Critical Loads and Exceedances of Acid Deposition and Associated Forest Growth in the Northern Hardwood and BorealConiferous Forests in Québec,Canada

We used the Québec forest monitoring network (`Réseaud'Étude et de Surveillance des Écosystèmes Forestiers' or RESEF)along with its atmospheric monitoring stations to assess criticalS and N loads and their combined soil acidification exceedancesfor natural ecosystems of the northern hardwood and borealconiferous forests in Québec, Canada. Critical loads (CL) forforest soil acidification were calculated using the simple mass-balance (SMB) approach and with the steady-state PROFILE model.Atmospheric deposition rates for water, S, N, Ca, Mg, Na, and K,for the years 1989–1993, and detailed, plot-specific forest andsoil characteristics were used as input. The SMB model alsorequired information regarding nutrient uptake and storage in theaboveground woody biomass. The CL calculations indicated that,from the 31 RESEF plots, 18 received atmospheric acidic inputs inexcess of their CL (55 and 61% of the hardwood and coniferousplots, respectively). The range of CL exceedance varied from 60to 470 eq ha^-1 yr^-1 for the hardwood stands, and from 10to 590 eq ha^-1 yr^-1 for the coniferous stands. The standswith CL exceedance were mainly located in the western and centralpart of the province. Stand growth associated with exceedanceclass of acidity was determined using the RESEF plots along withselected permanent forest survey plots having similar sitecharacteristics, but for which longer growth records wereavailable. We found a significant negative correlation betweenforest growth rates and critical soil acidification exceedancefor both the northern hardwood and the boreal conifer sites.Specifically, plots with critical load exceedances were found tohave a growth reduction of about 30% during the 1974–1982 andthe 1972–1990 measurement (plots with no soil acidificationexceedance served as a control). While this correlation is notnecessarily causal, it is nevertheless consistent with theexpectation that increased losses of soil base cations on accountof increased soil acidification should and could lead todeteriorating forest health conditions.     

Effects of Long-Term Application of Ammonium Sulphate on Nitrogen Fluxes in a Beech Ecosystem at Solling, Germany

To study the effects of elevated inputs of acidity and nitrogen (N), 1000 mmol m^-2 a^-1 of ammonium sulphate (NH₄NO₃) equivalent to an input of potential acidity of 2000 mmol m^-2 a^-1 was applied annually for 11 yr between 1983 and 1993 in a beech forest at Solling, Germany. Most of the applied NH₄ ⁺ was nitrified in the litter layer and in the upper mineral soil. N in soil leachate quickly responded to the elevated input, but most of the applied N was stored in the soil or left the ecosystem via pathways other than soil output. Leaching of N from the soil increased until the last year of N addition. After the last N application, N fluxes decreased rapidly to low values. The buffering of acidity produced by the nitrification of the applied NH₄ ⁺ was caused mainly by three different processes: (i) sulphur (S) retention, (ii) release of aluminium, (iii) release of base cations. Retention of S took place mostly in the subsoil. 72% of the S input was recovered in output after 14 years of the experiment. Due to the increased fluxes of mobile anions with soil solution, outputs of cations increased drastically.     

Nitrate Leaching From a Mountain Forest Ecosystem with Gleysols Subjected to Experimentally Increased N Deposition

Nitrate leaching was measured over seven years of nitrogen (N) addition in a paired-catchment experiment in Alptal, central Switzerland (altitude: 1200 m, bulk N deposition: 12 kg ha^-1 a^-1). Two forested catchments (1500 m² each) dominated by Picea abies) were delimited by trenches in the Gleysols. NH₄NO₃ was added to one of the catchments using sprinklers. During the first year, the N addition was labelled with ¹⁵N. Additionally, soil N transformationswere studied in replicated plots. Pre-treatment NO₃ ^--N leaching was 4 kg ha^-1 a^-1 from both catchments, and remained between 2.5 and 4.8 kg ha^-1 a^-1 in the control catchment. The first year of treatment induced an additional leaching of 3.1 kg ha^-1, almost 90% of which was labelled with ¹⁵N, indicating that it did not cycle through the large N pools of the ecosystem (soil organic matter and plants). These losses partly correspond to NO₃ ^- from precipitation bypassing the soil due to preferential flow. During rain or snowmelt events, NO₃ ^- concentration peaks as the water table is rising, indicating flushing from the soil. Nitrification occurs temporarily along the water flow paths in the soil and can be the source of NO₃ ^- flushing. Its isotopic signature however, shows that this release mainly affects recently applied N, stored only between runoff events or up to a few weeks. At first, the ecosystem retained 90% of the added N (2/3 in the soil), but NO₃ ^- losses increased from 10 to 30% within 7 yr, indicating that the ecosystem became progressively N saturated.     

Development and Application of Critical,Target and Monitoring Loads for the Management of Acid Deposition in Alberta,Canada

The Alberta, Canada acid deposition management framework, based upon the application of critical, target and monitoring loads, is described. This framework is the culmination of four years work by stakeholders brought together within Alberta's Clean Air Strategic Alliance (CASA). The elements of the framework include scientific aspects of measurement, model estimation, and monitoring of acid deposition, assessment of receptor sensitivity, and management processes to reduce emissions and deposition (should reductions become necessary). All loads are applied to grid cells measuring 1°latitude × 1° longitude, with each cell being categorized as sensitive, moderately sensitive or of low sensitivity on the basis of the sensitivities of the soil and water systems within the cell. Critical loads have been set at 0.25, 0.50 and 1.00 keq H⁺ ha^-1 yr^-1 for grid cells that are categorized as sensitive, moderately sensitive, and of low sensitivity, respectively. Target loads, the environmental management objectives, have been set at 0.22, 0.45, and 0.90 keq H⁺ ha^-1 yr^-1, and monitoring loads, a new concept in acid deposition management, have been set at 0.17, 0.35 and 0.70 keq H⁺ ha^-1 yr^-1 for the three sensitivity classes. When model prediction indicates that deposition may be exceeding the monitoring load, deposition monitoring and receptor sensitivity studies are to be initiated to confirm the deposition model prediction, and to ensure that the sensitivity of the recipient systems is understood. In this manner, management actions that occur in the event of a target load exceedance will be based upon receptor sensitivity and deposition data rather than upon model prediction alone. A process for stakeholder involvement in the evaluation of the framework and its application in long-term, long-range management of emissions and deposition is also included within the framework.     

Freshwater Critical Loads in Wales

The Steady State Water Chemistry (SSWC) modeland the Diatom model have been used to calculate criticalloads for acidity using annual mean chemistry for 102 acidupland streams in Wales sampled as part of the Welsh AcidWaters Survey (WAWS) in 1995. Diatom critical loads werelower than SSWC values reflecting the higher effective[ANC]_limit of the Diatom model compared to the[ANC]_limit of zero used in the SSWC model. The WAWSstream sites were all located within 41 10 × 10 km squares andeach square was assigned the lowest critical load value fromamongst the sites located within it. Comparison with valuesassigned under the UK national critical loads mappingprogramme (UKCLMAP) showed that WAWS critical loads were lowerthan UKCLMAP values in approximately 40% of squares.Differences in critical load class were variable, but exceeded2 keq ha^-1 yr^-1 in up to a maximum of seven squares.It cannot be assumed, therefore, that reducing acid depositionto the currently mapped UKCLMAP critical load will protect allstreams occurring within a given 10 × 10 km grid square inWales. The limited number of sample sites means that even inthose squares where all WAWS sites will be protected, theremay be other, more acid sensitive freshwaters with lowercritical loads. This has important implications for theinterpretation and use of critical loads data for regional andlocal environmental planning.     

Critical Loads of Acidity for Surface Waters in South-Central Ontario, Canada: Regional Application of the First-Order Acidity Balance (FAB) Model

Major sulphur emission control programs have been implemented in North America, resulting in current emissions being ～30% less than those in 1980. However, the level of acidic deposition remaining is still unlikely to promote widespread recovery of aquatic ecosystems. The First-order Acidity Balance (FAB) model has been applied to south-central Ontario (285 lakes in the Muskoka River Catchment) to evaluate the need for further reductions in emissions. As a result of the past decline in deposition, the proportion of lakes with critical loads exceedance has dropped substantially; however, further reductions in sulphur and nitrogen emissions are required to eliminate critical loads exceedance. Based on bulk deposition of sulphate and nitrogen (41.1 mmol_c m^-2 yr^-1 and 62.5 mmol_c m^-2 yr^-1, respectively) for the period 1995–1999, 166 lakes (58.3%) exceedcritical loads. Even with full implementation of SO₂ abatementprograms in Canada (achieved in 1994) and the United States (legislated for 2010), critical loads will be exceeded in a large proportion (46.6%) of the study lakes.     

Modeling Acidification Recovery on Threatened Ecosystems: Application to the Evaluation of the Gothenburg Protocol in France

To evaluate the acid deposition reduction negotiated for 2010 within the UNECE LRTAP Gothenburg Protocol, sulphur and nitrogen deposition time-series (1880–2100) were compared to critical loads of acidity on five French ecosystems: Massif Central basalt (site 1) and granite (2); Paris Bassin tertiary sands (3); Vosges mountains sandstone (4) and Landes eolian sands (5). The SAFE model was used to estimate the response of soil solution pH and ratio to the deposition scenario. Among the five sites, critical loads were exceeded in the past at sites 3, 4 and 5. Sites 3 and 4 were still expected to exceed in 2010, the Protocol year. Further reduction of atmospheric deposition, mainly nitrogen, would be needed to achieve recovery on these ecosystems. At sites 3, 4 and 5, the delay between the critical load exceedance and the violation of the critical chemical criterion was estimated to be 10 to 30 years in the top soil and 50 to 90 years in the deeper soil. At site 5, a recovery was expected in the top soil in 2010 with a time lag of 10 years. Unexpectedly, soil pH continued to decrease after 1980 in the deeper soil at sites 2 and 5. This time lag indicated that acidification moved down the soil profile as a consequence of slow base cation depletion by ion exchange. This delayed response of the soil solution was the result of the combination of weathering rates and vegetation uptake but also of the relative ratio between base cation deposition and acid compounds.     

Fluxes of NO3-, NH4+, NO,NO2, and N2O in an Old Danish Beech Forest

The fluxes of the major nitrogen compounds havebeen investigated in many ecosystem studies over the world.However, only in few studies has attention been drawn to theimportance of the fluxes of minor gaseous nitrogen compoundsto complete the nitrogen cycle. In Denmark a detailed study onthe nitrogen cycle in an old beech forest has been implementedin 1997 at Gyrstinge near Sorø, Zealand. The study includesthe fluxes of the gases NO, N₂O and water mediatedtransport of NO₃ ^- and NH₄ ⁺. Measurementsof the fluxes of the gaseous compounds are performed withmicro-meteorological methods (eddy-correlation and gradient)and with chambers. Water mediated fluxes encompass rain,throughfall, stem-flow and leaching from the root zone. Thehydrological model is verified by TDR measurements. The findings show that the total water mediated N input tothe forest floor with throughfall and stemflow was 25.6 kg Nha^-1 yr ^-1, and open field wet deposition withprecipitation was 19.0 kg N ha^-1 yr ^-1. The internalcycling of N in the ecosystem measured as turnover oflitterfall and plant uptake was 100 kg N ha^-1 yr ^-1and 14 kg N ha^-1 yr ^-1, respectively. The fluxes ofthe gaseous N compounds NO and N₂O were of minorimportance for the total N turnover in the forest, NO_xemission being <1 kg N ha^-1 yr ^-1 and N₂Oemission from the soil being 0.5 kg N ha^-1 yr ^-1 withno significant difference between wet and dry soils.Concentrations of NO₃ ^- and NH₄ ⁺ in thesoil solution beneath the rooting zone are very small andconsequently the N leaching is almost negligible. It isconcluded that the nitrogen mass balance of this old beechforest ecosystem mainly is controlled by the input by dry andwet deposition and a large internal N cycle with a fast litterturnover. The nitrogen input tothe forest ecosystem which currently exceeds the critical loadby 5 kg N ha^-1 yr ^-1is mainly accumulated in the soil and no significant nitrateleaching is occurring.     

Nitrogen Turnover and Nitrate Leaching after Bark Beetle Attack in Mountainous Spruce Stands of the Bavarian Forest National Park

More than 85% of the mountainous spruce forest of the Bavarian Forest National Park died after bark beetle attack during the last decade. The elemental budget of intact stands and of different stages after the dieback was investigated. N-fluxes in throughfall of intact stands were lower (12–16 kg ha^-1 a^-1) than in an earlier study in an intact mountainous spruce stand in the Bavarian Forest National Park and were reduced in the first years after the dieback (3–5 kg N ha^-1 a^-1). Nitrate-N fluxes by seepage water of intact stands at 40 cm depth, which is below the main rooting zone, were moderate (5–9 kg ha^-1 a^-1). After the dieback of the stands, NH₄ ⁺ concentrations were increased in humus efflux as were NO₃ ^- concentrations in mineral soil. Due to the relatively high precipitation, dilution of the elemental concentrations in seepage was considerable.Therefore, NO₃ ^- concentrations were usually below the level of drinking water (806 μmol NO₃ ^- L^-1), with lowest concentrations after the snowmelt and highest in autumn. Nitrate concentrations were elevated from the first year until the 7th year after the dieback. Total NO₃ ^--N losses by seepage until the 7th year after the dieback equalled 543 kg N ha^-1. Aluminium fluxesafter the dieback were enhanced in the mineral soil from 55 to 503 mmol_c m^-2 a^-1 (average of 8 yr), K⁺ fluxes from 8 to 37 mmol_c m^-2 a^-1, and Mg²⁺ fluxes from 13 to 35 mmol_c m^-2 a^-1. The consequences for the nutritional status of the ecosystem, the hydrosphere, and forest management are discussed in the paper.     

Post-Glacial Lead Dynamics in a Forest Soil

The use of alkyl-Pb additives in gasoline during the 20th century resulted in widespread Pb pollution. The objective of this study was to determine the relative importance of atmospherically deposited Pb and Pb released through weathering to soil Pb pools at the Hubbard Brook Experimental Forest, New Hampshire. We employed a selective extraction method to estimate the amount of Pb that was: water-soluble + exchangeable (EX); inorganically bound (IB); organically bound (ORG); bound to amorphous oxides (AMOX); and bound in crystalline minerals (RES). After normalizing crystalline-Pb concentrations to the immobile element Ti, we estimated that 14.1 kg ha^-1 of Pb has been weathered from Hubbard Brook soils in the 12,000–14,000 yr since deglaciation – a long-term average release of 1.0–1.2 g ha^-1 a^-1. Analysis of Ti-normalized total Pb concentrations indicated a net post-glacial decrease of 7.2 kg ha^-1 in the total Pb pool – consisting of a net accumulation of 4.9 kg ha^-1 in the O horizon, and a net loss of 12.1 kg ha^-1 from mineral soil. Atmospheric deposition of Pb between 1926 and 1989 (estimated as 8.7 kg ha^-1) was a major source of Pb in the post-glacial period. Together, long-term weathering release and 20th century atmospheric deposition could account for all of the Pb in the EX, IB, ORG, and AMOX fractions. Lead from gasoline appears to constitute a major fraction of the total Pb burden in Hubbard Brook soils. Periodic analysis of soil Pb fractions may be useful in monitoring the fate of Pb in forest soils.     

Levels and Characteristics of TOC in Throughfall, Forest Floor Leachate and Soil Solution in Undisturbed Boreal Forest Ecosystems

Total organic carbon (TOC) concentrations and fluxes in throughfall, forest floor leachate, soil solution (15 and 35 cm depths), and groundwater for coniferous forest sites in the boreal zone throughout Finland are described. Eight upland forest stands and one peatland forest stand are included in the study and the samples were collected during 1991–1997. Carbon (C) pools in the living tree biomass and soil compartments are presented, and the hydrophobic/hydrophilic and acidic components of dissolved organic carbon (DOC) in samples collected in autumn 1999 and spring 2000 from two of the sites are compared. Biomass (aboveground and belowground) pools of C averaged 88 Mg ha^-1 and soil (humus layer + 20 cm soil layer) averaged 55 Mg ha^-1. Stand throughfall TOC monthly mean concentrations ranged from 4.0 to 18.6 mg L^-1 and annual fluxes averaged 4.0 g m^-2 yr^-1. TOC concentrations in the water passing through the forest floor and soil decreased with depth. Plot mean concentrations at 35 cm depth values ranged from 4.1 to 21.2 mg L^-1 and fluxes averaged 3.7 g m^-2 yr^-1. Throughfall TOC concentrations were lowest during the winter, snowfall period and highest during the growing season. No monotonic trends in throughfall TOC concentrations over the 1991–1997 period were found. Soil solution TOC concentrations varied considerably, both within and between years. DOC in throughfall, forest floor, and soil solutions and in both autumn and spring seasons was dominated by hydrophobic fractions, particularly acids. Spruce canopies and litter appear to be important sources of soluble organic carbon, particularly acidic and hydrophobic compounds. Further studies on the nature and dynamics of organic carbon fluxing through coniferous, boreal forest ecosystems are needed.     

A Comparison of Loch Chemistry from 1955 and 1999 in the Cairngorms,N.E. Scotland

The Cairngorms in north-east Scotland is remote from pollutant sources although it currently receives ca. 10 kg ha¹ yr¹ S and ca. 11 kg ha¹ yr¹ N deposition from the atmosphere.In 1955, 15 lochs (lakes) at a range of altitudes were sampled and analysed for major ion concentrations. A new survey of these and an additional 23 lochs and their catchment soils was conducted in 1999 to determine the impact of acid deposition, and the changes in loch chemistry since the 1955 survey. The bedrock geology of this region has a strong influence on the loch chemistry. Surface waters were generally more acidic in high altitude areas due to predominantly poorly buffered, thin alpine soils developed on granitic parent material (mean acid neutralising capacity (ANC) for 23 lochs = 30 eq L¹). At lower altitudes where the geology is dominated by Dalradian metamorphic rocks surface waters are comparatively base rich and have higher ANC (mean ANC for 15 lochs = 157 eq L¹). Surface water nitrate concentrations show a negative relationship with soil C:N status, in that higher nitrate only occurs at low soil C:N ratios. A comparison of data for 1955 and 1999 shows that sulphate concentrations are significantly lower (67.8 and 47.5 eq L¹, respectively), and pH has improved (pH 5.6 and 5.9) in response to decreased S deposition since the mid 1970s. However, mean nitrate concentrations were found to increase from 2.48 >eq L¹ in 1955 to 5.65 eq L¹ in 1999. Differences in the sampling and laboratory methods from 1955 and 1999 are acknowledged in the interpretation of data.     

Linking field experiments to long-term simulation of impacts of nitrogen deposition on heathlands and moorlands

The results from three long-term field manipulation studies of the impacts of increased nitrogen deposition (0–120 kg N ha^?1 yr^?1) on lowland and upland heathlands in the UK were compared, to test if common responses are observed. Consistent increases in Calluna foliar N content and decreases in litter C:N ratios were found across all sites, while increases in N leaching were not observed at any site over the range 0–80 kg ha^?1 yr^?1. However, the response of Calluna biomass did vary between sites, possibly reflecting site differences in nutrient status and management histories. Five versions of a simulation model of heathland responses to N were developed, each reflecting different assumptions about the fate and turnover of soil N. Model outputs supported the deduction from mass balance calculations at two of the field sites that N additions have resulted in an increase in immobilisation; the latter was needed to prevent the model overestimating measured N leaching. However, this version of the model significantly underestimated Calluna biomass. Model versions, which included uptake of organic N by Callunaand re-mobilisation of N from the soil organic store provided some improvement in the fit between modelled and field biomass data, but re-mobilisation also led to an overestimation of N leaching. Quantification of these processes and their response to increased N deposition are therefore critical to interpreting experimental data and predicting the long-term impacts of atmospheric deposition on heathlands and moorlands.     

A Novel Method for Mapping Critical Loads Across a River Network: Application to the River Dart,Southwest England

To date, estimates of freshwater critical loads have beenbased on a single sample site within a given area, in theUK the `most sensitive' surface water in each 10 km gridsquare. The critical loads obtained are thus highlydependent on the sites chosen, and at a relatively coarsespatial resolution. To produce a higher resolutioncritical load assessment, the PEARLS (Prediction ofAcidification and Recovery on a Landscape Scale)procedure has been used to estimate critical loads acrossa large (248 km²), partially acid-sensitivecatchment in Southwest England. PEARLS utilises availablesoils and land-use databases, and sampled streamchemistry data, to derive characteristic runoffcompositions for a set of landscape types. Mixingequations are then used to calculate runoff chemistry,and subsequently critical loads, throughout the streamnetwork. Results show major spatial variability, withcritical loads lowest in streams draining peat-moorlandheadwaters, and generally increasing downstream asagricultural land contributes an increasing proportion ofrunoff. The 5th percentile freshwater critical loadfor the catchment is estimated at 0.29 keq H⁺ ha^-1yr^-1,and critical loads are exceeded for around 40% of totalstream length. The PEARLS methodology provides a novelopportunity to assess the spatial variability infreshwater critical loads, and to provide estimates ofexceedance at whole catchment scale. It has potentialapplication in the assessment of surface watersensitivity to acidification across wider areas in the UKand elsewhere.     

Linking field experiments to long-term simulation of impacts of nitrogen deposition on heathlands and moorlands

The results from three long-term field manipulation studies of the impacts of increased nitrogen deposition (0–120 kg N ha^–1 yr^–1) on lowland and upland heathlands in the UK were compared, to test if common responses are observed. Consistent increases in Calluna foliar N content and decreases in litter C:N ratios were found across all sites, while increases in N leaching were not observed at any site over the range 0–80 kg ha^–1 yr^–1. However, the response of Calluna biomass did vary between sites, possibly reflecting site differences in nutrient status and management histories. Five versions of a simulation model of heathland responses to N were developed, each reflecting different assumptions about the fate and turnover of soil N. Model outputs supported the deduction from mass balance calculations at two of the field sites that N additions have resulted in an increase in immobilisation; the latter was needed to prevent the model overestimating measured N leaching. However, this version of the model significantly underestimated Calluna biomass. Model versions, which included uptake of organic N by Callunaand re-mobilisation of N from the soil organic store provided some improvement in the fit between modelled and field biomass data, but re-mobilisation also led to an overestimation of N leaching. Quantification of these processes and their response to increased N deposition are therefore critical to interpreting experimental data and predicting the long-term impacts of atmospheric deposition on heathlands and moorlands.     

Pools and Fluxes of Cations,Anions and Doc in Two Forest Soils Treated With Lime and Ash

The effect of liming and ash treatment on pools, fluxes and concentrations of major solutes was investigated at two forestedsites (Norway spruce) in S. Sweden. One site was treated 15 yrprior to sampling (Hasslöv-Hs; dolomite: 3.45 and 8.75 t ha^-1) and the other 4 yr before (Horröd-Hd; dolomite: 3.25 t ha^-1; wood ash: 4.28 t ha^-1). Effects of limingwere most pronounced in the O horizon solutions where higher pH,elevated Ca (120–700 M) and Mg (50–600 M) were observed as compared to control plots. The impact on the mineralsoil was more moderate. Soil solution concentrations were combined with modelled hydrological flow to calculate mass flows,which largely followed the trends of the solution composition. Liming also resulted in large increases of both exchangeable Caand Mg as well as effective cation exchange capacity (CEC_E;2–5 times the controls). The base saturation (BS%) was raised to 60–100% in the O horizon while in the mineral soil elevated values were only seen at the Hs site (20–60%; down to 10–15 cm depth for 8.75 t ha^-1). Ash treatment did notaffect either the soil solution nor the exchangeable pool to thesame extent as lime. In general, the impact at the Hd site was less pronounced especially in the mineral soil, which might be due to shorter treatment time (4 vs. 15 yr) and also differentthickness of the O horizon. Budget calculations for Ca and Mg originating from the lime showed that a major part of the Ca (40–100%) was retained in the top 30 cm of the soil, of which30–95% was present in the O horizon. The mobility of Mg wasgreater and it was estimated that a significant part had been leached from the profile (30 and 50 cm depth) after 15 yr. Increased mass flows of NO₃ ^- due to nitrification resulting from liming at the Hs site were calculated in the range120–350 mmol m^-2 yr^-1 (or 1.2–3.5 kmol ha^-1 yr^-1). There was significant leaching of Al (25–60 mmol m^-2 yr^-1), of which about 70% was inorganic, in thelower B horizon at both sites with no influence of liming.     

Some limitations of sluiced fly ash as a liming material for acidic soils

Innovative methods are currently being sought to safely utilize and reduce the amount of sluiced fly ash stored in on-site facilities at thermal electric power facilities in Ontario, Canada. The objective of this study was to investigate the possibility of utilizing sluiced fly ash as a liming material on acidic soils. A greenhouse experiment was established to examine the influence of the ash on soil pH values and the yield and chemical composition of corn, soybeans, wheat and alfalfa grown on amended soil.The sluiced ash had an initial pH value of 9.1 (1:1 H₂O) and contained an equivalent CaCO₃ content of 11.5% (±0.2%) based on total content of Ca and Mg in the ash. Fly ash was applied at rates of up to 167 tonnes ha⁻¹ to samples of the surface horizon (0–15 cm) of an acidic clay textured soil found local to the power plant. Application of fly ash increased soil pH values but a high concentration of boron in the added ash limited plant growth. Results indicated that the sluiced ash could be used as a liming material at application rates of up to 110 tonnes ha⁻¹, on acidic, clay textured soils provided boron tolerant crops such as alfalfa were grown. Further research is required to establish the potential for leaching of boron to ground water and to determine the potential for plant uptake of other trace elements such as As and Se.     

The Importance of Selecting Appropriate Criteria for Calculating Acidity Critical Loads for Terrestrial Ecosystems Using the Simple Mass Balance Equation

The Simple Mass Balance (SMB) equationis commonly used throughout Europe for thecalculation of acidity critical loads for forestsoils. Different criteria can be set in themodel depending on whether the receptor (e.g. treeroots) is more sensitive to the toxic effects ofaluminium or to unfavourable pH conditions. Thispaper examines the effects on critical loadscalculations of using different criteria andcritical limits, and demonstrates the importanceof selecting the most appropriate and justifiablecriteria for the chosen receptor, since they caneffect the critical loads values obtained. Abrief review of the range of different criteriaand limits used throughout Europe is included. In addition, the gibbsite equilibrium constant,used in the SMB equation to represent therelationship between dissolved aluminium andhydrogen ions in soil solution, is discussed. This relationship is not generally described inthe literature as a criterion in the equation,but this work highlights the effects differentgibbsite values have on critical loadcalculations and the importance of applying themost appropriate value for the soil in therooting zone of the receptor.     

An automated wet deposition system to compare the effects of reduced and oxidised N on ombrotrophic bog species: Practical considerations

Critical N loads for ombrotrophic bogs, which often contain rare and N-sensitive plants (especially those in lower plant groups: lichens, mosses and liverworts), are based on very few experimental data from measured, low background N deposition areas. Additionally the relative effects of reduced versus oxidised N are largely unknown. This paper describes an automated field exposure system (30 km S. of Edinburgh, Scotland) for treating ombrotrophic bog vegetation with fine droplets of oxidised N (NaNO₃) and reduced N (NH₄Cl). Whim Moss exists in an area of low ambient N deposition (ca. 8 kg N ha^?1 y^?1), the sources and quantification of which are described. The wet N treatment system is run continuously, and is controlled/activated by wind speed and rainfall to provide a unique simulation of “real worl” treatment patterns (no rain=no treatment). Simulated precipitation is supplied at ionic concentrations below 4 mM in rainwater collected on site. Treatments provide a replicated dose response to 16, 32 and 64 kg N ha^?1 y^?1 adjusted for ambient deposition (8 kg N ha^?1 y^?1). The 16 and 64 kg N ha^?1 y^?1 are duplicated with a P+K supplement. Baseline soil chemistry and foliar nutrient status was established for all 44 plots for Calluna vulgaris, Sphagnum capillifolium, Hypnum jutlandicum and Cladonia portentosa.     

Acidification Trends and the Evolution of Neutralization Mechanisms through Time at the Bear Brook Watershed in Maine (BBWM), U.S.A.

The paired catchment study at the forested Bear Brook Watershed in Maine (BBWM) U.S.A. documents interactions among short- to long-term processes of acidification. In 1987–1989, runoff from the two catchments was nearly identical in quality and quantity. Ammonium sulfate has been added bi-monthly since 1989 to the West Bear catchment at 1800 eq ha^-1 a^-1; the East Bear reference catchment is responding to ambient conditions. Initially, the two catchments had nearly identical chemistry (e.g., Ca²⁺, Mg²⁺, SO₄ ^2-, and alkalinity ≈82, 32, 100, and 5 μeq L^-1, respectively). The manipulated catchment responded initially with increased export of base cations, lower pH and alkalinity, and increased dissolved Al,NO₃ ^- and SO₄ ^2-. Dissolved organic carbon and Si have remained relatively constant. After 7 yr of treatment, the chemical response of runoff switched to declining base cations, with the other analytes continuing their trends; the exports of dissolved and particulate Al, Fe, and P increased substantially as base cations declined. The reference catchment has slowly acidified under ambient conditions, caused by the base cation supply decreasing faster than the decrease of SO₄ ², as pollution abates. Export of Al, Fe and, P is mimicking that of the manipulated watershed, but is lower in magnitude and lags in time. Probable increasing SO₄ ^2- adsorption caused by acidification has moderated the longer-term trends of acidification of both watersheds. The trends of decreasing base cations were interrupted by the effects of several short-term events, including severe ice storm damage to the canopy, unusual snow pack conditions, snow melt and rain storms, and episodic input of marine aerosols. These episodic events alter alkalinity by5 to 15 μeq L^-1 and make it more difficult to determine recovery from pollution abatement.