Acid Forest Soils Research Articles

Nitrate, Sulfate, and Biphosphate Retention in Acid Forest Soils Affected by Natural Dissolved Organic Carbon

Natural dissolved organic carbon (DOC) may restrict the sorption of inorganic anions in acid forest soils due to the competition for binding sites. Competitive effects of DOC on the sorption of H 2 PO 4 - , SO 4 2- , and NO 3 - were tested in experiments with intact soil cores taken from different soil types. Solutions were applied containing the inorganic anions at concentrations of 0.06, 0.24, and 0.12 mmol L -1 , respectively, and the concentrations of DOC of the initial solutions varied between 0 and 5.6 mmol L -1 . Most soil horizons showed only a slight decrease of the H 2 PO 4 - sorption as the DOC concentration increased, indicating a higher sorption affinity of H 2 PO 4 - than of DOC. Increase of the DOC concentration resulted in a strong decrease of the binding of SO 4 2- for all soils investigated. Thus, sorption of SO 4 2- seems to be weaker than that of DOC. At the highest DOC concentrations, a net release of SO 4 2- nearly equal to the approximate amount of sorbed carboxyls was noticed for most soils. Thus, DOC may inhibit the sorption of SO 4 2- to soils completely or even remove SO 4 2- stored in soils. Because NO 3 - was found not to sorb on soils, no competitive effects of DOC were observed. The results indicate DOC to be a factor controlling the storage and the mobility especially of SO 4 2- in acid forest soils, whereas its effect on the sorption of phosphate and NO 3 - seems to be less important.

Release of complexing organic acids by rhizosphere fungi as a factor in Norway spruce yellowing in acidic soils

A field experiment in the Vosges (a low mountain range in north-eastern France) had shown that a CaMg amendment of an acidic forest soil reduced the total number of fungal propagules per gramme dry soil, while the number of species considerably increased. Some rhizosphere fungi isolated from the amended and/or control plots were grown in three media with ammonium or nitrate as the sole source of nitrogen, and with or without aluminium. Complexing organic acids released by the fungi and likely to interfere with tree nutrition were analyzed in the culture medium after fourteen days incubation. The largest amounts of complexing organic acids are released by fungi isolated from the acid control plot. These rhizospheric fungi take part in soil acidification, which results in the long-term decrease of soil mineral reserves in a forest soil subjected to permanent leaching. Therefore, we conclude from this work that fungi might contribute to the deficient mineral nutrition of the Norway spruce on acidic soils, by impoverishment of the root environment in essential cations. However, among the causes of the Norway spruce yellowing, rhizosphere deleterious fungi act as secondary agents or contributing factors rather than primary causes of decline.

Influence of logging traffic on the hydromorphic degradation of acid forest soils developed on loessic loam in middle Belgium

Modifications of soil physical parameters (bulk density, pore space pattern and clay dispersibility) induced by compaction in wheel-rutted areas due to mechanized forest exploitation was studied in loamy soils developed under beech stands in the loessic belt of middle Belgium. The consequences of surface waterlogging on pedological processes (especially the geochemical behaviour of iron and aluminium) were also investigated. The results show that rutted areas have: (1) a significant decrease of total porosity in the eluvial upper layers of the soil (0–30 cm depth), associated with an important lowering of the transmission pore volume and a decrease in macroporosity to about 6% of the total soil volume, i.e. to less than the current threshold value for root viability (10%); (2) a reduction of macropore space to about 9.5% in the lower illuvial horizons (30–50 cm depth); (3) a decrease of redox potential due to temporary waterlogging, inducing (a) the occurrence of reduced forms of iron, (b) a strong leaching of iron oxyhydroxides from the E g to the B tg horizon, (c) an increase of organically complexed iron forms in both the E g and B tg horizons, and (d) high solubility of Goethite and even of ferromagnesium minerals in the eluvial horizons; (4) an increase of clay dispersibility in both the E g and B tg horizons. These results provide clear evidence that on loessic materials soil compaction due to logging operations leads to rapid soil degradation through active hydromorphic processes.

Reconstruction of recent forest dynamics based on pollen analysis and micromorphological studies of young acid forest soils under Scots pine plantations

Reconstructions of vegetation development of forest ecosystems have traditionally drawn on pollen samples of thick mor-humus profiles. In The Netherlands, however, well developed acid forest soils are scarce, especially under Scots pine (Pinus sylvestris). Scots pines have been planted from ad 1800 onwards, since when an acid soil profile has developed. This paper aims to show that pollen analysis of young acid forest soils not only can be used for the reconstruction of vegetation development in forests but also, combined with micromorphological observations, reveals information about forest dynamics in terms of litter production, decomposition processes and soil development. For that purpose four sites were selected in Scots pine stands, planted between ad 1900 and ad 1940 on drift sand deposits. At each site samples for pollen analysis were obtained from the entire organic layer and the top of the mineral soil. The pollen records of young acid forest soils show a shift in pollen assemblages, caused by a change from an open heath landscape towards a closed canopy after forest plantation. Micromorphological observations by thin sections of the undisturbed acid soil profile enhanced the interpretations of the pollen records. Micromorphological studies of these forest soils revealed the presence of non-burrowing soil animals and the preservation of pollen grains in the excrements of these soil animals. The absence of soil mixing animals and the pollen preservation in soils results in a pollen stratification, representing the recent local vegetation development. Furthermore, the pollen concentration curves show the stabilization periods of drift sand. The incidental increase of the pollen concentrations in the mineral soil indicates alternations of active sand drifting and stabilization periods, while the continuous increase of pollen concentrations towards the top of the mineral soil points to the final stabilization of the drift sand by Scots pine and the development of an AE-horizon.

A simple model of soil organic matter complexation to predict the solubility of aluminium in acid forest soils

SummaryA simple equilibrium model for competitive binding of Al and protons to soil organic matter shows that Al activities in soil solutions of acid mineral soils are controlled by complexation reactions with soil organic matter. The model successfully explains the relation between pH and the activity of dissolved Al in several forest soils. Furthermore, we found evidence that pools of organically bound Al may be depleted fairly quickly. Kinetically controlled dissolution of inorganic soil Al compounds, which may be considerable, is the main cause for the re‐supply of the organically bound Al in the soil. The previously reported decrease in Al solubility that accompanies measured decline in organically bound Al in three Dutch soils was found quantitatively consistent with the model for organically bound Al proposed here.

Soil algae from acid rain impacted forest areas of the Krušné hory Mts. 1. Algal communities

The soil algal communities of forested and reforested, limed and unlimed plots were compared in the acid rain impacted Kru sne hory Mountains. The floristic composition of unlimed plots is similar to that found in acid forest soils with a naturally low pH and no effect of the acid rain on these communities can be detected. Chlorophyceae are the only group present in these soils. In limed areas with a higher soil pH, algal diversity is significantly increased while algal densities remain similar. Chlorophyceae, while still the dominant group, are accompanied in these soils by Bacillariophyceae, Xanthophyceae, Eustigmatophyceae and Cyanophyceae. Total vegetation cover and thus light hitting the soil surface seems to be most important in determining the algal biomass.

C and N mineralization in an acid forest soil along a gap-stand gradient

Using aerobic long-term incubations of 174 days, carbon and nitrogen mineralization, pools of potentially mineralizable C and N, microbial C and N, and leaching of organic CaCl 2-extractable C and N were determined in forest floor material (O F and O H) and mineral soil (0–5 cm) collected along a gap-stand gradient. Sample locations were the centre of gaps, the northern part of gaps, the northern edge between gaps and stand, and a mature European beech stand. Conditions changed along the gradient in (a) soil moisture, which was highest in the gap centre, (b) soil temperature, which was elevated in the northern part of the gap and at the northern edge, and (c) active tree roots, which were absent from the gap centre and the northern part. Samples were collected 21 months after gaps (30 m dia) had been created. Because N mineralization in the field during the first year after gap creation was higher in the gap than in the stand but lower in the second year, it was hypothesized that mineralizable substrate had been partially depleted under gap conditions. No differences were found between the gap and the stand in forest floor mass, or forest floor and mineral soil C and N concentrations. In the forest floor, C and N mineralization, and consequently the potentially mineralizable C and N, as derived from first order mineralization kinetics, decreased in the order stand > northern edge > gap centre > gap north. Amounts mineralized were 1053, 861, 761 and 610 g N g −1 and 24.4, 22.6, 22.5 and 20.2 mg C g −1 respectively. In mineral soil, these values were highest in the gap centre. A substantial decrease in microbial biomass occurred in samples from the gap centre during incubation. Other samples showed no marked decrease in microbial biomass during incubation. In the forest floor a shorter turnover time of mineralizable N was observed for gap locations, whereas the turnover of mineralizable C was slower in the northern part of the gap and the northern edge. This suggests that potentially-mineralizable pools of C and N were affected differently by either rooting, moisture or temperature. Similar turnover times (4–11 weeks) for potentially-mineralizable N in both forest floor and mineral soil suggested similar sources of mineralized N. Potentially-mineralizable C consisted of more recalcitrant fractions in mineral soil than in forest floor. Nitrogen immobilization in the form of CaCl 2-extractable organic compounds was unimportant in this soil. Organic N (N org) in CaCl 2-leachates was 4–12% of mineral N. Kinetic parameters indicated that N mineralization in the second year after gaps had been created was lower in the gaps, because mineralizable C and N pools had decreased.

Effect of the inhibitors nitrapyrin and sodium chlorate on nitrification and N2O formation in an acid forest soil

An acid forest soil from beech forest gaps, which were either limed or unlimed, and the undisturbed forest was investigated for the type of nitrifying populations and the process of N2O evolution. To see whether nitrifiers were of heterotrophic or autotrophic origin, the nitrification inhibitors nitrapyrin and sodium chlorate were applied to disturbed soil samples which underwent laboratory incubations. Nitrapyrin inhibits autotrophic nitrification. In different studies, sodium chlorate has been identified as an inhibitor either of autotrophic or of heterotrophic nitrification. In the samples investigated only nitrapyrin inhibited the autotrophic nitrification occurring in the limed soil. Sodium chlorate effectively inhibited heterotrophic nitrification. In the limed forest floor samples, where most autotrophic nitrification occured, sodium chlorate showed no inhibitory effect. In another laboratory incubation experiment, N2O evolution from undisturbed soil columns, to which the above inhibitors were applied, was investigated. In those samples, in which nitrification had been reduced, neither inhibitor significantly reduced N2O evolution. Thus it was concluded that the contribution of nitrification to N2O losses is negligible, and that N2O evolution arises from the activity of denitrifying organisms. Microbial biomass and respiration measurements showed that the inhibitors did not affect microflora negatively.

Effect of the inhibitors nitrapyrin and sodium chlorate on nitrification and N2O formation in an acid forest soil

Effect of the inhibitors nitrapyrin and sodium chlorate on nitrification and N2O formation in an acid forest soil

Water, NaHCO3−, NaH2PO4− and NaCl‐extractable SO42− in acid forest soils

AbstractA variety of different methods have been used for the determination of inorganic soil SO42− in the past, which makes it difficult to compare SO42− contents of soils.Sulfate was extracted with the four commonly used extraction solutions 0.5 M NaHCO3, 0.02 M NaH2PO4, 0.1 M NaCl and H2O from A‐, Bw‐ and Bs‐horizons of six acid forest soils. 5 g of field moist soil were percolated with a flow rate of 5 ml/h and percolations were repeated as long as SO42− was detectable in the percolate (> 0.5 mg SO4·l−1).NaCl and NaHCO3 extracted highest amounts of total inorganic SO42− in A‐horizons, but NaHCO3 caused analytical problems. NaHCO3 and NaH2PO4 yielded highest amounts in B‐horizons. With the exception of Bs‐horizons more than 70% of the total inorganic SO42− was H2O‐soIuble. Thus, if H2O‐soluble SO42− is defined as reversibly bound, the greater part of the inorganic SO42− in the investigated acid forest soils was reversibly bound. This SO42− fraction can potentially be released, if SO42− deposition decreases.

Cation exchange capacity variations in acidic forest soils from Sutton, Québec, Canada

Abstract Single values of the cation exchange capacity (CEC) are widely used in modeling soil solution chemistry in soil and water ecosystems. Our aim was to determine the CEC as a function of pH and ionic strength in an acidic forest soil. We examined the cation exchange of two Humo‐Ferric Podzols (Haplorthods) equilibrated with artificial soil solutions of different concentrations. Aliquots of acid (HC1) or base (NaOH) were added to generate a pH range of 3 to 6. The CEC, determined by displacement with BaCl2 showed little or no increase with increasing pH and a definite increase with lower pH. This anomalous behavior was attributed to the precipitation of aluminum (Al) at high pH and to its dissolution at low pH.

Ion dynamics in acid forest soils as affected by addition of Ca fertilizers

Ca fertilizers may be used as tools to improve the chemical status of acidic, base poor forest soils. The downward movement of ions after addition of CaCO 3, CaCO 3 + MgO, and CaS0 4 · 2H 2O was studied by monitoring the leachate composition from three types of columns — A1/B, mull/A1/B and moder/A1/B — reconstructed from an acid brown forest soil profile. The fertilizers were surface-applied at rates of 0, 0.56, 2.80 and 5.60 t ha −1 equivalent CaO, and the treated columns were leached with local rainfall in an open-air nursery during a 20-month period. For all column types and rates of application, the leaching of Ca increased in the order: control < CaCO 3 + MgO < CaCO 3 < CaSO 4 · 2H 2O. Retention of solubilized Ca from gypsum was mainly achieved by simple exchange with resident cations, with no major pH change relative to the control. The elution of Ca and desorbed cations with predominantly SO 4 2− anions resulted in an important increase in total cationic mobilization compared to the control. Due to kinetic restraints and strong buffering by the soil, the downward movement of Ca (from CaCO 3) and Mg (from MgO) ions was greatly limited. The mobility of Mg ions from CaCO 3 + MgO was however much greater than that of Ca, despite considerably larger inputs of the latter. In the leachates from the columns without humus, total Al concentrations decreased exponentially with increase in pH. For the other column types the reduced Al leaching, when observed, was not associated to any major pH increase. Gypsum application depressed nitrification but increased DOC leaching relative to the control. Liming generally had no or little influence on either NO 3 − or DOC leaching, but increased SO 4 2− mobilization in some cases. Total cation leaching from the non gypsum-treated columns was chiefly affected by NO 3 − dynamics. Varying the rate of fertilizer application did not modify the mechanisms involved in the downward movement of the various fertilizers, but could influence the intensity of the processes as well as the rate of soil chemical evolution.

Mineralization of carbon and nitrogen, and nitrification in Scots pine forest soil treated with fast- and slow-release nitrogen fertilizers

We studied the effects of fast- and slow-release organic N fertilizers (urea and urea-formaldehyde, Nitroform) on mineralization, nitrification, and N leaching in an acid, poor forest soil. We also studied the effects of a nitrification inhibitor (dicyandiamide) applied together with urea. Net nitrification, mineralization of N and C were determined by aerobic laboratory incubation of soil samples taken one and three growing seasons after N application. Numbers of autotrophic nitrifiers were estimated by a most probable number method three growing seasons after the treatment. Urea increased the CO2 production immediately after application, but after three growing seasons, CO2 production was the lowest in the urea-treated soils. In the nitroform-treated soils, the concentration of exchangeable NH inf4 sup+ after the first and third growing seasons was of the same magnitude, in contrast to the urea-treated soils, where hydrolysis took place immediately. Three growing seasons after application, the highest amount of NH inf4 sup+ accumulated during the laboratory incubation was in the nitro-form-treated soils. Unlike urea, nitroform did not increase the production of NO inf3 sup- or the number of NH inf4 sup+ oxidizers. In the urea+dicyandiamide-treated soils there was less NO inf3 sup- and a lower number of nitrifiers than in the urea-treated soils. The results showed that a slow-release N fertilizer, such as nitroform, increases the availability of mineral N in acid forest soils without increasing nitrification and hence the risk of NO inf3 sup- leaching.

Chemical significance of aluminium extracted from three horizons of an acid forest soil, using chloride salt solutions

SummaryAluminium is usually important in exchange reactions in acid soil, but amounts extracted are strongly affected by procedure. We examined the impact of proton release, following salt addition, on Al removal from three horizons (Oe, Oa and A12) of an acid, brown forest soil.The Al was first extracted with either 1 M KC1, 1 m NH4C1 or 0.5 m CuCl2 on the three horizons limed to pH 5.5. For any lime addition, both the pH drop following salt addition and the amounts of extracted Al were greater with CuCl2 than with KC1 or NH4C1. For the Oa and A12 horizons there was a single inverse linear relation between amount of Al extracted and the pH of final extract, independent of the extracting cation. In the Oe horizon CuCl2 extracted constant amounts of Al, whereas the quantity of Al removed by KC1 or NH4C1 increased linearly as pH declined below pH 3.6.Extra Al was mobilized following unbuffered KC1 extraction, a side effect induced by the extracting procedure itself.Al mobilization increased with increasing H+ additions. In the Oe horizon, equilibrium was established rapidly (≤ 24 h), and decreasing the pH of the soil‐KCl suspensions to 2.0 resulted in extracted Al amounts equal to or slightly greater than those obtained with CuCl2 solutions. Al amounts extracted with acidified KC1 solutions from the A12 were close to those obtained using CuCl2 solutions at similar pH in the extract. In the samples from the A12 layer, increasing the equilibration time resulted in increasing proton consumption and equivalent release of Al ions in the extract.

Threshold Application Rates of Wood Ash to an Acidic Forest Soil

AbstractChanges in soil and soil solution chemistry were studied for 2 yr after application of wood ash to an acidic forest Spodosol. Application rates to four 50 m2 plots were: 0 (control), 6, 13, and 20 Mg CaCO3 equivalents per hectare (Mg ha−1). In soil solution, the 13 and 20 Mg ha−1 treatments caused large but generally transient increases in base cations, pH, and anions. At 20 Mg ha−1, solution concentrations of H, acid neutralizing capacity (ANC), K, and SO4 remained elevated for 20 mo. There was minimal response in soil solutions at 6 Mg ha−1, suggesting that this rate may represent a threshold below which there is only minor response in soil solution chemistry. Loss of soil N into soil solution (as NO3) at the two higher application rates may have decreased available‐N in the already N‐limited soil. In the O horizon, all treatments resulted in increased pH and higher exchangeable Ca, K, Mg, CEC, and percent base saturation; Mn and Al decreased. In the B horizon, exchangeable Ca, K, Mg, and percent base saturation were higher than in the control. Responses in soil chemistry were not generally proportional to the ash application rate. Furthermore, the responses in soil chemistry at different application rates were generally not distinguishable from each other after 25 mo. The treatment rate of 6 Mg ha−1 ash was sufficient to favorably alter soil exchange chemistry, without seriously affecting solution chemistry. Above 6 Mg ha−1, soil exchange sites were unable to buffer ash amendments, resulting in substantial changes in solution chemistry.

Variability of N mineralization and nitrification in a simple, simulated microbial forest soil community

It is generally accepted that N transformations in natural terrestrial ecosystems are regulated by: (1) climatological and geomorphological conditions; (2) soil physics and chemistry; and (3) quality and quantity of soil organic matter. Consequently, spatial variability of N transformations within experimental plots have been related to fine-scale heterogeneity of these regulating factors, e.g. spatial differences in micro-climate. However, it has also been argued that spatial differences in the composition of the microbial community, which can be the result of stochastic events, may be an important source of spatial heterogeneity of decomposition processes. The aim of our study was to detect to what extent net N mineralization and nitrification can vary in soil samples as a result of differences in the composition of a simple microbial community of acid forest soils. This community consisted of two species of chitin decomposers, a mycelium-forming fungus and a rod-like bacterium, and two species of nitrifying bacteria. Characterization of the chitin decomposers in liquid cultures demonstrated two important differences between the fungus and the bacterium: (1) chitin-N mineralization by the bacterium was much slower than that by the fungus, and (2) the bacterium showed an antibiotic-type of inhibition against the nitrifying bacteria whereas the fungus did not. The effect of differences in the composition of the chitinolytic community on N mineralization and nitrification was studied using environmentally controlled incubations of the microorganisms in Petri-dishes containing purified sand with solid chitin or its soluble monomer, N-acetylglucosamine, as substrate. As seen in liquid cultures, the differences in chitin-N mineralization between series of sand incubations of either the fungus or the bacterium were considerable. Surprisingly, chitin-N mineralization in sand that had been inoculated with both chitinolytic decomposers, was relatively slow. This was most likely due to mycolytic activity of the bacterium against the fungus. Variation of N mineralization within series of identical inoculation, consisting of 10 replicates, was generally low (CV < 15%). In contrast, nitrification was extremely variable within many sand series (CV > 200%), especially those in which the chitinolytic bacterium was present. This high variability of nitrification was most likely due to instability of the antibiotic compound is sand. In conclusion, the results show that spatial variability in the composition of a simple microbial community and interactions therein can be an important source of small-scale heterogeneity of N transformations. The relevance of these results for the field situation is discussed.

Equilibrium solution composition and exchange properties of disturbed and undisturbed soil samples from an acid forest soil

Microscalic heterogeneity of soil chemical properties caused by soil structure has been reported for several soils. We investigated exchange properties and soil solution composition of disturbed and undisturbed samples of an acid forest soil lacking visible structure. Cation concentrations in the soil solution resulting from two extraction procedures and two analytical methods were compared. The effective cation exchange capacity (CECe) of the undisturbed sample represented 56–69% of the bulk soil CECe. Base saturation of undisturbed samples equalled that of disturbed samples for EA, Bhs, and Bsh horizons, and was higher for the Bw horizon. Contradicting the results of other authors, soil pore solution obtained by percolating soil cores under conditions of low water tension offered more favourable conditions for plant roots when compared to the equilibrium soil solution of the bulk soil sample in all except the Bsh horizon. Ca2+/Al3+ molar ratios were higher and fractions of H+ + Al3+ on total cationic charge were lower in the soil pore solution. These results were obtained employing soil: solution ratios of about 1:0.5 during the extraction of soil pore solution, and by determination of free cations. Other authors used a water extraction with soil:solution ratios up to 1:2 and took total metal for ion concentrations. The combination of the latter extraction and analytical method in our study, too, led to unfavourable Ca2+/Al3+ ratios and high tractions of H+ + Al3+. The choice of analytical and extraction method are thus decisive for the valuation of the soil solution composition in view of plant nutrition.

A system for microscale tensiometry and lysimetry

Growing roots, soil structure and soil horizons may induce microscale heterogeneities of soil solution chemistry and soil water content. To investigate heterogeneities on the centimeter scale, micro ceramic cells suitable for high resolution tensiometry and lysimetry have been developed, the construction of which is described in detail. The cells consist of a ceramic capillary with 1 mm diameter, connected to 1 16 ″ (1.59 mm) PEEK tubing. When combined with a pressure transducer the micro ceramic cells act as tensiometer, when coupled to a vacuum collecting device they can be used as micro suction cups. Also a combined tensio-lysimeter was constructed, but this application is limited to lower soil matrix potentials. The micro suction cups showed a quick response to changes in the outer solution (breakthrough volume < 80)μl). In initial field studies, the pH of the soil solution of an acid forest soil was measured in high spatial resolution using a matrix of micro-lysimeters.

Modelling Al-solubility controls in an acid forest soil, Solling, Germany

Hypotheses for controls of the Al solubility in acid forest soils were tested, using the large data set of soil- and solution chemistry of the Solling Spruce site (Germany). The observed low Al 3+-H + reaction stoichiometries (±0.3) as well as the measured increase of CEC in the surface soil with time, support the hypothesis of Al solubility control by complexation to soil organic matter (SOM). Incorporation of SOM-Al equilibrium in a comprehensive soil acidification model improves simulations of seasonal and long-term pH changes in soil solutions at Solling.

Aluminium chemistry of the soil solution in an acid forest soil as influenced by percolation rate and soil structure

SummaryThe predicted activity of Al in the soil solutions of acid forest soils often differs from that observed in the field. We have investigated the influence of soil structure and flow rate of the soil solution on the aluminum release to explain this divergence. Disturbed and undisturbed samples of soil were collected from the A and B horizons of a dystric cambisol at Waldstein (Fichtelgebirge, Germany). The samples were irrigated with solutions mixed according to field data on throughfall or soil solution composition with pH 3.5 with flow rates of 4 mm d−1, 12 mm d−1 and 36 mm d−1. The percolates were analysed for major ions. Resulting relations between pH and pAl were compared with batch experiments. In neither the A horizon nor in the B horizon did soil structure influence the relation between pH and pAl. The apparent equilibrium between pH and pAl was described as the pKapp value with pKapp= pAl—a pH (where a is an empirical constant). It was found that the pKapp values for the column percolates were in the range of variation of those found in batch experiments.Flow rate had no influence on pKapp at 4 and 12 mm d−1. At 36 mm d−1 a significant increase of pKapp was observed. This relative undersaturation of Al was more pronounced in the A horizon than in the B horizon. When flow is fast Al release into the percolating soil solution might be limited by diffusion.