Cation Pools Research Articles

Cycling of nutrient base cations in a twelve year old Sitka spruce plantation in upland mid-Wales

Abstract. The effects of the early growth of plantation forestry on the biogeochemical cycling of potassium, calcium and magnesium have been investigated in a stand of 12 year old Sitka spruce and adjacent moorland growing on acid peaty podzol soils in mid Wales. Element budgets have been calculated for both systems using measurements of soil and vegetation base cation pools and fluxes. In the moorland, the magnesium budget is approximately at steady-state with no net change to the soil store whilst the soil is accumulating potassium. The calcium budget is approximately balanced but contains significant uncertainties due to between plot variability in calcium leaching losses. Afforestation has greatly increased the above-ground living biomass which holds 7 to 15 times more nutrients compared to the living aerial biomass in the moorland. With the exception of magnesium, the base cation stores within the forest soil are being depleted as the increase in atmospheric deposition due to the forest canopy provides only a small offset to the much larger accumulation of base cations within the trees. The current net rate of change in the soil store of calcium is sustainable for only 65 years. However, as the trees mature, their demand for calcium will be reduced and they should be able to ‘tap' deeper sources of calcium in the soil profile as well as in the drift and regolith material. Keywords: Forestry, base cations, element cycling, calcium, magnesium, potassium

Biogeochemical Cycling of Calcium and Magnesium by Ceanothus and Chamise

Vegetation has long been recognized as a fundamental factor in soil formation, but vegetation and soils commonly covary in response to other environmental factors, confounding the specific effects of vegetation on soil properties. The lysimeter installation at the San Dimas Experimental Forest in southern California offers a rarely found opportunity for quantifying cation‐cycling processes in a setting where all factors except vegetation are kept constant. The lysimeters were filled in 1937 with homogenized, fine sandy loam and planted in 1946 with chamise (Adenostoma fasciculatum Hook. and Arn.) and ceanothus (Ceanothus crassifolius Torr.). Comparison of the chamise and ceanothus lysimeters was best achieved by using the Ca/Mg ratio of the different cation pools and fluxes as an index. In 1987, the ceanothus exchangeable soil pool contained proportionally more Ca than Mg compared with chamise; that is, the Ca/Mg ratio in the ceanothus exchangeable soil pool was higher than that in chamise. Strong evidence supports vegetation influence on intra‐system fluxes (weathering and biocycling) as the basis for these differences. First, more Ca than Mg was released by weathering under ceanothus than under chamise. Second, the ceanothus aboveground biomass exhibited a higher Ca/Mg ratio than the chamise. Third, differences between vegetation types widened with time since construction of the lysimeter installation in both the aboveground biomass and exchangeable soil pools. Differences in cation storage measured for the lysimeter chamise and ceanothus stands appear representative of natural chaparral communities throughout California, and may result in distinct Ca and Mg biogeochemical processes in associated ecosystems.

Direct Oxidative Carbon−Carbon Bond Formation Using the “Cation Pool” Method. 1. Generation of Iminium Cation Pools and Their Reaction with Carbon Nucleophiles

We have developed a method that involves the generation of a “cation pool” using low-temperature electrolysis, and then its reaction with nucleophiles under non-oxidative conditions. This one-pot method solves problems associated with conventional oxidative generation of cations and their in situ reaction with nucleophiles, and provides an efficient method for direct oxidative carbon−carbon bond formation. As an example of this method, generation of cation pools from carbamates by low-temperature electrolysis (−72 °C) and their reactions with carbon nucleophiles such as allylsilanes, enol silyl ethers, and enol acetates were examined and the desired products were obtained in good yields. Aromatic compounds and 1,3-dicarbonyl compounds can also be utilized as carbon nucleophiles. The present method was also applied to combinatorial parallel synthesis using a robotic synthesizer.

Effects of biomass removal in thinnings and compensatory fertilization on exchangeable base cation pools in acid forest soils

The effects of whole-tree harvesting and fertilization at first thinning on the exchangeable pools of base cations in the soil were examined on four sites five years after thinning. One site with Norway spruce ( Picea abies (L.) Karst.) is located in southwestern Sweden and three other sites with Scots pine ( Pinus sylvestris L.) are located in southeastern, south-central and northern Sweden. On all sites, the stands were thinned, and the following treatments were applied in a randomized block design ( n = 3): (i) stem-only harvest, logging residues left on site; (ii) whole-tree harvesting; (iii) whole-tree harvesting combined with compensatory fertilization (N–P–K were applied one year after thinning in the form of inorganic fertilizers, and the amounts were equal to the nutrient content of the harvested logging residues); (iv) stem-only harvesting in combination with N (pine sites) or NP (spruce site) fertilization (ammonium nitrate, 150 kg N ha −1, superphosphate, 30 kg P ha −1) and (v) whole-tree harvesting and nitrogen fertilization as in treatment (iv). There were no general treatment effects revealed from analysis across all sites. Effects of harvesting intensity were detected on two sites, indicating reduced pools of exchangeable K, Mg and Ca after whole-tree harvesting. The reductions were less pronounced than the effects observed in other studies from experiments with whole-tree harvesting at clear-felling of mature forests. Effects of application of nitrogen alone were more frequently observed than effects of harvesting of logging residues. In most cases, base cation pools were lower after N treatment than after other treatments, and this effect was probably due to exchange and translocation phenomena. However, Ca pools had increased after NP treatment on the spruce site due to high content of Ca in P fertilizer. No effect of compensatory fertilization with N–P–K was detected. A comparison between exchangeable nutrient pools before thinning and five years after indicated that K was generally lost from the soil profile.

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

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

CAN SITE-SPECIFIC TRENDS BE EXTRAPOLATED TO A REGION? AN ACIDIFICATION EXAMPLE FOR THE NORTHEAST

In the absence of true regional data on changes in the acid/base status of lakes in the northeastern United States, we explore the possibility of using site-specific trends information from a judgment sample of lakes to assess the efficacy of the Clean Air Act Amendments. A meta-analytical technique is used to combine trends results from 44 Long-Term Monitoring (LTM) lakes in the Northeast for the period 1982–1994, with the goal of producing estimates of overall trends in the region. The lakes are subdivided into subpopulations (High ANC, Intermediate Till Drainage, Thin Till Drainage and Perched Seepage lakes) on the basis of their expected response to changes in acidic deposition, and they appear to represent the most acid-sensitive of these lake classes well. While the overall tendencies in the trends are as expected (e.g., most of the recovery is observed in the most sensitive subpopulations), there is significant trend heterogeneity among the lakes within most of the subpopulations; this heterogeneity prohibits the summarizing of trends at the regional level (i.e., for all of the Northeast). This heterogeneity is explained by differences in the responses of lakes in two subregions (Adirondacks vs. New England), and we present trends results separately for each subpopulation within these two subregions. All subpopulations in both subregions showed decreasing trends in sulfate concentrations, probably a reflection of decreasing trends in sulfur deposition in the region. Few trends in nitrate concentrations were observed. Recovery (as evidenced by increasing trends in acid-neutralizing capacity) was evident in Thin Till and Intermediate Till Drainage lakes in New England, but not in the Adirondacks. Most groups of lakes exhibited downward trends in base cations (Σ[Ca2+ + Mg2+ + Na+ + K+]); the magnitudes of these trends were always greater in Adirondack lakes than in similar New England lakes. This suggests that the depletion of soil cation pools in the Adirondacks may be responsible for some of the differences in recovery between Adirondack and New England lakes. While export of base cations may be the key difference producing different trends results in the two subregions, the site-specific nature of the trends, and their possible lack of regional representation, should be considered in interpreting the overall results.

Chrono-lume® and Magnesium Potentiate Aggregation of Canine but not Human Platelets in Citrated Platelet-Rich Plasma

The effects of Chrono-lume (CL) and magnesium sulfate (Mg2+), a component of this luciferin-luciferase reagent, on platelet aggregation were studied in platelet-rich plasma (PRP) obtained from blood anticoagulated with sodium citrate from humans, dogs, cats, horses, and cows. The final added Mg2+ concentration of both solutions ranged from 0.75-3.7 mM. CL and Mg2+ had no effect on maximum aggregation of platelets from humans induced by sub-threshold concentrations of collagen and ADP. In contrast, addition of CL or Mg2+ to canine PRP resulted in a dose-dependent and equal potentiation of platelet aggregation in response to sub-threshold concentrations of collagen, ADP, and thrombin in normal and thrombopathic dogs. The effect of CL on platelet aggregation induced by sub-threshold concentrations of agonists was less pronounced and varied in other species according to the agonist. The reason for the marked difference in sensitivity of human and canine platelets to CL or Mg2+ is not clear, although a difference in releasable cation pools of the platelets from these two species has been recognized. Platelet aggregation studies of animals with suspected thrombopathias should be performed without CL to prevent masking of a platelet function defect.

Changes in soil mineralogy and exchangeable cation pools in stands of Norway spruce planted on former pasture land

Chemical and mineralogical properties of the soils in 35- and 70-year-old stands of Norway spruce (Picea abies (L.) Karst), planted on former pasture and were studied at Asa Experimental Forest, southern Sweden. Remnant deciduous forests bordering the spruce stands were used as controls to assess possible tree-species-related effects on soil development. All soils are acid with little difference in soil pH between the spruce and deciduous stands. However, the saturation of the exchange complex with Mg is lower beneath spruce and the total exchangeable Mg pool in the upper meter of these soils is one third of the Mg store beneath the deciduous stands. Amphibole, biotite and chlorite are the major sources of Mg in the parent soil. The clay fraction of the topsoil beneath spruce has been depleted of all these easily weatherable ferromagnesian minerals. Apart from weathering-resistant primary silicates, the clay fraction consists almost exclusively of expandable, smectitic mixed-layer minerals, which are believed to be the products of advanced stages of biotite weathering. In contrast, vermiculite is the dominant secondary mineral in the A-horizon in the deciduous stands, and some chlorite has survived. Moreover, a greater depth of in situ weathering is indicated for the soil of the old spruce stands where biotite/vermiculite mixed-layers have formed in the C-horizon as products of early stages of biotite weathering. Thus, differences between the paired sites in soil solution chemistry are supported by the qualitative differences in soil mineralogy, and are believed to reflect divergent biotic and/or abiotic processes in the different stand types.

Effects of Intermittent Desiccation on Nutrient Economy and Growth of Two Ecologically Contrasted Mosses

The mossesBrachythecium rutabulum(Hedw.) B., S. & G. andPseudoscleropodium purum(Hedw.) Fleisch. were cultivated for more than 50 d in a growth cabinet with or without weekly drying interludes of 24 h. Some plants also received applications of a dilute NPK nutrient solution at weekly intervals. The continuously hydrated plants showed appreciably more biomass production than those receiving intermittent desiccation. Desiccation led to some bleaching of the green tissues inB. rutabulumbut not inP. purumwhich appeared more desiccation-tolerant. NPK addition caused a further significant growth stimulation in continuously hydrated plants, but not in intermittently desiccatedB. rutabulum. Pseudoscleropodium purumshowed NPK-induced growth stimulation even when intermittently desiccated. Net uptake of N was similar in desiccated and hydrated plants in both species. Considerable net uptake of P and K+occurred in continuously wetB. rutabulum, but uptake was much reduced in intermittently desiccated plants. Net uptake of P and K+byP. purumwas similar in desiccated and hydrated samples. Intracellular K+, leaked from the cells during the desiccation treatment, was retained by cation exchange on the negatively charged cell walls in both species. Levels of intracellular K+and Mg2+in the new growth were maintained at the expense of the pool of exchangeable cations. The growth stimulation and the net uptake of nutrients under intermittent desiccation was greatest when the NPK application was made at the start of rehydration, possibly because of accentuated uptake in the early stages of recovery. The results support the hypothesis thatP. purumhas a lower nutrient requirement thanB. rutabulumand highlight the importance of continuous hydration for the latter's more productive plant life strategy. The data also show that considerable new growth of bryophyte tissues is possible without additional nutrient absorption.

At What Temporal Scales Does Disturbance Affect Belowground Nutrient Pools?

We monitored the effects of both harvesting aboveground biomass and Hurricane Hugo on soil chemical and physical properties, and live and dead root biomass over 6 yr in a subtropical wet forest in Puerto Rico. Our goal was to determine how belowground processes changed at different temporal scales including the immediate period prior to revegetation (9 wk), the intermediate period of initial regrowth (9 mo), and the longer-term reorganization of the vegetation and biogeochemical cycling (6 yr). Harvesting resulted in temporary increases in the availability of exchangeable nutrients, but forest floor and soil nutrient pools had generally returned to pre-harvest values over a 9 wk period. Significant amounts of K moved through the soil over this time period, amounting to 29-46 kg/ha-1, and resulting in a reduction in the size of the exchangeable soil K pool. The hurricane deposited approximately 845 kg/ha-1 of forest floor mass and considerable nutrients on the soil surface, and increased soil N03-N and exchangeable K pools, but in all cases, pool sizes had returned to pre-hurricane values within 9 mo. Examination of the data on an annual time step over the 6 yr period revealed an increase in soil cation pools and a significant decrease in soil pH. No change in soil organic matter was detected at any time step following the disturbances. Live fine root biomass was dramatically reduced as a result of the hurricane, and was only beginning to show signs of recovery near the end of the 6 yr experiment.

Simulated Responses of Red Spruce Forest Soils to Reduced Sulfur and Nitrogen Deposition

Implications of reducing S and N deposition on red spruce forests of the southern Appalachians are explored using the Nutrient Cycling Model (NuCM). We hypothesized that reducing deposition would cause (i) large reductions in soil solution NO{sub 3}{sup {minus}}, SO{sub 4}{sup 2{minus}}, Al and Ca/Al ratios, but (II) small changes in exchangeable base cation reserves. Hypothesis (I) was supported in part: simulated reductions in atmospheric deposition had substantial and nearly immediate effects upon soil solution mineral acid anions, Ca{sup 2+}, and Al{sup 3+} concentrations. Ca/Al molar ratios were much less sensitive to changes in deposition and soil solution ionic strength than either Ca{sup 2+} or Al{sup 3+} separately. Hypothesis (II) was not supported: although the increases in base saturation and exchangeable cation pools were small relative to cation exchange, they were large relative to initial exchangeable base cation pools. These changes in base cation pools and base saturation were of sufficient magnitude to affect simulated soil solution composition during a very short time. 29 refs., 11 figs., 2 tabs.

Application of the magic model to the Norway spruce stand at Solling, Germany

magic (Model of Acidification of Groundwaters In Catchments) is a long-term process-oriented model for soil and water acidification. The model consists of a set of chemical equilibria between soil and soil solution and an accounting of annual fluxes and pools of major anions and cations in the ecosystem. magic was applied to the dataset from the 105-year-old Norway spruce stand at Solling, central Germany. The primary aim was to evaluate the relative roles of forest management and acid deposition on soil acidification as expressed by the pool of exchangeable base cations (base saturation). Future scenarios of management and deposition reduction were investigated, and the model indicated that with a 75% reduction in deposition, soils would recover with respect to present day values if the forest is left, or removed and converted to grassland, but would continue to acidify if the forest was removed and replanted.

Acid-base status of soils in groundwater discharge zones — relation to surface water acidification

Critical load calculations have suggested that groundwater at depth of 2 m in Sweden is very sensitive to acid load. As environmental isotope studies have shown that most of the runoff in streams has passed through the soil, there is a risk in the near future of accelerated acidification of surface waters. To assess the importance of the last soil horizon of contact before discharge, the upper 0–0.2m of soils in seven discharge zones were analysed for pools of base cations, acidity and base saturation. The sites were about 3–4 m 2 in size and selected from two catchments exposed to different levels of acid deposition. The soils in the seven sites had high concentrations of exchangeable base cations and consequently high base saturation. The high correlation ( r 2 = 0.74) between base saturation in the soils of the discharge zones and mean pH of the runoff waters suggested that the discharge zone is important for surface water acidification. The high pool of exchangeable base cations will buffer initially against the acid load. As the cation exchange capacity (meq dm −3) and base saturation were lower in the sites from the catchment receiving lower deposition, these streams may be more vulnerable to acidification in the near future. The high concentration of base cations in non-exchangeable fractions may also buffer against acidification as it is likely that some of these pools will become exchangeable with time.

Simulated effects of atmospheric deposition, harvesting, and species change on nutrient cycling in a loblolly pine forest

The effects of atmospheric deposition (current, 50% sulfur (S) deposition, and 200% nitrogen (N) deposition), harvest, and species change (from loblolly pine ( Pinus taeda L.) to mixed oak) on nutrient cycling were simulated for Duke Forest, North Carolina, using the Nutrient Cycling Model. Based upon the literature, we hypothesized that: (1) changes in S and N deposition would cause only minor changes in leaching rates and soil exchangeable cation pools because of ecosystem buffering of S and N; (2) replacement of loblolly pine by hardwoods would cause depletion of exchangeable Ca 2+, reduced Ca 2+ leaching and increased Mg 2+ leaching because of high rates of Ca uptake by hardwoods; (3) total Ca export via leaching and whole-tree harvesting in the hardwood and loblolly pine stands would be similar because lower Ca removal in harvest would be offset by greater Ca 2+ loss by leaching in the loblolly pine stand. Hypothesis I was supported by the results of the simulations: atmospheric deposition scenarios caused minor (typically < 5%) changes in leaching rates and soil exchangeable cation pools. However, changes in atmospheric deposition did cause large changes in simulated soil solutions and exchangeable cation concentrations in surface horizons, where total pool sizes were relatively small. Thus, although the simulation results suggest that the changes in S and N deposition will have little effect upon total leaching losses and nutrient pool sizes, they do not imply that atmospheric deposition has negligible effects upon exchangeable base cation concentrations. Hypothesis II was supported by the simulation results. At the end of the 20 year simulations, soil exchangeable Ca 2+ and Ca 2+ leaching were greatly reduced by hardwoods. However, Hypothesis III was not supported: cumulative Ca 2+ leaching in the hardwood stand was only slightly (16%) less than in the loblolly stand, and total Ca export via leaching and whole-tree harvesting was considerably greater in the hardwood stand than in the loblolly pine stand. The patterns in Ca and Mg distribution and leaching in the simulations were very similar to the field results from near Oak Ridge, Tennessee. The simulation results suggest that nutrient budget analysis using only a few years of leaching data extrapolated over a full rotation can be very misleading because of changes in leaching rate with time.

Relationships among crown condition, growth, and stand nutrition in seven northern Vermont sugarbushes

We compared growth with soil and foliar elemental composition in seven stands of sugar maple (Acersaccharum Marsh.) in northern Vermont characterized by high or low incidence of crown dieback over the period 1989–1992. In stands with low-quality crown conditions, such as elevated crown dieback, long-term basal area growth rates were approximately half of those in stands with higher crown quality. Average annual basal area growth was 17.5 cm2 for dominant trees in these stands during the period 1953–1992 compared with 32.3 cm2 for trees in higher quality stands. The occurrence of elevated crown dieback was apparently unrelated to stand characteristics such as stand age, basal area, stem density, elevation, or aspect among the stands sampled. Stands with elevated crown dieback were found on soils characterized by low pH ( &lt;4.0), low base cation pools (particularly Ca and Mg), and higher Al in soil surface horizons than higher quality stands. Over 4 years, sugar maple stands with elevated crown dieback exhibited significantly lower (P &lt; 0.01) foliar Ca concentrations and somewhat lower foliar N and Mg than higher quality stands, while soil and foliar K were similar in both dieback classes. Among survey plots sampled in 1989, soil pH and Ca were strongly correlated with foliar Ca and K (P &lt; 0.001). Soil pH, soil Ca, and foliar Ca were also strongly correlated with the level of crown dieback among plots and stands (P &lt; 0.0001). Our results indicate that podzolic soils in northern Vermont may present marginal conditions for sugar maple growth in terms of soil pH and soil Ca availability, but that P and K pools appear to be sufficient for growth. While a lack of historical data prevents identification of long-term trends in nutrient availability in these soils, factors promoting losses of base cations from acidic, base-cation-poor Podzols may also exacerbate foliar nutrient deficiencies and thus affect the crown condition of sugarbushes in northern Vermont.

Base saturation, nutrient cation, and organic matter increases during early pedogenesis on landslide scars in the Luquillo Experimental Forest, Puerto Rico

We evaluate data from a chronosequence study of landslide scars, ranging in age from 1–55+ yr, in the Luquillo Experimental Forest (LEF) a subtropical lower montane wet forest (sensu Holdridge) in eastern Puerto Rico. Surface mineral soil (0–10 cm) base saturation index (BSI) values increase during primary succession in the LEF ( R = 0.85, P < 0.001). Both BSI values and major nutrient cation concentrations are extremely low on new landslide scars. During the course of the 55+ yr chronosequence, major nutrient cation concentrations are positively correlated with soil organic matter (SOM) content ( P = 0.079) and not with clay content ( P = 0.794). When data collected from plots representing the two dominant late-succession vegetation associations are added to the analysis, nutrient cation concentrations correlate significantly with both SOM ( P = 0.001) and clay content ( P = 0.033). Our data show that when initial conditions are oligotrophic, both nutrient cation pools and BSI values can increase in the surface mineral soil during early pedogenesis. We discuss exogenic litter input, substrate weathering, and precipitation as potential sources for nutrient cations in these soils. We further suggest that production and decomposition of SOM is the dominant process controlling capture, retention and intra-ecosystem cycling of nutrient cations in these forests.

Influence of tree species on acidification and mineral pools in deciduous forest soils of South Sweden

The influence of tree species on chemical properties of the soil profile (0 to 70 cm), especially acidity and cation pools, was studied below canopies of 100 to 150 yr old individuals of Fagus sylvatica L., Quercus robur L., Carpinus betulus L., Tilia cordata Mill, and Acer platanoides L., in three mixed deciduous forests. The pH(H2O) range in the topsoil (0 to 5 cm) was 4.2 to 5.3 at the different sites and increased for all species by about 0.2 pH units to the upper C-horizon (50 to 70 cm). Carpinus had higher pH and base saturation, and the total exchangeable store of Ca (0 to 70 cm) at the ‘poorest’ site was ca 1.5 times greater than for Fagus and Quercus. Soils were less acid under crowns of Quercus than under Fagus and Carpinus in the upper 20 cm of the soil profile in the ‘richest’ site, but more acid at the deeper levels. The total exchangeable pools (0 to 70 cm) of K and Zn were 2 and 1.6 times greater under Quercus at the ‘richest’ site. Tilia had 10 to 20% units higher base saturation values and twice as much exchangeable Ca and Mg than the other species. It was concluded that out of all tree species studied, Tilia acidified the soil the least. The current annual total proton load (TPL) on the soil was 1.3 to 3.0 kEq ha−1 yr−1. The annual acid load due to cation excess accumulation in biomass was 30 to 50% of TPL. The remainder originated from atmospheric deposition. Quercus had the highest annual TPL at all sites, mainly because of greater input of atmospherically derived acidity, but also by a greater cation excess accumulation in biomass. Carpinus, compared to Fagus and Quercus, tended to have the lowest TPL at all sites, and Tilia had the lowest TPL at the intermediate site.

Identifying sources of snowmelt acidification with a watershed mixing model

We used ionic tracers to estimate the volume of old (soil and ground) water interacting with snowmelt in eleven Adirondack, NY watersheds. The contribution of old water varied from 66 to 90%, with no general relationship between old water % and soil depth to till. This approach also discriminated between watershed retention and release of particular ions to lake outlet water during snowmelt. Most watersheds released NO3 − during snowmelt, in addition to the snowpack NO3 −. Nitrification of snowpack NH4 + explained part of the additional NO3 − in lake out outlet water, but some NO3 − was likely mineralized nitrogen from soil organic matter. All watersheds retained NH4 + as well. Nitrogen release was greatest in the acidic watersheds in the southwestern Adirondacks, a region thought to be impacted by anthropogenic deposition. During snowmelt, Ca2+ and Mg2+ ions (presumably from soil exchange sites) were also released from most watersheds. In watersheds with acidic (minimum pH<4.6) lake outlet water, Al was also released during snowmelt. Thus, lake outlet water acidification during snowmelt was both buffered by cation release, and intensified by NO3 − release. If the soil exchangeable cation pools were not replenished prior to snowmelt, or NO3 − mobilization were increased, acidification during snowmelt would intensify.

Determination of soil exchangeable-cation loss and weathering rates using Sr isotopes

To assess the response of forests to a changing chemical environment, a means is needed for separating the total cation export from the watershed into a component derived from mineral weathering reactions and a component due to the removal of exchangeable (plant-available) cations in the soil1–3. We show that this separation may be possible by using 87Sr/86Sr ratios as a tracer of cation sources in stream water. Our measurements from a high-elevation forest ecosystem in the Adirondack mountains, New York, indicate that mineral weathering reactions contribute about 70% and soil cation-exchange reactions about 30% of annual strontium exports. Based on these results and the ratios of major cations to strontium in the local glacial till, we estimate the release of Ca2+, Mg2+, K+ and Na+ owing to weathering. The present weathering rate seems adequate to replace annual losses of cations from the total soil exchangeable pool, suggesting that the watershed is not in immediate danger of acidification from atmospheric deposition. But as our strontium isotope data indicate that 50–60% of the strontium in the organic-soil-horizon exchangeable and vegetation cation pools has an atmospheric origin, reduction of atmospheric cation inputs4 coupled with continued strong-acid anion inputs5 may result in significant depletion of this cation reservoir.

Base cation nutrition for pine stands on lithic soils near Stockholm, Sweden

This study reports on base cation nutrition for pine stands growing on lithic soils, i.e. areas of exposed bedrock where very little or no mineral soil cover exists. Since there is little soil, the exchangeable pool of base cations must be very small and the aim was to determine the source of nutrients for the trees— from atmospheric input or from mineral weathering. The method used Sr isotopes as analogues for Ca. Because the 87 Sr 86 Sr distribution is specific for the source, Sr found in the biomass can be treated as a binary mixture of Sr from deposition and weathered bedrock minerals. The bedrock comprised gneisses or gneissic granite, but with varying chemistry. In four out of five sites, atmospheric deposition was found to be the dominant Sr (Ca) source. In one area, though, mineral weathering seems to be effectively the only source. The reason is assumed to be a different weathering rate. Only 3–16% of the Ca, Mg and K released by weathering and received by atmospheric deposition is used for biomass accumulation in the pine stands.