Exchangeable Cation Pools Research Articles

Carbon sequestration and nitrogen loss drive the evolution of French forest soils

The temporal change of soil chemistry in the forest floor and mineral soil down to a depth of 40 cm was assessed for the 102 permanent plots of the French Network for the Monitoring of Forest Ecosystems (RENECOFOR), over a 15-year period (from 1993–1995 to 2007–2012). In examining the separate and joint evolutions of a large set of parameters, many significant changes were detected reflecting the fact that French forest soils were not in a steady state. A significant increase in soil organic carbon (SOC) stocks was found, mainly in the surface soil (13.0% increase over the forest floor and the 0–10 cm layer). Conversely, the relative increase of the total nitrogen (Ntot) stocks was lower in the surface soil (4.8% increase), and a general and sharp decline of Ntot was detected between 10 and 40 cm depth (12.0% decrease). These results led to a substantial raise of C/N ratio over the whole soil profile. Another major finding is the difference in soil acidification recovery depending on the initial trophic level. In highly acidified contexts (top soil pH H2O < 4.5), increased soil acidification (pH and base saturation decrease, exchangeable Al increase) over the profile was observed while exchangeable base cation (Ca, Mg, K) pools increased. On the other hand, less acidic soils saw their global buffer capacity enhanced. These observations contrast with what is measured in other European inventories. While a previous study carried out on the same plots and over the same period highlighted SOC as a major driver of soil evolution in the top mineral soil, the possible mechanisms behind the large N decrease in the lower mineral soil remain to be confirmed.

Soil acidification and loss of base cations in a subtropical agricultural watershed

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H+) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha−1 yr−1). H+ production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K+, Ca2+, Na+, Mg2+, Al3+, NH4+, NO3−, SO42−, Cl−, and H+). The result showed that total H+ production in the watershed was 5152 molc ha−1 yr−1. N transformation was the most important H+ source (68%), followed by excess plant uptake of cations (25%) and H+ deposition (7%). Base cations exchange and weathering of minerals (3842 molc ha−1 yr−1) dominated H+ neutralization, followed by SO42− adsorption (1081 molc ha−1 yr−1), while H+ and Al3+ leaching amounted to 431 molc ha−1 yr−1, only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H+ addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230–1080 molc ha−1 yr−1). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification.

Interaction of land use history, earthworms, soil chemistry and tree species on soil carbon distribution in managed forests in Vermont, USA

Forest soils store a globally important pool of carbon (C) and reforestation has the potential to increase this pool. Past land use, tree species composition, and current management are known to affect carbon storage rates and amounts. Another important factor, especially in the vertical distribution of soil C, is the presence or absence of earthworms. We investigated these interactions in eighteen managed forest sites in Vermont that have varied land use history (cultivation, pasture, woodlot, continuous woodland) and a range of time since reforestation from 60 to 100+ years. Soils were sampled to about 1 m, or shallower if bedrock was reached, and both C and exchangeable cation pools quantified. Earthworms were enumerated to a depth of 20 cm and identified by species. Basal area by tree species was also measured and aboveground C calculated. Detailed land use history was documented using archival sources. Forest type was primarily northern hardwood, with a few lowland spruce-fir sites included. Eleven different species of earthworms were found, representing all ecotypes, and numbers ranged from 0 (at 8 of the 18 sites) to 319 individuals/m2. Carbon pools in the forest floor (1.5–30.1 Mg/ha) were greater with higher site elevation (range of 154–651 masl) and also greater with lower numbers of earthworms. Besides elevation, the best predictor of earthworm presence or absence, and overall numbers, was the soil exchangeable calcium (Ca) pool in the Oa and A horizons (range of 9.4–1288 cmolc/m2). Because soil exchangeable Ca and aluminum (Al) pools were inversely related, higher exchangeable Al was associated with lower earthworm numbers. In the mineral soil, the C pool (82.1–210.8 Mg/ha) was best explained by the thickness of the B horizons with the exchangeable Al pool as a positive, secondary factor. The thickness of the A horizon (0–16.3 cm), and its contribution to the entire profile C pool (0–54.7 Mg/ha), decreased relative to past land use in the order: cultivated < pasture < woodlot. No earthworm metric was strongly related to the C pool in either the mineral soil or the full soil profile. Dramatic differences in the vertical distribution of carbon were related to past land use and earthworm presence, which in turn was negatively correlated with elevation and the presence of conifers. While it is clear that earthworms and prior land-use have a long-lasting effect on the forest floor C pool, it is less clear whether or not there is an effect on full profile C sequestration over time.

Indicator value of trees for soil reaction in cool-humid forests

Acid rain, soil mineral weathering, and the interception of dust and sea salt still obscure the evaluation of tree effects on soil reaction in mature natural forests. The null hypothesis that tree species and size do not differentiate soil pH was tested in four ways: (i) comparison of an evergreen conifer ecosystem with a history of elevated exposure to acidifying emissions vs. a deciduous angiosperm ecosystem in a rural area; (ii) soil trenching to exclude mineral nutrient and water uptake by trees in the two ecosystems; (iii) comparisons of soil pH and the accumulation of decomposed organic material on the soil surface (Oh) beneath boreal angiosperms, boreal conifers, and temperate angiosperms at the landscape level; and, (iv) stand-level comparisons of large trees and small trees as a proxy for cumulative ecosystem effects. Results show for mature mixed forests on acid soils below pH 5 that tree species and size do not differentiate soil pH although varying tree species composition among forest stands differentiates the magnitude of acid rain impacts and indicates the disturbance history of forest vegetation at the landscape level. Low exchangeable base cation pools compared to annual turnover in foliage corroborates the notion of homoeostasis in mixed and structurally diverse natural forests. Natural conifer admixture in northern hardwoods moderates spring soil warming and thereby slows down element cycling. However, conifer admixture does not exacerbate biogenic soil acidification. Significantly reduced soil pH beneath ruderal angiosperms at the landscape level suggests a high vulnerability of cool-humid forests on acid soils to biogenic acidification caused by the destruction of the forest canopy.

Long-term changes of ecosystem services at Solling, Germany: Recovery from acidification, but increasing nitrogen saturation?

Time series of values of ingenious parameters indicating ecosystem services from European beech and Norway spruce ecosystems at Solling, Germany, were evaluated with respect to resilient or adaptive behaviour. Studied indicators comprise the use of monitoring data with up to more than 40 years of observation on deposition of potential acidity, sulphate (SO42−) budgets, exchangeable base cation pools, Bc/Al ratio in soil solution, nitrogen (N) budgets, foliar nutrition as indicated by the foliar Bc/N ratio, and defoliation. Deposition of potential acidity decreased considerably at both ecosystems. SO42− budgets reveal retention of sulphur in the soils affecting acid/base budgets. Exchangeable base cation pools decreased at both ecosystems by about 60%. Bc/Al ratio in soil solution in the mineral soil was mostly below critical limits indicating potential toxic stress to tree roots. N retention in the soils decreased from about 40kgha−1yr−1 in the 1970s to currently very low rates of 0–20kgha−1yr−1 indicating increasing N saturation. Foliar Bc/N ratio decreased at the spruce ecosystem indicating possible nutrient imbalances. Defoliation at both Solling ecosystems is on a high level compared to other forests in Germany, but reveals no distinct relation to soil acidification or N saturation. From the selected indicators, SO42− and N budgets reveal resilient behaviour, whereas indicators related to the acid/base status tend to adaptive behaviour.

Estimating weathering rates using base cation budgets in a Norway spruce stand on podzolised soil: Analysis of fluxes and uncertainties

Since forestry is often allocated to soils with a low weathering capacity, reliable estimates of weathering rates are crucial in analyses of sustainability, e.g. of whole-tree and stump harvesting. In the present study, weathering rates (kgha−1yr−1) for base cations were estimated using cation budgets in a replicated (n=4) experimental Norway spruce (Picea abies (L.) Karst.) plantation situated on a nutrient-poor glacial till in south-west Sweden and aged 25–39years during the study period. Weathering rates (central values) were 2.4, 1.4, 0.3 and 2.3kgha−1yr−1 for Ca, Mg, K and Na, respectively. However, weathering was a minor flux in the overall cycling of these cations in the ecosystem, and the confidence intervals of the weathering estimates had amplitudes that generally were greater than the central values. The overall uncertainties were divided into (i) regular standard errors of the mean, expressing spatial variability, sampling errors and random method-related errors in data from measurements replicated over the experimental plots (‘Type A’ uncertainties), and (ii) estimated standard uncertainties accounting for systematic errors of methods, and of uncertainties in variables, functions and factors not replicated over the plots (‘Type B’ uncertainties). For Ca and K, bioaccumulation dominated the overall uncertainty. Most (>90 %) of this uncertainty, in turn, was of Type A (between-plot variability in measured stem diameters and cation concentrations); the remainder resulted from Type B uncertainties in allometric functions etc. Hence, regular standard errors over plots yielded a correct level of uncertainty of weathering estimates for these ions at the studied site. For Mg and Na, however, deposition and leaching were large terms in the cation budget. Whereas the uncertainty in deposition was mostly taken into account by plot-wise replicated measurements of throughfall (Type A uncertainty), Type B uncertainties were crucial to the estimates of leaching. Due to the fact that uncertainties accumulate when terms are added and subtracted in a cation budget, it is difficult to predict the sustainability of the pools of exchangeable cations from estimated weathering rates; it may be better to measure them directly in the soil. However, in the studied ecosystem these had a rapid turnover (mean residence time, 1–4years), and underwent abrupt fluctuations over only a few years. A study performed during a limited time may therefore suffer from considerable temporal uncertainties, if the results are to be generalised for a longer period.

Forest restoration with Betula ssp. and Populus ssp. nurse crops increases productivity and soil fertility

The rapid re-establishment of forests following large disturbances is being seen as one option to increase the contribution of forests to climate change mitigation. The temporary inclusion of pioneer trees as nurse crops on disturbed sites can facilitate the establishment of target tree species and may additionally benefit productivity and soil fertility. In this study we compared productivity and nutrient cycling between stands of oak target species (Quercus robur and Quercus petraea) that were established with and without widely spaced Betula ssp. or Populus ssp. nurse crops. Simulation results for a full rotation of oaks (180years) indicated that both types of forests, with and without nurse crops, have a comparable total productivity. However, stands with nurse crops supplied 59–96Mgha−1 harvestable biomass after 20years, whereas the first harvest of biomass from stands without nurse crops would occur at least 30years later. Nutrient element costs associated with the removal of Betula ssp. wood were low compared to Populus ssp. Also, nurse crop stands had up to 2.5 times larger pools of exchangeable base cations in top mineral soils (0–30cm) compared to mono-specific oak stands. The high soil cation pools may have resulted from reduced leaching under nurse crops or the increased recycling of cations, also from deeper soil depth, via litter fall and fine-root turnover. Our results show that forest reestablishment with pioneer tree species may be a suitable tool for the rapid recovery of forest productivity and mitigation potential following disturbances while simultaneously helping to maintain or increase soil fertility.

Influence of afforestation on soil: The case of mineral weathering

Although concerns have been raised that increased nutrient demand by fast growing tree species could deplete soil nutrient pools, recent research suggests that some species are able to obtain nutrients via soil mineral weathering. Hybrid poplars, which are fast growing and nutrient demanding species, are increasingly used in intensive silvicultural settings. Understanding whether hybrid poplars have an effect on long term nutrient availability and can promote soil mineral weathering is therefore important. We investigated the levels of base cations (i.e. K, Ca, Mg, and Na) of surface soils (0–20cm) in 13 hybrid poplar plantations in Quebec, and compared the results with those of adjacent abandoned agricultural fields. To evaluate whether exchangeable base cation pools and non-exchangeable pools (i.e. those in the crystal lattice of minerals) were being depleted, we used a sequential leach with diluted salt (BaCl2 for exchangeable) and weak acid solutions (HCl and HNO3 for non-exchangeable). Levels of exchangeable and non-exchangeable cations were not statistically different between land use types. Exploratory analyses, however, revealed trends toward a greater depletion of Ca, Mg and Na in non-exchangeable forms following afforestation. The depletion of these non-exchangeable base cations due to afforestation occurred at sites where greater levels were initially present in soil. The results suggest increased soil mineral weathering due to greater amounts of minerals susceptible to dissolution and, in part, high clay content. Based on Ca, Mg and K concentrations of the different leaches and their molar ratios (Ca/ΣAl+Fe, Mg/ΣAl+Fe and K/ΣAl+Fe), we propose a lesser role of soil mineral weathering on Ca cycling than Mg and K, which could lead to faster depletion of exchangeable Ca pools of the surface soil due to fast growth and high Ca demand by the poplars.

Assessing Mg and Ca depletion from broadleaf forest soils and potential causes – A case study in the Morvan Mountains

Compared to conifers, broadleaf forests are less prone to soil and water acidification, because (1) they often have soils with larger exchangeable base cation pools, (2) the pollution scavenging capacity of deciduous trees is lower, and (3) they grow more slowly and are often less intensively managed. Since the 1980s, atmospheric deposition acidity has strongly decreased in forest ecosystems, so that the present acidification status of broadleaf forests should be improving. We used a 35year-old beech plot in the Morvan Mountains (Burgundy, France) to question past and present acidification processes in broadleaf ecosystems.Soil exchangeable Mg, Ca and K pools measured from samples collected in 1974 and 2001 were compared and input–output budgets were computed over the 2003–2008 period. The objectives were (i) to assess Mg and Ca pool size changes over 1974–2008, (ii) to discuss the potential causes of these changes and, (iii) discuss the limits of conventional methods to study nutrient pool size changes (soil data comparison and nutrient budgets).Soil exchangeable Mg pools decreased during the 1974–2001 period while Ca and K pools remained constant, and very small. Soil solution monitoring and input–output budgets over the 2003–2008 period suggested an ongoing loss of exchangeable Ca and Mg, partly due to the desorption of sulphate from the soil which induced Mg, Ca and K depletion. Given the very low concentration in exchangeable base cations, and assuming no change in soil spatial variability, we computed that resampling soils at 10years intervals may not unequivocally demonstrate a gain/loss of exchangeable base cations. Foliar Mg concentrations were continuously below the deficiency level, K and N concentrations decreased but Ca did not.From this, we discuss the validity/limits of the different approaches used, how the ecosystem can cope with such low levels of nutrients, and the fluxes and processes within the ecosystem that should be investigated in this perspective.

Hydro-Mechanical and Chemical-Mineralogical Analyses of the Bentonite Buffer from A Full-Scale Field Experiment Simulating a High-Level Waste Repository

AbstractThe effect of exposure to repository-like conditions on compacted Wyoming bentonite was determined by comparing the hydraulic, mechanical, and mineralogical properties of samples from the bentonite buffer of the Canister Retrieval Test (CRT) with those of reference material. The CRT, located at the Swedish Äspö Hard Rock Laboratory (HRL), was a full-scale field experiment simulating conditions relevant for the Swedish, so called KBS-3, concept for disposal of high-level radioactive waste in crystalline host rock. The compacted bentonite, surrounding a copper canister equipped with heaters, had been subjected to heating at temperatures up to 95°C and hydration by natural Na-Ca-Cl type groundwater for almost 5 y at the time of retrieval.Under the thermal and hydration gradients that prevailed during the test, sulfate in the bentonite was redistributed and accumulated as anhydrite close to the canister. The major change in the exchangeable cation pool was a loss in Mg in the outer parts of the blocks, suggesting replacement of Mg mainly by Ca along with the hydration with groundwater. Close to the Cu canister, small amounts of Cu were incorporated into the bentonite. A reduction of strain at failure was observed in the innermost part of the bentonite buffer, but no influence was noted on the shear strength. No change in swelling pressure was observed, while a modest decrease in hydraulic conductivity was found for the samples with the highest densities. No coupling was found between these changes in the hydro-mechanical properties and the montmorillonite — the X-ray diffraction characteristics, the cation exchange properties, and the average crystal chemistry of the Na-converted &lt;1 μm fractions provided no evidence of any chemical/structural changes in the montmorillonite after the 5 y hydrothermal test.

Calcium isotope constraints on the uptake and sources of Ca 2+ in a base-poor forest: A new concept of combining stable (δ 44/42Ca) and radiogenic ( εCa) signals

Plant-available reserves of major base cations, Ca 2+ and Mg 2+, decreased markedly in soils over the past century, thus posing a potential threat to forest ecosystem health. Trees are thought to obtain dissolved Ca 2+ ions mainly from an easily accessible soil–water reservoir also termed the ‘exchangeable cation pool’. The status of Ca reserves in this soil pool is sensitive to anthropogenic perturbations such as soil acidification induced by acid rain and/or excessive timber harvesting. Here we show that in a base-poor forest of the northeastern USA (i.e. Wachusett Mountain, Massachusetts) the ‘exchangeable Ca pool’ of deeper mineral soils has a unique isotope signature that is significantly enriched in the radiogenic 40Ca, due to the dissolution of K-rich silicate minerals such as biotite. Using a simple isotope mass balance, and assuming that the input of Ca from biotite has a ε Ca signature of ∼16, the results of our calculation indicate that the weathering of biotite may supply a sizeable fraction, up to 25%, of Ca 2+ ions into the ‘exchangeable cation pool’ of deeper mineral soils. Importantly, samples of local vegetation (i.e. woody tissues of red oak) show no detectable excess of the radiogenic 40Ca, and based on our model the upper limit of a possible biotite-derived Ca contribution in vegetation is estimated at ∼5%. We also found no evidence of the radiogenic 40Ca signal in the samples of forest floor and the uppermost organic-rich soils (0–15 cm depth), which in turn suggest that over the long-term development of the forest and its organic matter accumulation, the vegetation growth must have also relied primarily on the non-radiogenic Ca sources. Based on our experimental data, such sources may include (i) wet atmospheric deposition, (ii) the organically-complexed Ca in topsoil horizons, and (iii) chemical weathering and/or fungal-mediated dissolution of apatite and Ca-rich plagioclase. Hence, our stable and radiogenic Ca isotope data indicate that the studied base-poor forest is able to bypass the ‘exchangeable cation pool’ of deeper (i.e. below 15 cm) mineral soils, and still manages to meet its nutritional requirements with respect to Ca. Another important implication of this study is that the organically-complexed Ca in the topsoil horizon (0–15 cm depth) has to be tightly bound to the ion exchange sites, otherwise the large radiogenic 40Ca signatures present in the ‘exchangeable cation pool’ of deep mineral soils would be swamped by the downward gravitational flux of non-radiogenic Ca from the decaying organic matter and litterfall. Hence, the limited mobility of the organically-complexed Ca in soils and its tight biological cycling could explain the lack of a significant impact of vegetation on the Ca isotope systematics observed in large rivers.

Assessing Decadal Change in Mineral Soil Cation Chemistry at the Turkey Lakes Watershed

Mass balance studies and direct measurement of changes in soil chemistry have provided evidence of depletion of base cations in soils across decadal time scales. Losses of base cations have been attributed to leaching as a consequence of elevated levels of S and N deposition. Chemical properties of mineral soil horizons were determined at seven plots in a tolerant hardwood forest at the Turkey Lakes Watershed in 1986 and in 2003 and 2005. Archived soils from the original sampling were used to assess the impact of storage and changing methodology on results. Exchangeable cation concentrations were compared between the two sampling times, as were concentrations normalized for organic C. The effects of air‐dry storage were significant for some cations, but in absolute terms the changes were small when contrasted with the spatial variation in exchangeable cation concentrations for the samples collected in 2003 and 2005. There were no statistically significant declines in pH, exchangeable Ca, Mg, or K concentrations, or Ca, Mg or K concentrations normalized for organic C concentration during the 17‐ to 19‐yr sampling period across the watershed. Exchangeable Na and normalized Na concentrations decreased in deeper horizons of the soil profile. There were various patterns of cation change at the plot level, but these changes were unable to be attributed to the range of soil profile base status at the site. Mineral weathering inputs appeared to provide stability to the exchangeable base cation pool in spite of large leaching losses at the Turkey Lakes site.

Assessment of the optimal time interval for repeated soil surveys at intensively monitored forest plots

Statistical methods were developed to assess required changes in the contents and pools of major nutrients and exchangeable base cations in the organic layer and the mineral soil of European forest soils, to derive significant differences. Furthermore a simple element retention model is described and applied to assess the variation time periods, as a function of site and soil characteristics and atmospheric inputs, that are needed before repeating soil surveys in order to assess significant differences in element pools. Time periods that are needed to assess a significant difference have been limited to N in the organic layer and base cations in the mineral layer, since those pools are liable to change caused by nitrogen or acid deposition. Results showed that a time interval of 10 years, which is generally considered for a repetition of the soil survey, might give a significant difference in N and exchangeable base cation pools for approximately 25% and 10% of the plots, only.

Long Term Effects of Acid Irrigation at the Höglwald on Seepage Water Chemistry and Nutrient Cycling

In order to test the hypothesis of aluminium toxicity induced by acid deposition, an experimental acid irrigation was carried out in a mature Norway spruce stand in Southern Germany (Hoglwald). The experiment comprised three plots: no irrigation, irrigation (170 mm a−1), and acid irrigation with diluted sulphuric acid (pH of 2.6–2.8). During the seven years of acid irrigation (1984–1990) water containing 0.43 molc m−2 a−1 of protons and sulphate was added with a mean pH of 3.2 (throughfall + acid irrigation water) compared to 4.9 (throughfall) on both control plots. Most of the additional proton input was consumed in the organic layer and the upper mineral soil. Acid irrigation resulted in a long lasting elevation of sulphate concentrations in the seepage water. Together with sulphate both aluminium and appreciable amounts of base cations were leached from the main rooting zone. The ratio between base cations (Ca + Mg + K) and aluminium was 0.79 during acid irrigation and 0.92 on the control. Neither tree growth and nutrition nor the pool of exchangeable cations were affected significantly. We conclude that at this site protection mechanisms against aluminium toxicity exist and that additional base cation runoff can still be compensated without further reduction of the supply of exchangeable base cations in the upper mineral soil.

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.

Soil carbon, nitrogen, and base cation cycling 17 years after whole-tree harvesting in a low-elevation red spruce ( Picea rubens)-balsam fir ( Abies balsamea) forested watershed in central Maine, USA

Forest harvesting and regeneration may cause changes in soil and solution chemistry that adversely affect forest productivity and environmental quality. The objective of this study was to assess soil carbon (C), nitrogen (N), and base cation cycling 17 years following whole-tree harvesting in a low-elevation red spruce ( Picea rubens)-balsam fir ( Abies balsamea) watershed (regenerating watershed) in central Maine, USA. Results for forest floor and mineral soil nutrient concentrations and pools were compared to an adjacent non-harvested 77–85-year-old spruce-fir watershed (reference watershed) and to preharvest conditions obtained 17 years earlier. The total soil (forest floor + mineral soil) exchangeable magnesium (Mg) pool was 32% and 44% lower ( P ≤ 0.05), respectively, in the regenerating and reference watersheds compared to the preharvest condition. Nitrogen and exchangeable potassium (K) contents in the forest floor, however, were 26% and 65% lower ( P ≤ 0.05), respectively, for the preharvest condition compared to the reference watershed; no differences ( P > 0.05) occurred between the reference and regenerating watersheds. Mineral soil exchangeable calcium (Ca) and sodium (Na) concentrations and contents, effective cation exchange capacity (CEC e), and base saturation were at least twice as high ( P ≤ 0.05) in the regenerating compared to the reference watershed or preharvest conditions. Changes in the N and Mg pools indicate that atmospheric deposition may be a concern for Mg depletion and N saturation at this site. However, Ca depletion does not appear to be occurring at this site as a result of either acid deposition or whole-tree harvesting, although there are indications that the reference watershed may be in the initial stages of altered Ca cycling, which may lead to depletion. Soil solution and stream nutrients were compared between the regenerating and reference watersheds. The soil solution base cation and inorganic anion concentrations were two to four times higher ( P ≤ 0.05) in the reference compared to the regenerating watershed. In addition, bicarbonate (HCO 3 −) accounted for 63% of the cation:anion balance in soil solution of the regenerating watershed, but less than 1% in the reference watershed ( P ≤ 0.05). Soil solution Ca 2+ and Mg 2+ concentrations at 50-cm depth in the regenerating watershed showed a linear decrease ( P ≤ 0.05) 10–17 years after regeneration, whereas the stream Ca 2+ and Mg 2+ concentrations showed a linear increase ( P ≤ 0.05) during that timeframe. No consistent temporal trends ( P > 0.05) in stream Ca 2+ and Mg 2+ occurred in the reference watershed. Higher concentrations ( P ≤ 0.05) of soil air carbon dioxide (CO 2), soil solution dissolved inorganic C (DIC), and soil exchangeable Na, at or below 20 cm in the regenerating watershed suggest that these contrasting results were due to higher mineral weathering rates in the regenerating watershed. The soil elemental and exchangeable cation pools indicate that whole-tree harvesting may be a sustainable practice for at least one rotation. However, the soil solution and stream chemistry indicate that the regenerating watershed has not yet reached equilibrium for nutrient cycling.

Model Prognoses for Future Acidification Recovery of Surface Waters in Norway Using Long-Term Monitoring Data

During the past 20 years, acid deposition in Europe has decreased by more than 60%, yet still a large number of lakes and streams in southern Norway have not recovered to a water quality sufficient to support sustainable populations of trout or salmon. Long-term (30 years) monitoring data were used hereto constrain the calibration of the acidification model MAGIC to three Norwegian calibrated catchments. The model accounted for 60-80% of the variance in the year-to-year variations in concentrations of most of the major ions in streamwater. The results support the use of the lumped parameter acid neutralizing capacity (ANC) to link chemical parameters to biological response, as the calibration efficiency for ANC is considerably higher than for other biologically important parameters such as inorganic aluminum (Al(n+)) and pH. Three different scenarios for future deposition of sulfur were run: current legislation, maximum feasible reductions, and an illustrative scenario removing all anthropogenic deposition. These analyses show that much of the potential improvement in water quality has already occurred and that only limited further improvement can be expected from the current legislation. The current legislation is unlikely to produce ANC values sufficiently high to allow self-reproducing populations of trout at two of the three sites. Most of the response in water chemistry to reduced acid deposition has been rapid; the water chemical responses largely occur the same year or a few years after reduction in the input. The soil pool of exchangeable base cations depleted during 150 years of acid deposition, however, requires several centuries for replenishment. The uncertainties in future predictions come from several factors, such as future nitrogen dynamics and impacts from changes in seasalt and precipitation events. The differences in future water chemistry predicted from changed seasalt deposition or nitrogen dynamics are larger that the differences between different deposition scenarios. Hence, these factors must be included in future assessments of recovery from acidification.

Influence of landscape position and vegetation on long-term weathering rates at the Hubbard Brook Experimental Forest, New Hampshire, USA

The spatial variability of long-term chemical weathering in a small watershed was examined to determine the effect of landscape position and vegetation. We sampled soils from forty-five soil pits within an 11.8-hectare watershed at the Hubbard Brook Experimental Forest, New Hampshire. The soil parent material is a relatively homogeneous glacial till deposited ∼14,000 years ago and is derived predominantly from granodiorite and pelitic schist. Conifers are abundant in the upper third of the watershed while the remaining portion is dominated by hardwoods. The average long-term chemical weathering rate in the watershed, calculated by the loss of base cations integrated over the soil profile, is 35 meq m −2 yr −1—similar to rates in other ∼10 to 15 ka old soils developed on granitic till in temperate climates. The present-day loss of base cations from the watershed, calculated by watershed mass balance, exceeds the long-term weathering rate, suggesting that the pool of exchangeable base cations in the soil is being diminished. Despite the homogeneity of the soil parent material in the watershed, long-term weathering rates decrease by a factor of two over a 260 m decrease in elevation. Estimated weathering rates of plagioclase, potassium feldspar and apatite are greater in the upper part of the watershed where conifers are abundant and glacial till is thin. The intra-watershed variability across this small area demonstrates the need for extensive sampling to obtain accurate watershed-wide estimates of long-term weathering rates.

Experimental Acidification Causes Soil Base‐Cation Depletion at the Bear Brook Watershed in Maine

There is concern that changes in atmospheric deposition, climate, or land use have altered the biogeochemistry of forests causing soil base‐cation depletion, particularly Ca. The Bear Brook Watershed in Maine (BBWM) is a paired watershed experiment with one watershed subjected to elevated N and S deposition through bimonthly additions of (NH4)2SO4 Quantitative soil excavations in 1998 measured soil pools of exchangeable base cations 9 yr after treatments began. Stream sampling at the weirs on a weekly and event basis, and weekly precipitation sampling, were used for input‐output estimates. The treated watershed had lower concentrations of exchangeable Ca and Mg in all horizons, with evidence for the greater depletion in the O horizon compared to underlying mineral soils, and in softwoods compared to hardwoods. This difference between watersheds is interpreted to be treatment‐induced base‐cation depletion, which was reinforced by model simulations. The difference between watersheds was 66 and 27 kg ha−1 of exchangeable Ca and Mg, respectively, after accounting for soil mass differences between watersheds. This was comparable with the total cumulative excess stream Ca and Mg export in West Bear after 9 yr of treatment of 55 and 11 kg ha−1, respectively. Model simulations of watershed response to treatments predicted excess soil exchangeable Ca and Mg losses in the treated watershed of 47 and 9 kg ha−1, respectively. These results indicate that the response to a step‐increase in N and S deposition during the first decade of treatments in this experimental forested watershed was to invoke cation‐exchange buffering, resulting in a net decline in soil exchangeable base cations.

The soil acid–base status of boreal black spruce stands after whole-tree and stem-only harvesting

The effect of different harvesting practices on soil acid–base status was evaluated in black spruce (Picea mariana (Mill.) BSP) forests of Quebec by comparing soil from five pairs of whole-tree and stem-only harvested plots 3 years after harvest. Stem-only harvesting contributed to the enrichment of the exchangeable base cation pool, particularly in the forest floors of the pairs where whole-tree harvested plots showed the highest exchangeable Al3+. In the mineral soil, divergence between treatments was low, perhaps because these acidic soils were strongly saturated with Al3+(about 90%), which did not favour cationic exchange reactions. Although the effects of treatment may not persist over time, improved base cation nutrition may benefit stands during the early stages of development. Over a forest rotation of about 85 years, the estimated loss of alkalinity due to whole-tree harvesting was estimated to be low (less than 20%) when compared with the effect of acidic deposition.