Atmospheric models and acidification: Summary and conclusions of the Warsaw II meeting on atmospheric computations to assess acidification in Europe

Three topics are discussed in this report: sensitivity/uncertainty analysis of long range transport models, the interface between atmospheric models of different scales, and linkage between atmospheric and ecological models. In separate analyses of long range transport models, it was found that uncertainty of annual S deposition was mostly affected by uncertainty of wind velocity, mixing height and wet deposition parameterization. Uncertain parameters collectively caused S deposition errors of around 10–25% (coefficient of variation) in the models examined. The effect of interannual meteorological variability on computed annual S deposition was relatively small. Different methods were presented for combining models of regional and interregional scale. It was found to be more important to include interregional information in regional-scale models for annual computations compared to episodic computations. A variety of linkage problems were noted between atmospheric and ecological models. The vertical distribution of pollutants and ‘forest fittering’ of pollutant deposition were found to be important in ecological impact calculations but lacking in the output of most interregional atmospheric models.     

Metals and acidification: An overview

Both upland soils and lake sediments appear to retain atmospherically deposited trace metals (e.g. Pb) even under acid conditions. The abundant, local, mineralogically derived metals (e.g. Al and Mn) are exported from upland soils under acidic conditions, but are usually retained by lake sediments. Acidification, however, reduces the extent of retention in lake sediments and soils, potentially inducing elevated metal levels in lake water. Only under conditions of extreme acidification and with the more mobile metals do lakes become net exporters of metals. Surface depletion of metals in sediment cores may not be the result of recent acidification.     

Mobilization of Cd upon acidification of agricultural soils: column study and field modelling

The potential effect of acidification of contaminated sandy soils on Cd transport in the unsaturated zone was assessed. Forty‐eight soil profiles were sampled at five depths in a polluted field that was set aside in 1992. The Cd concentration in the top 30 cm of this field was, on average, 10 mg kg⁻¹. A column experiment was carried out with one of the topsoil samples. Homogeneously packed columns were leached with 0.001 m CaCl₂, adjusted to pH 3 or pH 5.7, at a pore water velocity of 6 cm day⁻¹. The Cd and proton transport was predicted with coupled transport equations. The Cd transport was modelled by assuming local equilibrium and by using sorption parameters derived from batch experiments, while acidification was modelled with a kinetic approach, on the assumption that proton buffering was due to cation exchange and mineral weathering. Organic matter was the main contributor to the cation exchange capacity of these soils. Observed and predicted pH and Cd profiles in the columns agreed well. With the same model, the proton and Cd transport at field scale was calculated for each of the 48 profiles sampled (‘grid model’). It was predicted that the field‐averaged Cd concentration in the seepage water will increase from 6 μg litre⁻¹ at present to 200 μg litre⁻¹ over 260 years, which greatly exceeds the maximum permissible concentration (MPC) in groundwater of 5 μg litre⁻¹. Predictions of Cd transport using field‐averaged soil properties yielded a later breakthrough time and a larger peak Cd concentration than predicted with the grid model, which illustrates the impact of spatial variability on solute transport. Continuation of liming practices is a possible solution to prevent breakthrough of Cd at concentrations far in excess of the MPC.     

Causes of soil acidification: a summary

Abstract. A review of recent data shows that (i) dissolved CO₂ has its greatest acidifying effect in soils with pH values above about 6.5, (ii) fertilizers containing NH₋₁⁺ ions or urea will acidify soil whether the ions are taken up directly by plants or are first nitrified, (iii) oxidation of nitrogen and sulphur in soil organic matter causes acidification especially after deforestation, and (iv) the acidifying effect of rainfall and dry deposition is due to sulphuric and nitric acids, SO₂ and NH₋₁⁺ ions. A table is given showing the order of magnitude of each source of acidification.     

Geographical extrapolation of forest soil acidification using pH(KCl) data: A case study

Soil acidification over the last three decades in three forest areas situated in northern Belgium (Region of Flanders), is assessed by comparing recent data of soil analysis with historical data. It is investigated whether acidification values of sites evenly spread over the region, averaged per soil group, can be used to assess acidification in specific forest areas. Results suggest that data obtained by this kind of geographical extrapolation do not differ significantly from measured data. In sandy soils however, a stop of acidification as measured by the pH(KCl), is observed at pH(KCl) values lower than 3.3.     

Zooplankton responses to acidification: A review of laboratory bioassays

This literature review summarizes laboratory bioassays of the effects of acidification on 32 zooplankton taxa. Low pH adversely affected survival, longevity, reproduction, Na flux, heart rate, growth rate, feeding or filtering rate, and respiration rate. The organisms most studied have been daphnids, specifically Daphnia magna and D. pulex. Recommendations for further research include analyses of sublethal effects on zooplankters other than daphnids, comparative studies of different taxa acclimated and tested under similar conditions, studies of the effects of acclimation conditions and genetic differences between zooplankton populations on acid sensitivity, and laboratory bioassays conducted under realistic conditions and supplemented with field data.     

Biological indicators of lake acidification

Indicator taxa are identified, based on both synoptic surveys and whole lake acidification experiments, for lake acidification in the pH 6.0 to 5.0 range. Acidobiontic diatoms (e.g Asterionella ralfsii, Fragillaria acidobiontica, etc.), periphyton (Mougeotia and related species), macroinvertebrates (e.g. Hyalella azteca, Orconectes sp., etc.), leeches, and cyprinid fishes (e.g. Pimephales promelas, Notropis cornutus, etc.) are identified as target organisms during early phases of lake acidification.     

Soil acidification and aluminium mobility

Abstract. Natural acidification processes result in increasing solubility of aluminium as soils become more acid. Exchangeable aluminium provides a large reserve that can be mobilized by percolating acids or salts, with solution pH determining the upper limit of its solubility. Aluminium can also be mobilized within soils and into drainage waters in soluble complexes with silica or fluoride, and in organically complexed forms.     

安溪铁观音茶园土壤重金属分布及污染评价

  目的  茶叶产量和品质与茶园土壤养分状况及肥力质量密切相关。然而,目前针对茶园肥力状况的调查评价依旧不足。  方法  本研究对福建省安溪县13个主要产茶乡镇共38个铁观音生产茶园进行土壤样品采集与理化指标检测,并通过对土壤pH值以及关键营养元素分析,建立土壤肥力综合指数（IFI）。  结果  结果表明,茶园土壤pH值平均值为4.0,有近55%样点茶园土壤酸化严重;茶园土壤的有机质、全氮、全磷、全钾、碱解氮、有效磷、速效钾等指标相对优质标准较高;综合各评价指标,得到IFI值范围在0.52 ~ 1.00,平均值为0.84,有84%样点的茶园处于 Ⅰ 级优质标准。  结论  安溪县铁观音茶园土壤大半已严重酸化,但安溪茶园土壤整体养分状况优良,茶园土壤肥力综合指数IFI较高,表明安溪铁观音茶园土壤总体较健康。     

Soil acidification gradients: Mode of development,status quo and classification

Studies on soil profiles down to a depth of 3.5 m were carried out in the catchment area of the Söse River (western Harz Mts. Germany). The results show that the presence of an acidification front in the soil or unsaturated zone is a normal occurrence. Acidification fronts can be classified on the basis of water flow paths: Type A: front developed under vertical flow conditions, Type B: front developed under conditions of different flow directions, Type C: front developed in the presence of layers of different permeability. Knowledge of the type of acidification front and its depth allows a simple ion-budget calculation to be used to predict whether and when acidification of fresh waters will take place.     

Reaction of lotic periphyton to experimental acidification

Two hypotheses that could explain increases in the biomass and production of lotic periphytic communities in acidified habitats were tested: 1) a greater bioavailability of P due to its greater release from sediments at low pH, and 2) utilization of S from the H₂SO₄ addition. The experiments were performed in semi-natural conditions by submitting the periphyton to either continuous or episodic acidification. The uptake of P and S by periphyton were determined at different times during the treatments. The increases in P and S uptake, in acidified habitats, are compatible with both hypotheses and could account for the increase in periphytic biomass and production.     

Mechanisms of soil acidification reducing bacterial diversity

A central goal in soil microbial ecology research is to identify the biodiversity patterns and reveal the underlying mechanisms. Long-term soil acidification is known to reduce soil bacterial diversity, but the mechanisms responsible for this pattern have not been well explored. Soil acidification may reduce bacterial richness through ecological filtering (EF). In contrast, two types of processes may promote the maintenance of bacterial richness: species may adapt to the acidic pressure through evolution, and endemic species already adapted to the acidic pressure can colonize the acidified soils through dispersal. To identify the relative contribution of EF and evolution/dispersal (ED), we collected soils with a pH range of 4–7 from different ecosystems, conducted an acidification experiment with a similar pH range in a neutral soil, and proposed a conceptual framework that could distinguish the three potential types of mechanism (neither EF nor ED operate; EF operates alone; ED counteracts some effect of EF). We found that the entire bacterial domain was driven by the third type of mechanism, with ED counteracting about 42.4% (95% confidence interval: 32.7–50.4%) effect of EF. Meanwhile, different bacterial phyla/classes were governed by different types of mechanisms, and the dominant was the third type. Our results highlight the importance of both ecological and evolutionary mechanisms for regulating soil bacterial communities under environmental changes.     

Predicting watershed acidification under alternate rainfall conditions

The effect of alternate rainfall scenarios on acidification of a forested watershed subjected to chronic acidic deposition was assessed using the model of acidification of groundwater in catchments (MAGIC). The model was calibrated at the Panola Mountain Research Watershed, near Atlanta, Georgia, U.S.A. using measured soil properties, wet and dry deposition, and modeled hydrologic routing. Model forecast simulations were evaluated to compare alternate temporal averaging of rainfall inputs and variations in rainfall amount and seasonal distribution. Soil water alkalinity was predicted to decrease to substantially lower concentrations under lower rainfall compared with current or higher rainfall conditions. Soil water alkalinity was also predicted to decrease to lower levels when the majority of rainfall occurred during the growing season compared with other rainfall distributions. Changes in rainfall distribution that result in decreases in net soil water flux will temporarily delay acidification. Ultimately, however, decreased soil water flux will result in larger increases in soil-adsorbed sulfur and soil-water sulfate concentrations and decreases in alkalinity when compared to higher water flux conditions. Potential climate change resulting in significant changes in rainfall amounts, seasonal distribution of rainfall, or evapotranspiration will change net soil water flux and, consequently, will affect the dynamics of the acidification response to continued sulfate loading.     

A protocol for determining lake acidification pathways

The chemistry of 282 sampled low pH (<6.0) lakes in the U.S. E.P.A. Eastern Lake Survey (ELS) was evaluated in an attempt to assess why these systems have low pH. Evaluations were made using a decision protocol for classifying lakes according to several hypothesized acidifying mechanisms: acidic deposition, presence of wetlands and organic soils, acid mine drainage, watershed S sources, salt driven acidification, and changes in land use. The algorithm evaluates lakes in three steps: (1) initial exclusion criteria exclude from consideration lakes with pH greater than 6.0 or subject to strong confounding influences (e.g., road salt); (2) a general classification discriminates between lakes according to anion dominance; and (3) a secondary classification of lakes within each anion dominant class determines the most likely acidification pathway, using preliminary quantitative criteria designed to discriminate among competing hypotheses. Results computed for sampled lakes were scaled-up to produce regional population estimates, using the statistical framework of the ELS. Acidic deposition appears to be the most likely cause of low pH conditions in about two-thirds of the non-excluded lakes in the ELS low pH target population. Organic acidity arising from wetlands or land use changes appears to be primarily responsible for the low pH status of one quarter of these lakes. Watershed S sources and acid mine drainage appear to be of negligible importance, though further information on dry deposition rates and/or watershed soils is required to confirm this.     

亚热带典型人工林土壤酸化特征及其生物学机理初步分析

对亚热带典型人工林土壤pH和酸化生物学机理的研究表明:荒草地种植马尾松和木荷20年以后土壤的pH发生了变化,在0 ～ 60 cm处降低了0.12～0.47个单位;垂直方向上在距离树干10 cm处马尾松0～20cm土层pH低于木荷,20～60 cm土层pH大于木荷;水平方向上随着与树干距离的增加土壤pH受树木影响的程度逐渐降低,距离树干20 cm处土壤pH受树木的影响最大.树干茎流雨可能是树干基部附近土壤加速酸化的重要因素,根系的分布及其对盐基阳离子的吸收是造成马尾松和木荷土壤剖面出现酸化差异的主导因素.     

Regional monitoring of lake acidification in Finland

The primary objective of this monitoring is to detect long-term Long-Range Transboundary Air Pollution (LRTAP) induced changes in the water quality of small lakes, throughout Finland, with low conductivity. The monitored lakes (n=171, sampled every autumn since 1990 and in 1987) have a smaller watershed (usually headwater or seepage lakes), a larger lake/catchment ratio, and lower base cation concentrations, alkalinity and pH than Finnish lakes on average. The monitoring network provides background data for air pollution dose/response studies, critical load calculations and for the modelling of acidification scenarios. The declining sulphate deposition seems to be reflected in small headwater lakes all over southern and central Finland as a lowering of the sulphate concentration in the waters. Nitrate concentrations in these lakes have been typically low in the autumn. The base cation concentration is not generally declining, as it is in deposition in many areas. The sulphate concentration in lakes has declined more than base cations. Hydrologically, the recent years have been quite variable because of varying annual precipitation. The variation in alkalinity and pH in typical Finnish lakes is dependent on the content of humic material derived from catchments. The monitoring period is too short to reveal consistent trends in major ion chemistry. However, signs of improvement in recent years can be seen; in comparing 1993 to 1987, years with similar organic acidity and base cations, it seems that the sulphate decline in lakes monitored is compensated by a significant rise in both alkalinity and pH.     

Natural and anthropogenic components of soil acidification

The following 8 theses are theoretically founded and experimentally quantified. 1. Rocks contain only bases and no acid precursors. Therefore, with the exception of sulfide containing rocks, soils cannot acidify as a result of atmospheric rock weathering. 2. A consumption of protons in rocks and soils results in a decrease of their acid neutralizing capacity (ANC) and can result in the buildup of a base neutralizing capacity (BNC). Strong soil acidification leads to the formation of stronger acids from weaker acids in the solid phase; this may be connected with a decrease in the BNC. 3. Weak acids (carbonic acid) lead in geological times to the depletion of bases without a larger accumulation of labile cation acids. Strong acids (HNO₃, organic acids, H₂SO₄) can lead within a few decades to soil acidification, i.e. to leaching of nutrient cations and the accumulation of labile cation acids. 4. The acid input caused by the natural emission of SO₂ and NO_x can be buffered by silicate weathering even in soils low in silicates. 5. The cause of soil impoverishment and soil acidification is a decoupling of the ion cycle in the ecosystem. 6. Acid deposition in forest ecosystems which persists over decades leads to soil acidification. 7. Formation and deposition of strong acids with conservative anions (SO₄, NO₃) shifts soil chemistry into the Al or Al/Fe buffer range up to great soil depth. In such soils eluvial conditions prevail throughout the solum and even in upper part of the C horizon: in connection with the decomposition of clay minerals, Al and eventually Fe are being eluviated. The present soil classification does not include this soil forming process. 8. In the long run, soil acidification by acid deposition results in the retraction of the root system of acid tolerant tree species from the mineral soil, and in water acidification.     

Bioelement content and biomass in scots pine: Effect of acidification and liming

Dry weights and bioelement contents in biomass of Scots pine (aboveground) were estimated on some differently treated plots from one acidification experiment in North Sweden. Dry weight estimates of Scots pine biomass showed relatively small differences between treatments. The content of N, P, K, Ca, Mg, Mn, and S showed significant differences in a number of cases. The amount of N in different crown components increased; fertilizer-N recovered in the biomass was up to 10% of that added (in total, the NPK-plots received 1260 kg N ha^?1). The concentration of other elements (P, K, Ca, Mg, Mn and S) showed some effect of the treatments, but was not as marked as that of N. Especially interesting is a decrease in the level of Mg in needles and shoots on the NPK-treated plots. The concentration of S was influenced by application of acid, but much more by NPK-fertilization throughout the period.