Exchangeable Cation Pools Research Articles

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.

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.

The importance of soil acidity, moisture, exchangeable cation pools and organic matter solubility to the cationic composition of beech forest (Fagus sylvatica L.) soil solution

AbstractHumus horizons of dystric cambisols were sampled six times during 1990–1992 at 66 points along a beech forest transect in Scania, s. Sweden. Cation concentrations of soil solutions obtained by centrifugation of sifted samples at field moisture were related to pH, DOC, exchangeable pools of the cations and soil moisture. Soil solution Al was speciated in free ionic (easily reacting) Alr and organically complexed Alorg. Two or three variables accounted for a large share (70–90 %) of the cation variability between sampling points. Exchangeable soil pools were the most important variables for K, Mg. Ca, and Mn and contributed more when calculated on C. E. C. than on soil dry weight. Some function of pH was also of importance to most cation concentrations. Alr correlated well with both Alorg(+) and pH(‐). Soil moisture was positively related to DOC and K, negatively to H‐ion concentration. pH measured by different methods were closely correlated (r = 0.93–0.97), pHkcl and pH being ca. 0.5 unit lower, pH ca. 0.3 unit higher than soil solution pH, which varied between 3.5 and 5.6.

Simulated effects of atmospheric sulfur deposition on nutrient cycling in a mixed deciduous forest

The effects of three S deposition scenarios — 50% reduction, no change, and 100% increase — on the cycles of N, P, S, K, Ca, and Mg in a mixed deciduous forest at Coweeta, North Carolina, were simulated using the Nutrient Cycling model (NuCM). The purpose of this exercise was to compare NuCM's output to observed soil and streamwater chemical changes and to explore NuCM's response to varying S deposition scenarios. Ecosystem S content and SO42− leaching were controlled almost entirely by soil SO42− adsorption in the simulations, which was in turn governed by the nature of the Langmuir isotherm set in the model. Both the simulations and the 20-year trends in streamwater SO42− concentration suggest that the ecosystem is slowly becoming S saturated. The streamwater data suggest S saturation is occurring at a slower rate than indicated by the simulations, perhaps because of underestimation of organic S retention in the model. Both the simulations and field data indicated substantial declines in exchangeable bases in A and BA soil horizons, primarily due to vegetation uptake. The correspondence of model output with field data in this case was a result of after-the-fact calibration (i.e. setting weathering rates to very low values) rather than prediction, however. Model output suggests that soil exchangeable cation pools change rapidly, undergoing annual cycles and multi-decade fluctuations.

SAMPLE SIZE REQUIREMENTS FOR THE DETERMINATION OF CHANGES IN SOIL NUTRIENT POOLS

In long-term field studies, the number of soil samples required to detect a specified change is often determined by using the standard error of a soil property from a pilot study. When a soil property is highly variable, estimates of its standard error will also be highly variable. Therefore, it is useful to have confidence limits for the standard error in sample size calculations. We explored the empirical distribution of the standard error for several soil properties (total carbon pools, exchangeable cation pools, and forest floor element pools) using data collected from a northern hardwood forest site in central New Hampshire. We used a bootstrapping routine to simulate data sets for sample sizes ranging from 6 to 60 soil pits and computed the mean, 95%, and 98% confidence limits for the percentage change detectable by a specified number of sample observations. Using the 95% confidence limit for standard error resulted in as much as a twofold increase in the sample size requirement compared with the average standard error. For normally distributed data, the bootstrap confidence limits of standard error agree with those computed analytically. In the presence of modest nonnormality in the data, however, the analytic confidence limits do not agree with the bootstrap limits. Since the bootstrap procedure is free of parametric assumptions, it is a useful tool for general application in soil science.

Weathering rates of gneiss and depletion of exchangeable cations in soils under environmental acidification

Budgets of Na, K, Mg, Ca, Al and Si in two forested basins in central Europe indicate that the weathering rate of gneiss and the depletion of exchangeable calcium and magnesium are increased due to acidification caused by high deposition of SO 2 . The production of the pool of exchangeable cations is slower than its depletion. There is a coincidence of a very short time necessary to deplete the topsoil of the exchangeable Ca 2+ and Mg 2+ and the die-back of spruce. These conclusions are derived from a new mass-balance approach that enables a separation of hydrolysis of primary minerals, depletion of exchangeable ions and production of the exchangeable pool of cations in soils from data on elemental budgets in small hydrological basins.

The Impact of Nitrification on Soil Acidification and Cation Leaching in a Red Alder Ecosystem

AbstractThe objectives of this study were to investigate the impacts of internal nitrification on soil and soil solution acidity and on the rate of nutrient export through NO3− mediated leaching. This was achieved by comparing soil chemical properties and soil solution composition within a naturally N‐rich red alder (Alnus rubra Bong.) ecosystem to those of an adjacent Douglas‐fir [Pseudotsuga menziesil (Mirbel) Franco] forest where soil N levels were significantly lower and no measurable HNO3 production could be observed. In the red alder system, where &gt; 100 kg ha−1 yr−1 of N were added through symbiotic N2 fixation, the net annual NO3− leaching past the 40‐cm soil depth amounted to 3460 mol charges ha−1, and NO3− concentrations in the solutions collected below 40 cm periodically exceeded drinking water standards of 10 mg L−1. The H+ and NO3− release was most pronounced in the forest floor and top 10 cm of the soil under alder occupancy and caused significant acidification of percolating solutions. Less than 1% of the total H+ input from internal (nitrification) and external (atmospheric) sources leached below the 40‐cm depth, which was indicative for the strong buffering capacity of this particular soil. The cation displacement reactions involved in this pH buffering caused a 15% decline in base saturation and a significant acidification of the upper part of the soil profile. The presence of large amounts of mobile NO3− in solution triggered accelerated cation leaching, causing a selective redistribution of primarily exchangeable Ca2+ from the A to the B horizon. These field studies lead us to conclude that the rate and the selectivity of NO3− mediated leaching in a red alder system could significantly lower the exchangeable cation pool in the rooting zone or cause nutrient imbalance, if a site is managed for repeated rotations of red alder.