Effects of repeated (NH4)2SO4 application on sulfur pools in soil,soil microbial biomass,and ground vegetation of two watersheds in the Black Forest/Germany

The effect of repeated (NH₄)₂SO₄ applications (3 × 700 kg ha^–1 in 1988, 1991, and 1994, respectively) on S pools in soil, soil microbial biomass, and ground vegetation was studied at two Norway spruce (Picea abies L. [Karst.]) sites in the Black Forest/Germany. In both eco-systems, most of the total S pool was located in the soil. The soil also was the predominant compartment for retention of applied SO₄ ^2--S. The fractions of organic and inorganic S forms in the initial soil S content, and the retention of experimentally applied S was different for both sites. In the podzol Schluchsee, organic S accounted for 92% of total S. In the cambisol Villingen, the S pool consisted of 33% organic S and 67% inorganic S. The retention of applied S in various compartments of both ecosystems reflected these proportions. Only minor amounts of fertilized S (<1%) was retained in the spruce trees, ground vegetation, and soil microbial biomass. However, between 51% (Villingen) and 72% (Schluchsee) of the applied S was retained in the soil. In the Schluchsee podzol, 75% of retained fertilizer S was accumulated as ester sulfate, whereas SO₄ ^2-adsorption and precipitation of Al hydroxy sulfates were restricted by dissolved organic matter in the soil solution. In the Villingen cambisol, SO₄ ^2- adsorption was the dominant process of S retention, although 20% of the fertilized S again was retained as ester sulfate. The significant relevance of organic S forms in the retention of fertilizer S in both soils emphasizes the need for models which include the formation and re-mineralization of organic S compounds, especially of ester sulfates, for correctly simulating and predicting the retention and remobilization of S in acid forest soils subject to changing atmospheric N and S deposition.     

The biogeochemistry of sulfur at Hubbard Brook

A synthesis of the biogeochemistry of S was done during 34 yr(1964–1965 to 1997–1998) in reference and human-manipulated forestecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. There have beensignificant declines in concentration (–0.44µmol/liter-yr) and input (–5.44mol/ha-yr)of SO₄ ^2– in atmospheric bulk wet deposition, and inconcentration(–0.64 µmol/liter-yr) an d output (–3.74mol/ha-yr) of SO₄ ^2– in stream water ofthe HBEF since 1964. These changes arestrongly correlated with concurrent decreases in emissions of SO₂from the source area for the HBEF. The concentration and input ofSO₄ ^2– in bulk deposition ranged from a low of 13.1µmol/liter (1983–1984) and 211 mol/ha-yr(1997–1998) to a high of 34.7 µmol/liter(1965–1966) and 479 mol/ha-yr (1967–1968), with along-term mean of 23.9 µmol/liter and 336mol/ha-yr during 1964–1965 to 1997–1998. Despiterecentdeclines in concentrations, SO₄ ^2– is the dominantanion in both bulk deposition and streamwater at HBEF. Dry deposition is difficult to measure, especially inmountainousterrain, but was estimated at 21% of bulk deposition. Thus, average totalatmospheric deposition was 491 and 323 mol/ha-yr during1964–1969 and 1993–1998, respectively. Based on the long-term³⁴S pattern associated with anthropogenic emissions,SO₄ ^2– deposition at HBEF is influenced by numerousSO₂sources, but biogenic sources appear to be small. Annual throughfall plusstemflow in 1993–1994 was estimated at 346 molSO₄ ^2–/ha. Aboveground litterfall, for thewatershed-ecosystemaveraged about 180 mol S/ha-yr, with highest inputs (190 molS/ha-yr) in the lower elevation, more deciduous forest zone. Weatheringrelease was calculated at a maximum of 50 mol S/ha-yr. Theconcentration and output of SO₄ ^2– in stream waterranged from a low of 42.3µmol/liter (1996–1997) and 309 mol/ha-yr(1964–1965), to a high of 66.1 µmol/liter(1970–1971) and 849 mol/ha-yr (1973–1974), with along-term mean of 55.5 µmol/liter and 496mol/ha-yr during the 34 yrs of study. Gross outputs ofSO₄ ^2– in stream water consistently exceeded inputsin bulkdeposition and were positively and significantly related to annualprecipitationand streamflow. The relation between gross SO₄ ^2–output and annual streamflow changed with time asatmospheric inputs declined. In contrast to the pattern for bulk depositionconcentration, there was no seasonal pattern for streamSO₄ ^2– concentration. Nevertheless, stream outputs ofSO₄ ^2– were highly seasonal, peaking during springsnowmelt, andproducing a monthly cross-over pattern where net hydrologic flux (NHF) ispositive during summer and negative during the remainder of the year. Nosignificant elevational pattern in streamwaterSO₄ ^2– concentration was observed. Mean annual,volume-weightedsoil water SO₄ ^2– concentrations were relativelyuniform by soil horizon andacross landscape position. Based upon isotopic evidence, much of theSO₄ ^2– entering HBEF in atmospheric depositioncycles throughvegetation and microbial biomass before being released to the soil solution andstream water. Gaseous emissions of S from watershed-ecosystems at HBEF areunquantified, but estimated to be very small. Organic S (carbon bonded andestersulfates) represents some 89% of the total S in soil at HBEF. Some 6% exists asphosphate extractable SO₄ ^2– (PSO₄).About 73% of the total S in the soilprofile at HBEF occurs in the Bs2 horizon, and some 9% occurs in the forestfloor. The residence time for S in the soil was calculated to be 9 yr, butonly a small portion of the total organic soil pool turns over relativelyquickly. The S content of above- and belowground biomass is about 2885mol/ha, of which some 3–5% is in standing dead trees. Yellowbirch, American beech and sugar maple accounted for 89% of the S in trees, with31% in branches, 27% in roots and 25% in the lightwood of boles. The pool of Sin living biomass increased from 1965 to 1982 due to biomass accretion, andremained relatively constant thereafter. Of current inputs to the availablenutrient compartment of the forest ecosystem, 50% is from atmospheric bulkdeposition, 24% from net soil release, 11% from dry deposition, 11% from rootexudates and 4% is from canopy leaching. Comparing ecosystem processes for Sfrom 1964–1969 to 1993–1998, atmospheric bulk deposition decreasedby 34%, stream output decreased by 10%, net annual biomass storage decreased by92%, and net soil release increased by 184% compared to the 1964–1969values. These changes are correlated with decreased emissions of SO₂from the source area for the HBEF. Average, annual bulk deposition inputsexceeded streamwater outputs by 160.0 ± 75.3 SD molS/ha-yr,but average annual net ecosystem fluxes (NEF) were much smaller, mostlynegativeand highly variable during the 34 yr period (–54.3 ± 72.9 SDmol S/ha-yr; NEF range, +86.8 to –229.5). While severalmechanisms may explain this small discrepancy, the most likely are netdesorption of S and net mineralization of organic S largely associated with theforest floor. Our best estimates indicate that additional S from dry depositionand weathering release is probably small and that desorption accounts for about37% of the NEF imbalance and net mineralization probably accounts for theremainder (60%). Additional inputs from dry deposition would result fromunmeasured inputs of gaseous and particulate deposition directly to the forestfloor. The source of any unmeasured S input has important implications for therecovery of soils and streams in response to decreases in inputs of acidicdeposition. Sulfate is a dominant contributor to acid deposition at HBEF,seriously degrading aquatic and terrestrial ecosystems. Because of the strongrelation between SO₂ emissions and concentrations ofSO₄ ^2– in both atmospheric deposition and streamwater at HBEF,further reductions in SO₂ emissions will be required to allowsignificant ecosystem recovery from the effects of acidic deposition. Thedestruction or removal of vegetation on experimental watershed-ecosystems atHBEF resulted in increased rates of organic matter decomposition andnitrification, a lowering of soil and streamwater pH, enhancedSO₄ ^2– adsorption on mineral soil and smallerconcentrations andlosses of SO₄ ^2– in stream water. With vegetationregrowth, this adsorbedSO₄ ^2– is released from the soil, increasingconcentrations andfluxes of SO₄ ^2– in drainage water. Streamwaterconcentration ofSO₄ ^2– and gross annual output ofSO₄ ^2–/ha are essentially the same throughout theHubbard BrookValley in watersheds varying in size by about 4 orders of magnitude, from 3 to3000 ha.     

Simulated effects of atmospheric sulfur deposition on nutrient cycling in a mixed deciduous forest

The effects of three S deposition scenarios — 50% reduction, no change, and 100% increase — on the cycles of N, P, S, K, Ca, and Mg in a mixed deciduous forest at Coweeta, North Carolina, were simulated using the Nutrient Cycling model (NuCM). The purpose of this exercise was to compare NuCM's output to observed soil and streamwater chemical changes and to explore NuCM's response to varying S deposition scenarios. Ecosystem S content and SO₄ ^2– leaching were controlled almost entirely by soil SO₄ ^2– adsorption in the simulations, which was in turn governed by the nature of the Langmuir isotherm set in the model. Both the simulations and the 20-year trends in streamwater SO₄ ^2– concentration suggest that the ecosystem is slowly becoming S saturated. The streamwater data suggest S saturation is occurring at a slower rate than indicated by the simulations, perhaps because of underestimation of organic S retention in the model. Both the simulations and field data indicated substantial declines in exchangeable bases in A and BA soil horizons, primarily due to vegetation uptake. The correspondence of model output with field data in this case was a result of after-the-fact calibration (i.e. setting weathering rates to very low values) rather than prediction, however. Model output suggests that soil exchangeable cation pools change rapidly, undergoing annual cycles and multi-decade fluctuations.Varying S deposition had very little effect upon simulated vegetation growth, nutrient uptake, or N cycling. Varying S deposition strongly affected simulated Ca²⁺. Mg²⁺, K⁺, and P leaching but caused little change in soil exchangeable pools of Ca²⁺ K⁺, or P because soil exchangeable pools were large relative to fluxes. Soil exchangeable Mg²⁺ pools were reduced by high rates of S deposition but remained well above levels sufficient for tree growth. Although the total soil pools of exchangeable Ca²⁺ and K⁺ were only slightly affected by S deposition, there was a redistribution of Ca²⁺ and K⁺ from upper to lower horizons with increasing S deposition, causing increased base saturation in the deepest (BC) horizon. The 100% increased S deposition scenario caused increasing peaks in simulated Al³⁺ concentrations in A horizons after 25 years as a result of large seasonal pulses of SO₄ ^2– and lowered base saturation. Simulated soil solution Al³⁺ concentrations remained well below toxicity thresholds for selected tree species at the site.     

Forest soil solutions: Acid/base chemistry and response to calcite treatment

Soil solution chemistry was investigated at a forested watershed draining into Woods Lake. N.Y. as part of the Experimental Watershed Liming Study (EWLS). The objective of this study was to assess the response of soil water to watershed treatment of calcite (CaCO₃). This material was applied in an effort to mitigate the effects of acidic atmospheric deposition. Soil solutions draining Oa and Bs horizons in reference subcatchments were characterized by low pH and acid neutralizing capacity (ANC) due to elevated concentrations of SO ₄ ^2– , NO ₃ ^– and organic anions relative to the sum of base cation (C_B Ca²⁺, Mg²⁺, Na⁺, K⁺) concentrations. Seasonal and spatial variation of pH andANC in soil solutions appeared to belargely controlled by variations in the concentrations of dissolved organic acids which, in turn, were regulated by reactions of Al with soil organic matter. Nitrate was positively correlated and SO²⁺ was negatively correlated with Ca²⁺ and Al concentrations in reference soil solutions, indicating that changes in NO ₃ ^– influences spatial and seasonal variations in Ca²⁺ and Al concentrations. On this basis, NO ₃ ^– appears to be important in soil acidification and the dynamics of drainage water acidity. Comparison of our results with historical data for the site showed declines in concentrations of SO ₄ ^2– , which are consistent with decreases in emissions of SO₄, in the eastern U.S. and atmospheric deposition of SO ₄ ^2– , to the Adirondack region. Mineral soil solutions have shown large increases in concentrations of NO ₃ ^– . Declines in concentrations of C_B and increases in concentrations of Al have occurred over the last ten years, suggesting depletion of soil pools of exchangeable basic cations and increased sensitivity to acidic deposition. Calcite (CaCO₃) treatment of 6.89 Mg/ha resulted in a significant increase of Ca²⁺, ANC and pH in both Oa and Bs horizon soil solutions. Soil water response to CaCO₃ addition was most evident during the first year after treatment, apparently due to macropore transport of particulate and dissolved CaCO₃ However, increases in ANC and pH in the mineral soil waters were not sustained and appeared insufficient to result in substantial improvement in surface water quality over the 43 month study period.     

Acid-base chemistry and aluminum transport in an acidic watershed and pond in New Hampshire

Cone Pond is one of the few acidic, clear-water ponds in the White Mountains of New Hampshire, a region dominated by high inputs of strong acids from atmospheric deposition and low base content of bedrock. Monitoring was conducted for 13 months to compare and contrast the acid-base chemistry of the terrestrial and aquatic portions of the watershed. Variations in Al concentration and speciation in drainage waters were correlated with changes in the supply of naturally occurring organic ligands. During the study period, the pond retained 28% of Al inputs, including nearly half of the inputs of organically complexed Al. Chemical equilibrium calculations indicated that the entire water-column was oversaturated with respect to the solubility of synthetic gibbsite during summer, as was the hypolimnion during winter. Retention of Al resulted from an increase in pH in the hypolimnion concomitant with SO₄ ^2– reduction, and from loss of organic anions in epilimnetic waters. Acid neutralizing capacity (ANC) generated in the pond primarily through SO₄ ^2– reduction and base cation (C _B ) release was balanced by ANC consumed as a result of Al retention.     

The experimental watershed liming study: Comparison of lake and watershed neutralization strategies

The Experimental Watershed Liming Study (EWLS) was initiated to evaluate the application of CaCO₃ to a forested watershed in an effort to mitigate the acidification of surface water. The objective of the EWLS was to assess the response of the Woods Lake watershed to an experimental addition of CaCO₃. During October 1989, 6.89 Mg CaCO₃/ha was applied by helicopter to two subcatchments comprising about 50% (102.5 ha) of the watershed area. The EWLS involved individual investigations of the response of soil and soil water chemistry, forest and wetland vegetation, soil microbial processes, wetland, stream and lake chemistry, and phytoplankton and fish to the CaCO₃ treatment. In addition, the Integrated Lake/Watershed Acidification (ILWAS) model was applied to the site to evaluate model performance and duration of the treatment. The results of these studies are detailed in this volume. The purposes of this introduction and synthesis paper are to: 1) present the overall design of the EWLS, 2) discuss the linkages between the individual studies that comprise the EWLS, and 3) summarize the response of the lakewater chemistry to watershed addition of CaCO₃ and compare these results to previous studies of direct lake addition. An analysis of lake chemistry revealed the watershed treatment resulted in a gradual change in pH, acid neutralizing capacity (ANC) and Ca²⁺ in the water column. This pattern was in contrast to direct lake additions of CaCO₃ which were characterized by abrupt changes following base addition and subsequent rapid reacidification. Over the three-year study period, the supply of ANC to drainage waters was largely derived from dissolution of CaCO₃ in wetlands. Relatively little dissolution of CaCO₃ occurred in freely draining upland soils. The watershed treatment had only minor effects on forest vegetation. The watershed treatment eliminated the episodic acidification of streamwater and the near-shore region of the lake during snowmelt, a phenomenon that occurred during direct lake treatments. Positive ANC water in the near-shore area may improve chemical conditions for fish reproduction, and allow for the development of a viable fish population. The watershed CaCO₃ treatment also decreased the transport of Al from the watershed to the lake, and increased the concentrations of dissolved organic carbon (DOC) and dissolved silica (H₄SiO₄) in stream and lakewater. The watershed treatment appeared to enhance soil nitrification, increasing concentrations of NO₃ ^– in soilwater and surface waters. However, the acidity associated with this NO₃ ^– release was small compared to the increase in ANC due to CaCO₃ addition and did not alter the acid-base status of Woods Lake. Acid neutralizing capacity (ANC) budgets for 12-month periods before and after the watershed treatment showed that the lake shifted from a large source of ANC to a minor source due to retention of SO₄ ^2–, NO₃ ^–, Al and the elevated inputs of Ca²⁺ associated with the watershed CaCO₃ application. In contrast to the direct lake treatments, Ca²⁺ inputs from the watershed application were largely transported from the lake.     

Processes controlling DOC in pore water during simulated drought cycles in six different UK peats

The effect of episodic drought on dissolved organic carbon (DOC) dynamics in peatlands has been the subject of considerable debate, as decomposition and DOC production is thought to increase under aerobic conditions, yet decreased DOC concentrations have been observed during drought periods. Decreased DOC solubility due to drought-induced acidification driven by sulphur (S) redox reactions has been proposed as a causal mechanism; however evidence is based on a limited number of studies carried out at a few sites. To test this hypothesis on a range of different peats, we carried out controlled drought simulation experiments on peat cores collected from six sites across Great Britain. Our data show a concurrent increase in sulphate (SO₄) and a decrease in DOC across all sites during simulated water table draw-down, although the magnitude of the relationship between SO₄ and DOC differed between sites. Instead, we found a consistent relationship across all sites between DOC decrease and acidification measured by the pore water acid neutralising capacity (ANC). ANC provided a more consistent measure of drought-induced acidification than SO₄ alone because it accounts for differences in base cation and acid anions concentrations between sites. Rewetting resulted in rapid DOC increases without a concurrent increase in soil respiration, suggesting DOC changes were primarily controlled by soil acidity not soil biota. These results highlight the need for an integrated analysis of hydrologically driven chemical and biological processes in peatlands to improve our understanding and ability to predict the interaction between atmospheric pollution and changing climatic conditions from plot to regional and global scales.     

The biogeochemistry of potassium at Hubbard Brook

A synthesis of the biogeochemistry of K was conducted during 1963–1992 in the reference and human-manipulated watershed-ecosystems of the Hubbard Brook Experimental Forest (HBEF), NH. Results showed that during the first two years of the study (1963–65), which coincided with a drought period, the reference watershed was a net sink for atmospheric inputs of K. During the remaining years, this watershed has been a net source of K for downstream ecosystems. There have been long-term declines in volume-weighted concentration and flux of K at the HBEF; however, this pattern appears to be controlled by the relatively large inputs during the initial drought years. Net ecosystem loss (atmospheric deposition minus stream outflow) showed an increasing trend of net loss, peaking during the mid-1970s and declining thereafter. This pattern of net K loss coincides with trends in the drainage efflux of SO₄ ^2– and NO₃ ^–, indicating that concentrations of strong acid anions may be important controls of dissolved K loss from the site. There were no long-term trends in streamwater concentration or flux of K. A distinct pattern in pools and fluxes of K was evident based on biotic controls in the upper ecosystem strata (canopy, boles, forest floor) and abiotic controls in lower strata of the ecosystem (mineral soil, glacial till). This biological control was manifested through higher concentrations and fluxes of K in vegetation, aboveground litter, throughfall and forest floor pools and soil water in the northern hardwood vegetation within the lower reaches of the watershedecosystem, when compared with patterns in the high-elevation spruce-fir zone. Abiotic control mechanisms were evident through longitudinal variations in soil cation exchange capacity (related to soil organic matter) and soil/till depth, and temporal and disturbance-related variations in inputs of strong-acid anions. Marked differences in the K cycle were evident at the HBEF for the periods 1964–69 and 1987–92. These changes included decreases in biomass storage, net mineralization and throughfall fluxes and increased resorption in the latter period. These patterns seem to reflect an ecosystem response to decreasing rates of biomass accretion during the study. Clearcutting disturbance resulted in large losses of K in stream water and from the removal of harvest products. Stream losses occur from release from slash, decomposition of soil organic matter and displacement from cation exchange sites. Elevated concentrations of K persist in stream water for many years after clearcutting. Of the major elements, K shows the slowest recovery from clearcutting disturbance.     

Chemical composition of soil solutions extracted from New Zealand beech forests and West German beech and spruce forests

Concentrations of ions were measured in soil solutions from beech (Nothofagus) forests in remote areas of New Zealand and in solutions from beech (Fagus sylvatica) and Norway spruce (Picea abies) forests in North-East Bavaria, West Germany, to compare the chemistry of soil solutions which are unaffected by acid deposition (New Zealand) with those that are affected (West Germany). In New Zealand, soil solution SO₄ ^2– concentrations ranged between <2 and 58 mol L^–1, and NO₃ ^– concentrations ranged between <1 and 3 mol L^–1. In West Germany, SO₄ ^2– concentrations ranged between 80 and 700 mol L^–1, and NO₃ ^– concentrations at three of six sites ranged between 39 and 3750 mol L^–1, but was not detected at the remaining three sites. At all sites in New Zealand, and at sites where the soil base status was moderately high in West Germany, pH levels increased, and total Al (Al_t) and inorganic monomeric Al (Al_i) levels decreased rapidly with increasing soil depth. In contrast, at sites on soils of low base status in West Germany, pH levels increased only slightly, and Al levels did not decline with increasing soil depth.Under a high-elevation Norway spruce stand showing severe Mg deficiency and dieback symptoms in West Germany, soil solution Mg²⁺ levels ranged between 20 and 60 mol L^–, and were only half those under a healthy stand. Al_t and Al_i levels were substantially higher the healthy stand than under the unhealthy stand, indicating that Al toxicity was not the main cause of spruce decline.     

Comparisons of watershed sulfur budgets in southeast Canada and northeast US: new approaches and implications

Most of eastern North America receives elevated levels of atmospheric deposition of sulfur (S) that result from anthropogenic SO₂ emissions from fossil fuel combustion. Atmospheric S deposition has acidified sensitive terrestrial and aquatic ecosystems in this region; however, deposition has been declining since the 1970s, resulting in some recovery in previously acidified aquatic ecosystems. Accurate watershed S mass balances help to evaluate the extent to which atmospheric S deposition is retained within ecosystems, and whether internal cycling sources and biogeochemical processes may be affecting the rate of recovery from decreasing S atmospheric loads. This study evaluated S mass balances for 15 sites with watersheds in southeastern Canada and northeastern US for the period 1985 to 2002. These 15 sites included nine in Canada (Turkey Lakes, ON; Harp Lake, ON; Plastic Lake, ON; Hermine, QC; Lake Laflamme, QC; Lake Clair, QC; Lake Tirasse, QC; Mersey, NS; Moosepit, NS) and six in the US (Arbutus Lake, NY; Biscuit Brook, NY; Sleepers River, VT; Hubbard Brook Experimental Forest, NH; Cone Pond, NH; Bear Brook Watershed, ME). Annual S wet deposition inputs were derived from measured bulk or wet-only deposition and stream export was obtained by combining drainage water fluxes with SO₄ ^2? concentrations. Dry deposition has the greatest uncertainty of any of the mass flux calculations necessary to develop accurate watershed balances, and here we developed a new method to calculate this quantity. We utilized historical information from both the US National Emissions Inventory and the US (CASTNET) and the Canadian (CAPMoN) dry deposition networks to develop a formulation that predicted SO₂ concentrations as a function of SO₂ emissions, latitude and longitude. The SO₂ concentrations were used to predict dry deposition using relationships between concentrations and deposition flux derived from the CASTNET or CAPMoN networks. For the year 2002, we compared the SO₂ concentrations and deposition predictions with the predictions of two continental-scale air quality models, the Community Multiscale Air Quality (CMAQ) model and A Unified Regional Air-quality Modeling System (AURAMS) that utilize complete inventories of emissions and chemical budgets. The results of this comparison indicated that the predictive relationship provides an accurate representation of SO₂ concentrations and S deposition for the region that is generally consistent with these models, and thus provides confidence that our approach could be used to develop accurate watershed S budgets for these 15 sites. Most watersheds showed large net losses of SO₄ ^2? on an annual basis, and the watershed mass balances were grouped into five categories based on the relative value of mean annual net losses or net gains. The net annual fluxes of SO₄ ^2? showed a strong relationship with hydrology; the largest net annual negative fluxes were associated with years of greatest precipitation amount and highest discharge. The important role of catchment hydrology on S budgets suggests implications for future predicted climate change as it affects patterns of precipitation and drought. The sensitivity of S budgets is likely to be greatest in watersheds with the greatest wetland area, which are particularly sensitive to drying and wetting cycles. A small number of the watersheds in this analysis were shown to have substantial S sources from mineral weathering, but most showed evidence of an internal source of SO₄ ^2?, which is likely from the mineralization of organic S stored from decades of increased S deposition. Mobilization of this internal S appears to contribute about 1?C6 kg S ha^?1 year^?1 to stream fluxes at these sites and is affecting the rate and extent of recovery from acidification as S deposition rates have declined in recent years. This internal S source should be considered when developing critical deposition loads that will promote ecosystem recovery from acidification and the depletion of nutrient cations in the northeastern US and southeastern Canada.     

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.     

Nitrogen cycling in an acid forest ecosystem in the Netherlands under increased atmospheric nitrogen input

Within a long-term research project studying the biogeochemical budget of an oak-beech forest ecosystem in the eastern part of the Netherlands, the nitrogen transformations and solute fluxes were determined in order to trace the fate of atmospherically deposited NH₄ ⁺ and to determine the contribution of nitrogen transformations to soil acidification.The oak-beech forest studied received an annual input of nitrogen via throughfall and stemflow of 45 kg N ha^–1 yr^–1, mainly as NH₄ ⁺, whereas 8 kg N ha^–1 yr^–1 was taken up by the canopy. Due to the specific hydrological regime resulting in periodically occurring high groundwater levels, denitrification was found to be the dominant output flux (35 kg N ha^–1 yr^–1). N₂0 emmission rate measurements indicated that 57% of this gaseous nitrogen loss (20 kg N ha^–1 yr^–1) was as N₂O. The forest lost an annual amount of 11 kg N ha^–1 yr^–1 via streamwater output, mainly as N0₃ ^–.Despite the acid conditions, high nitrification rates were measured. Nitrification occurred mainly in the litter layer and in the organic rich part of the mineral soil and was found to be closely correlated with soil temperature. The large amount of NH₄ ⁺ deposited on the forest floor via atmospheric deposition and produced by mineralization was to a large extent nitrified in the litter layer. Almost no NH₄ ⁺ reached the subsurface soil horizons. The N0₃ ^– was retained, taken up or transformed mainly in the mineral soil. A small amount of N0₃ ^– (9 kg N ha^–1 yr^–1) was removed from the system in streamwater output. A relatively small amount of nitrogen was measured in the soil water as Dissolved Organic Nitrogen.On the basis of these data the proton budget of the system was calculated using two different approaches. In both cases net proton production rates were high in the vegetation and in the litter layer of the forest ecosystem. Nitrogen transformations induced a net proton production rate of 2.4 kmol ha^–1 yr^–1 in the soil compartment.     

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.     

Long-term trends in soil solution and stream water chemistry at the Hubbard Brook Experimental Forest: relationship with landscape position

In acid-sensitive watersheds of the northeastern US, decreases in SO₂ emissions and atmospheric deposition of sulfur have not been accompanied by marked changes in pH and acid neutralizing capacity (ANC). To better understand this phenomenon, we investigated the long-term trends in soil solution (1984–1998) and stream water (1982–2000) chemistry along a natural soil catena at the Hubbard Brook Experimental Forest, New Hampshire, USA. Significant declines in strong acid anion concentrations were accompanied by declines in base cation concentrations in soil solutions draining the Oa and Bs soil horizons at all elevations. The magnitude of change varied with position in the landscape. Recovery, as indicated by increasing ANC (mean 2.38µEqL^–1year^–1) and decreasing concentrations of inorganic monomeric Al (mean 1.03µmolL^–1year^–1), was confined to solutions draining the Bs horizon at mid-to-higher elevations. However, persistently low Ca²⁺/Al_i ratios (<1) in Bs soil solutions at these sites may be evidence of continuing Al stress to trees. In Bs soil solution at a lower elevation site and in Oa soil solutions at all sites, declines in base cations (mean 3.71µEqL^–1year^–1) were either similar to or exceeded declines in strong acid anions (mean 3.25µEqL^–1year^–1) resulting in no change in ANC. Changes in the chemistry of stream water reflected changes in soil solutions, with the greatest improvement in ANC occurring at high elevation and the rate of increase decreasing with decreases in elevation. The pH of soil solutions and stream waters either declined or did not change significantly. Therefore pH-buffering processes, including hydrolysis of Al and possibly the deprotonation of organic acids, have prevented increases in drainage water pH despite considerable reductions in inputs of strong acids.     

Atmospheric deposition and sulphur cycling in chalk grasslandA mechanistic model simulating field observations

Sulphate fluxes in bulk deposition, throughfall and soil solution were monitored during two years, and integrated within a model describing the cycling of S in a chalk grassland ecosystem. Throughfall fluxes were strongly determined by interceptive properties of the grassland canopy. Seasonal variation in Leaf Area Index resulted in dry deposition velocities for SO₂ varying between 0.1 cm.s^–1 (snow cover, almost no aerodynamic resistance) to 0.9–1.8 cm.s^–1 in periods with a fully developed canopy. On an annual basis net canopy exchange (assimilation of SO₂ minus foliar leaching) was estimated to be –15% of net throughfall. Simulated soil solution concentrations, being the result of throughfall input, leaching, adsorption, biomass uptake and mineralization, closely fitted actual values (r > 0.92; p > 0.001). Actual and simulated leaching were 1.74 ± 0.03 and 2.00 keq.-ha^–1.yr^–1, respectively. Sulphur budgets for the soil showed net accumulation from April to October and net losses from October to April. Annual budgets for the ecosystem showed atmospheric input (2.02keq.ha^–1.yr^–1) and actual output (2.05keq.ha^–1.yr^–1) to be almost balanced. Apart from increased soil solution concentrations, additional input of sulphate (3.55 keq.ha^–1.yr^–1) to experimental plots resulted in additional accumulation in the ecosystem of 0.62 keq.ha^–1.yr^–1     

Leaching of nitrate and ammonium in heathland and forest ecosystems in Northwest Germany under the influence of enhanced nitrogen deposition

To study the impact of high atmospheric nitrogen deposition on the leaching of NO₃^– and NH₄⁺ beneath forest and heathland vegetation, investigations were carried out in adjacent forest and heathland ecosystems in Northwest Germany. The study area is subjected to high deposition of nitrogen ranging from 15.9 kg ha^–1 yr^–1 in bulk precipitation to 65.3 kg ha^–1 yr^–1 beneath a stand of Pinus sylvestris L. with NH₄–N accounting for 70–80% of the nitrogen deposited. Considerable leaching of nitrogen compounds from the upper horizons of the soil, mostly as nitrate, occurred at most of the forest sites and below a mixed stand of Calluna vulgaris (L.) Hull. and Erica tetralix, but was low in a Betula pubescens Ehrh. swamp forest as well as beneath Erica tetralix L. wet heath and heath dominated by Molinia caerulea(L.) Moench. Ground water concentrations of both NO₃–N and NH₄–N did not exceed 1 mg L^–1 at most of the sites investigated.     

Growth,net photosynthesis and leaf nutrient status of <Emphasis Type="Italic">Fagus crenata</Emphasis> seedlings grown in brown forest soil acidified with H<Subscript>2</Subscript>SO<Subscript>4</Subscript> or HNO<Subscript>3</Subscript> solution

To obtain basic information for evaluating critical loads of acid deposition for protecting Japanese beech forests, growth, net photosynthesis and leaf nutrient status of Fagus crenata seedlings grown for two growing seasons in brown forest soil acidified with H₂SO₄ or HNO₃ solution were investigated. The whole-plant dry mass of the seedlings grown in the soil acidified by the addition of H₂SO₄ or HNO₃ solution was significantly less than that of the seedlings grown in the control soil not supplemented with H⁺ as H₂SO₄ or HNO₃ solution. However, the degrees of reduction in the whole-plant dry mass and net photosynthetic rate of the seedlings grown in the soil acidified by the addition of H⁺ as H₂SO₄ solution at 100 mg l^–1 on the basis of air-dried soil volume (S-100 treatment) were greater than those of the seedlings grown in the soil acidified by the addition of H⁺ as HNO₃ solution at 100 mg l^–1 (N-100 treatment). The concentrations of Al and Mn in the leaves of the seedlings grown in the S-100 treatment were significantly higher than those in the N-100 treatment. A positive correlation was obtained between the molar ratio of (Ca+Mg+K)/(Al+Mn) in the soil solution and the relative whole-plant dry mass of the seedlings grown in the acidified soils to that of the seedlings grown in the control soil. Based on the results, we concluded that the negative effects of soil acidification due to sulfate deposition are greater than those of soil acidification due to nitrate deposition on growth, net photosynthesis and leaf nutrient status of F. crenata, and that the molar ratio of (Ca+Mg+K)/(Al+Mn) in soil solution is a suitable soil parameter for evaluating critical loads of acid deposition in efforts to protect F. crenata forests in Japan.     

Role of SO4 adsorption and desorption in the long-term S budget of a coniferous catchment on the Canadian Shield

The hypothesis that SO₄ desorption can explain apparent long term net SO₄-S losses (5 kg·ha^–1·yr^–1 on average) at the Lake Laflamme catchment from 1982 to 1991 is examined. Field observations show that SO₄ concentrations in the soil solution are strongly buffered during percolation through the Bf horizon. In the Bf horizon, SO₄ exchange reactions between the adsorbed and aqueous compartments are rapid (hours). Most (60%) of the adsorbed SO₄ may be readily desorbed with deionized water. These observations and the presence of an important adsorbed SO₄-S reservoir in the Bf horizon (113 kg·ha^–1) as compared with annual wet SO₄-S deposition (7 kg·ha^–1), suggest that on the short-term, adsorption and desorption reactions can control dissolved SO₄ concentration in the Bf horizon. To examine whether SO₄ adsorption/desorption could explain long-term SO₄-S losses by the catchment, an aggregated Langmuir isotherm for the Bf horizon was used to calculate the catchment's resilience to changing SO₄-S loads. The results indicate that the soil should adjust rapidly (within 4 years) to changing SO₄-S loads and that SO₄ desorption alone cannot explain long-term net SO₄-S losses. Other possibilities, such as an underestimation of dry deposition or the weathering of S-bearing minerals also appear unlikely. Our results suggest a net release of SO₄-S from the soil organic S reservoirs (1230 kg·ha^–1) present in the catchment.     

Processes regulating temporal and longitudinal variations in the chemistry of a low-order woodland stream in the Adirondack region of New York

Watershed processes influence the acid neutralizing capacity of surface waters by mediating changes in concentration of ionic solutes. Acidification of surface waters by atmospheric deposition of mineral acids and the extent to which ecosystem transformations neutralize this acidity are of particular concern. Seasonal variations in flow paths of water through soil and biological processes result in short-term changes in chemistry that may be critical to surface water ecology. In this study, we assessed longitudinal and temporal variations in the chemistry of a low-order stream, Pancake-Hall Creek, located in the west-central Adirondack region of New York. By quantifying changes in ionic solute concentration (e.g. Ca²⁺, Al^a+, SO ₄ ^2– , NO ₃ ^– ) we were able to evaluate processes responsible for short-term fluctuations in acid/base chemistry.In the headwater sites, stream water was acidic; changes in pH, acid neutralizing capacity (ANC) and Al were primarily due to seasonal variations in basic cation and NO ₃ ^– concentrations. At the downstream sites, water migrated through a large beaver impoundment and thick till resulting in higher pH, acid neutralizing capacity and basic cation concentrations, and lower concentrations of Al. Neutralization of acidity was particularly evident during the low flow summer period and coincided with retention of SO ₄ ^2– in the beaver impoundment. During the high flow non-summer (October to June) period, depressed pH and ANC, and elevated Al concentrations were observed in the downstream sites. Acidic conditions during the non-summer period were not due to the oxidation of reduced sulfur deposits (e.g. SO ₄ ^2– events) but rather the resumption of conservative SO ₄ ^2– transport through the beaver impoundment (e.g. minimal SO ₄ ^2– retention) coupled with increases in NO ₃ ^– .     

Tracing Sources of Streamwater Sulfate During Snowmelt Using S and O Isotope Ratios of Sulfate and <Superscript>35</Superscript>S Activity

The biogeochemical cycling of sulfur (S) was studied during the 2000 snowmelt at Sleepers River Research Watershed in northeastern Vermont, USA using a hydrochemical and multi-isotope approach. The snowpack and 10 streams of varying size and land use were sampled for analysis of anions, dissolved organic carbon (DOC), ³⁵S activity, and δ³⁴S and δ¹⁸O values of sulfate. At one of the streams, δ¹⁸O values of water also were measured. Apportionment of sulfur derived from atmospheric and mineral sources based on their distinct δ³⁴S values was possible for 7 of the 10 streams. Although mineral S generally dominated, atmospheric-derived S contributions exceeded 50% in several of the streams at peak snowmelt and averaged 41% overall. However, most of this atmospheric sulfur was not from the melting snowpack; the direct contribution of atmospheric sulfate to streamwater sulfate was constrained by ³⁵S mass balance to a maximum of 7%. Rather, the main source of atmospheric sulfur in streamwater was atmospheric sulfate deposited months to years earlier that had microbially cycled through the soil organic sulfur pool. This atmospheric/pedospheric sulfate (pedogenic sulfate formed from atmospheric sulfate) source is revealed by δ¹⁸O values of streamwater sulfate that remained constant and significantly lower than those of atmospheric sulfate throughout the melt period, as well as streamwater ³⁵S ages of hundreds of days. Our results indicate that the response of streamwater sulfate to changes in atmospheric deposition will be mediated by sulfate retention in the soil.