Using Multiple Chemical Criteria for Critical Loads of Acidity in Maritime Regions

Critical loads have become a well-establishedpart of the work programme of the UnitedNations Economic Commission for Europe (UNECE) Conventionon Long-Range Transboundary Air Pollution (CLRTAP). Thelinking of ecosystem response to deposition level is thecentral principal of the critical loads approach. Foreach ecosystem, a biological indicator is chosen, asuitable chemical criterion selected and a criticalchemical limit assigned. The Bc:Al ratio is the mostwidely used chemical criterion for setting criticalloads. However, critical loads based on this criterionare very sensitive to marine deposition. In regions whichreceive high depositions of marine-derived base cations,such as the Republic of Ireland (ROI) and the UnitedKingdom (UK), critical loads based on the Bc:Al criterionwill inevitably be high. Therefore, it is proposed thatcritical loads are estimated using multiple chemicalcriteria with appropriate critical limits to protect thechosen biological indicators. The range of publishedchemical criteria have been applied to the ROI and theUK. The chemical criterion corresponding to the mostsensitive critical load have been mapped and thecontribution of each to the final maps investigated. Thesimulations indicate that the most sensitive criteria forsetting critical loads are based on specifying critical Hor Al concentrations. However, the choice of critical limits andmodel parameters will ultimately effect the criticalloads. Therefore, it is important that appropriate criticallimits are chosen to protect the biological indicator andreceptor ecosystem from long-term damage.     

Soil Acidity Parameters and Defoliation Degree in Six Norway Spruce Stands in Finland

Soil acidity parameters (pH, basesaturation, exchangeable Al) in the organic and mineralsoil layers and in soil water (pH, dissolved organiccarbon, total Al, Al³⁺ and molar Ca/Al ratios) insix Norway spruce stands in different parts of Finlandwere compared. An attempt was also made to relate thedegree of defoliation in the tree stand to N and Sdeposition and soil parameters. No relationship was foundbetween soil acidity parameters and defoliation in thesix stands. Defoliation was positively correlated withstand age and the C/N ratio of the organic layer, andnegatively with the cation exchange capacity. The plotlocated on a so-called sulphate soil on the west coast ofFinland had very low soil pH values, and extremely highAl and SO₄ ^2- concentrations and molar Ca/Alratios of well below 1.0 in soil water. Despite the highnatural acidity in the soil on this plot, defoliation inthe spruce stand was the lowest (mean 8.6%) of all sixplots. The results of this study indicate that soilacidity is not a major factor affecting stand conditionin these spruce stands, and that the variation in soilacidity parameters is closely related to climatic factorsand natural soil formation processes.     

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.     

Critical Loads of Acidity for Forest Soils: Tentative Modifications

We reviewed the current methods for calculatingcritical loads of acidity for forest soils. The consequencesof four sets of assumptions concerning the soil modelstructure, parameter values and the critical loads criterionwere explored by comparing the values of the averageaccumulated exceedance (AAE) calculated for Finland withdeposition values for the year 1995. The AAE index is given inthe unit of deposition and is a measure of how far a region isfrom being protected in terms of fulfilling a certaincriterion, taking into account the size of the ecosystem areas.Using a critical limit for the molar ratio of theconcentrations of base cations to aluminium in soil solutiongave the lowest average accumulated exceedance. Assumingorgano-aluminium complexes and leaching of organic anions gaveAAE = 4 eq ha^-1 a^-1, which was close to the valueobtained with the standard approach used in Finland, assuminggibbsite equilibrium and no leaching of organic anions,yielding AAE = 5 eq ha^-1 a^-1. With a critical basesaturation limit, instead of the concentrations criterion, theAAE index was 17 eq ha^-1 a^-1. The highest averageaccumulated exceedance (AAE = 25 eq ha^-1 a^-1),corresponding to the lowest critical load, was obtained whenthe effects-based criterion (critical concentration or criticalbase saturation) was substituted with one restricting thedeterioration of the neutralizing capacity of the soil, ANC _le(crit) = 0. These tests illustrate the variabilityof the critical load values for acidity that can be introducedby changing the criterion or by varying the calculation method,without, however, representing the extreme values of criticalloads that could be derived.     

An Analysis of the Structure of the Simple Mass Balance Equation: Implications for Testing National Critical Loads Maps

The critical loads concept is used by the UN-ECEConvention on Long Range Transboundary Air Pollution(CLRTAP) for setting pollution reduction targets.Increasing numbers of countries are adopting the SimpleMass Balance equation (SMB) to calculate critical loads ofacidifying S and N for forest soils. The equation is madeup of a series of mass balances each of which is used tocalculate a leaching flux. The assumptions in the SMBequation were investigated by testing its ability topredict current sulphur load and by comparing each of thecalculated leaching fluxes to measured rates. It was notpossible to predict current sulphur load at our sites usingthe SMB equation. The leaching tests demonstrated that,primarily due to its steady state assumptions, the SMBequation generates critical loads that are theoretical longterm estimates of risk, and are untestable. Thesimplifying assumptions sometimes lead to illogicalresults. Some of these can be improved by adding a final,simple but dynamic, calculation step to determine theexpected time until effects are observed. Theacceptability of combining annual average data, which bestapproximates steady state, with a biological indicator isquestionable. It is not possible to test critical loadscalculated using the SMB equation when applied with all ofits assumptions but it is possible to test its fundamentalapproach using non steady state data.     

Effects of Acidification and its Mitigation with Lime and Wood Ash on Forest Soil Processes: A Review

Anthropogenic acid deposition causes forest soil acidification and perturbation of the soil forming processes. The impact of soil acidification on tree growth is discussed in view of the role of mycorrhizal fungi in weathering and nutrient uptake. A review has been carried out of experiments involving treatments of forest soil by lime and wood ash, where soil properties and soil solution composition have been investigated. Results from these experiments in Europe and North America are summarized. In general, the content of C in the mor layer decreased as a result of treatment due to higher microbial activity and soil respiration as well as increased leakage of DOC. In addition, the content of N in the mor layer, in general, decreased after treatment and there are occasional peaks of high NO₃concentrations in soil solution. In nearly all reviewed investigations the pH of the deep mineral soil solution decreased and Al, SO₄and NO₃concentrations increased after treatment. These effects are probably due to the high ionic strength and increased microbial activity as a consequence of the treatments. In the soil, pH, CEC and base saturation increased in the upper horizons, but decreases in the upper mineral soil are also reported. In general, there was no increase in tree growth as a result of these treatments. The positive effects of the treatments on soil processes and tree growth are therefore questionable. In view of these conclusions, an investigation was carried out on the soil and soil solution chemistry and the role of mycorrhizal fungi in a spruce stand treated with two doses of lime and another treated with lime/ash in southern Sweden. The results of this investigation is reported in this volume.     

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.     

Effects of Acidification and its Mitigation with Lime and Wood Ash on Forest Soil Processes in Southern Sweden. A Joint Multidisciplinary Study

A joint multidisciplinary investigation was undertaken to studythe effects of lime and wood ash applications on two Norway spruce forest Spodosolic soils. The two sites, typical for southern Sweden, were treated in 1994 with either 3.25 t ha^-1 dolomite or 4.28 t ha^-1 wood ash (Horröd site) or in 1984 with either 3.45 or 8.75 t ha^-1 dolomite (Hasslöv site). Both sites show signs of acidification by atmospheric anthropogenic deposition and possessed low soil pH(4.3) and high concentrations of inorganic Al (35 M) in theupper illuvial soil solution. The prevailing soil conditions indicated perturbed soil processes. Following treatment with lime or wood ash, the soil conditions were dramatically altered. Cation exchange capacity (CEC) and base saturation (BS) was considerable increased after addition. Four years after application most of the added Ca and Mg was still present in the mor layer. Fifteen years after application,Mg in particular, became integrated deeper in the soil profile with a greater proportion lost by leaching incomparison to Ca. The concentrations of these ions were greatestin the mor layer soil solutions and Mg had higher mobility givinghigher concentrations also deeper in the profile. Four years after treatment, the application of wood ash and limeresulted in lower pH values and higher inorganic Al in mineral subsoil solutions compared to the untreated soil. We hypothesize that this was probably due to an increased flow of hydrogen ionsfrom the upper soil as a result of displacement by Ca and Mg ionsin the enlarged exchangeable pool. In contrast, fifteen years after lime and wood ash application, the mineral subsoil horizonspossessed a higher pH and lower soil solution Al content than theuntreated plots.Liming promoted soil microbial activity increasing soil respiration 10 to 36%. This is in the same range as net carbon exchange for forests in northern Sweden and could potentially have a climatological impact. The turnover of low molecularweight organic acids (LMWOA) by the soil microbial biomass werecalculated to contribute 6 to 20% to this CO₂ evolution.At Horröd, citrate and fumarate were the predominant LMWOAs with lowest concentrations found in the treated areas. In contrast, at the Hasslöv site, propionate and malonate were the most abundant LMWOAs. Higher microbial activity in the upper soil horizons was also theprobable cause of the considerably higher DOC concentrations observed in the soil solution of ash and lime treated areas. Thelime-induced increase in DOC levels at Hasslöv could be attributed to increases in the 3–10 kDa hydrophobic size fraction. Liming also promoted nitrification with high liming doses leading to extreme concentrations of NO₃ ^- (1 mM) in soil solution.At Hasslöv the community of mycorrhizal fungi was dramatically changed by the addition of lime, with only four of 24 species recorded being common to both control and treated areas.Many of the observed effects of lime and ash treatment can be viewed as negative in terms of forest sustainability. After fouryears of treatment, there was a decrease in the pH of the soil solution and higher concentrations of inorganic Al and DOC. Increased organic matter turnover, nitrification and NO₃ ^-leakage were found at Hasslöv. Considering that the weathering rate and the mineral nutrient uptake by trees is mostprobably governed by mycorrhizal hyphae etchingmineral grains in the soil, it is important to maintain this ability of the mycorrhizal fungi. The lime and ash-induced changed mycorrhizal community structure may significantly affect this capability. In light of this investigation and others, as reviewed by Lundström et al. (2003), the implications ofliming on forest health are multifaceted with complex relationships occurring over both space and time.     

Is The Ca + K + Mg/Al Ratio in the Soil Solution a Predictive Tool for Estimating Forest Damage?

The ratio between (Ca +K +Mg) and Al in nutrientsolution has been suggested as a predictive tool forestimating tree growth disturbance. However, the ratiois unspecific in the sense that it is based on severalelements which are all essential for plant growth;each of these may be growth-limiting. Furthermore,aluminium retards growth at higher concentrations. Itis therefore difficult to give causal and objectivebiological explanations for possible growthdisturbances. The importance of the proportion ofbase-cations to N, at a fixed base-cation/Al ratio, isevaluated with regard to growth of Picea abies.The uptake of elements was found to be selective;nutrients were taken up while most Al remained insolution. Biomass partitioning to the roots increasedafter aluminium addition with low proportions of basecations to nitrogen. We conclude that the low growthrates depend on nutrient limitation in thesetreatments. Low growth rates in the high proportionexperiments may be explained by high internal Alconcentrations. The results strongly suggest thatgrowth rate is not correlated with the ratio in therooting medium and question the validity of usingratios as predictive tools for estimating forestdamage. We suggest that growth limitation of Picea abies in the field may depend on lowproportions of base cations to nitrate. It istherefore important to know the nutritional status ofthe plant material in relation to the growth potentialand environmental limitation to be able to predict andestimate forest damage.     

Element Removal in Harvested Tree Biomass: Scenarios for Critical Loads in Wallonia,South Belgium

The critical load concept is nowwidely used as a tool for developing emissioncontrol policies in Europe. As a signatorycountry of the Convention of Long-RangeTransboundary Air Pollution, critical loads foracidity, nutrient nitrogen, nitrogen and sulphurhave been calculated for the Flemish and Walloonregions in Belgium. This paper describes themethodology used for estimating critical loadsfor forest soils in the Walloon region accordingto the Steady-State Mass Balance equations. As anexample the methodology was applied to thecatchment `Waroneu', situated in a sensitive areaof the Haute Ardenne. Main input parameters tothe equations were derived from precipitation andrunoff data of the catchment study. Improvedestimates of nitrogen uptake (Nu) and base cationuptake (BCu) were obtained by intensive samplingof Picea abies and Quercus roburtrees. Nutrient contents (Ca, Mg, K, N) andnutrient to nitrogen ratios of Picea abiesreflected the poor soil quality at a site withhigh N deposition. Quercus robur nutrientcontents increased from stem to higher orderbranches with a high proportion of nutrientslocated in the bark. However the simulation ofstem only harvesting had a minor effect oncritical loads. Measured wood densities werelower than reported literature values with a maineffect on Nu and BCu estimates. The use ofrecommended default values and/or data derivedfrom the experimental site resulted in a widerange of critical loads, some of which werelargely overestimated. Results demonstrated theimportance of site specific data for criticalload calculations.     

Base Cation Losses from a Coniferous Catchment in Central Ontario,Canada

Long-term monitoring of a predominantlyconiferous catchment (PC-1) in central Ontario has enabledmass budgets of base cations to be estimated between 1983and 1998. During this period, sulphur deposition decreasedby approximately 30%, although this region still receivesacid deposition that exceeds the critical load with respectto acidity for forest soils. Between 1983 and 1998 therewas a net loss of 76.3 kg ha^-1 Ca and 13.7 kg ha^-1Mg from PC-1, and a net retention of K of 55.7 kg ha^-1. A net loss of Ca and Mg occurred every year during the studyperiod (except 1986/87 for Mg), although annual losses of Caand Mg have been generally lower in recent years. On anannual basis, net losses of Ca and Mg were extremelyvariable and were strongly related to export of SO₄,which in turn appeared to be strongly influenced by climatefactors. Measured losses of Ca and Mg over the 16-yearperiod represented 37% and 59% of their respectiveexchangeable pools measured in the upland soils in 1983.These values probably overestimate base cation losses fromthe upland however, because losses from organic soils in thecatchment were proportionately greater during yearsfollowing El Niño events. There was no change in basal areabetween 1983 and 1998, although there was a shift towardless nutrient-demanding species (white pine, hemlock). Pools of Ca and Mg in tree biomass are approximately doubletheir exchangeable pools in podzols, and so losses from soilmay be offset by changes in forest structure (size,composition) in the short-term (decades). If net losses ofCa and Mg continue due to harvesting and/or acid depositionand estimates of weathering and exchangeable pools arecorrect, then the long-term sustainability of the uplandforest at PC-1 must be in doubt.     

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.     

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.     

Impact of Harvest Intensity on Long-Term Base Cation Budgets in Swedish Forest Soils

The effects of harvesting on the long-term mass balances of calcium (Ca), magnesium (Mg) and potassium (K) were evaluated on a regional level in Sweden. A new high-resolution weathering database was used together with estimates of total deposition, losses through harvest and leaching. Estimates were made for pine and spruce separately and for two harvesting intensity scenarios: stem harvesting and whole-tree harvesting. The mass balance calculations showed net losses of Ca and Mg in almost the whole country for both scenarios. The losses were smaller for pine than for spruce. The K balances were mainly positive for pine but negative for spruce. Leaching was a main factor in the mass balances, especially for Ca and Mg. Whole-tree harvesting in spruce forests led to substantially higher net losses of K and Ca than stem harvesting, according to the calculations. In the whole-tree harvesting scenario in spruce forests the estimated yearly net losses of Ca, Mg and K corresponded to at least 5%, 8% and 3% of the pools of exchangeable base cations, respectively, at 25% of the analysed sites. If losses of this magnitude continue the depletion of the pools of Ca, Mg and K may lead to very low base saturation of the soils, possibly accompanied by negative effects on soil fertility, runoff water quality, tree vitality and tree growth within a forest rotation in parts of Sweden. Avoiding whole-tree harvesting can improve the situation substantially for K, but the losses of Ca and Mg will still be significant.     

On the Calculation and Interpretation of Target Load Functions

In this study critical load functions and target load functions of nitrogen and sulphur deposition with respect to acidity and minimum base cation to aluminium ratio were calculated with the SAFE model using three different averaging strategies: (1) averaging based on current forest generation, (2) averaging based on next generation and (3) averaging based on the entire simulation period. From the results it is evident that although target load calculation and indeed critical load calculation is straight forward, there is a problem in translating a predicted recovery according to the target load calculation back to a site-specific condition. We conclude that a policy strategy for emission reductions that ensures recovery, according to calculated target load functions, is likely to be beneficial from an ecosystem point of view. However, such a strategy may not be sufficient to achieve actual non-violation of the chemical criteria throughout the seasonal or rotational variations. To address this issue we propose a method for calculating dynamic critical loads which ensures that the chosen criteria is not violated.     

Does Soil Acidification Affect Spruce Needle Chemical Composition and Tree Growth?

In 1994, a large survey of soil chemistry was undertaken in thecounty of Värmland in central Sweden (Lundström et al., 1998).The southern part of the county was affected by soilacidification whereas there were no such indications in thenorthern part. To investigate the influence of soil chemistryon the trees at the specific sites, the survey was continued byan analysis of needle chemistry (Norway spruce) which wasundertaken at 150 of the 180 sites, and of tree growth at 65 ofthe 180 sites. Growth was expressed as a ratio between expectedgrowth, estimated with a national, empirical growth model, andthe growth observed in the field. In statistical analyses,using rank correlation, PCA and PLS, there were only weakindications of an influence of soil chemistry on needlechemistry and on tree growth. A moderate correlation betweennitrogen and sulphur in needles was found, which wasinterpreted as an effect of deposition and of processes in thetree canopy. No obvious regional pattern of the growth ratiowas found, in contrast to the clear pattern of soilacidification. The statistical analysis could not with anycertainty point out any of the soil chemistry variables asespecially important for the tree growth ratio.     

Uncertainties in Large Scale Assessments of Critical Load Exceedances

In the 1999 Gothenburg protocol to the UN/ECE LRTAP Convention andin the negotiations for an EU acidification strategy the area withexceedances of critical loads has been the preferred measure forenvironmental impacts. The aim of this study has been to assessthe influence of the uncertainty and spatial variation of both thecritical loads and deposition values on the calculated area withexceedances of critical loads. This has been done on a nationalscale for Denmark and on the European scale based on the dataincluded in the RAINS model. It is demonstrated that includinguncertainty and spatial variation in exceedance calculations, ingeneral gives larger exceeded area for the critical load ofacidity, CL(A). The picture for the critical load of nutrientnitrogen, CL_nut(N), is more mixed because of the higherproportion of exceeded areas. A further point of interest is thepossibility of validating relationships between critical loadexceedances and observable damage based on large scale monitoringand model data. It is demonstrated that it will probably not bepossible to use exceedance calculations on European scale as basisfor validation exercises, linking exceedances to observable damage.     

Biomass and Nutrient Dynamics of Restored Neotropical Forests

Restoring species-rich tropical forests is an important activity because it helps mitigate land deforestation and degradation. However, scientific understanding of the ecological processes responsible for forest restoration is poor. We review the literature to synthesize the current state of understanding of tropical forest restoration from a biogeochemical point of view. Aboveground biomass and soil carbon accumulation of restored tropical forests are a function of age, climate, and past land use. Restored forests in wet life zones accumulate more biomass than those in moist or dry life zones. Forests restored on degraded sites accumulate less aboveground biomass than forests restored on pastures or agricultural land. Rates of aboveground biomass accumulation in restored forests are lower than during natural succession, particularly during the first decades of forest establishment. Rates of litterfall, biomass production, soil carbon accumulation, and nutrient accumulation peak during the first few decades of restored forest establishment and decline in mature stages. Changes in species composition and canopy closure influence the rate of primary productivity of older restored stands. Species composition also influences the rate and concentration of nutrient return to the forest floor. The ratio of primary productivity to biomass is high in young restored forests and low in mature stands irrespective of climate. The ratio is low when past land use has little effect on biomass accumulation, and high when past land uses depresses biomass accumulation. This effect is due to a high rate of litterfall in restored forests, which helps restore soil by circulating more nutrients and biomass per unit biomass accumulated in the stand. The degree of site degradation and propagule availability dictates the establishment and growth of tree species. Reestablishment of forest conditions and the enrichment of sites by plant and animal species invasions lead to faster rates of succession, aboveground primary productivity, and biomass accumulation in restored forests. Our review demonstrates that nutrient cycling pathways and nutrient use efficiency are critical for interpreting the suitability of tree species to different conditions in forest stands undergoing restoration.     

Modification of Soil Solid Aluminium Phases During an Extreme Experimental Acidification of A Horizons of Forest Soils from Southwest Europe

Four umbric A horizons from acid forest soils were acidified in a batch type experiment and its effect in the Al pools of the solid phase analysed by means of selective dissolution methods. The results showed that Al release accounted for the consumption of 85–99% of the added protons, and causes a decrease of 2–33% of the ‘reactive’ Al pool of the soil solid phase. In these A horizons, inorganic non-crystalline Al and high stability Al-humus complexes are the main sources of the dissolved Al. The contribution of the complexes with intermediate stability only was relevant in the more acid horizon developed from phyllites (P18-A). The increase of equilibration time from 96 to 720 h did not caused significant differences in the decrease of the ‘reactive’ Al pool suggesting the acid neutralising reactions occurred in less than 96 h. In most cases the quantity of released Al is in agreement with the decrease of the different reactive Al pools of the solid phase.     

Metal releases from a municipal solid waste incineration air pollution control residue mixed with compost

The influence of 10 wt.% mature compost was tested on the heavy metal leachate emissions from a calcium-rich municipal solid waste incineration air pollution control residue (MSWI APC). Apart from elongated columns (500 and 1250 mm), an otherwise norm compliant European percolation test setup was used. More than 99% of the metals Al, As, Cd, Cr, Cu, Fe and Ni were left in the APC residue after leaching to a liquid-to-solid ratio (L/S) of 10. Apparent short-term effects of elevated leachate DOC concentrations on heavy metal releases were not detected. Zn and Pb leachate concentrations were one order of magnitude lower for L/S 5 and 10 from the pure APC residue column, which suggests a possible long-term effect of compost on the release of these elements. Prolonging the contact time between the pore water and the material resulted in elevated leachate concentrations at L/S 0.1 to L/S 1 by a factor of 2. Only Cr and Pb concentrations were at their maxima in the first leachates at L/S 0.1. Equilibrium speciation modelling with the PHREEQC code suggested portlandite (Ca(OH)2) to control Ca solubility and pH.