Release Of Base Cations Research Articles

A concept of CO2 Georeactor sequestration at the Ogachi HDR site, NE Japan

Abstract A field experimental study of CO 2 sequestration into the Ogachi hot dry rock (approximately 200 grad C, mainly consists of granitic rocks) have been conducted to explore safe and permanent CO 2 storage by creating carbonate minerals in the subsurface. In the CO 2 -Injection Experiment by single well testing, CO 2 dissolved water with a tracer was injected by free flowing injection through a deep geothermal well (OGC-2) to the reservoir. And chemical monitoring with a wireline sampler in batch has been enforced for the estimation of the release of base cations from reservoir rock, reaction of CO 2 with cations. Resultant CO 2 –Tracer balance suggests that carbonates might precipitate in a few days. In addition, the crystal-growth testing in the borehole using a newly developed chemical reactor sonde was implemented to ascertain CO 2 fixation as carbonates under in-situ conditions and it was indicated that a spiral growth rate of calcite was order of mm/year. Laboratory experiments and simulations for CO 2 –water–rock interaction also have been executed to make long-term evaluation of the CO 2 storage process and optimization of reservoir modeling.

Relationship between chemical and mineralogical properties and the rapid response to acid load of soils in humid Asia: Japan, Thailand and Indonesia

It is essential to determine the relationship between soil chemical and mineralogical properties and soil response to acid load to understand the acid-neutralizing capacity and cation behavior of different ecosystems. For 46 soil samples from a subsurface horizon in humid Asia, that is, Japan, Thailand and Indonesia, exchangeable cations, total bases and oxalate-extractable Al (Alo) were determined, and acid titration was conducted to investigate the rapid soil response to acid load. The acid titration experiment indicated three types of soil response: (1) the release of base cations (particularly Ca and Mg) strongly correlated with exchangeable bases, which dominated the tropical soil samples, (2) the release of Al correlated with Alo content, which dominated the Japanese soil samples, (3) acid and anion adsorption in soil samples with low acid-neutralizing capacity. To gain further information on the source of soil alkalinity, a column experiment with HCl was conducted using eight selected soil samples in which first-order kinetics were assumed to simulate the time-courses of cation release. In the column experiment, the amounts of Ca and Mg released were close to the exchangeable amounts, and Alo dissolved more rapidly than Al in crystalline minerals. The rate constants of cation release were large for Ca and Mg, and small for Al, clearly indicating a difference between the exchange and dissolution reactions. Thus, rapid soil response to acid load differed among the soils. A cation exchange reaction was dominant in the tropical soils. In some tropical soils, Ca and Mg were present in exchangeable forms at a higher ratio in the total amounts and they were considered to be easily utilized by plants, but leached out from the soils. In the Japanese soils, including the Andisols, secondary mineral dissolution was conspicuous, resulting in a large acid-neutralizing capacity. In both the tropical and Japanese soils with low acid-neutralizing capacity, anion adsorption mainly contributed to acid neutralization.

Mineralogical sources of the buffer capacity in a granite catchment determined by strontium isotopes

Abstract The role of different minerals in base cation release and thus the increase of buffering capacity of groundwater against acid deposition is controversially discussed in the literature. The 87Sr/86Sr ratios and base cation concentration were investigated in whole rock leachates, mineral separates, precipitation, soil solution, groundwater and stream water samples in the Lehstenbach catchment (Germany) to identify the weathering sequence of the granite bedrock. Three different approaches were followed in parallel. It was assumed that the contribution of different minerals to base cation supply of the groundwater with increasing weathering intensity would be observed by investigating (1) unweathered rock leachates, deep groundwater and shallow groundwater, (2) groundwater samples from new groundwater wells, reflecting the initial weathering of the drilled bedrock, and groundwater from wells that were drilled in 1988, (3) stream water during baseflow, dominated by deep groundwater, and stream water during high flow, being predominantly shallow groundwater. Whereas the first approach yielded consistent patterns, there was some evidence that groundwater from the new wells initially reflected contamination by the filter gravel rather than cation release in an initial stage of weathering. Time series samples of stream water and groundwater solute concentrations and isotope ratios turned out to reflect varying fractions of soil water and precipitation water at baseflow and high flow conditions rather than varying contributions of different minerals that prevail at different stages of granite weathering. The early phase of weathering was clearly dominated by base cation release from biotite weathering and to a lesser extent by apatite dissolution. Feldspars contributed to base cation release as well, but could not be regarded as endmembers of the mixing diagram. There was no clear evidence for a contribution from calcite. Correspondingly, base cation input by atmospheric deposition and liming had only minor effects on groundwater buffer capacity.

A comparison of weathering rates for acid-sensitive catchments in Nova Scotia, Canada and their impact on critical load calculations

Critical loads are strongly dependent on the rate of release of base cations from the soil matrix. This study compares five commonly used methods for estimating weathering rates at five acid-sensitive catchments across Nova Scotia, Canada. Three of the methods (Zr Depletion, Clay Content, and the PROFILE model) are based on soil profiles and consider only the rooting zone, whereas the two remaining methods (the soil acidification model MAGIC and catchment Mass Balance Deficit) are catchment-based, and account for contributions from all soils within a catchment. Each weathering estimate method resulted in similar values among the five catchments, indicating similar sensitivity to acidic deposition among the study areas. Base cation weathering estimates were very low using the three soil profile-based methods, with rates varying from 3 to 13 mmol c m − 2 a − 1 . In contrast, catchment-based methods predicted base cation weathering rates an order of magnitude higher (60 to 155 mmol c m − 2 a − 1 ), possibly due to spatial heterogeneity of the soil deposits, and contributions from deeper soil (till). Critical load (sulphur and nitrogen) estimates using the profile-based weathering rates indicate that critical loads for forest soils are currently exceeded at all catchments by 23 to 61 mmol c m − 2 a − 1 . Predicted future reductions in acidic deposition should reduce the magnitude of critical load exceedance, but will not result in the catchments reaching a non-exceeded state.

Soil Nutrient Dynamics after Harvesting and Slash Treatments in Boreal Aspen Stands

The effects of harvesting and slash treatments on soil nutrient dynamics were assessed in boreal aspen stands growing on mesic clayey sites. Stem‐only harvested stands were compared with unharvested controls according to a complete block design with three replications. Within harvested areas, four slash treatments (stem‐only harvesting [SOH], whole‐tree harvesting [WHT], wood chip application, and slash burns) were compared. Treatments created a gradient of slash that ranged from 52.3 Mg ha−1 in stem‐only to 13.8 Mg ha−1 in control stands. The amount of slash had no effect on wood decomposition rates but was strongly associated with higher forest floor organic C, Kjeldahl N and base cation concentrations (Cae and Mge), base saturation, pH, and effective cation exchange capacity (CEC) and lower microbial C/N when control stands were compared with stem‐only harvested stands. Slash burn severity was too low to significantly reduce slash loads and induce base cation release but severe enough to reduce forest floor microbial C and N concentrations (−48 and −55%, respectively) for at least one complete growing season. Slash burn also induced increases in forest floor available P (54%) concentration compared with other slash treatments. Chipping reduced forest floor microbial N concentration by 25% and increased microbial C/N by 28% but had no impact on nutrient availability. Differences between WTH and SOH were linked to the abundance of slash. Finally, the results illustrate that whatever the treatment, the amount of slash left on the ground is the main factor found to affect soil microbial community characteristics and soil nutrient availability.

Base cation mineral weathering and total release rates from soils in three calibrated forest watersheds on the Canadian Boreal Shield

Total release rates of base cations (Ca, Mg, K, and Na) from soils and from watersheds were evaluated using three methods. Three methods, one of which is new, were also used to evaluate mineral weathering rates of soils for three calibrated forest watersheds in the forest ministry’s monitoring network (Réseau d’étude et de surveillance des eco ystems forestiers: Quebec Forest Ecosystem Research and Monitoring Network; RESEF) on the Canadian Boreal Shield. We also compiled an extensive literature review of forest soil base cation release rates, focussed on northeastern North American forest soils of granitic lithology. With the exception of the total release and mineral weathering of Ca from soils at the Lake Laflamme Watershed site, and the total release of K from the three watersheds, soils and watershed release rates for the three watersheds were within the confidence interval of release rates compiled for forest ecosystems with similar granitic environment (compiled data for solum [mmol (+) m-2 yr-1 ± 95% CI], Ca: 33.8 ± 16.3, Mg: 16.8 ± 4.2, K: 13.0 ± 5.6, Na: 11.1 ± 3.0, and sum of base cations (BC): 61.2 ± 11.0; compiled data from watersheds, Ca: 82.8 ± 24.6, Mg: 50.8 ± 17.0, K: 7.8 ± 2.2, Na: 44.7 ± 12.8, and BC: 186.0 ± 49.9). Given the uncertainties associated with the calculations, there was little overall difference between total release rates and weathering release rates from soils for two of the three watersheds. Key words: Weathering rate, forest soil, granitic environment, watershed, PMB method, compositional trends

Base cation, aluminum, and phosphorus release potential in Danish forest soils

AbstractLong‐term nutrient supply in forest ecosystems is due to the dissolution of primary and secondary minerals in soils. The potential of nutrient release in 19 forest soils in a cool humid climate was examined. The soil profiles are classified as Alfisols (10), Spodosols (2), Entisols (4), Ultisols (1), and Mollisols (2), thus covering a gradient in soil fertility. Short‐term and long‐term release of calcium, magnesium, potassium, phosphorus, and aluminum was evaluated by a batch extraction using dilute nitric acid (0.1 M) for 2 hours, followed by 2 days (48 h), and 7 days (168 h). The solution was renewed after 2 and 50 hours extraction time. Nutrient pools expressed as g m–2 to soil depth 100 cm, and a base index (Ca2++ Mg2++ K+ (molc m–2) : Ca2+ + Mg2+ + K+ + Al3+ (molc m–2)) were interpreted in relation to soil texture classes. Subsoil texture classes: Coarse: < 5 % clay; medium 5–10 % clay or (> 5 % silt or > 50 % fine sand), or fine > 10 % clay were evaluated as an indicator of forest soil quality. Base cation and phosphorus release decreased in the order fine > medium > coarse. Texture classes explained base cation release by about 80 % of total variation, and phosphorus release by 40–50 %. The base index generally increased by extraction time for sandy soils and decreased for loamy soils. This indicated that sandy soils released accumulated reactive aluminum in the 0–2 hour extraction. Subsoil texture class is suggested as a pedotransfer function for long‐term nutrient release potential in Danish forest soils.

Intensive monitoring of forest ecosystems in Europe: 2: Atmospheric deposition and its impacts on soil solution chemistry

In order to gain a better understanding of the effects of air pollution and other stress factors on forests, a Pan-European programme for intensive and continuous monitoring of forest ecosystems has been implemented in 1994. Results of this intensive monitoring programme presented in this paper are related to the surveys on atmospheric deposition and soil solution chemistry, focusing on data for 1997. Both bulk deposition and throughfall of N was higher than that of S at nearly all the plots, especially in Western Europe. The average calculated total N deposition was approximately twice as large as S. Approximately, 55% of the considered plots received an N input above 1000 mol c ha −1 yr −1 (14 kg ha −1 yr −1), being a deposition level above which the species diversity of the ground vegetation may decrease. The total input of acidity ranged mostly between 200 and 4000 mol c ha −1 yr −1. The deposition of SO 4, NO 3 and Ca was significantly higher in the central/eastern part of Europe, but NH 4 was slightly higher in western Europe. A highly significant positive correlation of atmospheric deposition and rainfall was observed for most ions. The concentrations of NO 3 in soil solution exceeded the EU ground water quality criterion of 800 mmol c m −3 in the subsoil at 9% of the plots. Al/(Ca+Mg+K) ratios exceeded a critical ratio of 1.0 in approximately 30–39% of the plots, depending on the layer considered. The concentration of potentially toxic Al in the subsoil was strongly related to the concentration of SO 4 and NO 3 in soils with a base saturation below 25% or a pH below 4.5, indicating that Al release is the dominant buffering process in acid soils. Above a base saturation of 25% and a pH level of 4.5, there was a strong relationship between Ca and strong acid anions, indicating that the acidity is mainly neutralised by the release of base cations in less acidified soils. The variation in concentrations of major ions in the soil solution could be, to a large extent, explained by differences in atmospheric deposition and meteorological conditions.

H + budget in oligotrophic Quercus pyrenaica forests: atmospheric and management-induced soil acidification?

The aim of the present work was to quantify the importance of the atmospheric deposition and the internal processes in soil acidification in oligotrophic Quercus pyrenaica forests, and determine the mechanism(s) involved in the neutralization of the acidity through the canopy and the soil. Four permanent oak forest plots were monitored in the Sierra de Gata mountains (Central-Western Spain): pH and mineral elements were determined in bulk precipitation, dry deposition, throughfall, stemflow, surface runoff, and in the soil, the water flowing through the humic (Ah) and mineral (Bw) horizons. Soil acidification has been estimated from the balance of the elements (atmospheric input minus output by surface runoff and deep drainage) and the net accumulation of nutrients by biomass. The results showed that rainwater pH was 5.5, and dry deposition of H+ was lower than bulk deposition (20 vs. 31 eq H+ ha−1 y−1). Throughfall was slightly less acid (pH 5.6) than the incident rain; however, stemflow was more acid than rain (pH 5.2). As a result, only 22 eq H+ ha−1 y−1 reached the soil surface, and the rest was taken up by the canopy (29 eq H+ ha−1 y−1), which could only explain between 4 and 8% of the cation leaching. The pH of water was increased slightly when percolating through the soil (from pH 5.6 to 5.8). Atmospheric deposition of free H+ and N-compounds had a minor contribution to the soil acidification (3 and 12%, respectively). The rest, 77% of H+, were produced by cation uptake, and 8% by mobilization of anion from mineral weathering and by precipitation or adsorption of cations. Almost all the acidity generated by the external and internal sources, was buffered by the soil (2.40 keq ha−1 y−1 on average), and losses from the soil were less than 1% of total H+. Neutralization occurred mainly by release of base cations from exchange surfaces and weathered minerals (71%); aluminium mobilization contributed to only 8%. If the forest reaches its degenerative phase, the lost acid-neutralizing capacity (ANC) could be completely restored by release of cations during biomass mineralization. However, if biomass is exported from these forests, further acidification of these already oligotrophic oak coppices will result in the future.

Analysis of Proton Generation and Consumption of Forest Surface Soils in Hokkaido, Northern Japan

We determined proton budgets of surface soils in a deciduous forest (Df) and a coniferous forest (Cf) of Volcanogenous Regosols in Tomakomai, Hokkaido of northern Japan. The total H+ source was 12.9 and 11.6 kmolc ha−1 y−1 at Df and Cf respectively, and the external H+ was 1% at Df and 2% at Cf. The primary H+ sources were vegetation uptake of base cations and nitrification, while the major H+ sinks were release of base cations and NO3+ uptake by vegetation. Leaching incubation experiments using A horizon soils including Df and Cf with NH4+ solutions (5.3, 15.9 mg N L−1) showed that H+ from nitrification was generally higher in the Df soil than Cf soil, and nitrification of Tomakomai Df soil was the highest in both treatments. Results of multiple regression analyses suggested that pHkCl and exchangeable Ca2+ contributed to the H+ generation via nitrification. Leaching experiments with dilute HCl (pH 3.3) revealed that cation release (mainly Ca2+) occurred, and the proportion of release by decrease of exchangeable cations was higher than that by mineral weathering. Mineral weathering in the Tomakomai soil was higher than the other soils.

Proton Budget for a Japanese Cedar Forest Ecosystem

The proton budget for a Japanese cedar (Cryptomeria japonica) forest in Gunma Prefecture, Japan, was studied by estimating biogeochemical fluxes. The proton budgets were estimated for three individual compartments of the ecosystem: vegetation canopy, and the upper (O horizon + 0–10 cm) and lower (10–100 cm) soil layers. The dominant proton sources in the compartments were atmospheric deposition (1.2 kmol ha−1 yr−1), nitrification (5.1 kmol, ha−1 yr−1) and base-cation uptake by vegetation (8.0 kmol, ha−1 yr−1) respectively. These proton sources were neutralized almost completely within the individual compartments mainly by base-cation release from the canopy or the soil. The sum of internal proton sources was five times as large as that of external ones. Nitrogen input from the atmosphere was 2.2 kmol ha−1 yr−1, whereas its output from the lower soil layer was 3.9 kmol ha−1 yr−1, indicating that a net loss of nitrogen occurred in the ecosystem. However, this did not cause the acidification of soil leachates because of a sufficient release rate of base cations from the soil.

Stream chemistry modeling of two watersheds in the Front Range, Colorado

We investigated the hydrologic, geochemical, and biogeochemical controls on stream chemical composition on the Green Lakes Valley and Andrews Creek watersheds using the alpine hydrochemical model (AHM). Both sites had comparable data sets from 1994 and 1996, including high‐resolution spatial data and high‐frequency time series of hydrology, geochemistry, and meteorology. The model of each watershed consisted of three terrestrial subunits (soil, talus, and rock), with the routing between the subunits determined by spatial land cover data. Using 1994 data for model calibration and 1996 data for evaluation, AHM captured the dominant processes and successfully simulated daily stream chemical composition on both watersheds. These results confirm our procedure of using spatial and site‐specific field and laboratory data to generate an initial catchment model and then calibrating the model to calculate effective parameters for unmeasured processes. A net source of nitrogen was identified in the Andrews Creek watershed during the spring snowmelt period, whereas nitrogen was immobilized in the Green Lakes Valley. This difference was most likely due to the larger and more dominant area of talus in the Andrews Creek watershed. Our results also indicate that routing of snowmelt through either soil or talus material is sufficient for retention of H+ and release of base cations but that N retention is more important on areas mapped as soil. Owing to the larger ionic pulse and larger fraction of surface runoff the Green Lakes Valley was more sensitive to a doubling of wet deposition chemistry than the Andrews Creek watershed.

THE pH DEPENDENCE OF HORNBLENDE DISSOLUTION

Hornblende minerals are important in proton neutralization and base cation release processes in many soils. This study was conducted to determine the pH dependence of hornblende dissolution rates and the rate of proton consumption during weathering under laboratory conditions. Hornblende was crushed mechanically and purified by magnetic and density separation. The samples were initially weathered (22 days) in pH 4.0, 0.01mol/L HOAc-LiOAc buffer to minimize the effect of grinding on dissolution rates. The pH range investigated was from 3.4 to 4.6 at approximately 0.2 pH unit intervals. The hornblende dissolution experiment was conducted in a 0.01 mol/L HOAc-LiOAc buffer in batch-type reactors for 30 days. The rate of proton consumption was determined for both the initially weathered samples and for samples not initially weathered in 0.01 mol/L LiCl at pH near 4.0. The dissolution was nonstoichiometric during the study period, with preferential release of Al, Fe, and Mg relative to Si. Nearly linear kinetics were observed for the release of Si, but the release rates of Al, Fe, and Mg decreased with time. Hornblende dissolution rates in the pH range studied decreased as pH increased. The overall apparent linear reaction orders, with respect to solution pH, were found to be 0.71, 0.39, 0.42, and 0.79 for Al, Fe, Mg, and Si release, respectively. The reaction orders at pH values between 3.4 and 4.2 were lower than those between pH 4.2 and 4.6. The quantity of proton consumed was equivalent to the total charges of the cations released. The proton consumption rate of samples not initially weathered was five times higher than that of the initially weathered samples. Dissolution rates obtained in acetate buffer were 3 to 4 times higher than those obtained in unbuffered LiCl solution at comparable pH and ionic strengths. The rate of hornblende dissolution is a function of solution pH and concentration of organic ligand.

Modelling impacts of atmospheric deposition, nutrient cycling and soil weathering on the sustainability of nine forest ecosystems

To assess the impact of acid deposition on the long-term sustainability of nine oak, pine and spruce stands on sandy to loamy sandy parent material in Sweden, Denmark and The Netherlands, a dynamic soil acidification model (ReSAM) was applied. Two deposition scenarios for the period 1990–2090 were used: a business as usual scenario (BAU) and a restrictive critical load scenario (CL). The BAU scenario leads to a strong decrease in both Al concentrations and pH in the topsoil of the Dutch and the Danish sites due to a decrease in the amount of amorphous Al compounds. The decline in pH leads to an enhanced release of base cations by silicate weathering. Despite the ongoing acidification, base saturation increases during the simulation period, due to both the increase in base cation weathering and an increased input from mineralization with the ageing of the tree stands. No change in Al concentration is predicted for northern Sweden as deposition levels are below critical loads. Soil chemistry at the recently replanted Swedish sites is dominated by changes in N cycling instead of by deposition. The CL scenario leads, especially after 2010, to a stronger decline in Al concentration compared with the BAU scenario, which is mainly caused by a reduction of the acid input. Up to 2010, a considerable acid input to the soil system is maintained as N supply is larger than the consumption of N by the trees. Despite the reduction of the deposition of S and N to critical loads, the readily available cation pools are still declining on the Danish and Dutch sites in 2090. It is concluded that deposition levels above critical loads lead to exhaustion of the pool of amorphous Al compounds and a decline in pH. Base saturation does not decline due to an increase in mineralization with stand age and an increase in weathering rate due to the decline in pH.

Primary mineral weathering in the central Appalachians: a mass balance approach

We use a mass balance approach to calculate long-term weathering rates for three forested watersheds in the southeastern USA. One watershed (Shaver Hollow) is underlain by Precambrian granodiorite, and two (White Oak Run, Deep Run) are underlain by Cambrian metasedimentary units. Each of the study areas receives high levels of acidic precipitation, and each watershed was almost completely defoliated by gypsy moth larvae for at least two consecutive years during the study period. Our analysis uses stream and precipitation chemistry obtained weekly from 1987 to 1993 for Shaver Hollow and from 1980 to 1993 for White Oak Run and Deep Run. Mass balance calculations for the granodiorite watershed indicate that plagioclase feldspar is the dominant reactant and that basalt dikes which comprise a small fraction of the catchment make a significant contribution to the base cation budget. Corresponding calculations for the metasedimentary bedrock watersheds indicate contributions from plagioclase feldspar, muscovite, and biotite. At best, the mass balance models provide an approximation of weathering processes because the soils are immature and contain few stoichiometric clay minerals such as kaolinite. Annual cation release rates from Shaver Hollow suggest soil profile ages of ∼4 ka, consistent with the observed surficial geology and the geological history of the area. Base cation efflux patterns during the period of severe defoliation differ markedly between the three watersheds and reflect a continuum of ecosystem sensitivities. The Pedlar granodiorite system is relatively robust to the effects of disturbance: calculated weathering rates and proportions change by <10% during defoliation. The Chilhowee Formation watershed White Oak Run is moderately acid-sensitive and is unstable with respect to defoliation: annual base cation export by roughly 30% during each year of larval infestation. The Deep Run watershed is superficially robust to defoliation, as base cation efflux increased by only 5% during the disturbance. However, seasonal patterns of base cation uptake and release both prior to and during defoliation indicate this system is severely stressed, presumably as a result of chronic acid deposition. Mass balance calculations suggest that base cation availability in this watershed is low enough to inhibit normal biomass aggradation even in the absence of extreme defoliation.

Determination of mineral surface area in relation to the calculation of weathering rates

The release of base cations by the weathering of soil minerals provides the primary buffer against the detrimental impact of acid deposition on the soil. The rate of release of base cations will be governed by mineral composition and the exposed surface area of mineral grains. In theory it is possible to calculate the surface area of a mixture of grains on the basis of particle size and geometry. An alternative is the use of correlations. It has been suggested (Sverdrup et al. 1990, Nordic Council of Ministers 98) that mineral surface area can be related to soil particle size distribution via a simple empirical equation. This contribution compares the surface areas of soils as calculated by geometric equations and Sverdrup's equation with actual BET measurements. The equations tested do not appear to be applicable to the soils used in the current work. Modified versions of the equation suggested by Sverdrup, applied to subsets of our data, based on sites and soil types, yield better results for the soils used in the current work. Statistically significant improved fits are obtained when soil mineralogy as well as soil texture is taken into account.

Base Cation Supply in Spruce and Beech Ecosystems of the Strengbach Catchment (Vosges Mountains, N-E France)

In spruce and beech stands, mineral budgets for a rotation period were calculated from measured element fluxes. The release of base cations by mineral weathering was calculated with the steady state soil chemistry model PROFILE. The calcium release rate by weathering of the mineral fine earth was extremely low. For the period of one rotation, mineral weathering cannot provide enough Ca to compensate timber harvesting and leaching. Forest sustainability depends strongly on the amounts of Ca gained from deposition and lost by biomass removal. Magnesium was supplied by atmospheric deposition and mineral weathering. Calculated weathering rates were close to present soil losses. However, as the model assumes that all dissolution reactions are congruent, the computed release rate of Mg from illite might be too high. Main inputs of K to the soil solutions were primarily attributed to canopy leaching and litterfall in upper horizons and to mineral weathering in deeper horizons. The cation budget of the beech stand was much more equilibrated than that of the spruce stand. Given possible changes in silviculture and deposition chemistry, the sustainability of the present stands is rather improbable with respect to their mineral supply.

A Comparison of Soil Sensitivity to Acidification Based on Laboratory-Determined Short-Term Acid Buffering Capacity and the Skokloster Classification

The sensitivity of mineral soils to anthropogenically-induced acidification may be assessed using the Skokloster classification or by considering the short-term acid buffering capacity (STABC). The Skokloster classification is based on the assumption that the majority of acidification neutralization is due to base cation release from mineral weathering. It therefore considers the long-term neutralizing capacity of a soil on a scale of decades. The STABC of a soil is due largely to the adsorption and exchange of H+ ions by soil components, and acts on the scale of single years. The United Kingdom maps generated to identify soils that are sensitive to acidification use the Skokloster classification. A comparison ofthe laboratory-determined STABC ofthe different horizons from soil profiles with the Skokloster classes of soils shows that there is generally no correlation between the two. Maps showing soil sensitivity to acidification based on measured STABC and Skokloster classes are markedly different. The STABC measure has the potential for quantitatively assessing a soil’s sensitivity to acidification, while the Skokloster approach is empirical. It maybe possible to combine the two approaches to produce a dynamic method of assessing soil recovery from acidification.

The acidification of the Herrenwieser See, Black Forest, Germany, before and during industrialisation

Past trends in the biology and chemistry of the Herrenwieser See in the base-poor Bunter Sandstone Black Forest were reconstructed from a sediment core. Diatom inferred pH indicated pre-industrial acidification periods with subsequent recovery. For the most recent acidification, multiple regression indicated a pH decline by 0.78 pH units to a minimum of pH 3.91 in the early 1980s; weighted averaging also indicated a pH minimum in the 1980s, but the lowest value was pH 4.58 and the overall decline only 0.30 units. The most important indicator species were Asterionella ralfsii for moderate acidification and Tabellaria quadriseptata for extreme acidification. Community diversity H′ and floristic composition, as shown by cluster analysis, were sensitive to pH changes in the lake. Chlorophyll pigments offered another indicator for pH trends since periods of acidification were accompanied by higher concentrations of chlorophyll derivatives. Acidic inputs into the lake were indirectly detected via analysis of persistent pollutants, metals and polycyclic aromatic hydrocarbons. Characteristic chemical changes indicated a metal burden from atmospheric deposition, and the release of base cations and aluminium from weathering. PAH concentrations rose simultaneously with the pH decline in the 19th century. After the input maximum in the 1960s the flux rates for PAHs decreased and are now < 20% of their peak value. The simultaneous trends in biological indicators and persistent pollutants demonstrate that recent acidification was caused by atmospheric acid deposition. So far there has been no significant reverse trend despite signs of recent changes indicated by the diatoms. However, pre-industrial acidification and recovery were also related to acid deposition, and indicate that reversal is possible in this lake.

Evidence from chemical changes in incubated soils for effects of sea salt deposition on Scottish upland soils

Evidence in the literature supporting the hypothesis that inputs of sea salts from the atmosphere increase soil pH and exchangeable Na + and Mg 2+ concentrations in soils in which the release of base cations by mineral weathering is slow is briefly reviewed. It is postulated that if a sea salt effect indeed occurs, long-term incubation of such soils in a field moist state should result in a decline in soil pH and in the relative importance of exchangeable Na + and Mg 2+ on the cation exchange sites. The postulate is confirmed by examining changes in pH and base cation ratios in soils incubated over 3 months at 4°C or at room temperature or for 20 months at ambient outdoor temperatures in Aberdeen. The results obtained during the shorter incubation period support the hypothesis. However, the soil changes observed after 20 months of incubation are more variable, and interpretation is complicated by the high degree of oxidation of organic C observed.