Base Cation Inputs Research Articles

Long-term changes in soil composition in unmanaged central European mountain spruce forests after decreased acidic deposition and a bark beetle outbreak

We studied the effect of tree mortality on nutrient concentrations and pools in soils of two unmanaged mountain forest catchments (Plešné, PL; and Čertovo, CT; Czech Republic) recovering from acidification due to decreasing acidic atmospheric deposition since the late 1980s. Both catchments were situated at similar elevation, and before a bark beetle outbreak were covered by healthy mature Norway spruce (Picea abies) forests. However, differing bedrock [granite (PL) vs mica-schist and quartzite (CT)] resulted in lower concentrations of base cations (except for magnesium), but higher concentrations of aluminium and iron oxyhydroxides in the CT soils, enabling higher phosphorus (P) accumulation. Despite these differences, soils were similarly acidic, with pHH2O from 3.5 to 4.3 and cation exchange capacity (CEC) dominated by exchangeable protons (H+ex) and aluminium in both catchments in 2000. In the PL catchment, >75 % of mature spruce trees died after a bark beetle infestation between 2004 and 2008, and all dead biomass was left on site. Forest damage in the CT catchment was minor. Soil concentrations and pools of exchangeable calcium (Ca2+ex), magnesium (Mg2+ex), potassium (K+ex), and H+ex were tightly related to concentrations of organic carbon (C). Following the tree dieback in the PL catchment, we observed significant (p < 0.05) increases in (1) concentrations and pools of Ca2+ex, Mg2+ex, and K+ex, (2) base saturation (from 30 to ∼ 40 %), (3) CEC:C ratios (i.e., CEC of soil organic carbon), (4) pHH2O, (5) mineral P forms, and (6) total nitrogen (N) in the upper (O and A) soil horizons. Similar changes were less pronounced or negligible in the less impacted CT soils. The elevated litterfall after tree dieback, resulting in the increased input of base cations to forest floor and increased CEC of soil organic C, played a more important role in the decreasing acidity of the PL soils than the continuing decrease in acidic deposition.

A century of change: Reconstructing the biogeochemical history of Hubbard Brook

AbstractEcosystems constantly adjust to altered biogeochemical inputs, changes in vegetation and climate, and previous physical disturbances. Such disturbances create overlapping ‘biogeochemical legacies’ affecting modern nutrient mass balances. To understand how ‘legacies’ affected watershed‐ecosystem (WEC) biogeochemistry during five decades of studies within the Hubbard Brook Experimental Forest (HBEF), we extended biogeochemical trends and hydrologic fluxes back to 1900 to provide an historical framework for our long‐term studies. This reconstruction showed acid rain peaking at HBEF in the late 1960s‐early 1970s near the beginning of the Hubbard Brook Ecosystem Study (HBES). The long‐term, parabolic arc in acid inputs to HBEF generated a corresponding arc in the ionic strength of stream water, with acid inputs generating increased losses of H+ and soil base cations between 1963 and 1969 and then decreased losses after 1970. Nitrate release after disturbance is coupled with previous N‐deposition and storage, biological uptake, and hydrology. Sulfur was stored in soils from decades of acid deposition but is now nearly depleted. Total exports of base cations from the soil exchange pool represent one of the largest disturbances to forest and associated aquatic ecosystems at the HBEF since the Pleistocene glaciation. Because precipitation inputs of base cations currently are extremely small, such losses can only be replaced through the slow process of mineral weathering. Thus, the chemistry of stream water is extremely dilute and likely to become even more dilute than pre‐Industrial Revolution estimates. The importance of calculating chemical fluxes is clearly demonstrated in reconstruction of acid rain impacts during the pre‐measurement period. The aggregate impact of acid rain on WEC exports is far larger than historical forest harvest effects, and even larger than the most severe deforestation experiment (Watershed 2) at HBEF. A century of acid rain had a calcium stripping impact equivalent to two W2 experiments involving complete deforestation and herbicide applications.

Effects of Moderate Nitrate and Low Sulphate Depositions on the Status of Soil Base Cation Pools and Recent Mineral Soil Acidification at Forest Conversion Sites with European Beech (“Green Eyes”) Embedded in Norway Spruce and Scots Pine Stands

High N depositions of past decades brought changes to European forests including impacts on forest soil nutrition status. However, the ecosystem responses to declining atmospheric N inputs or moderate N depositions attracted only less attention so far. Our study investigated macronutrient (N, S, Ca2+, Mg2+, K+) pools and fluxes at forest conversion sites over 80 years old in Central Germany with European beech (so-called “Green Eyes” (GE)). The GE are embedded in large spruce and pine stands (coniferous stands: CS) and all investigated forest stands were exposed to moderate N deposition rates (6.8 ± 0.9 kg ha−1 yr−1) and acidic soil conditions (pHH2O &lt; 4.7). Since the understanding of forest soil chemical and macronutrient status is essential for the evaluation of forest conversion approaches, we linked patterns in water-bound nutrient fluxes (2001–2018) and in predicted macronutrient storage in the herbaceous and tree layer to patterns in litter fall (2016–2017) and in forest floor and mineral soil macronutrient stocks at GE and CS assessed in 2018. Our results exhibited 43% (Nt) and 21% (S) higher annual throughfall fluxes at CS than at GE. Seepage water at 100 cm mineral soil depth (2001–2018) of CS is characterized by up to fivefold higher NO3− (GE: 2 ± 0.7 µmolc L−1; CS: 9 ± 1.4 µmolc L−1) and sevenfold higher SO42− (GE: 492 ± 220 µmolc L−1; CS: 3672 ± 2613 µmolc L−1) concentrations. High base cation (∑ Ca2+, Mg2+, K+) concentrations in CS mineral soil seepage water (100 cm depth: 2224 ± 1297 µmolc L−1) show significant positive correlations with SO42−. Tree uptake of base cations at GE is associated especially with a Ca2+ depletion from deeper mineral soil. Foliar litter fall turns out to be the main pathway for litter base cation return to the topsoil at GE (&gt;59%) and CS (&gt;66%). The litter fall base cation return at GE (59 ± 6 kg ha−1 yr−1) is almost twice as large as the base cation deposition (30 ± 5 kg ha−1 yr−1) via throughfall and stemflow. At CS, base cation inputs to the topsoil via litter fall and depositions are at the same magnitude (24 ± 4 kg ha−1 yr−1). Macronutrient turnover is higher at GE and decomposition processes are hampered at CS maybe through higher N inputs. Due to its little biomass and only small coverage, the herbaceous layer at GE and CS do not exert a strong influence on macronutrient storage. Changes in soil base cation pools are tree species-, depth- and might be time-dependent, with recently growing forest floor stocks. An ongoing mineral soil acidification seems to be related to decreasing mineral soil base cation stocks (through NO3− and especially SO42− leaching as well as through tree uptake).

The Mineral Fertilizer-Dependent Chemical Parameters of Soil Acidification under Field Conditions

Soil acidification in agroecosystems is a natural process that could be accelerated, mainly by the inappropriate application of mineral fertilizers, or prevented, by sustainable management practices. On the basis of a three-year field study in a grassland agroecosystem, the impact of different rates of fertilization with nitrogen (N), phosphorus (P), and potassium (K) on soil chemical parameters related to soil acidity was evaluated. It was found that high-rate fertilization with ammonium nitrate accelerated the soil acidification process, which was additionally intensified by the application of superphosphate and potassium salt. The sum of exchangeable base cations, the values of base saturation and hydrolytic acidity in the soil reflected the interactions between the applied NPK-fertilizer levels. Considering chemical parameters related to soil acidity studied in this experiment, it seems that the best strategies for mitigating soil acidification in grasslands are reducing nitrate leaching, changing fertilizer types and increasing the input of base cations.

The effect of canopy exchange on input of base cations in a subalpine spruce plantation during the growth season

Canopy exchange is one of the most important processes involved in the internal transfer of elements in forest ecosystems. However, little information is available on how canopy exchange influences the input of base cations in subalpine forests. Therefore, the concentrations and fluxes of base cations in throughfall and stemflow were investigated from August 2015 to July 2016 (except for the frozen season) in a representative subalpine spruce plantation in the eastern Tibet Plateau. Our results showed that the mean concentrations of K, Ca, Na and Mg were higher in the stemflow than in the throughfall and precipitation. The total input fluxes of K, Ca, Na and Mg in the internal forest were lower than those in the non-forest. Moreover, the results from the canopy budget model indicated that the canopy exchange fluxes of K, Ca and Mg were higher than the dry deposition fluxes, and Ca and Mg were uptaken, whereas K was leached when precipitation passed through the canopy. Therefore, the results suggested that the input of base cations is mainly controlled by canopy exchange during precipitation in subalpine forest ecosystems, and the canopy could alter the sinks and sources of base cations from precipitation.

Model-Based Analysis of the Long-Term Effects of Fertilization Management on Cropland Soil Acidification.

Agricultural soil acidification in China is known to be caused by the over-application of nitrogen (N) fertilizers, but the long-term impacts of different fertilization practices on intensive cropland soil acidification are largely unknown. Here, we further developed the soil acidification model VSD+ for intensive agricultural systems and validated it against observed data from three long-term fertilization experiments in China. The model simulated well the changes in soil pH and base saturation over the last 20 years. The validated model was adopted to quantify the contribution of N and base cation (BC) fluxes to soil acidification. The net NO3- leaching and NO4+input accounted for 80% of the proton production under N application, whereas one-third of acid was produced by BC uptake when N was not applied. The simulated long-term (1990-2050) effects of different fertilizations on soil acidification showed that balanced N application combined with manure application avoids reduction of both soil pH and base saturation, while application of calcium nitrate and liming increases these two soil properties. Reducing NH4+ input and NO3- leaching by optimizing N management and increasing BC inputs by manure application thus already seem to be effective approaches to mitigating soil acidification in intensive cropland systems.

Cloud water interception and element deposition differ largely between Norway spruce stands along an elevation transect in Harz Mountains, Germany

AbstractCloud deposition can contribute a significant amount of water and elements to montane forests and thus play an important role in the hydrological and biochemical cycles of these ecosystems. However, elevational pattern in cloud and throughfall input to forest ecosystems and the associated chemical element fluxes are poorly understood so far. We studied gross precipitation, throughfall and cloud water deposition, and the associated fluxes of elements (N, P, Ca, K, Mg and Na) in five mature stands of Norway spruce along an elevation transect from lowlands to the alpine treeline (420–1060 m) at Mt Brocken (Harz Mountains, Germany). We tested the hypotheses that water input increases with elevation despite a reduction in intercepting plant surfaces, leading to higher inputs of nitrogen and other elements to high‐elevation forests than in lower‐elevation stands. Gross precipitation (only snow‐free period) rose by 75% towards the treeline, but water input with throughfall increased by 350% because of a large increase in cloud water deposition (from 2 mm at 420 m to 160–200 mm at 1000 m in the study period). Element fluxes with gross precipitation were not uniformly related to elevation across all six elements. However, element fluxes via throughfall showed a positive correlation with elevation for all elements. Thus, the upper montane spruce forests received ~50–150% higher atmospheric inputs of base cations and inorganic N and ~200% higher P inputs than lower‐elevation forests. We conclude that elevation does have a large influence on the atmospheric water and element inputs in temperate mountain forests. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Modelling boreal lake catchment response to anthropogenic acid deposition

A dynamic hydrogeochemical model of water acidification (MAGIC: Model of Acidification of Groundwater in Catchments) was applied to two catchments with contrasting hydrological influences in the Athabasca Oil Sands Region of Alberta to predict catchment response to elevated levels of acidic deposition. Key processes that determine catchment response to atmospheric deposition, including groundwater base cation inputs and retention of sulphur (S) in peatland complexes were parameterized in the model. Although deposition of S and nitrogen (N) in the region has increased over the last 40 years, levels are low at the study sites relative to impacted areas of eastern North America. Model forecasts for the period 2005–2100 were run under constant 2005 deposition levels (base case) and at acid deposition double this level. Simulated past and future soil base saturation was constant over the course of the 200 year (1900–2100) modelled period. At the lake with high charge balance acid neutralizing capacity (ANCCB), where large base cation sources dominate lake chemistry, little change in surface water chemistry was predicted under either forecast scenario. Under the double acid forecast scenario at the low ANCCB lake, simulated lake ANCCB decreased in response to elevated S deposition, but the magnitude of decrease was comparable to the range in observational data. The simulations suggest limited risk of acidification, primarily due to S retention in the catchments, but the potential for drought-induced episodic depression of ANCCB may be important on this landscape.

Uncertainties in critical loads and target loads of sulphur and nitrogen for European forests: Analysis and quantification

An analysis of the uncertainties in critical loads and target loads of sulphur (S) and nitrogen (N) for 182 European forest soils was carried out using the Very Simple Dynamic (VSD) model. The VSD model was calibrated with a Bayesian approach using prior probability functions for model parameters based on literature data, data from 200 Dutch forest sites and from simulated denitrification rates from a detailed ecosystem model. The calibration strongly improved the fit of the model to observed soil and soil solution concentrations, especially for pH and base saturation. Calibration also narrowed down the ranges in input parameters. The uncertainty analysis showed which parameters contribute most to the uncertainty in the critical loads and target loads. Base cation weathering and deposition and the parameters describing the H–Al equilibrium in the soil solution determine the uncertainty in the maximum critical loads for S, CLmax(S), when based on the aluminium to base cation (Al/Bc) criterion. Uncertainty in CLmax(S) based on an acid neutralizing capacity (ANC) criterion is completely determined by base cation inputs alone. The denitrification fraction is the most important source of uncertainty for the maximum critical loads of N, CLmax(N). N uptake and N immobilisation determine the uncertainties in the critical load for N as a nutrient, CLnut(N). Calibration of VSD reduced the uncertainty: the coefficient of variation (CV) was reduced for all critical loads and criteria. After calibration, the CV for CLmax(S) was below 0.4 for almost all plots; however for CLmax(N) high values occurred for plots with high denitrification rates. Model calibration also improved the robustness of target load estimates: after calibration, no target loads were needed in any of the simulations for 40% of the plots, with the uncalibrated model there was a positive probability for the need of a target load for almost all plots.

Alkalinity and acidity cycling and fluxes in an intermediate fen peatland in northern Ontario

Peatlands are important to global carbon (C) sequestration and surface water acid–base status, both of which are affected by peatland alkalinity and acidity cycling. Relationships among sulfate (SO4 2−), nitrate (NO3 −), organic acids (OA−), base cations (i.e., Ca2+, Mg2+, K+, and Na+), proton (H+) acidity, and bicarbonate (HCO3 −) alkalinity were investigated in an intermediate fen peatland in northern Ontario during 2004 (an average precipitation year) and 2005 (a dry summer). Potential evapotranspiration was higher and the water table, groundwater input from the uplands, and runoff were lower during 2005. Net inputs of base cations, HCO3 −, SO4 2−, and OA−, and to a lesser degree NO3 −, were lower during the drier year, mainly due to lower groundwater transfer to the fen. Fen porewater HCO3 − concentration and net output were also lower in the drier year, whereas Ca2+, Mg2+, and SO4 2− concentrations and net output were higher. During the climatically average year, N immobilization, carbonic acid (H2CO3) dissociation, and OA dissociation were equally important H+-producing reactions. Peat cation exchange accounted for 50% of the H+ sink, while SO4 2− reduction and denitrification accounted for an additional 20 and 25% of the H+ sink, respectively. During the dry year, S oxidation accounted for 55% of the H+ net production, while that for H2CO3 dissociation was 70% lower than that during the climatically average year. Peat cation exchange consumed three times the acidity, and accounted for 92% of the H+ consumption during the dry year compared to the climatically average year. This was consistent with a three-fold higher net base cation export from the fen during the dry year. Based on the study results, a conceptual model was developed that describes the role of acidity formation and its implications to intermediate fen acidification.

Holocene Environmental and Climate History of Trettetjørn, a Low-alpine Lake in Western Norway, Based on Subfossil Pollen, Diatoms, Oribatid Mites, and Plant Macrofossils

ABSTRACTHolocene lake and catchment environmental history and regional climate are reconstructed from lake sediments at Trettetjørn, a small lake situated close to the present-day treeline in western Norway. Sediments began to accumulate in the lake ca. 8575 (± 115) cal yr BP. Pollen-inferred mean July temperatures (Tjul) fluctuated below 12°C with two cooler phases ca. 8400 and 8200 cal yr BP. The pollen-inferred annual precipitation (Pann) was lowest during the early Holocene and varied around 1600 mm yr−1. At the same time the highest diatom-inferred pH values (6.8) were reconstructed, probably due to input of base cations from the immature catchment soils. Betula pubescens became established around the lake ca. 8270 cal BP, soon followed by Pinus sylvestris. Maximum Tjul of ca. 12.5°C occurred from 7760 to 5200 cal yr BP. The oribatid mite assemblages confirm the vegetation development from semiopen grassland to forest. Inferred Tjul was variable after 5200 cal yr BP and declined markedly around 4175 cal yr BP. At the same time more oceanic conditions are inferred with changes in the vegetation and mite assemblages suggesting the expansion of mires. After ca. 5000 cal yr BP and towards the present, the diatom concentration in the core becomes low and variable, thickness of the valves within the same taxon varies, and sometimes diatom frustules are completely absent from the sediment column. The diatom valve dissolution has affected both the diatom assemblages and the diatom-inferred pH up to the present-day. All proxies suggest that human impact affected the catchment after ca. 2000 cal yr BP followed by more intensive impact after ca. 1555 cal yr BP. The enlargement of the settlement at Upsete during the construction of the Bergen–Oslo railway (1894–1909 AD) is reflected by a charcoal peak and the presence of spheroidal carbonaceous particles coincides with a change in the diatom assemblages.

Dry deposition of particles and canopy exchange: Comparison of wet, bulk and throughfall deposition at five forest sites in Italy

The contribution of dry deposition to the total atmospheric input of acidifying compounds and base cations is of overwhelming importance. Throughfall measurements provide an estimate of the total deposition to forest soils, including dry deposition, but some uncertainties, related to the canopy interaction processes, affect this approach. We compared the concentrations and the fluxes of the main ions determined in wet-only, bulk and throughfall samples collected at five forest sites in Italy. The contribution of coarse particles deposited onto the bulk samplers was of prime importance for base cations, representing on average from 16% to 46% of the bulk deposition. The extent of this dry deposition depended on some geographical features of the sites, such as the distance from the sea and the annual rainfall. The possibility of applying specific bulk/wet ratios to estimate the wet deposition proved to be limited by the temporal variability of these ratios, which must be considered together with the spatial variability. A direct comparison of the dry contribution deriving from the bulk–wet and the throughfall–wet demonstrated that an extensive natural surface (forest canopy) performs better than a small synthetic surface (funnel of the bulk sampler) in collecting dry deposition of SO 4 2−, NO 3 − and Na +. The canopy exchange model was applied to both bulk and wet data to estimate the contribution of dry deposition to the total input of base cations, and the uncertainty associated to the model discussed. The exclusive use of bulk data led to a considerable underestimation of base cation dry deposition, which varies among the study sites.

Modelling the effects of climate change on an acidic upland stream

Most modelling studies of soil and surface water recovery from acidification assume a constant influence of climate over the simulation period. Given the likelihood of future climate change, and recent identification of links between climatic fluctuations and surface water trends on decadal time scales, an attempt is made to simulate a number of climate-related effects on the recovery of an acidified stream in mid-Wales. Empirical relationships were derived to model past and future variations in (i) runoff, as a function of rainfall and temperature; (ii) stream chloride concentration and sea-salt deposition, as a function of the North Atlantic Oscillation Index (NAOI); and (iii) stream dissolved organic carbon (DOC) concentration, as a function of summer temperatures and sulphur deposition. Incorporating these relationships in the MAGIC dynamic model reproduced a significant part of the observed variation in streamwater chemistry. Current scenarios of rising UK temperature and NAOI, and falling rainfall and acid deposition, were used to forecast effects of climate change on soil and water recovery from acidification. Results suggest that increasing DOC would have a significant negative impact on soil recovery, with elevated organic acidity lowering soil water pH and severely depleting soil base saturation. The resulting increase in soil base cation export would, however, have a positive impact on streamwater recovery. For increased sea-salt deposition, effects are essentially the opposite; increased base cation inputs enhance the recovery of soil base saturation, but displace acidity to the stream. However, simulated effects of raising DOC considerably outweighed those of increased sea-salt deposition. It is argued that greater consideration of these, and other, climatic effects on biogeochemical cycles needs to be taken when predicting the recovery of soils and surface waters from acidification.

Assessment of the spatial heterogeneity of weathering rates in upland catchments using the sodium dominance index and its significance in integrated catchment management

The sodium dominance index was developed to quantify weathering rates and critical loads in Scotland, where atmospheric aerosols of maritime origin dominate over biogeochemical weathering in providing base cation inputs to catchment soils and drainage waters. High sodium dominance in river or lake water indicates low weathering rate. Here, this concept is evaluated using intensive temporal and spatial sampling strategies in two substantial catchments, one in Scotland and the other in central England, with particular reference to detection of groundwater inputs, and to possible problems from road salting in the calibration. In the Dee network, the spatial distribution of sodium dominance reflects the distribution of soil parent material geology, but land use also influences the equations. It is postulated that road density, via winter road salting, influences the sodium dominance calibration in lowland agricultural areas. Although road salting can also be problematic in some upland areas, the index still can provide clear indication of the likely severity of acid flush events in remote upland streams. In the Etherow catchment, sodium dominance varies markedly, sometimes over relatively small distances, reflecting soil type distribution, the occurrence of ground-water inputs to streams, and the influence of water in tributaries above the sampling point.

The application of an integrated biogeochemical model (PnET‐BGC) to five forested watersheds in the Adirondack and Catskill regions of New York

AbstractPnET‐BGC is an integrated biogeochemical model formulated to simulate the response of soil and surface waters in northern forest ecosystems to changes in atmospheric deposition and land disturbances. In this study, the model was applied to five intensive study sites in the Adirondack and Catskill regions of New York. Four were in the Adirondacks: Constable Pond, an acid‐sensitive watershed; Arbutus Pond, a relatively insensitive watershed; West Pond, an acid‐sensitive watershed with extensive wetland coverage; and Willy's Pond, an acid‐sensitive watershed with a mature forest. The fifth was Catskills: Biscuit Brook, an acid‐sensitive watershed. Results indicated model‐simulated surface water chemistry generally agreed with the measured data at all five sites. Model‐simulated internal fluxes of major elements at the Arbutus watershed compared well with previously published measured values. In addition, based on the simulated fluxes, element and acid neutralizing capacity (ANC) budgets were developed for each site. Sulphur budgets at each site indicated little retention of inputs of sulphur. The sites also showed considerable variability in retention of NO3−. Land‐disturbance history and in‐lake processes were found to be important in regulating the output of NO3− via surface waters. Deposition inputs of base cations were generally similar at these sites. Various rates of base cation outputs reflected differences in rates of base cation supply at these sites. Atmospheric deposition was found to be the largest source of acidity, and cation exchange, mineral weathering and in‐lake processes served as sources of ANC. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Continuous cover forestry: possible implications for surface water acidification in the UK uplands

Abstract. The effects of widespread conifer afforestation on the acidity of lakes and streams in the acid sensitive uplands of the UK has been researched extensively and has contributed to the development and implementation of national forest management guidelines (e.g. Forest and Water Guidelines; Forestry Commission, 1993). However, a recent policy document (Woodlands for Wales; National Assembly for Wales, 2000) has proposed a major shift in the management of 50% of the Forestry Commission estate in Wales from the current system of patch clearfelling to Continuous Cover Forestry (CCF). This scale of change is without precedent in the UK; no studies in the UK forest environment have examined the likely environmental impacts of CCF. However, the wealth of environmental data from studies of UK forests managed by patch clearfelling enables an assessment of the impact of a change to CCF on three issues of particular relevance to surface water acidification in the uplands; forest harvesting, soil base cation depletion and atmospheric pollutant deposition. Whilst there is uncertainty as to how even-aged stands will be transformed to CCF in the UK, guiding principles for CCF on acidic and acid sensitive sites should focus on those aspects of management which minimise nitrate leaching, encourage base cation retention within the soil-plant system and enhance base cation inputs from external (atmospheric) and internal sources (weathering). CCF may provide opportunities to achieve this by reducing the scale of clearfelling, increasing species diversity, changing the structure of plantation forests and maintaining uninterrupted woodland cover. Keywords: acidification, forestry, continuous cover forestry, clearfelling

Acidification and recovery of aquatic ecosystems in south central Ontario, Canada: regional application of the MAGIC model

Abstract. The dynamic model MAGIC was applied to 25 lakes in south-central Ontario, Canada using a regional modelling methodology. Soil and lake water chemistry for each lake catchment was simulated for the period 1850–2050. Sulphate (SO42–) deposition forecasts were derived from recently proposed emission reductions, which correspond roughly to a 50% reduction in SO42– deposition by 2010 from the 2000 baseline. Changes in SO42– deposition had a significant impact on lake chemistry. Simulated lake water chemistry showed a recovery potential under the current deposition scenario; by 2050 concentration levels recovered to values predicted for the early 1900s. Moreover, simulated future lake water chemistry showed significant recovery compared to 1975 levels. However, although regional simulations predict that base cation losses have decreased in recent years, soils in the region will continue to acidify with Ca2+ losses dominating depletion of the exchangeable pool. Base cation losses from the exchangeable pool are currently buffering lakes against the impacts of acid deposition; ultimately base cation inputs into the lakes will decrease as exchangeable base cation pools become depleted. Further emission reductions are necessary to ensure continued recovery from acidification. Keywords: regional dynamic model, sulphate, acid neutralising capacity, critical loads, lake chemistry, soil base saturation

Weathering and atmospheric deposition signatures of base cations in upland soils of NE Scotland: their application to critical load assessment

Although the importance of atmospheric base cation deposition to soils is increasingly acknowledged, field data demonstrating the signatures of atmospheric and weathering-derived cations are seldom presented. This paper explores how combined atmospheric and internal base cation inputs are reflected in exchangeable and soil solution base cation compositions of upland soils, ranging from acidic to basic parent materials. Although total atmospheric base cation deposition exceeded weathering inputs for most soils, relative proportions of exchangeable base cations were controlled by parent materials. Calcium and magnesium concentrations were most closely related to geology, but were likely affected by the affinity of humic material for divalent cations, which resulted in elevated concentrations in surface horizons. In contrast, soil solution compositions deviated little from those of precipitation, showing minimal geochemical influence. A soil-based weathering rate index utilizing the dominance of the base cation Na was explored with the goal of enhancing spatial resolution in critical load assessment. However, the calibration of the relative proportions of exchange site Na against weathering rates, determined by three conventional critical load calculation methods, was inconclusive. Although power relationships were suggested, these were poor and largely insignificant, with a considerable range of %Na values at low weathering rates. It was not resolved whether the data available was insufficient to obtain accurate weathering rates, or that the base cation chemistry was affected by factors other than weathering alone.

A review of air pollution and atmospheric deposition dynamics in southern Saxony, Germany, Central Europe

Abstract This is the first comprehensive compilation of air pollution history in the Erzgebirge region. It presents selected data sets from more than 20 years of continuous research, chosen after rigorous quality control. Gases and particulates, and wet deposition are discussed in sequence. The gases include SO 2 , NO x , O 3 , and fluorine compounds. SO 2 -concentrations declined from about 80 to 120 μg m 3 until 1990 to 5–10 μg SO 2 m 3 air today, while NO x shows little change, and O 3 steadily increases as of 1990. Fluoride deposition decreased with SO 2 . Particle deposition is differentiated by sampling methods and grain sizes, and their chemical composition. The total amount of aerosols has decreased in the past 12 years, and many trace constituents now show 10% of their previous concentrations. Precipitation is represented by wet only, throughfall, fog and cloud water deposition, including major and minor chemical compounds. As with the gases, S-compounds decreased considerably. While NO 3 -N-concentrations show a slight decline (from 13 to 10–11 kg ha −1 a −1 in throughfall deposition); no trend is visible for NH 4 -N. As of the mid-1990s, continuously lower base cation inputs are being measured, and pH-values are on steady increase (now 4.8). The final synopsis rounds up the experience gained from those valuable data sets and can be used for many regions worldwide.

ACCUMULATION AND DEPLETION OF BASE CATIONS IN FOREST FLOORS IN THE NORTHEASTERN UNITED STATES

Loss of base cations from forest soils can be accelerated by acid rain, by forest regrowth following harvest removals, and by declining inputs of base cations from atmospheric deposition. Calcium losses from forest floors have been reported at several sites in the northeastern United States. To test for loss of base cations from forest floors at the Hubbard Brook Experimental Forest in New Hampshire (USA), we analyzed samples collected on seven dates between 1976 and 1997. Calcium and magnesium contents of the forest floor did not decline significantly; a change >0.9%/yr would have been detectable. Concentrations of Ca were 40% higher in 1969-1970 than in the current study, but the difference is partly due to changes in collection methods. Magnesium concentrations were too variable to detect a loss of <47% over the 21-yr interval. To determine whether base- cation losses were associated with forest growth, we resampled a chronosequence of north- ern hardwood stands in the White Mountains of New Hampshire. The 13 stands did not show consistent changes in Ca, Mg, and potassium over the 15-yr interval. Losses of these cations were most pronounced in stands logged more than 25 yr earlier. Younger stands, contrary to our expectation that rapid forest growth should cause cation depletion, all gained base cations in the forest floor. Early in stand development these forest floors appeared to accumulate biomass along with living vegetation, rather than serving as a net source of nutrients. Finally, in a regional survey of 28 mature stands in the northeastern United States, some lost significant forest-floor Ca and Mg between 1980 and 1990, while others gained. The average change in Ca and Mg content was not significant; a loss of 1.4%/yr would have been detectable. Forest floors in the region are not currently experiencing rapid losses of base cations, though losses may have preceeded the onset of these three studies.