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A soil acidification study using the PROFILE model on two contrasting regions in Switzerland
Chemical Geology 170 Ž2000. 243–257 www.elsevier.comrlocaterchemgeo
A soil acidification study using the PROFILE model on two contrasting regions in Switzerland Urs Eggenberger a,) , Daniel Kurz b a
Institute of Mineralogy and Petrology, UniÕersity of Berne, Baltzerstrasse 1, 3012 Bern, Switzerland b Eggenberger & Kurz Geo-Science, Ralligweg 10, 3012 Bern, Switzerland Received 15 January 1998; accepted 15 December 1999
Abstract The steady-state soil chemistry model PROFILE was used to calculate the chemical status of forest soils under present deposition loads for two areas with dissimilar ecosystem properties. Two regions in Switzerland, with contrasting bedrock geology were selected to be investigated in detail: 88 locations in the Jura Mountains, representative for carbonate bedrock and 91 locations in the Ticino Area dominated by metamorphic crystalline host rocks. Weathering rates calculated for the key regions cover the tremendous range between 0.013 and 25 keq hay1 yry1. In the Ticino Area, the effect of increased abundance of relatively fast weathering silicates Žepidote, hornblende and plagioclase. on the weathering rate is apparently masked by the total effects of the physical conditions applied and by the variation in the deposition load. In the Jura Mountains, generally high weathering rates occur with about 50% of the sites yielding rates above 1 keq hay1 yry1. In many of the sites investigated, however, carbonates have already been dissolved completely in the soil horizons of interest resulting in very low weathering rates. The critical load of actual acidity was calculated according to: CL Acidity s R Weathering y ANC Leaching , where alkalinity leaching is estimated by keeping the base cation to aluminum molar ratio at the critical limit of 1 at steady-state. The minimum critical load calculated was 0.2 keq hay1 yry1 and the maximum was 6.2 keq hay1 yry1. Comparing the cumulative frequency distributions of critical loads of actual acidity for forest soils in the individual areas it can be seen that the differences between the key regions are less substantial than with the weathering rates. Critical loads of acidity for the Ticino Area range from 1 to 3.9 keq hay1 yry1. Sites yielding the lowest critical loads of acidity are observed in the Jura Mountains. Among these apparent sensitive soils are soils with intermediate or high weathering rates, although it has depleted topsoil layers. Within the context of this model application, it becomes apparent that the sensitivity of these soils with respect to acidification is also governed by the alkalinity leaching term and not only by the susceptibility of its minerals to weathering. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Acidification; Critical load; Chemical weathering; Forest soil; Deposition; PROFILE model
1. Introduction
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Corresponding author. Tel.: q31-631-8781; fax: q31-6314843. E-mail addresses: [email protected] ŽU. Eggenberger., [email protected] ŽD. Kurz..
Anthropogenically induced soil acidification causes increased depletion of base saturation of soils and increased aluminum and hydrogen concentrations in the soil solution. Additionally, it disturbs the
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nutrient status of soils by inducing strong changes in the availability of essential elements. This has raised concern about adverse effects of changes in the chemical status of soils and soil solution with respect to forest health. It has often been argued that continuous acidic input to forest ecosystems may reduce tree growth and enhance tree mortality in the long term ŽSverdrup and Warfvinge, 1993.. Relating the effects of acidification of a soil system to forest health, the stage at which acidification of soils will harm trees and the forest ecosystems had to be defined. The United Nations Economic Commission for Europe ŽUN ECE. adapted within the work program under the Convention on Long Range Transboundary Air Pollution ŽLRTAP. the ‘critical load’ concept to assess the environmental impact of acidifying compound emission. Critical load in general was defined as ‘‘a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified elements of the environment do not occur according to present knowledge.’’ With respect to soil acidification, the critical load of acidity is ‘‘the maximum deposition of acidifying compounds that will not cause chemical changes leading to long-term harmful effects on ecosystem structure and function.’’ The molar ratio between the sum of base cations calcium ŽCa2q ., magnesium ŽMg 2q . and potassium ŽKq. , lumped into a divalent component Bc, and aluminum in the soil solution is often used as critical chemical parameter, assuming that a BcrAlG 1 will prevent forest ecosystems from damage ŽSverdrup and Warfvinge, 1993.. Relating environmental properties to a single quantitative value entails the use of models. Regional estimates of critical loads of acidity are currently assessed by a static modeling approach. In this approach, which is usually referred to as steady-state mass balance method, a steady-state is assumed for ecosystem relevant chemical processes, such that all sources of acidity are balanced by sources of alkalinity. In practice, the objective is to avoid the selected critical chemical value being exceeded at critical load. At present critical loads are calculated by various implementations of the steady-state mass balance approach for soil and water chemistry, e.g., the simple mass balance model ŽSMB. ŽCCE, 1993.. SMB model variants have been employed by all European countries. Since 1993,
Switzerland has additionally been applying a fourlayer steady-state mass balance model called PROFILE ŽWarfvinge and Sverdrup, 1992; Kurz et al., 1995.. The current study focuses on steady-state chemistry of forest soils under present deposition loads, on critical loads of acidity and exceedance of critical loads of acidity as predicted by the PROFILE model. It discusses differences in the results of two selected areas in respect of dissimilarities in their ecosystem properties. 2. Materials and methods 2.1. The PROFILE model Steady-state implies circumventing past acidification processes and directly evaluating the soil’s condition at the post-acidification state. By definition, net depletion of base saturation is zero at steady-state. Acidic input to soils is balanced by internal sources of alkalinity such as weathering and net nitrogen uptake, or by export of acidity from the system through runoff. PROFILE operates on the basis of a number of internally homogeneous horizons within the soil profile, corresponding to the natural stratification of soils. Processes are represented by mass balance equations for acid neutralizing capacity ŽANC., base cations, nitrate and ammonium and by kinetic equations for chemical weathering and nitrification. Temperature dependence is considered for all individual processes including aqueous equilibrium and dissolution reactions. Phases and reactants in the system interact via the soil solution. Soil solution ANC is controlled by the auto-protolysis of water, the carbonate system, the acid–base reactions of organic acids and the aluminum system. The hydrogen ion is treated as being dependent on the variable aqueous ANC. Aluminum species concentrations are calculated assuming equilibrium with the solid model phase gibbsite. The complete set of equilibrium equations and the corresponding coefficients are described in detail by Warfvinge and Sverdrup Ž1992.. The major components of the model are listed in Table 1. Field weathering rates are calculated using laboratory dissolution rates for individual minerals Žfor
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257 Table 1 Major components of the PROFILE model Žappropriate equations, see Sverdrup, 1992. Mass balances
Aqueous equilibrium systems
Kinetic control Input quantities
ANCaq base cations ŽCa, Mg, K. nitrate ammonium carbonic acid aluminum–water system weak organic acids water autoprotolysis chemical weathering nitrification climatic conditions water balance atmospheric deposition nutrient uptake soil properties
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The only biological process that is modeled in terms of a kinetic rate expression is nitrification. Production of ANC is considered by the uptake of ammonia and nitrate, as well as by nitrification. The values of the kinetic rate coefficient and the Michaelis–Menten saturation constant are discussed in Sverdrup et al. Ž1990.. From the complete mathematical derivation ŽWarfvinge and Sverdrup, 1992., it becomes evident that the net reaction of nitrogen, nitrification and nitrogen uptake, always generates a significant contribution to ANC. Nitrogen uptake and base cation uptake have to be given as input. Base cation uptake consumes ANC, for each keq base cation taken up 1 keq ANC. 2.2. Site description
reaction orders and rate coefficients, see Sverdrup, 1990. and by taking into account soil hydrology, soil physical properties, soil mineralogy and mineral composition. The overall rate is calculated as the sum of the rates of the individual mineral dissolving reactions, whereby simultaneous product inhibition is realized by rate reduction factors. Dissolution reactions with hydrogen ion, water, carbon dioxide and organic acids are considered ŽSverdrup, 1990.. The rate R We athering is treated as proportional to the exposed surface area of the minerals and to the soil moisture saturation. The temperature dependence of the weathering reactions is considered using the Arrhenius function. Exchange reactions between exchangeable base cations and acidity are modeled as perfectly reversible chemical reactions. The rate of such reactions, however, is limited by the transport of base cations between the bulk of the soil solution and active sites on the exchanger surface. The pseudo steady-state rate equation for such a process was derived from Fick’s first law. The change in exchangeable bases X BC , will be proportional to the flux of base cations to the exchanger, which is driven by the concentration difference between the surface of the exchangeable complex and the concentration in the bulk of the soil solution. In this study, the flow of nutrient cations, nitrogen compounds, and ANC between the soil solution and the roots of growing vegetation has been neglected.
In the present study, two regions in Switzerland with contrasting bedrock geology were selected to be investigated in detail. The selected area in the Jura Mountains is dominated by carbonate host rocks. The soils’ protolith of the area in the Southern Alps ŽTicino Area. is crystalline, metamorphic rocks. For the lithologies, soil types, vegetation and atmospheric deposition, data were available on a 1 = 1-km grid resolution. The model was applied to sites on intersections of a 4 = 4-km sub-grid of the 1 = 1-km grid and calculations were restricted to forested sites. In the Jura Mountains, 88 sites, and in the Ticino Area, 91 sites were processed. Detailed information on data collection and detailed input data derivation are described in SAEFL Ž1998.. The location of the two areas used in this study is shown in Fig. 1. 2.2.1. The Jura Mountains The area along the northern boundary of Switzerland covers the eastern and central Jura Mountains. The arched chain consists of more or less regular folds with a moderate relief in the central Folded Jura, where the south easternmost chain is generally the topographical highest one. The north easterly direction, adjoining Tabular Jura consists of Black Forest basement uplifts and their mostly undisturbed Mesozoic cover. Light colored Upper Jurassic limestones form the backbone of most of the Jura folds and build up most of the plateaus except in those parts of the Aargau and Basel Jura, where they have
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Fig. 1. Location of the two regions investigated. Both areas are comparable in area size and the number of sites. North direction towards top of page in all figures.
been truncated by pre-Eocene erosion. There, Middle Jurassic oolitic limestones cover a core region that is bounded to the north and the south by more argillaceous and marly Late Jurassic and Triassic formations. Partly siliciclastic sedimentary rocks of Molasse type are preserved in strike parallel intra. and Quaternary mountain basins Že.g., Delemont ´ deposits predominantly cover the Rhein Graben and associated parts around Basel. Most of the more closely investigated soils have a calcareous protolith Ž62%., 5% are developed on a siliciclastic or argillic sedimentary bedrock, and in 33%, the protolith of the soils constitutes of Quaternary material ŽFig. 2.. The dominant soil types in the Jura Mountains are therefore calcareous lithosol- and rendzina-type soils Ž81%., followed by fluvisol Ž12%. and calcareous regosol and calcareous cambisol Ž6%.. Due to the moderate relief of the Jura Mountains, the vegetation cover with respect to forest type is
rather heterogeneously distributed. Coniferous forests are primarily on topographic highs in particular on the backbones of the Jura folds and on the horsts of the Tabular Jura. Deciduous forests generally occur in topographical lower sites and are therefore more frequent in the eastern Jura Mountains. 2.2.2. The Ticino Area The key area in the Jura Mountains was compared with an area in the southern part of Switzerland mainly for two reasons. The area, largely identical with the Canton Ticino, lies entirely to the south of the major alpine watershed and experiences substantial deposition of acidifying compounds from industrialized northern Italy. The central and northern part of the area consists of crystalline basement nappes of pre-Carboniferous age, with varied ortho- and paragneisses predominating. Mesozoic metasediments occur as thin and often discontinuous bands between the gneiss units and accumulate in the northernmost
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Fig. 2. Bedrock lithology. Criteria for the simplified classification according to the Geotechnical Map of Switzerland are listed in SAEFL Ž1998..
part of the Lepontine Alps. To the south, separated from the Penninic nappes by the Insubric line, follow south Alpine basement rocks including high-grade metasediments and metaintrusives. In the Lago di Lugano area, basement rocks were intruded by Late Paleozoic volcanic rocks and to the east the basement is overlain by Liassic siliceous limestones. The lithology ŽFig. 2. derived from the geotechnical map of Switzerland is dominated by crystalline rocks such as biotite–muscovite schists Ž67.5%., sericite–chlorite schists Ž4%. and intrusives Ž1.5%.. Calcareous lithology consists of the Liassic siliceous limestones in the southernmost part of the area and . in the Penof calcareous schists ŽBundnerschiefer ¨ ninic nappes in the northern part of the investigated area. Criteria for the classification of the sites are listed in SAEFL Ž1998.. Bedrock lithology implies the presence of soils highly sensitive to acidification. In the central and northern parts of the area, poorly differentiated well-drained soils Ž63% lithosol, ranker. characterize the higher altitudes. Towards lower elevation, more evolved soils of podzolic tendency Ž24%. are present. Calcareous soil types only make up 6% of the total area, corresponding to the locations dominated by calcareous lithology. The pronounced relief in the Ticino Area permits the distinction of altitude dependent zones of vegeta-
tion. Deciduous forest vegetation covers the lower altitudes with Castanea satiÕa becoming frequent towards the south of the Insubric line. As the altitude rises towards the north, coniferous forests become more frequent in comparison to deciduous forests. 2.3. Input On the regional scale input data were acquired according to the UN ECE Task Force on Mapping Guidelines ŽSverdrup et al., 1990.. The data acquisition procedure is mainly based on regional survey information or single spot measurements. Transformation or extrapolation functions are then applied to the required regional grided or site-specific data ŽFOEFL, 1994; SAEFL, 1998.. For some input parameters Že.g., soil mineralogy., computerized submodels were used for their derivation ŽEggenberger, 1995.. An overview of the most relevant parameters is given in Table 2. The following discussion focuses on input parameters that are important in interpreting the model result. 2.3.1. Hydrology and atmospheric depositions Precipitation was derived from the Hydrological Atlas of Switzerland Ž1992.. Infiltration and runoff was calculated from precipitation by correcting for
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Table 2 Overview of the most important input parameters needed and the data acquisition strategy Parameter
Commentrsource
Precipitation Wet deposition Dry deposition Bc deposition Thickness of layers
Hydrological Atlas of Switzerland Ž1992.. Simplified corrections for interception and surface runoff. From altitude dependent rainwater concentration ŽFOEFL, 1994. and precipitation rates. Concentration of the pollutant in air multiplied with component specific deposition velocities ŽFOEFL, 1994.. From open-field bulk deposition using filtering factors for dry deposition as proposed by CCE Ž1991.. Layer thicknesses for 9 elevation classes was assessed using measurements from 172 soil profiles of the 1993–1995 NFI ŽZimmerman, EAFV, personal communication, 1995., or measured values. From particle size distribution. Total elemental analysis ŽSAEFL, 1998., normative mineral compositions and back-calculations methods ŽEggenberger, 1995., or measured ŽXRD, internal standard method. values Žcalcite, dolomite, K-feldspar, plagioclase Žan-content., albite, hornblende, pyroxene, epidote, garnet, biotite, muscovite, chlorite, vermiculite, kaolinite, apatite.. From water retention capacity ŽEJPD et al., 1980., water mass balance, and soil depth Žsee layer thickness.. Interpolated from a limited set of measurements ŽFOEFL, 1994.. Long-term annual means were used.
Surface area Mineralogy
Soil water content Soil temperature
surface runoff and evapotranspiration, respectively ŽFOEFL, 1994.. The two regions under investigation differ significantly in the amount of water that infiltrates into the soil, as shown in Fig. 3. The Ticino Area receives among the highest annual precipitation
in Switzerland ranging between 1000 and 2600 mm yry1 , whereas the Jura Mountains show moderate annual precipitation values, ranging from 650 to 1500 mm yry1 . The overall higher values in the Ticino Area are due to the geographic situation. The
Fig. 3. Infiltration in mm yry1 . Source: Hydrological Atlas of Switzerland Ž1992.. Cumulative distribution of infiltrated precipitation into the soil.
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Ticino incorporates the first significant topographic elevations towards the Alps that leads to the high precipitation values, strongly dependent on the precipitation–elevation relationship. Deposition of acidifying compounds was extrapolated from 1986–1989 field measurements Že.g., BUWAL, 1990. and are described in detail in SAEFL Ž1998.. Individual deposition models for wet, dry and gas deposition were then used to calculate the grided data set ŽFig. 4.. High deposition rates in the Ticino Area are a consequence of substantial emissions of local and north Italian industries. Acid deposition rates in lower regions, particularly in the Ticino Area, are dominated by dry deposition due to an inverse dependence of dry deposition rates on altitude. Generally, the effect of higher filtering interception and higher deposition velocities in coniferous forests at higher altitudes is smaller than the decrease in the pollutant concentration with increasing altitude. Wet acid deposition rates, on the other hand, increase with increasing precipitation rates towards higher elevation, this increase however being
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smaller than the decrease in the dry deposition rates. In general, the acid deposition rate pattern is therefore inverse to the precipitation and infiltration patterns ŽFig. 4., respectively. This also leads to a larger range of acid deposition inputs Ž1.8–5.4 keq hay1 yry1 . in the Ticino Area. In the Jura Mountains on the contrary, 99% of the rates lie in the range between 2.5 and 3.2 keq hay1 yry1 . Analogous to acid deposition, base cation deposition is also relatively high in the Ticino Area ŽFig. 5. and moderate in the Jura Mountains. Base cation deposition rates in the Ticino Area are in the range of 0.32–1.05 keq hay1 yry1 , compared to 0.25–0.85 keq hay1 yry1 in the Jura Mountains. Base cation deposition is often associated with episodic events Že.g., Sahara dust. and measurements show therefore large spatial and temporal spreads. The higher deposition rates in the eastern part of the Ticino Area is rather an artificial effect of the extrapolation method. Bulk deposition loads of base cations were supplied by the Coordination Center for Effects ŽCCE, 1991, 1993. with a 18 longitude= 1r28 latitude resolution.
Fig. 4. Acid deposition in keq hay1 yry1 . Cumulative distribution of acid deposition rates.
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Fig. 5. Base cation deposition in keq hay1 yry1. Source: bulk deposition rates according to CCE Ž1991.. Dashed line, see text. Cumulative distribution of base cation deposition rates.
The CCE suggests for open-field conditions in the deposition grid of the eastern part of the Ticino Area a value of 0.427 keq hay1 yry1 , whereas in the adjoining western grid, a value of 0.370 keq hay1 yry1 is estimated. The divide, marked in Fig. 5 with a dashed line, is located just west of the sites showing base cation deposition values higher than 1 keq hay1 yry1 . Higher deposition rates at more elevated sites are due to the higher filtering factors for dry deposition to coniferous forests Žmultiplication factor: 2.5.. The filtering factors decrease with decreasing occurrence of coniferous forests toward lower altitudes Žfiltering factor for deciduous forests: 1.0.. The general base cation deposition pattern therefore reflects the distribution of the tree species. The National Forest Inventory ŽEAFV, 1988. provided the data for tree species distribution as well as basic information to assess Bc and nitrogen uptake.
of reference soil profiles by means of a soil classification Žaccording to the Map of suitability of soils ŽBEK. in Switzerland; EJPD et al., 1988.. Moisture content was extrapolated from field capacity evaluated from the BEK classification and weighted according to the precipitation. Since soil mineralogy data was scarce, as only a few soil profiles were investigated in detail Ž3 from the Jura Mountains, 11 from the Ticino Area., soil mineral compositions was derived from chemical soil analyses using a normalization procedure ŽSAEFL, 1998..
2.3.2. Soil properties Soil properties including soil bulk density and mineral surface area were extrapolated from a series
Weathering rates calculated for the key regions cover a remarkable range between 0.013 and 25 keq hay1 yry1 ŽFig. 6.. Theoretical considerations imply
3. Results and discussion 3.1. Weathering rates
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Fig. 6. Calculated weathering rates in keq hay1 yry1 at present state. Cumulative distribution of calculated weathering rates. The X-axis is in log units.
that the three silicate groups’ epidote, hornblende and plagioclase feldspars are instrumental in the weathering rates observed for soils on silicate bedrock. Previous studies on weathering in soils confirm this implication in so far, as higher plagioclase and mafic mineral contents tend to give higher modeled weathering rates. In the Swiss key regions, the effect of increased abundance of these minerals is not directly correlated with weathering rates. Qualitatively, it appears that the Žregional. variation in the soil properties primarily is responsible for the scatter in the weathering rates. The effects of varying soil physico-chemical characteristics on weathering rates has been quantified, e.g., by Hodson et al. Ž1996.. In the region with calcareous bedrock, on the other hand, the presencerabsence of carbonates in the soil column primarily governs the magnitude of the weathering rate. Weathering rates calculated for 91 Ticino forest soils show a characteristic spatial variation ŽFig. 6.. Generally, more elevated sites yield weathering rates
predominantly higher than 0.5 keq hay1 yry1 , whereas low elevation sites in the Sotto-Ceneri area, in the Magadino plane, and in the Leventina valley are dominated by weathering rates lower than 0.5 keq hay1 yry1 . At the two south-westernmost sites, where about 1–5 wt.% of carbonates are present in the soil, only one site contributes to high weathering rates. The direct dependence of the weathering rate upon the mineralogical composition is exemplified solely at one location, where a carbonate content of 4–35 wt.% leads to the highest weathering rate of 2.2 keq hay1 yry1 in Ticino. Not all soils upon carbonate-bearing lithologies yield high weathering rates, as shown in the Jura Mountains. The region that is dominated by carbonate-bearing bedrock exhibits a rather heterogeneous spatial variation pattern of weathering. Soils producing very high weathering rates can be located next to soils with very low ones within the spatial resolution of the 4 = 4-km grid. At many of the sites examined, carbonates have already been dissolved in the soil
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layers considered. On the level of the cumulative frequency distribution ŽFig. 6., however, it becomes evident that generally higher weathering rates occur in the Jura Mountains. In this area 50% of the sites yield rates above 1 keq hay1 yry1 , whereas in the Ticino Area 95% of the sites show weathering rates below 1 keq hay1 yry1 . The model calculates very high weathering rates in calcareous soils. The question arose whether the soil solution chemistry fulfills equilibrium considerations usually adopted in geochemical modeling. Solution chemistry calculated with PROFILE was checked for possible oversaturation in respect to carbonates and possible secondary phases for three locations representative for very high, high, and intermediate weathering rates using PHREEQE ŽParkhurst et al., 1980.. Even at the very high weathering rate location Ž20 keq hay1 yry1 ., calcite saturation in the soil solution calculated with PROFILE was not yet reached according to PHREEQE results. The calculated Ca concentrations are therefore in accordance with independently determined calcite equilibrium considerations.
3.2. pH Solution pH calculated for the soils considered ranges from 4.0 up to 7.6. There is considerable spatial variation of pH, the pattern being comparable to the variation pattern of the weathering rate. The spread in pH is also larger in the Jura Mountains than in the Ticino Area. In the Ticino Area, calculated pH varies between 4.2 and 5.3 with only two sites having soil solution pH above 5.0. CaCl 2-pH of topsoils Žthe first 20 cm. has also been measured within the scope of the National Forest Inventory ŽEAFV, 1988.. The spatial variation pattern of both calculated and measured pH values is comparable. However, the absolute values may differ strongly. In the Ticino Area, measured pH is predominantly lower than calculated, whereas the opposite is common in the Jura Mountains. It is known that CaCl 2 extraction of acidified soils, as found in the Ticino Area, results lower pH-values in comparison to pure water extracts. The mobilization of the protons from the organic exchange sites results in a pH shift from y1r2 to y1 units. Differences in
Fig. 7. Calculated minimum molar BcrAl ratio. Cumulative distribution of minimum molar BcrAl. The X-axis is in log units.
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257
measured and calculated pH in the Jura Mountains are difficult to interpret, because there are sites where no carbonates were observed in the investigated soil compartments, but the measured pH shows values well above 5. Possible explanations are: Ž1. small amounts of carbonates eventually present in these soils were either not detected ŽXRD detection limit 1 wt.%. or not modeled Žuncertainty in the applied back-calculation of geochemical analyses., but control the water chemistry, Ž2. the comparability of soil extract and soil solution chemistry Žmodeled or measured. is limited in general, and Ž3. solution pH calculated for upper soil layers may be uncertain, due to simplifications adopted in the model Že.g., nutrient cycling not considered.. 3.3. Bc r Al molar ratio The solution BcrAl molar ratio provides a chemical assessment of the environmental impact of acidifying compound deposition. Above the threshold value of 1, trees should be protected from adverse
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effects of acidification. With a multi-layer model the minimum of the BcrAl molar ratios calculated for the individual soil layers within the rooting zone is often used as the relevant value for the soil profile. In Fig. 7, the distribution of these BcrAl ratio minima is shown. With current deposition loads, 43% of the sites in the Ticino Area and around 65% of the sites in the Jura Mountains develop minimum BcrAl molar ratios below 1. The calculated values indicate adverse influence of acid deposition on the soils’ chemical status, which increases the risk of forest damage at these sites. The spatial trend of the BcrAl molar ratio in the Ticino Area generally follows the trend of the weathering rate shown in Fig. 6. Sample sites at lower elevations in the core region of the Ticino Area show the lowest ratios. It was not expected that soils of the Jura Mountains would show even lower BcrAl molar ratios than the soils of the Ticino Area, where high acid deposition rates in combination with low weathering silicate rocks forecast unfavorable soil conditions. However, the weathering residuum of calcareous
Fig. 8. Critical load of acidity in keq hay1 yry1 . Cumulative distribution of critical load of acidity. The X-axis is in log units.
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soils lacks fast weathering minerals such as plagioclase feldspars or dark minerals and consequently prevailing low solution pH entails higher amounts of dissolved Al 3q, calculated on the basis of gibbsite equilibrium. Bc concentrations on the other hand remain low in these depleted soils resulting in low BcrAl ratios, if Bc sources other than weathering, e.g., nutrient cycling are neglected. 3.4. Critical load of acidity For the critical load assessment, the deposition rates are adjusted iteratively until the solution BcrAl molar ratio is equal to or larger than the threshold value of 1 throughout the soil profile. The criterion was applied to the top 30 to 50 cm, respectively, of the soils, assuming these to be the tree rooting depths of coniferous and deciduous trees. The critical load of actual acidity, representing the inherent capability of the system to neutralize acid deposition, was calculated according to CL Acidity s R Weathering y
ANC Leaching . In acid forest soils, the process of hydrogen and aluminum leaching govern the acidity loss that can be permitted. According to the criterion, these fluxes are calculated using base cation deposition, weathering rate and net base cation uptake to determine the amount of Bc available in the BcrAl molar ratio. The minimum critical load calculated was 0.2 keq hay1 yry1 and the maximum was 6.2 keq hay1 yry1 . These values quantify the total acidity input the system can tolerate regardless of where the acidity comes from. Comparing the cumulative frequency distributions of critical loads of actual acidity for forest soils in the individual areas it can be seen ŽFig. 8. that the differences between the key regions are less substantial than with the weathering rates. The distribution of the lower 40–50% of the critical loads estimated for sites on crystalline bedrock coincides with the distribution of the critical loads estimated for sites on potentially carbonate-bearing sedimentary substratum.
Fig. 9. Exceedance of critical load of acidity. Positive values indicate that acid deposition is higher than the soils buffering capacity. Cumulative distribution of the exceedance of the critical load of acidity. The X-axis is in log units.
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Critical loads of acidity according to steady-state calculations for the Ticino Area range from 1 to 3.9 keq hay1 yry1 . The spatial variation of the site values follows quite closely the pattern of the weathering rates, indicating that the central corridor and in particular the area between the southern slope of the Ticino valley and the Lago di Lugano comprise the sensitive soils. Despite generally lower weathering rates in the Ticino Area than in the Jura Mountains Žsee cumulative distribution function in Fig. 6., critical loads do not fall below 1 keq hay1 yry1 . Within the context of this model application, it becomes apparent that the sensitivity of soils with respect to acidification is also governed by the alkalinity leaching term and not only by the susceptibility of its minerals to weathering. Spots yielding the lowest critical loads of acidity are observed in the Jura Mountains. Among these apparent sensitive soils are soils with intermediate or high weathering rates, although with depleted topsoil layers.
Mountains and 63% of the sites in the Ticino Area received acid deposition in excess of the critical load. In the Ticino Area, the exceedance pattern mainly is the result of outstanding present deposition rates ŽS Dep q N Dep y non-marine Bc Dep . rather than of the sensitivity of soils regarding acidification. In the Jura Mountains, where the range of acid deposition rates is restricted and of critical loads extended, the exceedance pattern more explicitly is the result of the sensitivity of soils in respect of acidification. Areas in which the critical load of acidity is exceeded will experience BcrAl molar ratios in the soil solution below the limiting value of 1, if acid deposition continues at the present level ŽFig. 7.. Response data for a variety of tree species indicate that this chronic exceedance of the threshold load will adversely effect the forests in these areas ŽSverdrup and Warfvinge, 1993..
3.5. Exceedance of CL-acidity
4. Conclusions
Critical load of actual acidity indicates the total amount of acidity input of either anthropogenic or natural origin that an ecosystem may tolerate. The exceedance of the critical load is calculated by subtracting all acidity inputs to the soil from the critical load:
The application of the PROFILE model on a regional scale generates a large variability of achieved results as a consequence of the sensitivity of the model to most of the input parameters. An often used assumption in the assessment of soil acidification that single sites can be used to represent lithologically uniform areas, could not be confirmed. In the Ticino Area, as an example, sensitive soils tend to concentrate on low elevated areas in a central corridor. The surrounding areas appear not as sensitive, although the soils’ protolith consists of a similar gneiss suite. On the other side, sites yielding low critical loads of acidity are observed in the Jura Mountains. Among these apparent sensitive soils are sites on calcareous bedrock lithology although with already depleted topsoil layers. The assumption that the reactivity of different geological parent materials influences or even dominates the sensitivity of soils to acid precipitation, is strictly valid only for calcareous forest soils. At many of the investigated sites, physical and chemical soil properties are just as important as the soils’ parent materials Žmineralogy.. Nevertheless, minerals in the soil play an important role in the mitigation
Exceedance of CL Acidity s S Dep q N Dep y Bc Dep y CL Acidity y net Nuptake q net Bc uptake . Exceedance calculated for forest soils of the key regions scatter between y3.0 and q3.2 keq hay1 yry1 ŽFig. 9.. Currently, around 60% of the sites yield exceedances above zero, implying that the present level of acidic deposition is too high compared to forest ecosystem tolerance according to the BcrAl criterion. Fig. 9 shows the sites where reductions of the acidic deposition is required to protect forest soils. This refers besides some scattered sites primarily to part of the northwestern slope of the Jura Mountains and to the central region of the Ticino Area. Around 54% of the sites in the Jura
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of atmospherically deposited acidity in the long-term perspective. Even for soils which show very high weathering rates, the model seems to predict soil solution compositions which are in accordance to equilibrium considerations. However, in respect to soil acidification very high weathering soils may not be so important, since these soils are not endangered by acidification. Within the context of this model application, it also becomes evident that the sensitivity of soils with respect to acidification, expressed as critical load of acidity, is as well governed by the alkalinity leaching term. Most of the sites in Ticino Area show negative ANC leaching values in the runoff. The ability of soil minerals to supply base cations to the soil solution in regions of high acid deposition and relatively low weathering rates can be sufficient to buffer pH around 4.3, and may be sufficient to prevent the system from excessive Al leaching. However, the export of acidity in terms of hydrogen ions may be problematic for surface waters such as first order streams and alpine lakes ŽZobrist and Drever, 1990. as well as ground water. Calculations concerning the steady-state soil and soil solution chemistry, considering present deposition loads, may be used as a guide in which direction a forest soil evolves in future. Steady-state calculations, however, do not address time aspects and therefore give no information on the timing of the adverse evolution of the soil chemistry as well as of the chemistry of surface and ground waters due to acidity leaching from soils.
Acknowledgements We thank B. Achermann ŽSAEFL, Switzerland., P. Blaser and S. Zimmermann ŽFederal Institute for Forest, Snow and Landscape Research, Switzerland., S. Braun ŽInstitute for Applied Plantbiology, Switzerland., B. Rihm ŽMETEOTEST, Switzerland., H. Sverdrup and P. Warfvinge ŽLund University, Sweden. for their contributions and comments. The financial support of the Swiss NF ŽNo. 21-30207.90. and the SAEFL is gratefully acknowledged.
References BUWAL, 1990. Luftbelastung, 1989. Messresultate des Nationalen Beobachtungsnetzes fur ¨ Luftfremdstoffe ŽNABEL.. Bundesamt fur ¨ Umwelt Wald und Landschaft ŽBUWAL., Bern, Schweiz. CCE, 1991. Mapping critical loads for Europe. CCE Technical Report No. 1, RIVM Report No. 259101001. Coordination Center for Effects, National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. CCE, 1993. Calculation and mapping of critical loads in Europe. CCE Technical Report No. 2, RIVM Report No. 259101003. Coordination Center for Effects, National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. EAFV, 1988. Schweizerisches Landesforstinventar — Ergebnisse der Erstaufnahme 1982–1986. Berichte No. 305. Eidgenos¨ sische Anstalt fur ¨ das forstliche Versuchswesen ŽEAFV., Birmensdorf, Schweiz. Eggenberger, U., 1995. Mineral weathering in soils: experiments, field studies, and modeling. PhD Thesis. Institute of Geology, University of Berne, Switzerland. EJPD, EVD, EDI, 1980. Bodeneignungskarte der Schweiz ŽMap of suitability of soils in Switzerland. 1:200,000. Eidg. Drucksachen- und Materialzentrale, Bern, Schweiz. FOEFL, 1994. Critical loads of acidity for forest soils and alpine lakes — steady-state mass balance method. Environmental Series Air No. 234. Federal Office of Environment, Forests and Landscape ŽFOEFL., Bern, Switzerland. Hodson, M.E., Langan, S.J., Wilson, M.J., 1996. A sensitivity analysis of the PROFILE model in relation to the calculation of soil weathering rates. Appl. Geochem. 11, 835–844. Hydrological Atlas of Switzerland, 1992. Edited by the National Hydrological and Geological Survey on behalf of the Swiss Federal Council, Bern, Switzerland. Kurz, D., Eggenberger, U., Rihm, B., 1995. Evaluating critical loads of acidity for Swiss forest soils — comparison of two calculation methods. Water, Air, Soil Pollut. 85, 2533–2538. Parkhurst, D.L., Thorstenson, D.C., Plummer, L.N., 1980. PHREEQE — A Computer Program for Geochemical Calculations. U.S. Geological Survey, Reston, VA, USA, Žrevised edition 1990.. SAEFL, 1998. Critical loads of acidity for forest soils — regionalized PROFILE model. Environmental Documentation No. 88. Swiss Agency for the Environment, Forests and Landscape ŽSAEFL., Bern, Switzerland. Sverdrup, H., 1990. The Kinetics of Base Cation Release Due to Chemical Weathering. Lund Univ. Press, Lund, Sweden. Sverdrup, H.U., de Vries, W., Henriksen, A., 1990. Mapping critical loads — a guidance to the criteria, calculations, data collection and mapping of critical loads. Miljørapport 1990:14, Nord 1990:98. Nordic Council of Ministers Copenhagen, Denmark. Sverdrup, H., Warfvinge, P., 1993. The effect of soil acidification on the growth of trees, grass and herbs as expressed by the
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257 ŽCaqMgqK.rAl ratio. Reports in Ecology and Environmental Engineering No. 2. Lund University, Lund, Sweden. Warfvinge, P., Sverdrup, H., 1992. Calculating critical loads of acid deposition with PROFILE — a steady-state soil chemistry model. Water, Air, Soil Pollut. 63, 119–143.
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Zobrist, J., Drever, J.J., 1990. Weathering processes in Alpine watersheds sensitive to acidification. In: Johannessen, M. ŽEd.., Acidification Processes in Remote Mountain Lake. Air Pollution Report 20 CEC, Brussels.
A soil acidification study using the PROFILE model on two contrasting regions in Switzerland Urs Eggenberger a,) , Daniel Kurz b a
Institute of Mineralogy and Petrology, UniÕersity of Berne, Baltzerstrasse 1, 3012 Bern, Switzerland b Eggenberger & Kurz Geo-Science, Ralligweg 10, 3012 Bern, Switzerland Received 15 January 1998; accepted 15 December 1999
Abstract The steady-state soil chemistry model PROFILE was used to calculate the chemical status of forest soils under present deposition loads for two areas with dissimilar ecosystem properties. Two regions in Switzerland, with contrasting bedrock geology were selected to be investigated in detail: 88 locations in the Jura Mountains, representative for carbonate bedrock and 91 locations in the Ticino Area dominated by metamorphic crystalline host rocks. Weathering rates calculated for the key regions cover the tremendous range between 0.013 and 25 keq hay1 yry1. In the Ticino Area, the effect of increased abundance of relatively fast weathering silicates Žepidote, hornblende and plagioclase. on the weathering rate is apparently masked by the total effects of the physical conditions applied and by the variation in the deposition load. In the Jura Mountains, generally high weathering rates occur with about 50% of the sites yielding rates above 1 keq hay1 yry1. In many of the sites investigated, however, carbonates have already been dissolved completely in the soil horizons of interest resulting in very low weathering rates. The critical load of actual acidity was calculated according to: CL Acidity s R Weathering y ANC Leaching , where alkalinity leaching is estimated by keeping the base cation to aluminum molar ratio at the critical limit of 1 at steady-state. The minimum critical load calculated was 0.2 keq hay1 yry1 and the maximum was 6.2 keq hay1 yry1. Comparing the cumulative frequency distributions of critical loads of actual acidity for forest soils in the individual areas it can be seen that the differences between the key regions are less substantial than with the weathering rates. Critical loads of acidity for the Ticino Area range from 1 to 3.9 keq hay1 yry1. Sites yielding the lowest critical loads of acidity are observed in the Jura Mountains. Among these apparent sensitive soils are soils with intermediate or high weathering rates, although it has depleted topsoil layers. Within the context of this model application, it becomes apparent that the sensitivity of these soils with respect to acidification is also governed by the alkalinity leaching term and not only by the susceptibility of its minerals to weathering. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Acidification; Critical load; Chemical weathering; Forest soil; Deposition; PROFILE model
1. Introduction
)
Corresponding author. Tel.: q31-631-8781; fax: q31-6314843. E-mail addresses: [email protected] ŽU. Eggenberger., [email protected] ŽD. Kurz..
Anthropogenically induced soil acidification causes increased depletion of base saturation of soils and increased aluminum and hydrogen concentrations in the soil solution. Additionally, it disturbs the
0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 2 5 0 - 8
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nutrient status of soils by inducing strong changes in the availability of essential elements. This has raised concern about adverse effects of changes in the chemical status of soils and soil solution with respect to forest health. It has often been argued that continuous acidic input to forest ecosystems may reduce tree growth and enhance tree mortality in the long term ŽSverdrup and Warfvinge, 1993.. Relating the effects of acidification of a soil system to forest health, the stage at which acidification of soils will harm trees and the forest ecosystems had to be defined. The United Nations Economic Commission for Europe ŽUN ECE. adapted within the work program under the Convention on Long Range Transboundary Air Pollution ŽLRTAP. the ‘critical load’ concept to assess the environmental impact of acidifying compound emission. Critical load in general was defined as ‘‘a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified elements of the environment do not occur according to present knowledge.’’ With respect to soil acidification, the critical load of acidity is ‘‘the maximum deposition of acidifying compounds that will not cause chemical changes leading to long-term harmful effects on ecosystem structure and function.’’ The molar ratio between the sum of base cations calcium ŽCa2q ., magnesium ŽMg 2q . and potassium ŽKq. , lumped into a divalent component Bc, and aluminum in the soil solution is often used as critical chemical parameter, assuming that a BcrAlG 1 will prevent forest ecosystems from damage ŽSverdrup and Warfvinge, 1993.. Relating environmental properties to a single quantitative value entails the use of models. Regional estimates of critical loads of acidity are currently assessed by a static modeling approach. In this approach, which is usually referred to as steady-state mass balance method, a steady-state is assumed for ecosystem relevant chemical processes, such that all sources of acidity are balanced by sources of alkalinity. In practice, the objective is to avoid the selected critical chemical value being exceeded at critical load. At present critical loads are calculated by various implementations of the steady-state mass balance approach for soil and water chemistry, e.g., the simple mass balance model ŽSMB. ŽCCE, 1993.. SMB model variants have been employed by all European countries. Since 1993,
Switzerland has additionally been applying a fourlayer steady-state mass balance model called PROFILE ŽWarfvinge and Sverdrup, 1992; Kurz et al., 1995.. The current study focuses on steady-state chemistry of forest soils under present deposition loads, on critical loads of acidity and exceedance of critical loads of acidity as predicted by the PROFILE model. It discusses differences in the results of two selected areas in respect of dissimilarities in their ecosystem properties. 2. Materials and methods 2.1. The PROFILE model Steady-state implies circumventing past acidification processes and directly evaluating the soil’s condition at the post-acidification state. By definition, net depletion of base saturation is zero at steady-state. Acidic input to soils is balanced by internal sources of alkalinity such as weathering and net nitrogen uptake, or by export of acidity from the system through runoff. PROFILE operates on the basis of a number of internally homogeneous horizons within the soil profile, corresponding to the natural stratification of soils. Processes are represented by mass balance equations for acid neutralizing capacity ŽANC., base cations, nitrate and ammonium and by kinetic equations for chemical weathering and nitrification. Temperature dependence is considered for all individual processes including aqueous equilibrium and dissolution reactions. Phases and reactants in the system interact via the soil solution. Soil solution ANC is controlled by the auto-protolysis of water, the carbonate system, the acid–base reactions of organic acids and the aluminum system. The hydrogen ion is treated as being dependent on the variable aqueous ANC. Aluminum species concentrations are calculated assuming equilibrium with the solid model phase gibbsite. The complete set of equilibrium equations and the corresponding coefficients are described in detail by Warfvinge and Sverdrup Ž1992.. The major components of the model are listed in Table 1. Field weathering rates are calculated using laboratory dissolution rates for individual minerals Žfor
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257 Table 1 Major components of the PROFILE model Žappropriate equations, see Sverdrup, 1992. Mass balances
Aqueous equilibrium systems
Kinetic control Input quantities
ANCaq base cations ŽCa, Mg, K. nitrate ammonium carbonic acid aluminum–water system weak organic acids water autoprotolysis chemical weathering nitrification climatic conditions water balance atmospheric deposition nutrient uptake soil properties
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The only biological process that is modeled in terms of a kinetic rate expression is nitrification. Production of ANC is considered by the uptake of ammonia and nitrate, as well as by nitrification. The values of the kinetic rate coefficient and the Michaelis–Menten saturation constant are discussed in Sverdrup et al. Ž1990.. From the complete mathematical derivation ŽWarfvinge and Sverdrup, 1992., it becomes evident that the net reaction of nitrogen, nitrification and nitrogen uptake, always generates a significant contribution to ANC. Nitrogen uptake and base cation uptake have to be given as input. Base cation uptake consumes ANC, for each keq base cation taken up 1 keq ANC. 2.2. Site description
reaction orders and rate coefficients, see Sverdrup, 1990. and by taking into account soil hydrology, soil physical properties, soil mineralogy and mineral composition. The overall rate is calculated as the sum of the rates of the individual mineral dissolving reactions, whereby simultaneous product inhibition is realized by rate reduction factors. Dissolution reactions with hydrogen ion, water, carbon dioxide and organic acids are considered ŽSverdrup, 1990.. The rate R We athering is treated as proportional to the exposed surface area of the minerals and to the soil moisture saturation. The temperature dependence of the weathering reactions is considered using the Arrhenius function. Exchange reactions between exchangeable base cations and acidity are modeled as perfectly reversible chemical reactions. The rate of such reactions, however, is limited by the transport of base cations between the bulk of the soil solution and active sites on the exchanger surface. The pseudo steady-state rate equation for such a process was derived from Fick’s first law. The change in exchangeable bases X BC , will be proportional to the flux of base cations to the exchanger, which is driven by the concentration difference between the surface of the exchangeable complex and the concentration in the bulk of the soil solution. In this study, the flow of nutrient cations, nitrogen compounds, and ANC between the soil solution and the roots of growing vegetation has been neglected.
In the present study, two regions in Switzerland with contrasting bedrock geology were selected to be investigated in detail. The selected area in the Jura Mountains is dominated by carbonate host rocks. The soils’ protolith of the area in the Southern Alps ŽTicino Area. is crystalline, metamorphic rocks. For the lithologies, soil types, vegetation and atmospheric deposition, data were available on a 1 = 1-km grid resolution. The model was applied to sites on intersections of a 4 = 4-km sub-grid of the 1 = 1-km grid and calculations were restricted to forested sites. In the Jura Mountains, 88 sites, and in the Ticino Area, 91 sites were processed. Detailed information on data collection and detailed input data derivation are described in SAEFL Ž1998.. The location of the two areas used in this study is shown in Fig. 1. 2.2.1. The Jura Mountains The area along the northern boundary of Switzerland covers the eastern and central Jura Mountains. The arched chain consists of more or less regular folds with a moderate relief in the central Folded Jura, where the south easternmost chain is generally the topographical highest one. The north easterly direction, adjoining Tabular Jura consists of Black Forest basement uplifts and their mostly undisturbed Mesozoic cover. Light colored Upper Jurassic limestones form the backbone of most of the Jura folds and build up most of the plateaus except in those parts of the Aargau and Basel Jura, where they have
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Fig. 1. Location of the two regions investigated. Both areas are comparable in area size and the number of sites. North direction towards top of page in all figures.
been truncated by pre-Eocene erosion. There, Middle Jurassic oolitic limestones cover a core region that is bounded to the north and the south by more argillaceous and marly Late Jurassic and Triassic formations. Partly siliciclastic sedimentary rocks of Molasse type are preserved in strike parallel intra. and Quaternary mountain basins Že.g., Delemont ´ deposits predominantly cover the Rhein Graben and associated parts around Basel. Most of the more closely investigated soils have a calcareous protolith Ž62%., 5% are developed on a siliciclastic or argillic sedimentary bedrock, and in 33%, the protolith of the soils constitutes of Quaternary material ŽFig. 2.. The dominant soil types in the Jura Mountains are therefore calcareous lithosol- and rendzina-type soils Ž81%., followed by fluvisol Ž12%. and calcareous regosol and calcareous cambisol Ž6%.. Due to the moderate relief of the Jura Mountains, the vegetation cover with respect to forest type is
rather heterogeneously distributed. Coniferous forests are primarily on topographic highs in particular on the backbones of the Jura folds and on the horsts of the Tabular Jura. Deciduous forests generally occur in topographical lower sites and are therefore more frequent in the eastern Jura Mountains. 2.2.2. The Ticino Area The key area in the Jura Mountains was compared with an area in the southern part of Switzerland mainly for two reasons. The area, largely identical with the Canton Ticino, lies entirely to the south of the major alpine watershed and experiences substantial deposition of acidifying compounds from industrialized northern Italy. The central and northern part of the area consists of crystalline basement nappes of pre-Carboniferous age, with varied ortho- and paragneisses predominating. Mesozoic metasediments occur as thin and often discontinuous bands between the gneiss units and accumulate in the northernmost
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257
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Fig. 2. Bedrock lithology. Criteria for the simplified classification according to the Geotechnical Map of Switzerland are listed in SAEFL Ž1998..
part of the Lepontine Alps. To the south, separated from the Penninic nappes by the Insubric line, follow south Alpine basement rocks including high-grade metasediments and metaintrusives. In the Lago di Lugano area, basement rocks were intruded by Late Paleozoic volcanic rocks and to the east the basement is overlain by Liassic siliceous limestones. The lithology ŽFig. 2. derived from the geotechnical map of Switzerland is dominated by crystalline rocks such as biotite–muscovite schists Ž67.5%., sericite–chlorite schists Ž4%. and intrusives Ž1.5%.. Calcareous lithology consists of the Liassic siliceous limestones in the southernmost part of the area and . in the Penof calcareous schists ŽBundnerschiefer ¨ ninic nappes in the northern part of the investigated area. Criteria for the classification of the sites are listed in SAEFL Ž1998.. Bedrock lithology implies the presence of soils highly sensitive to acidification. In the central and northern parts of the area, poorly differentiated well-drained soils Ž63% lithosol, ranker. characterize the higher altitudes. Towards lower elevation, more evolved soils of podzolic tendency Ž24%. are present. Calcareous soil types only make up 6% of the total area, corresponding to the locations dominated by calcareous lithology. The pronounced relief in the Ticino Area permits the distinction of altitude dependent zones of vegeta-
tion. Deciduous forest vegetation covers the lower altitudes with Castanea satiÕa becoming frequent towards the south of the Insubric line. As the altitude rises towards the north, coniferous forests become more frequent in comparison to deciduous forests. 2.3. Input On the regional scale input data were acquired according to the UN ECE Task Force on Mapping Guidelines ŽSverdrup et al., 1990.. The data acquisition procedure is mainly based on regional survey information or single spot measurements. Transformation or extrapolation functions are then applied to the required regional grided or site-specific data ŽFOEFL, 1994; SAEFL, 1998.. For some input parameters Že.g., soil mineralogy., computerized submodels were used for their derivation ŽEggenberger, 1995.. An overview of the most relevant parameters is given in Table 2. The following discussion focuses on input parameters that are important in interpreting the model result. 2.3.1. Hydrology and atmospheric depositions Precipitation was derived from the Hydrological Atlas of Switzerland Ž1992.. Infiltration and runoff was calculated from precipitation by correcting for
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Table 2 Overview of the most important input parameters needed and the data acquisition strategy Parameter
Commentrsource
Precipitation Wet deposition Dry deposition Bc deposition Thickness of layers
Hydrological Atlas of Switzerland Ž1992.. Simplified corrections for interception and surface runoff. From altitude dependent rainwater concentration ŽFOEFL, 1994. and precipitation rates. Concentration of the pollutant in air multiplied with component specific deposition velocities ŽFOEFL, 1994.. From open-field bulk deposition using filtering factors for dry deposition as proposed by CCE Ž1991.. Layer thicknesses for 9 elevation classes was assessed using measurements from 172 soil profiles of the 1993–1995 NFI ŽZimmerman, EAFV, personal communication, 1995., or measured values. From particle size distribution. Total elemental analysis ŽSAEFL, 1998., normative mineral compositions and back-calculations methods ŽEggenberger, 1995., or measured ŽXRD, internal standard method. values Žcalcite, dolomite, K-feldspar, plagioclase Žan-content., albite, hornblende, pyroxene, epidote, garnet, biotite, muscovite, chlorite, vermiculite, kaolinite, apatite.. From water retention capacity ŽEJPD et al., 1980., water mass balance, and soil depth Žsee layer thickness.. Interpolated from a limited set of measurements ŽFOEFL, 1994.. Long-term annual means were used.
Surface area Mineralogy
Soil water content Soil temperature
surface runoff and evapotranspiration, respectively ŽFOEFL, 1994.. The two regions under investigation differ significantly in the amount of water that infiltrates into the soil, as shown in Fig. 3. The Ticino Area receives among the highest annual precipitation
in Switzerland ranging between 1000 and 2600 mm yry1 , whereas the Jura Mountains show moderate annual precipitation values, ranging from 650 to 1500 mm yry1 . The overall higher values in the Ticino Area are due to the geographic situation. The
Fig. 3. Infiltration in mm yry1 . Source: Hydrological Atlas of Switzerland Ž1992.. Cumulative distribution of infiltrated precipitation into the soil.
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Ticino incorporates the first significant topographic elevations towards the Alps that leads to the high precipitation values, strongly dependent on the precipitation–elevation relationship. Deposition of acidifying compounds was extrapolated from 1986–1989 field measurements Že.g., BUWAL, 1990. and are described in detail in SAEFL Ž1998.. Individual deposition models for wet, dry and gas deposition were then used to calculate the grided data set ŽFig. 4.. High deposition rates in the Ticino Area are a consequence of substantial emissions of local and north Italian industries. Acid deposition rates in lower regions, particularly in the Ticino Area, are dominated by dry deposition due to an inverse dependence of dry deposition rates on altitude. Generally, the effect of higher filtering interception and higher deposition velocities in coniferous forests at higher altitudes is smaller than the decrease in the pollutant concentration with increasing altitude. Wet acid deposition rates, on the other hand, increase with increasing precipitation rates towards higher elevation, this increase however being
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smaller than the decrease in the dry deposition rates. In general, the acid deposition rate pattern is therefore inverse to the precipitation and infiltration patterns ŽFig. 4., respectively. This also leads to a larger range of acid deposition inputs Ž1.8–5.4 keq hay1 yry1 . in the Ticino Area. In the Jura Mountains on the contrary, 99% of the rates lie in the range between 2.5 and 3.2 keq hay1 yry1 . Analogous to acid deposition, base cation deposition is also relatively high in the Ticino Area ŽFig. 5. and moderate in the Jura Mountains. Base cation deposition rates in the Ticino Area are in the range of 0.32–1.05 keq hay1 yry1 , compared to 0.25–0.85 keq hay1 yry1 in the Jura Mountains. Base cation deposition is often associated with episodic events Že.g., Sahara dust. and measurements show therefore large spatial and temporal spreads. The higher deposition rates in the eastern part of the Ticino Area is rather an artificial effect of the extrapolation method. Bulk deposition loads of base cations were supplied by the Coordination Center for Effects ŽCCE, 1991, 1993. with a 18 longitude= 1r28 latitude resolution.
Fig. 4. Acid deposition in keq hay1 yry1 . Cumulative distribution of acid deposition rates.
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Fig. 5. Base cation deposition in keq hay1 yry1. Source: bulk deposition rates according to CCE Ž1991.. Dashed line, see text. Cumulative distribution of base cation deposition rates.
The CCE suggests for open-field conditions in the deposition grid of the eastern part of the Ticino Area a value of 0.427 keq hay1 yry1 , whereas in the adjoining western grid, a value of 0.370 keq hay1 yry1 is estimated. The divide, marked in Fig. 5 with a dashed line, is located just west of the sites showing base cation deposition values higher than 1 keq hay1 yry1 . Higher deposition rates at more elevated sites are due to the higher filtering factors for dry deposition to coniferous forests Žmultiplication factor: 2.5.. The filtering factors decrease with decreasing occurrence of coniferous forests toward lower altitudes Žfiltering factor for deciduous forests: 1.0.. The general base cation deposition pattern therefore reflects the distribution of the tree species. The National Forest Inventory ŽEAFV, 1988. provided the data for tree species distribution as well as basic information to assess Bc and nitrogen uptake.
of reference soil profiles by means of a soil classification Žaccording to the Map of suitability of soils ŽBEK. in Switzerland; EJPD et al., 1988.. Moisture content was extrapolated from field capacity evaluated from the BEK classification and weighted according to the precipitation. Since soil mineralogy data was scarce, as only a few soil profiles were investigated in detail Ž3 from the Jura Mountains, 11 from the Ticino Area., soil mineral compositions was derived from chemical soil analyses using a normalization procedure ŽSAEFL, 1998..
2.3.2. Soil properties Soil properties including soil bulk density and mineral surface area were extrapolated from a series
Weathering rates calculated for the key regions cover a remarkable range between 0.013 and 25 keq hay1 yry1 ŽFig. 6.. Theoretical considerations imply
3. Results and discussion 3.1. Weathering rates
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257
251
Fig. 6. Calculated weathering rates in keq hay1 yry1 at present state. Cumulative distribution of calculated weathering rates. The X-axis is in log units.
that the three silicate groups’ epidote, hornblende and plagioclase feldspars are instrumental in the weathering rates observed for soils on silicate bedrock. Previous studies on weathering in soils confirm this implication in so far, as higher plagioclase and mafic mineral contents tend to give higher modeled weathering rates. In the Swiss key regions, the effect of increased abundance of these minerals is not directly correlated with weathering rates. Qualitatively, it appears that the Žregional. variation in the soil properties primarily is responsible for the scatter in the weathering rates. The effects of varying soil physico-chemical characteristics on weathering rates has been quantified, e.g., by Hodson et al. Ž1996.. In the region with calcareous bedrock, on the other hand, the presencerabsence of carbonates in the soil column primarily governs the magnitude of the weathering rate. Weathering rates calculated for 91 Ticino forest soils show a characteristic spatial variation ŽFig. 6.. Generally, more elevated sites yield weathering rates
predominantly higher than 0.5 keq hay1 yry1 , whereas low elevation sites in the Sotto-Ceneri area, in the Magadino plane, and in the Leventina valley are dominated by weathering rates lower than 0.5 keq hay1 yry1 . At the two south-westernmost sites, where about 1–5 wt.% of carbonates are present in the soil, only one site contributes to high weathering rates. The direct dependence of the weathering rate upon the mineralogical composition is exemplified solely at one location, where a carbonate content of 4–35 wt.% leads to the highest weathering rate of 2.2 keq hay1 yry1 in Ticino. Not all soils upon carbonate-bearing lithologies yield high weathering rates, as shown in the Jura Mountains. The region that is dominated by carbonate-bearing bedrock exhibits a rather heterogeneous spatial variation pattern of weathering. Soils producing very high weathering rates can be located next to soils with very low ones within the spatial resolution of the 4 = 4-km grid. At many of the sites examined, carbonates have already been dissolved in the soil
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layers considered. On the level of the cumulative frequency distribution ŽFig. 6., however, it becomes evident that generally higher weathering rates occur in the Jura Mountains. In this area 50% of the sites yield rates above 1 keq hay1 yry1 , whereas in the Ticino Area 95% of the sites show weathering rates below 1 keq hay1 yry1 . The model calculates very high weathering rates in calcareous soils. The question arose whether the soil solution chemistry fulfills equilibrium considerations usually adopted in geochemical modeling. Solution chemistry calculated with PROFILE was checked for possible oversaturation in respect to carbonates and possible secondary phases for three locations representative for very high, high, and intermediate weathering rates using PHREEQE ŽParkhurst et al., 1980.. Even at the very high weathering rate location Ž20 keq hay1 yry1 ., calcite saturation in the soil solution calculated with PROFILE was not yet reached according to PHREEQE results. The calculated Ca concentrations are therefore in accordance with independently determined calcite equilibrium considerations.
3.2. pH Solution pH calculated for the soils considered ranges from 4.0 up to 7.6. There is considerable spatial variation of pH, the pattern being comparable to the variation pattern of the weathering rate. The spread in pH is also larger in the Jura Mountains than in the Ticino Area. In the Ticino Area, calculated pH varies between 4.2 and 5.3 with only two sites having soil solution pH above 5.0. CaCl 2-pH of topsoils Žthe first 20 cm. has also been measured within the scope of the National Forest Inventory ŽEAFV, 1988.. The spatial variation pattern of both calculated and measured pH values is comparable. However, the absolute values may differ strongly. In the Ticino Area, measured pH is predominantly lower than calculated, whereas the opposite is common in the Jura Mountains. It is known that CaCl 2 extraction of acidified soils, as found in the Ticino Area, results lower pH-values in comparison to pure water extracts. The mobilization of the protons from the organic exchange sites results in a pH shift from y1r2 to y1 units. Differences in
Fig. 7. Calculated minimum molar BcrAl ratio. Cumulative distribution of minimum molar BcrAl. The X-axis is in log units.
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measured and calculated pH in the Jura Mountains are difficult to interpret, because there are sites where no carbonates were observed in the investigated soil compartments, but the measured pH shows values well above 5. Possible explanations are: Ž1. small amounts of carbonates eventually present in these soils were either not detected ŽXRD detection limit 1 wt.%. or not modeled Žuncertainty in the applied back-calculation of geochemical analyses., but control the water chemistry, Ž2. the comparability of soil extract and soil solution chemistry Žmodeled or measured. is limited in general, and Ž3. solution pH calculated for upper soil layers may be uncertain, due to simplifications adopted in the model Že.g., nutrient cycling not considered.. 3.3. Bc r Al molar ratio The solution BcrAl molar ratio provides a chemical assessment of the environmental impact of acidifying compound deposition. Above the threshold value of 1, trees should be protected from adverse
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effects of acidification. With a multi-layer model the minimum of the BcrAl molar ratios calculated for the individual soil layers within the rooting zone is often used as the relevant value for the soil profile. In Fig. 7, the distribution of these BcrAl ratio minima is shown. With current deposition loads, 43% of the sites in the Ticino Area and around 65% of the sites in the Jura Mountains develop minimum BcrAl molar ratios below 1. The calculated values indicate adverse influence of acid deposition on the soils’ chemical status, which increases the risk of forest damage at these sites. The spatial trend of the BcrAl molar ratio in the Ticino Area generally follows the trend of the weathering rate shown in Fig. 6. Sample sites at lower elevations in the core region of the Ticino Area show the lowest ratios. It was not expected that soils of the Jura Mountains would show even lower BcrAl molar ratios than the soils of the Ticino Area, where high acid deposition rates in combination with low weathering silicate rocks forecast unfavorable soil conditions. However, the weathering residuum of calcareous
Fig. 8. Critical load of acidity in keq hay1 yry1 . Cumulative distribution of critical load of acidity. The X-axis is in log units.
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soils lacks fast weathering minerals such as plagioclase feldspars or dark minerals and consequently prevailing low solution pH entails higher amounts of dissolved Al 3q, calculated on the basis of gibbsite equilibrium. Bc concentrations on the other hand remain low in these depleted soils resulting in low BcrAl ratios, if Bc sources other than weathering, e.g., nutrient cycling are neglected. 3.4. Critical load of acidity For the critical load assessment, the deposition rates are adjusted iteratively until the solution BcrAl molar ratio is equal to or larger than the threshold value of 1 throughout the soil profile. The criterion was applied to the top 30 to 50 cm, respectively, of the soils, assuming these to be the tree rooting depths of coniferous and deciduous trees. The critical load of actual acidity, representing the inherent capability of the system to neutralize acid deposition, was calculated according to CL Acidity s R Weathering y
ANC Leaching . In acid forest soils, the process of hydrogen and aluminum leaching govern the acidity loss that can be permitted. According to the criterion, these fluxes are calculated using base cation deposition, weathering rate and net base cation uptake to determine the amount of Bc available in the BcrAl molar ratio. The minimum critical load calculated was 0.2 keq hay1 yry1 and the maximum was 6.2 keq hay1 yry1 . These values quantify the total acidity input the system can tolerate regardless of where the acidity comes from. Comparing the cumulative frequency distributions of critical loads of actual acidity for forest soils in the individual areas it can be seen ŽFig. 8. that the differences between the key regions are less substantial than with the weathering rates. The distribution of the lower 40–50% of the critical loads estimated for sites on crystalline bedrock coincides with the distribution of the critical loads estimated for sites on potentially carbonate-bearing sedimentary substratum.
Fig. 9. Exceedance of critical load of acidity. Positive values indicate that acid deposition is higher than the soils buffering capacity. Cumulative distribution of the exceedance of the critical load of acidity. The X-axis is in log units.
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257
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Critical loads of acidity according to steady-state calculations for the Ticino Area range from 1 to 3.9 keq hay1 yry1 . The spatial variation of the site values follows quite closely the pattern of the weathering rates, indicating that the central corridor and in particular the area between the southern slope of the Ticino valley and the Lago di Lugano comprise the sensitive soils. Despite generally lower weathering rates in the Ticino Area than in the Jura Mountains Žsee cumulative distribution function in Fig. 6., critical loads do not fall below 1 keq hay1 yry1 . Within the context of this model application, it becomes apparent that the sensitivity of soils with respect to acidification is also governed by the alkalinity leaching term and not only by the susceptibility of its minerals to weathering. Spots yielding the lowest critical loads of acidity are observed in the Jura Mountains. Among these apparent sensitive soils are soils with intermediate or high weathering rates, although with depleted topsoil layers.
Mountains and 63% of the sites in the Ticino Area received acid deposition in excess of the critical load. In the Ticino Area, the exceedance pattern mainly is the result of outstanding present deposition rates ŽS Dep q N Dep y non-marine Bc Dep . rather than of the sensitivity of soils regarding acidification. In the Jura Mountains, where the range of acid deposition rates is restricted and of critical loads extended, the exceedance pattern more explicitly is the result of the sensitivity of soils in respect of acidification. Areas in which the critical load of acidity is exceeded will experience BcrAl molar ratios in the soil solution below the limiting value of 1, if acid deposition continues at the present level ŽFig. 7.. Response data for a variety of tree species indicate that this chronic exceedance of the threshold load will adversely effect the forests in these areas ŽSverdrup and Warfvinge, 1993..
3.5. Exceedance of CL-acidity
4. Conclusions
Critical load of actual acidity indicates the total amount of acidity input of either anthropogenic or natural origin that an ecosystem may tolerate. The exceedance of the critical load is calculated by subtracting all acidity inputs to the soil from the critical load:
The application of the PROFILE model on a regional scale generates a large variability of achieved results as a consequence of the sensitivity of the model to most of the input parameters. An often used assumption in the assessment of soil acidification that single sites can be used to represent lithologically uniform areas, could not be confirmed. In the Ticino Area, as an example, sensitive soils tend to concentrate on low elevated areas in a central corridor. The surrounding areas appear not as sensitive, although the soils’ protolith consists of a similar gneiss suite. On the other side, sites yielding low critical loads of acidity are observed in the Jura Mountains. Among these apparent sensitive soils are sites on calcareous bedrock lithology although with already depleted topsoil layers. The assumption that the reactivity of different geological parent materials influences or even dominates the sensitivity of soils to acid precipitation, is strictly valid only for calcareous forest soils. At many of the investigated sites, physical and chemical soil properties are just as important as the soils’ parent materials Žmineralogy.. Nevertheless, minerals in the soil play an important role in the mitigation
Exceedance of CL Acidity s S Dep q N Dep y Bc Dep y CL Acidity y net Nuptake q net Bc uptake . Exceedance calculated for forest soils of the key regions scatter between y3.0 and q3.2 keq hay1 yry1 ŽFig. 9.. Currently, around 60% of the sites yield exceedances above zero, implying that the present level of acidic deposition is too high compared to forest ecosystem tolerance according to the BcrAl criterion. Fig. 9 shows the sites where reductions of the acidic deposition is required to protect forest soils. This refers besides some scattered sites primarily to part of the northwestern slope of the Jura Mountains and to the central region of the Ticino Area. Around 54% of the sites in the Jura
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of atmospherically deposited acidity in the long-term perspective. Even for soils which show very high weathering rates, the model seems to predict soil solution compositions which are in accordance to equilibrium considerations. However, in respect to soil acidification very high weathering soils may not be so important, since these soils are not endangered by acidification. Within the context of this model application, it also becomes evident that the sensitivity of soils with respect to acidification, expressed as critical load of acidity, is as well governed by the alkalinity leaching term. Most of the sites in Ticino Area show negative ANC leaching values in the runoff. The ability of soil minerals to supply base cations to the soil solution in regions of high acid deposition and relatively low weathering rates can be sufficient to buffer pH around 4.3, and may be sufficient to prevent the system from excessive Al leaching. However, the export of acidity in terms of hydrogen ions may be problematic for surface waters such as first order streams and alpine lakes ŽZobrist and Drever, 1990. as well as ground water. Calculations concerning the steady-state soil and soil solution chemistry, considering present deposition loads, may be used as a guide in which direction a forest soil evolves in future. Steady-state calculations, however, do not address time aspects and therefore give no information on the timing of the adverse evolution of the soil chemistry as well as of the chemistry of surface and ground waters due to acidity leaching from soils.
Acknowledgements We thank B. Achermann ŽSAEFL, Switzerland., P. Blaser and S. Zimmermann ŽFederal Institute for Forest, Snow and Landscape Research, Switzerland., S. Braun ŽInstitute for Applied Plantbiology, Switzerland., B. Rihm ŽMETEOTEST, Switzerland., H. Sverdrup and P. Warfvinge ŽLund University, Sweden. for their contributions and comments. The financial support of the Swiss NF ŽNo. 21-30207.90. and the SAEFL is gratefully acknowledged.
References BUWAL, 1990. Luftbelastung, 1989. Messresultate des Nationalen Beobachtungsnetzes fur ¨ Luftfremdstoffe ŽNABEL.. Bundesamt fur ¨ Umwelt Wald und Landschaft ŽBUWAL., Bern, Schweiz. CCE, 1991. Mapping critical loads for Europe. CCE Technical Report No. 1, RIVM Report No. 259101001. Coordination Center for Effects, National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. CCE, 1993. Calculation and mapping of critical loads in Europe. CCE Technical Report No. 2, RIVM Report No. 259101003. Coordination Center for Effects, National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. EAFV, 1988. Schweizerisches Landesforstinventar — Ergebnisse der Erstaufnahme 1982–1986. Berichte No. 305. Eidgenos¨ sische Anstalt fur ¨ das forstliche Versuchswesen ŽEAFV., Birmensdorf, Schweiz. Eggenberger, U., 1995. Mineral weathering in soils: experiments, field studies, and modeling. PhD Thesis. Institute of Geology, University of Berne, Switzerland. EJPD, EVD, EDI, 1980. Bodeneignungskarte der Schweiz ŽMap of suitability of soils in Switzerland. 1:200,000. Eidg. Drucksachen- und Materialzentrale, Bern, Schweiz. FOEFL, 1994. Critical loads of acidity for forest soils and alpine lakes — steady-state mass balance method. Environmental Series Air No. 234. Federal Office of Environment, Forests and Landscape ŽFOEFL., Bern, Switzerland. Hodson, M.E., Langan, S.J., Wilson, M.J., 1996. A sensitivity analysis of the PROFILE model in relation to the calculation of soil weathering rates. Appl. Geochem. 11, 835–844. Hydrological Atlas of Switzerland, 1992. Edited by the National Hydrological and Geological Survey on behalf of the Swiss Federal Council, Bern, Switzerland. Kurz, D., Eggenberger, U., Rihm, B., 1995. Evaluating critical loads of acidity for Swiss forest soils — comparison of two calculation methods. Water, Air, Soil Pollut. 85, 2533–2538. Parkhurst, D.L., Thorstenson, D.C., Plummer, L.N., 1980. PHREEQE — A Computer Program for Geochemical Calculations. U.S. Geological Survey, Reston, VA, USA, Žrevised edition 1990.. SAEFL, 1998. Critical loads of acidity for forest soils — regionalized PROFILE model. Environmental Documentation No. 88. Swiss Agency for the Environment, Forests and Landscape ŽSAEFL., Bern, Switzerland. Sverdrup, H., 1990. The Kinetics of Base Cation Release Due to Chemical Weathering. Lund Univ. Press, Lund, Sweden. Sverdrup, H.U., de Vries, W., Henriksen, A., 1990. Mapping critical loads — a guidance to the criteria, calculations, data collection and mapping of critical loads. Miljørapport 1990:14, Nord 1990:98. Nordic Council of Ministers Copenhagen, Denmark. Sverdrup, H., Warfvinge, P., 1993. The effect of soil acidification on the growth of trees, grass and herbs as expressed by the
U. Eggenberger, D. Kurzr Chemical Geology 170 (2000) 243–257 ŽCaqMgqK.rAl ratio. Reports in Ecology and Environmental Engineering No. 2. Lund University, Lund, Sweden. Warfvinge, P., Sverdrup, H., 1992. Calculating critical loads of acid deposition with PROFILE — a steady-state soil chemistry model. Water, Air, Soil Pollut. 63, 119–143.
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Zobrist, J., Drever, J.J., 1990. Weathering processes in Alpine watersheds sensitive to acidification. In: Johannessen, M. ŽEd.., Acidification Processes in Remote Mountain Lake. Air Pollution Report 20 CEC, Brussels.