Nitrogen and Sulfur Input-Output Budgets in the Experimental and Reference Watersheds, Bear Brook Watershed in Maine (BBWM)

Dry (NH4)2SO4 (1,800 eq-1 ha-1 yr-1) has been applied to the western of two contiguous 10 ha catchments at the Bear Brook Watershed in Maine (BBWM) since November, 1989. The initial rapid and significant response in both S and N in West Bear, compared to the reference East Bear, slowed after three years. Annual S retention of the total experimental treatment decreased from 86 to 34%, with a seven year cumulative retention of 59%. Hydrology influences the export flux of S; S is retained more in dry seasons and dry years. The annual retention of N has decreased from 96 to 81%, with a cumulative retention of 82%. The export of N from the reference watershed has declined from 178 to 23 eq-1 ha-1 yr-1 during the treatment period. The treatment N (as NH4) initially stimulated nitrification, and caused pre-existing N to be lost in runoff, rather than the treatment N. Retention of the treatment N has decreased to approximately 80%. The majority of the retained N is stored in the soil, but the reasons for the decreased flux from the reference watershed are not known.     

Comparing decadal responses of whole-watershed manipulations at the Bear Brook and Fernow experiments

The Bear Brook Watershed in Maine (BBWM), USA, and the Fernow Experimental Forest in West Virginia, USA, represent unique, long-term, paired, whole watershed, experimental manipulations focusing on the effects of nitrogen (N) and sulfur (S) deposition on temperate forests. Both watersheds began whole-ecosystem additions of N and S as (NH₄)₂SO₄ in the fall of 1989, and both are entering their third decade of chronic enrichment of the treated watersheds, while the reference watersheds offer unique opportunities to evaluate forest watershed responses to recovery. Differences between BBWM and Fernow in the history of atmospheric deposition, soil properties, and forest composition all contribute to different response trajectories in stream chemical exports over time. The four watersheds represent a spectrum of N enrichment and retention, ranging from ≈98% N retention in the reference watershed in Maine, to ≈20% N retention in the treated watershed in West Virginia. Despite these differences, there is evidence that mechanisms of response in base cation leaching and other processes are similar among all four watersheds. In both cases, the history to date of two decades of research and monitoring has provided new insights into ecosystem response not evident in more traditional short-term research.     

Ecological restoration of mined-out areas of dry tropical environment, India

The objective of the present study was to evince the long-term changes after natural revegetation and experimental revegetation of the coal mine spoils with respect to total plant biomass, available plant nutrients, nitrogen transformation and microbial biomass N (MBN) in dry tropical environment of India. Total plant biomass (above- and below-ground), plant available nitrogen, soil nitrogen mineralization and microbial biomass N (MBN) were studied for 2 years in 5 and 10 years old naturally vegetated and revegetated coal mine spoils, and dry tropical forest ecosystem of India. In forest ecosystem, the above ground biomass values ranged from 3,520 to 3,630 kg ha(-1) and belowground from 6,280 to 6,560 kg ha(-1). Plant available nitrogen ranged from 16.76 to 23.21 microg g(-1), net N-mineralization from 9.8 to 48.53 microg g(-1) month(-1) and MBN from 26.4 to 80.02 microg g(-1). In naturally revegetated mine spoil, the above ground biomass values ranged from 1,036 to 1,380 kg ha(-1) and belowground from 2,538 to 3,380 kg ha(-1). Plant available nitrogen ranged from 7.33-17.14 microg g(-1), net N-mineralization from 3.1 to 12.46 microg g(-1) month(-1) and MBN from 14.2 to 35.44 microg g(-1). In revegetated mine spoil, the above ground biomass values ranged from 1,224 to 1,678 kg ha(-1) and belowground from 2,870 to 4,130 kg ha(-1). Plant available nitrogen ranged from 9.4 to 18.83 microg g(-1), net N-mineralization from 4.2 to 16.2 microg g(-1) month(-1) and MBN from 21.6 to 42.6 microg g(-1). The mean plant biomass values in 5 and 10 years mine spoils was lower compared to forest ecosystem by 2.5 and 2 times, respectively. N-mineralization value in 5 year mine spoil was 3.5 times lower and in 10 years mine spoil 2 times lower compared to forest ecosystem. The MBN value was about 2 times lower in both 5 and 10 year mine spoils compared to native forest. MBN was positively related to the re-vegetation age of the mine spoil.     

Mineral Soil and Solution Responses to Experimental N and S Enrichment at the Bear Brook Watershed in Maine (BBWM)

Buried mineral soil-bags and natural solutions were studied as indicators of forest ecosystem response to elevated N and S inputs at the Bear Brook Watershed in Maine (BBWM). The BBWM is the site of a paired watershed manipulation experiment in a northern New England forested ecosystem. The study includes two small (10 ha each) catchments dominated by northern hardwood forests with red spruce in the upper elevations. Treatments consist of (NH4)2SO4 applied to the West Bear watershed six times per year, increasing N and S deposition 3× and 2× above ambient values, respectively. Buried mineral soil-bag changes over time reflected both the native soil environment and the treatments. Most of the treatment effects on mineral soils were evident as higher inorganic S found in the treated watershed soils. Adsorbed SO4 in the buried mineral soil-bags increased by approximately 40% under softwood stands and 50% under hardwood stands over the study period. Hardwood soil solutions responded with significant increases in NO3 and SO4 concentrations that resulted in accelerated cation leaching, primarily Ca and Al. Few differences that could be attributed to treatments were evident in soil solutions under softwoods. No treatment effects were evident in throughfall and stemflow chemistry.     

Mass Balances of Mercury and Nitrogen in Burned and Unburned Forested Watersheds at Acadia National Park, Maine, USA

Precipitation and streamwater samples were collected from 16 November 1999 to 17 November 2000 in two watersheds at Acadia National Park, Maine, and analyzed for mercury (Hg) and dissolved inorganic nitrogen (DIN, nitrate plus ammonium). Cadillac Brook watershed burned in a 1947 fire that destroyed vegetation and soil organic matter. We hypothesized that Hg deposition would be higher at Hadlock Brook (the reference watershed, 10.2 μg/m²/year) than Cadillac (9.4 μg/m²/year) because of the greater scavenging efficiency of the softwood vegetation in Hadlock. We also hypothesized the Hg and DIN export from Cadillac Brook would be lower than Hadlock Brook because of elemental volatilization during the fire, along with subsequently lower rates of atmospheric deposition in a watershed with abundant bare soil and bedrock, and regenerating vegetation. Consistent with these hypotheses, Hg export was lower from Cadillac Brook watershed (0.4 μg/m²/year) than from Hadlock Brook watershed (1.3 μg/m²/year). DIN export from Cadillac Brook (11.5 eq/ha/year) was lower than Hadlock Brook (92.5 eq/ha/year). These data show that ∼50 years following a wildfire there was lower atmospheric deposition due to changes in forest species composition, lower soil pools, and greater ecosystem retention for both Hg and DIN.     

The Bear Brook Watershed, Maine (BBWM), USA

The Bear Brook Watershed Manipulation project in Maine is a paired calibrated watershed study funded by the U. S. EPA. The research program is evaluating whole ecosystem response to elevated inputs of acidifying chemicals. The project consists of a 2.5 year calibration period (1987-1989), nine years of chemical additions of (NH4)2SO4 (15N- and 34S-enriched for several years) to West Bear watershed (1989-1998), followed by a recovery period. The other watershed, East Bear, serves as a reference. Dosing is in six equal treatments/yr of 1800 eq SO4 and NH4/ha/yr, a 200% increase over 1988 loading (wet plus dry) for SO4 and 300% for N (wet NO3 + NH4). The experimental and reference watersheds are forested with mixed hard- and softwoods, and have thin acidic soils, areas of 10.2 and 10.7 ha, and relief of 210 m. Thin till of variable composition is underlain by metasedimentary pelitic rocks and calc-silicate gneiss intruded by granite dikes and sills. For the period 1987-1995, precipitation averaged 1.4 m/yr, had a mean pH of 4.5, with SO4, NO3, and NH4 concentrations of 26, 14, and 7 eq/L, respectively. The nearly perrenial streams draining each watershed have discharges ranging from 0 (East Bear stops flowing for one to two months per year) to 150 L/sec. Prior to manipulation, East Bear and West Bear had a volume weighted annual mean pH of approximately 5.4, alkalinity = 0 to 4 eq/L, total base cations = 184 eq/L (sea-salt corrected = 118 eq/L), and SO4 = 100 to 111 eq/L. Nitrate ranged from 0 to 30 eq/L with an annual mean of 6 to 25 eq/L; dissolved organic carbon (DOC) ranged from 1 to 7 mg/L but was typically less than 3. Episodic acidification occurred at high discharge and was caused by dilution of cations, slightly increased DOC, significantly higher NO3, and the sea-salt effect. Depressions in pH were accompanied by increases in inorganic Al. The West Bear catchment responded to the chemical additions with increased export of base cations, Al, SO4, NO3, and decreased pH, ANC, and DOC. Silica remained relatively constant. Neutralization of the acidifying chemicals occurred dominantly by cation desorption and mobilization of Al.     

Nutrient Export from Watersheds on Mt. Desert Island, Maine, as a Function of Land Use and Fire History

A study of 13 small (less than 7.5 km²) watersheds on Mt. Desert Island, Maine, was conducted from January 1999 to September 2000 to determine nutrient export delivery to coastal waters around the island, and to determine whether a series of wildfires in 1947 have affected nutrient export in burned watersheds. Nutrient export (nitrate–nitrogen, total nitrogen, total phosphorus) was determined for each watershed during the study period, and was normalized by watershed area. The yield of nitrate–nitrogen (N) ranged from 10 to 140 kg/km²/year. Total N yield ranged from 42 to 250 kg/km²/year. Total phosphorus (P) yield ranged from 1.4 to 7.9 kg/km²/year. Watersheds entirely within Acadia National Park (lacking human land-based nutrient sources) exported significantly less total N and total P than watersheds that were partly or entirely outside the park boundary. Nitrate–N export was not significantly different in these two groups of watersheds, perhaps because atmospheric deposition is a dominant source of nitrate in the study area. No relation was observed between burn history and nutrient export. Any effect of burn history may be masked by other landscape-level factors related to nutrient export.     

Chemistry of Dissolved Organic Carbon at Bear Brook Watershed, Maine: Stream Water Response to (NH4)2SO4 Additions

This study was conducted to determine the response of stream water DOC and organic acidity to increased inputs of ammonium sulfate to a whole catchment. Precipitation, throughfall, soil solutions (from Spodosols) and stream waters were characterized for DOC concentrations and fractions (hydrophobic acids and neutrals, hydrophilic acids, bases, and neutrals) in both the control (East Bear) and the treatment (West Bear) catchments of Bear Brook Watershed, Maine (BBWM), a northern hardwood forest. In all solutions except precipitation, DOC was composed primarily of organic acids, with hydrophobic acids dominating (> 60% of DOC) in forest floor leachates (5000 mol C L-1), and a balance of hydrophobic and hydrophilic acids in deep B horizons and stream waters ( 150 mol C L-1). Stream waters had higher concentrations of DOC during storm or snowmelt events (high discharge), often reaching 300 to 400 mol C L-1. Forest floor leachate C was rapidly attenuated by the mineral soils under all flow conditions, indicating how important mineral soil sorption of DOC was in reducing the loss of C via surface water from BBWM. No differences occurred between control and treatment streams for concentration or composition of DOC due to treatment from 1989 through 1994. In 1995, West Bear Brook had much lower concentrations of DOC than East Bear for the first time. However, this occurred during a year of record low runoff, suggesting that hydrology may have affected export of C. Average annual export of DOC from the catchments was similar (1000 to 2000 mol C ha-1 yr-1). Organic anions in streamwaters increased slightly during high flow events (e.g., East Bear had means of 15 and 19 eq L-1 organic anions during base flow and high discharge in 1995). Treatment of West Bear caused a decrease in organic anions, both in concentration and contribution to overall anion composition (organic anions during high discharge as a percentage of total anions decreased from about 8 to 4% for 1987-89 and 1993-95 samples, respectively). This was probably due to decreased solution pH (greater protonation of organics) and higher concentrations of inorganic anions. Overall, there were no clear, detectable changes in stream water DOC, with only minor changes in organic anions, as a result of treatment with ammonium sulfate.     

Soil Type and Forest Vegetation Influences on Forest Floor Nitrogen Dynamics at the Bear Brook Watershed in Maine (BBWM)

The paired watershed experiment at the Bear Brook Watershed in Maine (BBWM) provided an opportunity to study changes in forest soil O horizon properties as a result of experimental, chronic N additions. The West Bear brook watershed received elevated N and S inputs since November 1989 as bimonthly applications of (NH4)2SO4. Forest floor samples (O horizon) were collected in July of 1992 from three dominant stand and five soil types at BBWM. The (NH4)2SO4 amendments in the treated watershed (West Bear) stimulated potential net nitrification, but significant increases were found only in hardwood O horizons after three years of treatment. Hardwood stand forest floor soil materials had the lowest C:N ratios (mean=23), compared with mixedwood (mean=27) and softwood stands (mean=33). NH4-N accounted for over 95% of the inorganic N in the forest floor. The lack of a strong relationship between soil type and potential net N mineralization at BBWM, coupled with conflicting results in the literature, suggested that stand characteristics were more important than conventional soil nomenclature based on pedogenetic features, or 2.5 years of treatments, in defining differences in soil N dynamics and responses to increased N inputs.     

Nitrogen mineralization in O horizon soils during 27 years of nitrogen enrichment at the Bear Brook Watershed in Maine,USA

Chronic elevated nitrogen (N) deposition has altered the N status of temperate forests, with significant implications for ecosystem function. The Bear Brook Watershed in Maine (BBWM) is a whole paired watershed manipulation experiment established to study the effects of N and sulfur (S) deposition on ecosystem function. N was added bimonthly as (NH₄)₂SO₄ to one watershed from 1989 to 2016, and research at the site has studied the evolution of ecosystem response to the treatment through time. Here, we synthesize results from 27 years of research at the site and describe the temporal trend of N availability and N mineralization at BBWM in response to chronic N deposition. Our findings suggest that there was a delayed response in soil N dynamics, since labile soil N concentrations did not show increases in the treated watershed (West Bear, WB) compared to the reference watershed (East Bear, EB) until after the first 4 years of treatment. Labile N became increasingly available in WB through time, and after 25 years of manipulations, treated soils had 10× more extractable ammonium than EB soils. The WB soils had 200× more extractable nitrate than EB soils, driven by both, high nitrate concentrations in WB and low nitrate concentrations in EB. Nitrification rates increased in WB soils and accounted for ~?50% of net N mineralization, compared to ~?5% in EB soils. The study provides evidence of the decadal evolution in soil function at BBWM and illustrates the importance of long-term data to capture ecosystem response to chronic disturbance.     

Background Mercury Concentrations in River Water in Maine, U.S.A.

Mercury concentrations in 58 rivers in Maine was measured to range from below detection up to 7.01 ng L^-1 and averaged 1.80±1.29 ng L^-1. The concentration gradient for mercury in rivers across the state was not uniform. Mercury strongly correlated with dissolved organic carbon (DOC) and aluminum, and less strongly with copper, lead, and zinc. Mercuryexhibited significant differences in correlations with chemical variables and local geology when partitioned by flow state (high or low). Mercury concentrations were greatest in rivers flowingacross either wacke-type bedrock at low metamorphic grade, or glacial-till deposits. Elevated concentrations of mercury formed a locus in northern Maine under both high and low-flow states while in southwestern Maine a locus formed only during high-flowstates. These regional differences were statistically significantwhen compared by geographical location. We suggest that there is a bedrock source of mercury in northeastern Maine that is dilutedduring periods of high runoff. The elevated concentrations detected under high-flow states, as noted in southwestern Maine, may reflect mercury released from storage in association withDOC during periods of high runoff. The association of mercury with flow state indicates that watershed processes and local geology can modulate the concentration of mercury in rivers.     

The evolution of the science of Bear Brook Watershed in Maine, USA

The Bear Brook Watershed in Maine (BBWM), USA is a paired watershed study with chemical manipulation of one watershed (West Bear = WB) while the other watershed (East Bear = EB) serves as a reference. Characterization of hydrology and chemical fluxes occurred in 1987–1989 and demonstrated the similarity of the ca. 10 ha adjacent forested watersheds. From 1989–2010, we have added 1,800 eq (NH₄)₂SO₄ ha^???1 y^???1 to WB. EB runoff has slowly acidified even as atmospheric deposition of SO $_{4}^{2-}$ has declined. EB acidification included decreasing pH, base cation concentrations, and alkalinity, and increasing inorganic Al concentration, as SO $_{4}^{2-}$ declined. Organic Al increased. WB has acidified more rapidly, including a 6-year period of increasing leaching of base cations, followed by a long-term decline of base cations, although still elevated over pretreatment values, as base saturation declined in the soils. Sulfate in WB has not increased to a new steady state because of increased anion adsorption accompanying soil acidification. Dissolved Al has increased dramatically in WB; increased export of particulate Al and P has accompanied the acidification in both watersheds, WB more than EB. Nitrogen retention in EB increased after 3 years of study, as did many watersheds in the northeastern USA. Nitrogen retention in WB still remains at over 80%, in spite of 20+ years of N addition. The 20-year chemical treatment with continuous measurements of critical variables in both watersheds has enabled the identification of decadal-scale processes, including ecosystem response to declining SO $_{4}^{-2}$ in ambient precipitation in EB and evolving mechanisms of treatment response in WB. The study has demonstrated soil mechanisms buffering pH, declines in soil base saturation, altered P biogeochemistry, unexpected mechanisms of storage of S, and continuous high retention of treatment N.     

Phosphorus run-off assessment in a watershed

The Watershed Assessment Model was used to simulate the runoff volume, peak flows, and non-point source phosphorus loadings from the 5870 km(2) Lake Okeechobee watershed as a case study. The results were compared to on-site monitoring to verify the accuracy of the method and to estimate the observed/simulated error. In 2008, the total simulated phosphorus contribution was 9634, 6524 and 3908 kg (P) y(-1) from sod farms, citrus farms and row crop farmlands, respectively. Although the dairies represent less than 1% of the total area of Kissimmee basin, the simulated P load from the dairies (9283 kg (P) y(-1) in 2008) made up 5.4% of the total P load during 2008. On average, the modeled P yield rates from dairies, sod farms and row crop farmlands are 3.85, 2.01 and 0.86 kg (P) ha(-1) y(-1), respectively. The maximum sediment simulated phosphorus yield rate is about 2 kg (P) ha(-1) and the particulate simulated phosphorus contribution from urban, improved pastures and dairies to the total phosphorus load was estimated at 9%, 3.5%, and 1%, respectively. Land parcels with P oversaturated soil as well as the land parcels with high phosphorus assimilation and high total phosphorus contribution were located. The most critical sub-basin was identified for eventual targeting by enforced agricultural best management practices. Phosphorus load, including stream assimilation, incoming to Lake Okeechobee from two selected dairies was also determined.     

Stream ecosystem response to chronic deposition of N and acid at the Bear Brook Watershed, Maine

The Bear Brook Watershed in Maine (BBWM) is a long-term, paired watershed experiment that addresses the effects of acid and nitrogen (N) deposition on whole watersheds. To examine stream response at BBWM, we synthesized data on organic matter dynamics, including leaf breakdown rates, organic matter inputs and standing stocks, macroinvertebrate secondary production, and nutrient uptake in treated and reference streams at the BBWM. While N concentrations in stream water and leaves have increased, the input, standing stocks, and breakdown rates of leaves, as well as macroinvertebrate production, were not responsive to acid and N deposition. Both chronic and acute increases of N availability have saturated uptake of nitrate in the streams. Recent experimental increases in phosphorus (P) availability enhanced stream capacity to take up nitrate and altered the character of N saturation. These results show how the interactive effects of multiple factors, including environmental flow regime, acidification, and P availability, may constrain stream response to chronic N deposition.     

Controls on Mercury and Methylmercury Deposition for Two Watersheds in Acadia National Park, Maine

Throughfall and bulk precipitation samples were collected for two watersheds at Acadia National Park, Maine, from 3 May to 16 November 2000, to determine which landscape factors affected mercury (Hg) deposition. One of these watersheds, Cadillac Brook, burned in 1947, providing a natural experimental design to study the effects of forest type on deposition to forested watersheds. Sites that face southwest received the highest Hg deposition, which may be due to the interception of cross-continental movement of contaminated air masses. Sites covered with softwood vegetation also received higher Hg deposition than other vegetation types because of the higher scavenging efficiency of the canopy structure. Methyl mercury (MeHg) deposition was not affected by these factors. Hg deposition, as bulk precipitation and throughfall was lower in Cadillac Brook watershed (burned) than in Hadlock Brook watershed (unburned) because of vegetation type and watershed aspect. Hg and MeHg inputs were weighted by season and vegetation type because these two factors had the most influence on deposition. Hg volatilization was not determined. The total Hg deposition via throughfall and bulk precipitation was 9.4 μg/m²/year in Cadillac Brook watershed and 10.2 μg/m²/year in Hadlock Brook watershed. The total MeHg deposition via throughfall and bulk precipitation was 0.05 μg/m²/year in Cadillac Brook watershed and 0.10 μg/m²/year in Hadlock Brook watershed.     

Impact of elevated O3 on visible foliar symptom, growth and biomass of Cinnamomum camphora seedlings under different nitrogen loads

The effects of elevated ozone (O(3)) and enhanced nitrogen (N) on the growth and biomass of Cinnamomum camphora, a subtropical evergreen broad-leaved tree species, were investigated. The seedlings, supplied with N (NH(4)NO(3) solution) at 0, 30 and 60 kg ha(-1) year(-1) (simplified as N0, N30, N60, respectively), were exposed to ambient (AA) or elevated O(3) (AA + 60 nmol mol(-1) and AA + 120 nmol mol(-1), designated as AA + 60 and AA + 120, respectively) for the 2009 and 2010 growing seasons. Symptomatic leaf percentages were significantly increased with O(3) concentration. AA + 120 significantly decreased the mean leaf size and chlorophyll content of both 2009- and 2010-emerged leaves, inhibited the growth of stem height and basal diameter, and reduced biomass accumulation of all plant parts except for leaves. By comparison, only the chlorophyll content of 2009-emerged leaves and root dry weight were significantly reduced under AA + 60. Specific leaf area, total leaf area and foliar biomass were not affected even at a higher O(3) level. On the other hand, N loads, especially N60, exerted significantly positive effects on all variables except mean leaf size and shoot/root ratio. No significant interactions between O(3) and N were detected, suggesting that the N supply at ≤60 kg ha(-1) year(-1) did not significantly modify the response of C. camphora to O(3) in terms of seedling growth and biomass accumulation.     

Soil chemical and physical properties at the Bear Brook Watershed in Maine, USA

Acidic deposition leads to the acidification of waters and accelerated leaching and depletion of soil base cations. The Bear Brook Watershed in Maine has used whole-watershed chemical manipulations to study the effects of elevated N and S on forest ecosystem function on a decadal time scale. The objectives of this study were to define the chemical and physical characteristics of soils in both the reference and treated watersheds after 17 years of treatment and assess evidence of change in soil chemistry by comparing soil studies in 1998 and 2006. Results from 1998 confirmed depletion of soil base cation pools and decreased pH due to elevated N and S within the treated watershed. However, between 1998 and 2006, during a period of declining SO $_{4}^{\,\,2-}$ deposition and continued whole-watershed experimental acidification on the treated watershed, there was little evidence of continued soil exchangeable base cation concentration depletion or recovery. The addition of a pulse of litterfall and accelerating mineralization from a severe ice storm in 1998 may have had significant effects on forest floor nutrient pools and cycling between 1998 and 2006. Our findings suggest that mineralization of additional litter inputs from the ice storm may have obscured temporal trends in soil chemistry. The physical data presented also demonstrate the importance of coarse fragments in the architecture of these soils. This study underscores the importance of long-term, quantitative soil monitoring in determining the trajectories of change in forest soils and ecosystem processes over time.     

The Bear Brook Watershed Manipulation Project: Watershed Science in a Policy Perspective

The Bear Brook Watershed Manipulation in Maine is a paired watershed experiment. Monitoring of the paired catchments (East Bear Brook — reference; West Bear Brook — experimental) began in early 1987. Chemical manipulation of West Bear Brook catchment began in November 1989. Process studies on the watershed, outflow observation and monitoring, and modeling simulations of predicted versus observed response, are yielding a wealth of information about the behavior of the paired catchments and their responses to the deposition of acidifying substances. Results from the studies are providing important information relevant to national policies on emissions controls.     

Soilaluminum, Iron, and Phosphorus Dynamics in Response to Long-Term Experimental Nitrogen and Sulfur Additions at the Bear Brook Watershed in Maine, USA

Atmospheric deposition of nitrogen (N) and sulfur (S) containing compounds affects soil chemistry in forested ecosystems through (1) acidification and the depletion of base cations, (2) metal mobilization, particularly aluminum (Al), and iron (Fe), (3) phosphorus (P) mobilization, and (4) N accumulation. The Bear Brook Watershed in Maine (BBWM) is a long-term paired whole-watershed experimental acidification study demonstrating evidence of each of these acidification characteristics in a northeastern U.S. forested ecosystem. In 2003, BBWM soils were studied using the Hedley fractionation procedure to better understand mechanisms of response in soil Al, Fe, and P chemistry. Soil P fractionation showed that recalcitrant P was the dominant fraction in these watersheds (49%), followed by Al and Fe associated P (24%), indicating that a majority of the soil P was biologically unavailable. Acidification induced mobilization of Al and Fe in these soils holds the potential for significant P mobilization. Forest type appears to exert important influences on metal and P dynamics. Soils supporting softwoods showed evidence of lower Al and Fe in the treated watershed, accompanied by lower soil P. Hardwood soils had higher P concentrations in surface soils as a result of increased biocycling in response to N additions in treatments. Accelerated P uptake and return in litterfall overshadowed acidification induced P mobilization and depletion mechanisms in hardwoods.     

Effects of chronic ammonium sulfate treatment on the forest at the Bear Brook Watershed in Maine

At the Bear Brook Watershed in Maine (BBWM), the forest tree composition was characterized and the effects of the chronic ammonium sulfate ((NH₄)₂SO₄) treatment on basal area growth, foliar chemistry, and gas exchange were investigated on forest species. The BBWM is a paired watershed forest ecosystem study with one watershed, West Bear (WB), treated since 1989 with 26.6 kg N ha^???1 year^???1 and 30 kg S ha^???1 year^???1applied bimonthly as (NH₄)₂SO₄, while the other watershed, East Bear (EB), serves as a reference. Tree species richness, density, and mortality were found to be similar between watersheds. Basal area increment was estimated from red spruce and sugar maple, showing that, for the first 7 years of treatment, it was significantly higher for sugar maple growing in WB compared to EB, but no differences were observed for red spruce between watersheds. However, the initial higher sugar maple basal area growth in WB subsequently decreased after 8 years of treatment. Foliar chemical analysis performed in trees, saplings, and ground flora showed higher N concentrations in the treated WB compared to the reference EB. But, foliar cation concentrations, especially Ca and Mg, were significantly lower for most of the species growing in WB compared with those growing in EB. For sugar maple, foliar N was higher on WB, but there were no differences in foliar Ca and Mg concentrations between treated and reference watersheds. In addition, only sugar maple trees in the treated WB showed significantly higher photosynthetic rates compared to reference EB trees.