept N via plant uptake and denitrification in the high runoff potential soils of the Chester basin. The high percentage of stormflow (40%) coupled with high stream slopes resulted in high soil erosion potential, which may explain the higher TP stream concentrations measured in the Chester compared to that in the Choptank. Differences in the hydrologic pathway appear to explain the different effects of forest on water quality in these two basins.     

Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate

Biochar produced by pyrolysis of biomass can be used to counter nitrogen (N) pollution. The present study investigated the effects of feedstock and temperature on characteristics of biochars and their adsorption ability for ammonium N (NH₄ ⁺-N) and nitrate N (NO₃ ⁻-N). Twelve biochars were produced from wheat-straw (W-BC), corn-straw (C-BC) and peanut-shell (P-BC) at pyrolysis temperatures of 400, 500, 600 and 700°C. Biochar physical and chemical properties were determined and the biochars were used for N sorption experiments. The results showed that biochar yield and contents of N, hydrogen and oxygen decreased as pyrolysis temperature increased from 400°C to 700°C, whereas contents of ash, pH and carbon increased with greater pyrolysis temperature. All biochars could sorb substantial amounts of NH₄ ⁺-N, and the sorption characteristics were well fitted to the Freundlich isotherm model. The ability of biochars to adsorb NH₄ ⁺-N followed: C-BC>P-BC>W-BC, and the adsorption amount decreased with higher pyrolysis temperature. The ability of C-BC to sorb NH₄ ⁺-N was the highest because it had the largest cation exchange capacity (CEC) among all biochars (e.g., C-BC400 with a CEC of 38.3 cmol kg⁻¹ adsorbed 2.3 mg NH₄ ⁺-N g⁻¹ in solutions with 50 mg NH₄ ⁺ L⁻¹). Compared with NH₄ ⁺-N, none of NO₃ ⁻-N was adsorbed to biochars at different NO₃ ⁻ concentrations. Instead, some NO₃ ⁻-N was even released from the biochar materials. We conclude that biochars can be used under conditions where NH₄ ⁺-N (or NH₃) pollution is a concern, but further research is needed in terms of applying biochars to reduce NO₃ ⁻-N pollution.     

Export of dissolved organic carbon and nitrogen from Gleysol dominated catchments – the significance of water flow paths

In this study, we estimated whether changes in hydrological pathwaysduring storms could explain the large temporal variations of dissolvedorganic carbon (DOC) and nitrogen (DON) in the runoff of threecatchments: a forest and a grassland sub-catchment of 1600m² delineated by trenches, and a headwater catchment of 0.7km².The average annual DOC export from the sub-catchments was 185 kg DOCha^–1 y^–1 for the forest, 108 kg DOCha^–1 y^–1 for the grassland and 84 kgDOC ha^–1 y^–1 for the headwatercatchment. DON was the major form of the dissolved N in soil and streamwater. DON export from all catchments was approximately 6 kg Nha^–1 y^–1, which corresponded to 60% ofthe total N export and to 50% of the ambient wet N deposition. DOC andDON concentrations in weekly samples of stream water were positivelycorrelated with discharge. During individual storms, concentrations andproperties of DOC and DON changed drastically. In all catchments, DOCconcentrations increased by 6 to 7 mg DOC l^–1 comparedto base flow, with the largest relative increment in the headwatercatchment (+350%). Concentrations of DON, hydrolysable amino acids, andphenolics showed comparable increases, whereas the proportion ofcarbohydrates in DOC decreased at peak flow. Prediction of DOC and DONconcentrations by an end-member mixing analysis (EMMA) on the base ofinorganic water chemistry showed that changes in water flow pathslargely explained these temporal variability. According to the EMMA, thecontribution of throughfall to the runoff peaked in the initial phase ofthe storm, while water from the subsoil dominated during base flow only.EMMA indicated that the contribution of the DOC and DON-rich topsoil washighest in the later stages of the storm, which explained the highestDOC and DON concentrations as the hydrograph receded. Discrepanciesbetween observed and predicted concentrations were largest for thereactive DOC compounds such as carbohydrates and phenolics. Theyoccurred at base flow and in the initial phase of storms. This suggeststhat other mechanisms such as in-stream processes or a time-variantrelease of DOC also played an important role.     

Change in water quality during the passage through a tropical montane rain forest in Ecuador

We studied five 20-m transects onthe lower slope under tropical lower montanerain forest at 1900–2200 m above sea level. We collectedsamples of soil and of weekly rainfall,throughfall, litter leachate, and stream waterbetween 14 March 1998 and 30 April 1999 anddetermined the concentrations of Al, totalorganic C (TOC), Ca, Cl^–, Cu, K, Mg, Mn,NH₄ ⁺-N, NO₃ ^–-N, total N (TN), Na, P, S, and Zn. The soils were shallowInceptisols; pH ranged 4.4–6.3 in the Ohorizons and 3.9–5.3 in the A horizons, totalCa (6.3–19.3 mg kg^–1) and Mgconcentrations (1.4–5.4) in the O horizon weresignificantly different between the transects.Annual rainfall was 2193 mm; throughfall variedbetween 43 and 91% of rainfall, cloud waterinputs were 3.3 mm a^–1 except forone transect (203). The volume-weighted mean pHwas 5.3 in rainfall and 6.1–6.7 in throughfall.The median of the pH of litter leachate andstream water was 4.8–6.8 and 6.8, respectively.The concentrations of Ca and Mg in litterleachate and throughfall correlatedsignificantly with those in the soil (r =0.76–0.95). Element concentrations inthroughfall were larger than in rainfallbecause of leaching from the leaves (Al, TOC,Ca, K, Mg), particulate dry deposition (TOC,Cu, Cl^–, NH₄ ⁺-N), and gaseousdry deposition (NO₃ ^–-N, total N, S).Net throughfall (= throughfall-rainfalldeposition) was positive for most elementsexcept for Mn, Na, and Zn. High-flow eventswere associated with elevated Al, TOC, Cu, Mn,and Zn concentrations.     

The role of monoterpenes in soil nitrogen cycling processes in ponderosa pine

The effects of select monoterpenes on nitrogen (N) mineralization and nitrification potentials were determined in four separate laboratory bioassays. The effect of increasing monoterpene addition was an initial reduction in NO₃ ^–-N production (nitrification inhibition), followed by a reduction in the sum of NH₄ ⁺-N and NO₃ ^–-N (inhibition of net N mineralization and net immobilization at high monoterpene additions. Monoterpenes could produce this pattern by inhibiting nitrification, reducing net N mineralization, enhancing immobilization of NO₃ ^–-N relative to NH₄ ⁺-N, and/or stimulating overall net immobilization of N by carbon-rich material.Initial monoterpene concentrations in the assay soils were about 5% of the added amount and were below detection after incubation in most samples.Potential N mineralization-immobilization, nitrification, and soil monoterpene concentrations were determined by soil horizon for four collections from a ponderosa pine (Pinus ponderosa) stand in New Mexico. Concentrations of monoterpenes declined exponentially with soil depth and varied greatly within a horizon. Monoterpene content of the forest floor was not correlated with forest floor biomass. Net N mineralization was inversely correlated with total monoterpene content of all sampled horizons. Nitrification was greatest in the mineral soil, intermediate in the F-H horizon, and never occurred in the L horizon. Nitrification in the mineral soil was inversely correlated with the amount of monoterpenes in the L horizon that contain terminal unsaturated carbon-carbon bonds (r ² = 0.37, P 0.01). This pattern in the field corresponded to the pattern shown in the laboratory assays with increasing monoterpene additions.     

Effect of a modular extensive green roof on stormwater runoff and water quality

Runoff quantity and quality from a 248 m² extensive green roof and a control were compared in Connecticut using a paired watershed study. Weekly and individual rain storm samples of runoff and precipitation were analyzed for TKN, NO₃ + NO₂-N, NH₃-N, TP, PO₄-P, and total and dissolved Cu, Pb, Zn, Cd, Cr, and Hg. The green roof watershed retained 51.4% of precipitation during the study period based on area extrapolation. Overall, the green roof retained 34% more precipitation than predicted by the paired watershed calibration equation. TP and PO₄-P mean concentrations in green roof runoff were higher than in precipitation but lower than in runoff from the control. The green roof was a sink for NH₃-N, Zn, and Pb, but not for TP, PO₄-P, and total Cu. It also reduced the mass export of TN, TKN, NO₃ + NO₂-N, Hg, and dissolved Cu primarily through a reduction in stormwater runoff. Greater than 90% of the total Cu, Hg, and Zn concentrations in the green roof runoff were in the dissolved form. The growing media and slow release fertilizer were probable sources of P and Cu in green roof runoff. Overall, the green roof was effective in reducing stormwater runoff and overall pollutant loading for most water quality contaminants.     

Nitrogen dynamics in a eutrophic lake sediment

Nitrogen flux from sediment of a shallow lake and subsequent utilization by water hyacinth (Eichhornia crassipes [Mart] Solms) present in the water column were evaluated using an outdoor microcosm sediment-water column. Sediment N was enriched with ¹⁵N to quantitatively determine the movement of NH₄-N from the sediment to the overlying water column. During the first 30 days. 48% of the total N uptake by water hyacinth was derived from sediment ¹⁵NH₄-N. This had decreased to 14% after 183 days. Mass balance of N indicates that about 25% sediment NH₄-N was released into the overlying water, but only 17% was assimilated by water hyacinth. NH₄-N levels in the water column were very low, with very little or no concentration gradients. NH₄-N levels in the interstitial water of the sediment were in the range of 30–35 mg L^–1 for the lower depths (> 35 cm), while in the surface 5 cm of depth NH₄-N levels decreased to 3.2 mg L^–1. Simulated results also showed similar trends for the interstitial NH₄-N concentration of the sediment. The overall estimated NH₄-N flux from the sediment to the overlying water was 4.8 µg cm^–2 day^–1, and the soluble organic N flux was 5.8 µg N cm^–2 day^–1. Total N flux was 10.6 µg N cm^–2 day^–1.     

Nitrate reduction in sediments of lowland tropical streams draining swamp forest in Costa Rica: An ecosystem perspective

Nitrate reduction and denitrification were measured in swamp forest streams draining lowland rain forest on Costa Rica's Atlantic slope foothills using the C₂H₂-block assay and sediment-water nutrient fluxes. Denitrification assays using the C₂H₂-block technique indicated that the full suite of denitrifying enzymes were present in the sediment but that only a small fraction of the functional activity could be expressed without adding NO₃ ^–. Under optimal conditions, denitrification enzyme activity averaged 15 nmoles cm^–3 sediment h^–1. Areal NO₃ ^– reduction rates measured from NO₃ ^– loss in the overlying water of sediment-water flux chambers ranged from 65 to 470 umoles m^–2 h^–1. Oxygen loss rates accompanying NO₃ ^– depletion averaged 750 umoles m^–2 h^–1. Corrected for denitrification of NO₃ ^– oxidized from NH₄ ⁺ in the sediment, gross NO₃ ^– reduction rates increase by 130 umoles m^–2 h^–1, indicating nitrification may be the predominant source of NO₃ ^– for NO₃ ^– reduction in swamp forest stream sediments. Under field conditions approximately 80% of the increase in inorganic N mass along a 1250-m reach of the Salto River was in the form of NO₃ ^– with the balance NH₄ ⁺ . Scrutiny of potential inorganic N sources suggested that mineralized N released from the streambed was a major source of the inorganic N increase. Despite significant NO₃ ^– reduction potential, swamp forest stream sediments appear to be a source of inorganic N to downstream communities.     

Atmospheric deposition and solute export in giant sequoia — mixed conifer watersheds in the Sierra Nevada,California

Atmospheric depostion and stream discharge and solutes were measured for three years (September 1984 — August 1987) in two mixed conifer watersheds in Sequoia National Park, in the southern Sierra Nevada of California. The Log Creek watershed (50 ha, 2067–2397 m elev.) is drained by a perennial stream, while Tharp's Creek watershed (13 ha, 2067–2255 m elev.) contains an intermittent stream. Dominant trees in the area include Abies concolor (white fir), Sequoiadendron giganteum (giant sequoia), A. magnifica (red fir), and Pinus lambertiana (sugar pine). Bedrock is predominantly granite and granodiorite, and the soils are mostly Pachic Xerumbrepts. Over the three year period, sulfate (SO₄ ^2–), nitrate (NO₃ ^–), and chloride (Cl^–1) were the major anions in bulk precipitation with volume-weighted average concentrations of 12.6, 12.3 and 10.0 eq/1, respectively. Annual inputs of NO₃-N, NH₄-N and SO₄-S from wet deposition were about 60 to 75% of those reported from bulk deposition collectors. Discharge from the two watersheds occurs primarily during spring snowmelt. Solute exports from Log and Tharp's Creeks were dominated by HCO₃ ^–, Ca²⁺ and Na⁺, while H⁺, NO₃ ^–, NH₄ ⁺ and PO₄ ^3– outputs were relatively small. Solute concentrations were weakly correlated with instantaneous stream flow for all solutes (r² <0.2) except HCO₃ ^– (Log Cr. r² = 0.72; Tharp's Cr. r² = 0.38), Na⁺ (Log Cr. r² = 0.56; Tharp's Cr. r² = 0.47), and silicate (Log Cr. r² = 0.71; Tharp's Cr. r² = 0.49). Mean annual atmospheric contributions of NO₃-N (1.6 kg ha^–1), NH₄-N (1.7 kg ha^–1), and SO₄-S (1.8 kg ha^–1), which are associated with acidic deposition, greatly exceed hydrologic losses. Annual watershed yields (expressed as eq ha^–1) of HCO₃ ^– exceeded by factors of 2.5 to 37 the annual atmospheric deposition of H⁺.     

Hydrochemical response during storm events in a South African mountain catchment: the influence of antecedent conditions

The influence of different antecedent conditions on hydrochemical response during storm events was investigated in a small, south-western Cape mountain catchment. Winter and summer storms (four in total) were sampled both before and after the catchment was deliberately burnt. During winter storms, discharge responded rapidly to rainfall, and direct runoff represented the major component of streamflow. Marked lags were observed between rainfall and discharge peaks during the summer storms, and streamflow was dominated by delayed interflow. Chloride, PO _inf4 ^sup3– -P and NO _inf3 ^sup– -N exhibited variable response to discharge according to seasonal variations in soil-moisture levels, whereas the response of HCO _inf3 ^sup– , H⁺ and NH _inf4 ^sup+ -N was not influenced by season. The movement of ions appears to be affected more by geochemical processes operating within the soil than by plant-uptake dynamics, as the prescribed burn appeared to have little effect on relationships between ionic concentration and discharge. The findings of the study highlight the complexity of relationships between solute concentration and discharge.     

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.     

Stream chemistry and hydrologic pathways during snowmelt in a small watershed adjacent Lake Superior

In regions with airborne contaminants and large snowpacks, there is concern over the impact that snowmelt chemical pulses — periods of sharp increase in meltwater solute concentration — could have on aquatic resources during spring runoff. A major variable in determining such an effect is the flow path of snowmelt solutes to the stream or lake. From December 1988, to late April 1989, the quality and quantity of precipitation, snowmelt, soil solution and streamwater were measured in a 176-ha gauged watershed on the south shore of Lake Superior. The main objectives were to (1) examine the change in flow path meltwaters take to the stream during distinct winter and spring hydrologic periods, (2) quantify ecosystem-level ion budgets prior to, during, and following snowmelt, and (3) examine if streamwater chemistry might be a sensitive indicator of change in ecosystem flow paths. Prior to peak snowmelt, groundwater made up 80% of stream discharge. During peak snowmelt, the groundwater level rose to the soil surface resulting in lateral water movement through near-surface macropores and as overland flow. Near the end of snowmelt, melt-waters exerted a piston action on deeper soil solution again increasing its relative contribution to streamwater discharge. Net groundwater drawdown during the study resulted in streamwater discharge about equal to precipitation inputs. Unfrozen soils and brief mid-winter thaws resulted in steady snowmelt throughout early and mid-winter. The snowpack lost > 50% of most ions prior to the period of major snowmelt and high stream discharge in late March and early April. Snowmelt and streamwater NO₃ ^– and NH₄ pulses occurred before the period of overland flow and peak streamwater discharge (April 4–24). During overland flow, stream discharge of total N, P, DOC, and AI peaked. Nutrient budgets computed for distinct hydrologic periods were much more helpful in explaining ecosystem pathways and processes than were changes in solute concentration. For the study period, watershed base cation (C_B) discharge was 23 times input and SO₄ ^2– discharge exceeded input by 42%. H⁺ was the most strongly conserved ion with output < 0.2% of input. Also conserved were NH₄ ⁺ with only 1.4% of input leaving the ecosystem and NO₃ ^– with output equal to 9.4% of input.