Impacts of changing climate and atmospheric deposition on N and S drainage losses from a forested watershed of the Adirondack Mountains, New York State

Biogeochemical responses to changing climate and atmospheric deposition were investigated using nitrogen (N) and sulfur (S) mass balances, including dry deposition and organic solutes in the Arbutus Lake watershed in the Adirondack Mountains, New York State. Long‐term monitoring of wet‐only precipitation (NADP/NTN, 1983–2001) and dry deposition (AIRMoN, 1990–2001) at sites adjacent to the watershed showed that concentrations of SO₄^2? in precipitation, SO₄^2? in particles,and SO₂ vapor all declined substantially (P<0.005) in contrast to no marked temporal changes observed for most N constituents (NH₄⁺ in precipitation, HNO₃ vapor, and particulate NO₃^?), except for NO₃^? in precipitation, which showed a small decrease in the late 1990s. From 1983 to 2001, concentrations of SO₄^2? in the lake outlet significantly decreased (?2.1 μeq L^?1 yr^?1, P<0.0001), whereas NO₃^? and dissolved organic N (DON) concentrations showed no consistent temporal trends. With the inclusion of dry deposition and DON fluxes into the mass balance, the retained portion of atmospheric N inputs within the main subcatchment increased from 37% to 60%. Sulfur outputs greatly exceeded inputs even with the inclusion of dry S deposition, while organic S flux represented another source of S output, implying substantial internal S sources. A significant relationship between the annual mean concentrations of SO₄^2? in lake discharge and wet deposition over the last two decades (r=0.64, P<0.01) suggested a considerable influence of declining S deposition on surface water SO₄^2? concentrations, despite substantial internal S sources. By contrast, interannual variations in both NO₃^? concentrations and fluxes in lake discharge were significantly related to year‐to‐year changes in air temperature and runoff. Snowmelt responses to winter temperature fluctuations were crucial in explaining large portions of interannual variations in watershed NO₃^? export during the months preceding spring snowmelt (especially, January–March). Distinctive response patterns of monthly mean concentrations of NO₃^? and DON in the major lake inlet to seasonal changes in air temperature also suggested climatic regulation of seasonal patterns in watershed release of both N forms. The sensitive response of N drainage losses to climatic variability might explain the synchronous patterns of decadal variations in watershed NO₃^? export across the northeastern USA.     

Element Fluxes and Landscape Position in a Northern Hardwood Forest Watershed Ecosystem

Chemical changes along headwater streams at the Hubbard Brook Experimental Forest in New Hampshire suggest that important differences exist in biogeochemical cycles along an altitudinal gradient within small watershed ecosystems. Using data collected during the period 1982–92, we have constructed element budgets [Ca, Mg, K, Na, Si, Al, dissolved organic carbon (DOC), S, and N] for three subcatchments within watershed 6, a forested watershed last logged around 1917–20. The biogeochemistry of the high-elevation spruce-fir–white birch subcatchment was dominated by processes involving naturally occuring organic compounds. Stream water and soil solutions in this zone had elevated concentrations of organic acidity, DOC, and organically bound monomeric aluminum (Al_o), relative to lower-elevation sites. The middle-elevation subcatchment, dominated by hardwood vegetation, had the greatest net production of inorganic-monomeric aluminum (Al_i), and exhibited net immobilization of DOC and Al_o. The low-elevation subcatchment, also characterized by deciduous vegetation, had the highest rates of net production of base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) among the subcatchments. Living biomass of trees declined slightly in the spruce-fir–white birch subcatchment during the study period, remained constant in the middle-elevation zone, and increased by 5% in the low-elevation subcatchment. Coupling the corresponding changes in biomass nutrient pools with the geochemical patterns, we observed up to 15-fold differences in the net production of Ca, Mg, K, Na, and Si in soils of the three subcatchments within this 13.2-ha watershed. Release of Ca, Na, and dissolved Si in the highest-elevation subcatchment could be explained by the congruent dissolution of 185 mol ha⁻¹ y⁻¹ of plagioclase feldspar. The rate of plagioclase weathering, based on the net output of Na, increased downslope to 189 and 435 mol ha⁻¹ y⁻¹ in the middle-elevation and low-elevation subcatchments, respectively. However, the dissolution of feldspar in the hardwood subcatchments could account for only 26%–37% of the observed net Ca output. The loss of Ca from soil exchange sites and organic matter is the most likely source of the unexplained net export. Furthermore, this depletion appears to be occurring most rapidly in the lower half of watershed 6. The small watersheds at the Hubbard Brook Experimental Forest occupy a soil catena in which soil depth and soil-water contact time increase downslope. By influencing hydrologic flowpaths and acid neutralization processes, these factors exert an important influence on biogeochemical fluxes within small watersheds, but their influence on forest vigor is less clear. Our results illustrate the sensitivity of watershed-level studies to spatial scale. However, it appears that much of the variation in element fluxes occurs in the first 10–20 ha of drainage area. Received 13 August 1998; accepted 7 September 1999.     

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.     

Atmospheric Deposition to the Turkey Lakes Watershed: Temporal Variations and Characteristics

We investigated the atmospheric concentrations and deposition fluxes of major ions to the Turkey Lakes Watershed (TLW) between 1980 and 1996. During that time, daily SO₄ ²⁻ concentrations in precipitation decreased markedly, while NO₃ ⁻, NH₄ ⁺, and H⁺ concentrations remained roughly constant. It appears that precipitation acidity did not decrease in spite of declining SO₄ ²⁻ concentrations due to a concurrent and counterbalancing decrease in the concentrations of Ca²⁺, Mg²⁺, and K⁺ in precipitation. The reasons for the decline in base cations are unknown, but this decline is probably related to decreasing emissions of soil-derived particles from agricultural, industrial, and road sources. A similar situation was seen during the same period in other parts of Canada, the eastern United States, and Europe. Wet, dry, and total (wet + dry) deposition fluxes of sulphur (S) and nitrogen (N) were estimated annually for the years 1980–96. The 17-year mean annual total (wet + dry) deposition of S to the watershed was estimated at 38.5 mmol m⁻² y⁻¹ (range 24.3–50.3). Total S deposition decreased by 35% from the early 1980s (1982–84) to the mid-1990s (1994–96), a decline consistent with the 23% decline in annual SO₂ emissions in eastern North America during the same period. In contrast, the annual total (wet + dry) deposition of oxidized N ranged from 39.8 to 60.4 mmol m⁻² y⁻¹, with a 15-year mean of 50.1 mmol m⁻² y⁻¹ and a net increase of 10% between the early 1980s (1983–85) and the mid-1990s (1994–96). This is in keeping with a 10% increase in NO_x emissions in eastern North America during the same period. For both S and N (oxidized), wet deposition dominated over dry deposition as the major mechanism for atmospheric input to the watershed. Annually, wet deposition accounted for approximately two-thirds of the total atmospheric deposition of both S and N. Dry S deposition was due more to gaseous SO₂ deposition (two-thirds of dry S deposition) than to particulate SO₄ ²⁻ deposition (one-third of dry S deposition). Dry deposition of oxidized N, however, was dominated (95%) by gaseous HNO₃ deposition, with minimal input from particulate NO₃ ⁻ deposition. Compared to several selected watershed/forest sites in Canada, the United States, and Europe, the estimated total deposition of S and N at the TLW was relatively high during the measurement period. Received 5 October 1999; accepted 1 March 2001.     

Spatial and temporal variation in phosphorus budgets for 24 watersheds in the Lake Erie and Lake Michigan basins

We estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for 1974, 1978, 1982, 1987, and 1992. NAPI quantifies all anthropogenic inputs of P (fertilizer use, atmospheric deposition, and detergents) as well as trade of P in food and feed, which can be a net input or output. Fertilizer was the dominant input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed. NAPI increased in relation to area of disturbed land (R² = 0.90) and decreased with forested and wetland area (R² = 0.90). Export of P by rivers varied with NAPI, especially for the 18 watersheds of LM (R² = 0.93), whereas the relationship was more variable among the six LE watersheds (R² = 0.59). On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. A comparison of our results with others as well as nitrogen (N) budgets suggests that fractional export of P may vary regionally, as has been reported for N, and the proportion of P inputs exported by rivers appears lower than comparable findings with N.     

Chemical limnology of soft water lakes in the Upper Midwest

Water samples from 36 lakes in northern Minnesota, Wisconsin, and Michigan were collected and analyzed during 1983–1984. All study lakes were dilute and had total alkalinities of less than 150 eq · L^–1. Minnesota lakes have hydrologic inputs from the watershed and inputs of base cations derived from the watershed. Study lakes in Minnesota had higher total alkalinities, dissolved organic carbon, and noncarbonate alkalinity as a result of watershed inputs. Lakes in Michigan and Wisconsin were precipitation-dominated seepage lakes that have lower concentrations of base cations than lakes in Minnesota. All of the study lakes have lower sulfate concentrations than expected, based on atmospheric wet deposition and evapotranspiration.Pore water samples collected from one of the study lakes—Little Rock Lake—in Wisconsin were used to calculate diffusive fluxes between the sediment and water column. According to these calculations, the sediments were a source of total alkalinity and Ca²⁺ and a sink for SO₄ ^2–. The sediment-water exchange of total alkalinity, Ca²⁺, and SO₄ ^2– appears to be important in the whole-lake budgets of these ions for Little Rock Lake.     

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     

Long-term sulphate and inorganic nitrogen mass balance budgets in European ICP Integrated Monitoring catchments (1990–2012)

Empirical evidence based on integrated environmental monitoring including physical, chemical and biological variables is essential for evaluating the ecosystem benefits of costly emission reduction policies. The international multidisciplinary ICP IM (International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems) programme studies the integrated effects of air pollution and climate change on ecosystems in unmanaged and calibrated forested catchments. We calculated site-specific annual input-output budgets for sulphate (SO₄) and total inorganic nitrogen (TINNO₃-N + NH₄-N) for 17 European ICP IM sites in 1990–2012. Temporal trends for input (deposition) and output (runoff water) fluxes and the net retention/net release of SO₄ and TIN were also analysed. Large differences in the input and output fluxes of SO₄ and TIN reflect important gradients of air pollution effects in Europe, with the highest deposition and runoff water fluxes at IM sites located in southern Scandinavia and in parts of Central and Eastern Europe and the lowest fluxes at more remote sites in northern European regions. A significant decrease in the total (wet + dry) deposition of non-marine SO₄ and bulk deposition of TIN was found at 90% and 65% of the sites, respectively. Output fluxes of non-marine SO₄ in runoff decreased significantly at 65% of the sites, indicating positive effects of the international emission abatement actions in Europe during the last 20 years. Catchments retained SO₄ in the early and mid-1990s, but this shifted towards a net release in the late 1990s, which may be due to the mobilization of legacy S pools accumulated during times of high atmospheric SO₄ deposition. Despite decreased deposition, TIN output fluxes and retention rates showed a mixed response with both decreasing (9 sites) and increasing (8 sites) trend slopes, and trends were rarely significant. In general, TIN was strongly retained in the catchments not affected by natural disturbances. The long-term annual variation in net releases for SO₄ was explained by variations in runoff and SO₄ concentrations in deposition, while a variation in TIN concentrations in runoff was mostly associated with a variation of the TIN retention rate in catchments. The net release of SO₄ from forest soils may delay the recovery from acidification for surface waters and the continued enrichment of nitrogen in catchment soils poses a threat to terrestrial biodiversity and may ultimately lead to a higher TIN runoff through N-saturation. Continued monitoring and further evaluations of mass balance budgets are thus needed.     

Nutrient budgets in a dry heathland watershed in northeastern Spain

Dissolved nutrient inputs in bulk precipitation and outputs in streamwater were measured during 3 years of contrasting hydrological conditions in a 6.3-ha, grazed heathland watershed on schists in the Montseny mountains (NE Spain), drained by an intermittent stream. On average, 39% of the precipitation became streamflow. Bulk precipitation delivered positive net alkalinity (mean 0.22 keq/ha/yr), sulphate input was moderate (9.0 kg SO₄-S/ha/yr), and the mean input of inorganic N was not exceptionally high (6.6 kg/ha/yr). Ion concentrations were relatively low in streamwater; SO₄ ^2- was the dominant anion. Most concentrations in streamwater varied seasonally, with maxima in late summer or early autumn and minima in spring. This pattern probably resulted from increased availability of ions for leaching due to decomposition of organic matter and chemical weathering during the warm period. Nitrate concentrations were relatively high in winter and dropped sharply in early spring, probably because of biological uptake. Annual element outputs in streamwater varied between years and seemed to be controlled by both the amount of annual streamflow and its seasonal distribution. Annual inputs exceeded outputs for dissolved inorganic N. The watershed accumulated H⁺ and Ca²⁺, had net losses of Na⁺ and Mg²⁺, and was close to steady state for K⁺, SO₄ ^2-, Cl^- and alkalinity. The chloride budgets gave no evidence of substantial dry deposition in this system. The cationic denudation rate was negative (-0.14 keq/ha/yr) because Ca²⁺ retention was higher than net exports of Na⁺ and Mg²⁺ from silicate weathering. Low nutrient export and little production of alkalinity suggest that this watershed has a low buffering capacity.     

Models of sulfur dynamics in forest and grassland ecosystems with emphasis on soil processes

The major S constituents in terrestrial ecosystems include inorganic SO ₄ ^2– , C-bonded S and ester sulfate with the organic fractions constituting the major soil S pools. Conceptual models of S dynamics link inorganic SO ₄ ^2– flux to organic sulfur transformations and other elements such as N and C. Mass balance models have been useful in ascertaining whether a system is at steady-state with respect to adsorption processes and/or nutritional demands of vegetation for S. Chemical equilibrium/surface complexation models have been used to evaluate the effects of a complex of factors, including effects of pH on SO₄ ^– adsorption and precipitation; these models have not generally been integrated into ecosystem models of S dynamics. Models such as ILWAS, Birkenes, Storgama, Trickle-Down and MAGIC were developed to ascertain surface water acidification processes within watersheds; these models incorporated SO₄ ^2– adsorption in some formulation combined with hydrological considerations. None of these models explicitly treat organic S transformations and fluxes. In contrast, grassland ecosystem models detail organic S transformations, but give little attention to adsorption and hydrologic factors. More detailed simulation models of S transformations in forest and grassland soils have recently been developed, but these results have yet to be incorporated into ecosystem and watershed models.     

Throughfall studies of deposition to forest edges and gaps in montane ecosystems

An extensive network of bottle/funnel collectors was used to measure hydrologic, SO₄ ^2–, and NO₃ ^– fluxes in rain events and in throughfall beneath the canopies of several high elevation forest stands in the Great Smoky Mountains National Park during 1989–1990. The throughfall fluxes were used as deposition surrogates to quantify trends in atmospheric inputs to sapling trees growing in forest gaps and to the mature forest canopy at the edge surrounding each gap. The paired gap/edge stands were located above (1940 m) and below (1720 m) the base of the clouds typically impacting this mountain. Total hydrologic and ion fluxes beneath the edge trees during the forest growing season exceeded fluxes beneath the adjacent gap saplings by nearly a factor of three (e.g. 230 vs 88 meq m^–2 for SO₄ ^2–) at both elevations. Water and SO₄ ^2– fluxes were up to two times greater beneath the forest edge at the cloud-prone 1940 m site than at 1720 m (e.g. 230 vs 110 meq m^–2 for SO₄ ^–2). However, throughfall NO₃ ^– fluxes were about 30% higher at 1720 m (17 vs 13 meq m^–2), because this lower site receives greater dry deposition of HNO₃ due to its ridgetop location and greater wind penetration. Estimates of SO₄ ^2–; deposition from cloud impaction were consistent with the net throughfall flux of SO₄ ^2– (throughfall flux minus rain flux) at the 1940 m forest edge, but greatly exceeded the net throughfall flux at 1940 m gap, suggesting differences in ion concentrations in cloud droplets impacting on mature edge trees and young saplings in forest gaps.     

Acidic deposition, nutrient leaching and forest growth

Studies in Germany and confirmed in North America established that the forest decline that developed in the late 1970's and 80's resulted from a deficiency in one or more of the nutrient cations: Ca²⁺, Mg²⁺, and K⁺. These nutrients are essential to the structure of the foliage, to photosynthesis and to the growth of the trees. The reactions and mechanisms involved in the entry of nutrients to the soil, their storage, and rate of transfer to the soil solution, and through it, to the fine roots and to the leaves at the top of the tree are reviewed. The continuing material balance studies carried out on a watershed at the Hubbard Brook Experimental Forest in New Hampshire allow a unique analysis of the changes caused in these nutrient transfers by acid rain. The nutrient cations are stored in the soil by adsorption on negatively charged clay, and the presence of an acid is required for their release to the soil solution. In pre-industrial times this acid was H₂CO₃, which was subsequently displaced from the soil solution by H₂SO₄ and HNO₃, as a result of acid deposition. The effect of the increased concentration of the negatively charged SO₄ ^2– and NO₃ ^– anions seeping through the soil, compared with that of the HCO₃ ^– that had been previously present, resulted in a substantially increased rate of transfer of an equivalent of Ca²⁺ and other positively charged nutrient cations from the soil to the soil solution. The increased concentration of Ca²⁺ in the soil solution resulted in both an initial increase in the rate of biomass growth and in a simultaneous increase in the rate of Ca²⁺ loss in the effluent soil solution from the watershed. It was found that this increased rate of removal of Ca²⁺ from the watershed soil had become greater than its rate of input to the soil from weathering and from dust and rain. As a result, the large Ca²⁺ inventory that had built up in the soil as a result of the reduced leaching in the years prior to the entry of acid rain, that started in about the1880's, was eventually depleted in the hardwood forest at Hubbard Brook in the 1980's, about 100 years later. With insufficient Ca²⁺ available for its continuing transfer, net biomass growth on the watershed stopped. This resulted from the rate of tree mortality becoming equal to that of the small incremental growth of a few trees on the watershed. The future growth of forests is at risk from the long-term effects of acid deposition. The fundamental nature of the reactions involved indicates that similar growth anomalies are occurring in other forests impacted by acid rain. These changes from normal biomass growth can affect the amount of CO₂ stored in the biomass, of importance to our understanding of Global Warming.     

Solute Sources in Stream Water during Consecutive Fall Storms in a Northern Hardwood Forest Watershed: A Combined Hydrological, Chemical and Isotopic Approach

Understanding the effects of climate change including precipitation patterns has important implications for evaluating the biogeochemical responses of watersheds. We focused on four storms in late summer and early fall that occurred after an exceptionally dry period in 2002. We analyzed not only the influence of these storms on episodic chemistry and the role of different water sources in affecting surface water chemistry, but also the relative contributions of these storms to annual biogeochemical mass balances. The study site was a well studied 135-ha watershed in the Adirondack Park of New York State (USA). Our analyses integrated measurements on hydrology, solute chemistry and the isotopic composition of NO₃⁻(δ¹⁵N and δ¹⁸O) and SO₄²⁻(δ³⁴S and δ¹⁸O) to evaluate how these storms affected surface water chemistry. Precipitation amounts varied among the storms (Storm 1: Sept. 14–18, 18.5 mm; Storm 2: Sept. 21–24, 33 mm; Storm 3: Sept. 27–29, 42.9 mm; Storm 4: Oct. 16–21, 67.6 mm). Among the four storms, there was an increase in water yields from 2 to 14%. These water yields were much less than in studies of storms in previous years at this same watershed when antecedent moisture conditions were higher. In the current study, early storms resulted in relatively small changes in water chemistry. With progressive storms the changes in water chemistry became more marked with particularly major changes in C_b (sum of base cations), Si, NO₃⁻, and SO₄²⁻, DOC and pH. Analyses of the relationships between Si, DOC, discharge and water table height clearly indicated that there was a decrease in ground water contributions (i.e., lower Si concentrations and higher DOC concentrations) as the watershed wetness increased with storm succession. The marked changes in chemistry were also reflected in changes in the isotopic composition of SO₄²⁻ and NO₃⁻. There was a strong inverse relationship between SO₄²⁻ concentrations and δ³⁴S values suggesting the importance of S biogeochemical redox processes in contributing to SO₄²⁻ export. The isotopic composition of NO₃⁻ in stream water indicated that this N had been microbially processed. Linkages between SO₄²⁻ and DOC concentrations suggest that wetlands were major sources of these solutes to drainage waters while the chemical and isotopic response of NO₃⁻ suggested that upland sources were more important. Although these late summer and fall storms did not play a major role in the overall annual mass balances of solutes for this watershed, these events had distinctive chemistry including depressed pH and therefore have important consequences to watershed processes such as episodic acidification, and the linkage of these processes to climate change.     

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.     

The biogeochemistry of two forested catchments in the Black Forest and the eastern Ore Mountains (Germany)

The biogeochemical input-output fluxes of two forested catchments with contrasting levels of atmospheric deposition were investigated in Germany. This paper focuses on the effects of recent changes in atmospheric inputs on the chemical composition in the soil solution and stream. The catchment 'Schluchsee' (Black Forest; SW Germany) is characterized by relatively low atmospheric inputs whereas 'Rotherdbach' (Ore Mountains; E Germany) received significant amounts of acid deposition (mainly originating from SO₂ emissions) until recent years. Both sites reveal decreases in H⁺ and S deposition during the 1990s. This pattern is typical when compared to trends in Europe. In response to the reduced S deposition, soil solution and streamwater SO₄ ^2– concentrations decreased significantly. A net release of SO₄ ^2– (output > input) was observed at both sites due to the release of S previously stored in the soil. The level of N deposition was more or less constant at both sites. At Schluchsee, NO₃ ^– concentration in streamwater remained more or less unchanged, whereas a decrease at Rotherdbach was observed. A recovery from acidification was found in seepage water as indicated by increasing acid neutralizing capacity (ANC). Streamwater ANC increased only in the permanently acidified Rotherdbach. No change of ANC was observed in the Schluchsee stream, which was characterized by episodic acidification during high-flow conditions. Nevertheless, the key factor controlling the recovery from surface water acidification was the type, amount and distribution of stored S pools in the ecosystem. Thus, time series analysis of long-term data of input-output chemistry can be a valuable instrument in order to improve the understanding of linked terrestrial-aquatic systems and give useful clues for modeling efforts.     

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.     

Calcium Additions and Microbial Nitrogen Cycle Processes in a Northern Hardwood Forest

Evaluating, and possibly ameliorating, the effects of base cation depletion in forest soils caused by acid deposition is an important topic in the northeastern United States. We added 850 kg Ca ha⁻¹ as wollastonite (CaSiO₃) to an 11.8-ha watershed at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in New Hampshire, USA, in fall 1999 to replace calcium (Ca) leached from the ecosystem by acid deposition over the past 6 decades. Soil microbial biomass carbon (C) and nitrogen (N) concentrations, gross and potential net N mineralization and nitrification rates, soil solution and stream chemistry, soil:atmosphere trace gas (CO₂, N₂O, CH₄) fluxes, and foliar N concentrations have been monitored in the treated watershed and in reference areas at the HBEF before and since the Ca addition. We expected that rates of microbial C and N cycle processes would increase in response to the treatment. By 2000, soil pH was increased by a full unit in the Oie soil horizon, and by 2002 it was increased by nearly 0.5 units in the Oa soil horizon. However, there were declines in the N content of the microbial biomass, potential net and gross N mineralization rates, and soil inorganic N pools in the Oie horizon of the treated watershed. Stream, soil solution, and foliar concentrations of N showed no response to treatment. The lack of stimulation of N cycling by Ca addition suggests that microbes may not be stimulated by increased pH and Ca levels in the naturally acidic soils at the HBEF, or that other factors (for example, phosphorus, or Ca binding of labile organic matter) may constrain the capacity of microbes to respond to increased pH in the treated watershed. Possible fates for the approximately 10 kg N ha⁻¹ decline in microbial and soil inorganic pools include components of the plant community that we did not measure (for example, seedlings, understory shrubs), increased fluxes of N₂ and/or N storage in soil organic matter. These results raise questions about the factors regulating microbial biomass and activity in northern hardwood forests that should be considered in the context of proposals to mitigate the depletion of nutrient cations in soil.     

Trends in cation, nitrogen, sulfate and hydrogen ion concentrations in precipitation in the United States and Europe from 1978 to 2010: a new look at an old problem

Industrial emissions of SO₂ and NO_x, resulting in the formation and deposition of sulfuric and nitric acids, affect the health of both terrestrial and aquatic ecosystems. Since the mid-late 20th century, legislation to control acid rain precursors in both Europe and the US has led to significant declines in both SO₄–S and H⁺ in precipitation and streams. However, several authors noted that declines in streamwater SO₄–S did not result in stoichiometric reductions in stream H⁺, and suggested that observed reductions in base cation inputs in precipitation could lessen the effect of air pollution control on improving stream pH. We examined long-term precipitation chemistry (1978–2010) from nearly 30 sites in the US and Europe that are variably affected by acid deposition and that have a variety of industrial and land-use histories to (1) quantify trends in SO₄–S, H⁺, NH₄–N, Ca, and NO₃–N, (2) assess stoichiometry between H⁺ and SO₄–S before and after 1990, and (3) examine regional synchrony of trends. We expected that although the overall efforts of developed countries to reduce air pollution and acid rain by the mid-late 20th century would tend to synchronize precipitation chemistry among regions, geographically varied patterns of fossil fuel use and pollution control measures would produce important asynchronies among European countries and the United States. We also expected that control of particulate versus gaseous emission, along with trends in NH₃ emissions, would be the two most significant factors affecting the stoichiometry between SO₄–S and H⁺. Relationships among H⁺, SO₄–S, NH₄–N, and cations differed markedly between the US and Europe. Controlling for SO₄–S levels, H⁺ in precipitation was significantly lower in Europe than in the US, because (1) alkaline dust loading from the Sahara/Sahel was greater in Europe than the US, and (2) emission of NH₃, which neutralizes acidity upon conversion to NH₄ ⁺, is generally significantly higher in Europe than in the US. Trends in SO₄–S and H⁺ in precipitation were close to stoichometric in the US throughout the period of record, but not in Europe, especially eastern Europe. Ca in precipitation declined significantly before, but not after 1990 in most of the US, but Ca declined in eastern Europe even after 1990. SO₄–S in precipitation was only weakly related to fossil fuel consumption. The stoichiometry of SO₄–S and H⁺ may be explained in part by emission controls, which varied over time and among regions. Control of particulate emissions reduces alkaline particles that neutralize acid precursors as well as S-containing particulates, reducing SO₄–S and Ca more steeply than H⁺, consistent with trends in the northeastern US and Europe before 1990. In contrast, control of gaseous SO₂ emissions results in a stoichiometric relationship between SO₄–S and H⁺, consistent with trends in the US and many western European countries, especially after 1991. However, in many European countries, declining NH₃ emissions contributed to the lack of stoichiometry between SO₄–S and H⁺.Recent reductions in NO_x emissions have also contributed to declines in H⁺ in precipitation. Future changes in precipitation acidity are likely to depend on multiple factors including trends in NO_x and NH₃ emission controls, naturally occurring dust, and fossil fuel use, with significant implications for the health of both terrestrial and aquatic ecosystems.     

Urban influences on the nitrogen cycle in Puerto Rico

Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha^?1 year^?1 in the Río Bayamón watershed to 32.9 kg ha^?1 year^?1 in the highly urbanized Río Piedras watershed and 33.3 kg ha^?1 year^?1 in the rural-agricultural Río Grande de Añasco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha^?1 year^?1. Data on wet deposition of inorganic N ( $\hbox{NH}_{4}^{+}+\hbox{NO}_{3}^{-}Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha⁻¹ year⁻¹ in the Río Bayamón watershed to 32.9 kg ha⁻¹ year⁻¹ in the highly urbanized Río Piedras watershed and 33.3 kg ha⁻¹ year⁻¹ in the rural-agricultural Río Grande de A?asco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha⁻¹ year⁻¹. Data on wet deposition of inorganic N () suggest that rates of atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and mobile sources of NO _x (NO+NO₂) and N₂O generated in the large urban centers may be responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively characterize the local N cycle. More research is required to assess rates of atmospheric N deposition, N fixation in natural and human-dominated landscapes, N-balance associated with food and feed trade, and denitrification.     

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

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO₃ ^–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO₃ ^– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO₃ ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO₃ ^-. At the regional scale, we observed high variability in the magnitude of NO₃ ^- losses: while six of the surveyed watersheds showed accelerated rates of NO₃ ^– loss, three did not. Moreover, in contrast to the strong linear relationship between NO₃ ^– loss and crown damage within HBEF watersheds [r ²: (W1 = 0.91, W6 = 0.85)], stream water NO₃ ^– concentrations were weakly related to crown damage (r ² = 0.17) across our regional sites. The efflux of NO₃ ^– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO₃ ^– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA.