Soil Science Society of America Journal 65:1455-1462 (2001)
© 2001 Soil Science Society of America
DIVISION S-5 - PEDOLOGY
Altered Podzolization Resulting from Replacing Heather with Sitka Spruce
Lone Mossin*,a,
Birgit Tejg Jensenb and
Per Nørnberga
a Dep. of Earth Sciences, Univ. of Aarhus, Ny Munkegade Bygn. 520, DK-8000 Aarhus, Denmark
b Dep. of Chemistry, Univ. of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark
* Corresponding author (mossin{at}geo.aau.dk)
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ABSTRACT
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Podzolization processes were studied under heather (Calluna vulgaris L. Hull) and Crowberry (Empetrum nigrum L.) and sitka spruce [Picea sitchensis (Bong.) Carr] planted on former heathland at Hjelm Hede, Jutland, Denmark. The aim was to compare changes in soil-water chemistry with changes in the solid-state chemistry after vegetation change. Fourteen profiles under heather and five profiles under spruce were examined and soil water was collected in lysimeter wells from the bottom of individual soil horizons A, E, Bh, and Bs from four representative profiles. The investigated area has been heathland for more than 2000 yr and part of it was planted with sitka spruce in 1933. The sitka spruce litter provided a more acidic soil environment, and the taller canopy increased the deposition of seasalts. The acidifying effects were increased by increased evapotranspiration from the plantation compared with the heathland. The investigation indicates disturbance of the podzolization process under spruce. Soil-water chemistry showed a lack of Al immobilization in the B horizon under spruce, contrary to the heather system. Analyses of the spruce soil reveal a depletion of pyrophosphate (PYR) extractable Al and Fe. The charge balance in soil water showed a high deficit of anions under spruce compared with the dissolved organic C (DOC) concentration. Therefore, the deficit cannot be exclusively ascribed to negative charge on DOC. Sources of anion deficit might include (i) dissociated organic acids (R-COO-), (ii) reduction in positive charge because of the existence of organo-metal complexes [R-(COO)2Al+], and (iii) reduction in positive charge because of an inorganic pH-dependent species distribution of Al [Al(OH)2+, Al(OH)+2.
Abbreviations: AAS, atomic absorption spectroscopy • DCB, dithionite citrate bicarbonate • DOC, dissolved organic C • PYR, pyrophosphate • SD, standard deviation
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INTRODUCTION
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The formation of the spectacular podzolic soils has been debated in the literature for decades. Two classic pathways of formation have been proposed and discussed, (i) formation and transport of soluble organic acids complexing with metals, especially Fe and Al (Petersen, 1976; De Conick, 1980; Lundström, 1993), and (ii) a downward transport of inorganic sols of Si and Al; the imogolite theory (Farmer and Fraser, 1982; Anderson et al., 1992). Another pathway of mobilizing metals in the E horizon has recently been ascribed to rock-eating fungi (Jongmans et al., 1997; van Breemen et al., 2000). The mechanism of immobilization of the metal–organic complexes has been ascribed to three pathways; (i) A saturation of the complexes with metals will neutralize their negative charge and they will precipitate (Petersen, 1976; De Coninck, 1980), (ii) immobilization can be because of adsorption of the complexes on positively charged surfaces (Boudot et al, 1989), and (iii) biodegradation of the dissolved organic acids will liberate the metals from the complexes, where after they form a Fe–Al–Si phase (Lundström and Giesler, 1995). In the B horizon several secondary mineral phases have been found, e.g., imogolite protoimogolite (Farmer et al., 1980), and ferrihydrite (Kodama and Wang, 1989). A thorough review of the podzolization process has recently been presented by Lundström et al., 2000.
However, very few authors have reported disturbances of the podzolization process. Disturbance of the processes is noticed from morphological changes because of biological and chemical changes caused by vegetation change from heather to oak (Quercus robur) (Nielsen and Nørnberg, 1987a,b). Usually coniferous forest soils are assumed to be more podzolized and have a higher extractable content of Fe and Al than profiles under heather and deciduous forests (De Kimpe and Martel, 1976; Nielsen et al., 1987a; Nielsen et al., 1999). It has been reported that when the processes are disturbed, the form of transported Al changes from organic to monomeric inorganic. This is because of a release of Al caused by a redissolution of the organically bound Al previously accumulated in the spodic horizon. The dissolution can be caused by acidity derived from atmospheric pollution (Boudot et al., 1996) whereby the complexing abilities of the organic acids are destroyed. Another investigation showed an absence of organic complexation of Al in the E horizon. This was interpreted as a termination of the podzolization process because metal-organo complexes are assumed to be essential for the podzolization process (Lundström and Giesler, 1995).
The chemistry of the solid phase, which reflects the accumulated results of processes that have occurred over centuries, is described in many papers (e.g., De Coninck, 1980; Jakobsen, 1991). Present day dynamics as deduced from soil-water analyses are reported by numerous authors. Lundström and Giesler (1995) observed high Al concentrations because of anthropogenic acidification. The role of proton donors in pedogenesis is described by Ugolini and Sletten (1991). Influence and characteristics of soluble humic substances are reported by Cronan and Aiken (1985). Guggenberger and Kaiser (1998) noted the influence of dissolved organic matter on transport of cations and acidity in forest soils. Soil-water chemistry and its dependence of sampling method are conveyed by Giesler et al. (1996). Lundström (1994) gave an account of the influence of organic acids on the podzolization process and weathering. As well as, David and Driscoll (1984) describing the species distribution of Al in soil water. Changes in the soil-water and solid-state chemistry of a podzolic soil because of vegetation changes are reported by Nørnberg et al. (1993); Nielsen et al. (1987); De Kimpe and Martel (1975) and Nielsen et al. (1999). Furthermore, changes of the soil dynamics because of alterations in atmospheric acid deposition are reported by Giesler et al., 1996.
Ecosystems in maritime regions are influenced by seasalt. The deposition of sea salt induces short-term acidification as a result of ion-exchange reactions (Farrell, 1995). Hydrogen and Al will be exchanged from the cation-exchange complex in the soil and replaced by Na and Mg (Pedersen and Bille-Hansen, 1995). As a consequence, pH in the soil water is lowered and the Al concentration increased. However, because of the low pH, no increase in base saturation is expected. The deposition rate depends on the height of the vegetation, leaf area, and whether it is deciduous or coniferous.
When charge balances are calculated for soil-water samples, a deficit of anions is normally observed. The shortage of a negative charge in the charge balance is ascribed to the negative charge on the DOC because of the dissociation of acid groups. A linear correlation is expected between DOC and the shortage of negative charge in the charge balance (Cronan and Aiken, 1985). This assumption has been questioned because of the existence of ionized organically complexed and colloidal species (Guggenberger and Kaiser, 1998). The negative charge on DOC is because of dissociation of the acid groups (e.g., carboxylic acids). The degree of dissociation depends on pKa of the acids and on pH of the solvent. The pKa of acid groups on DOC is estimated in the literature as 4.23 to 4.33 in a hardwood forest in central Maine (Vance and David, 1991), possibly below 2 for humic substance in natural waters (Perdue et al., 1984), 3 to 6 in water from peatlands (Urban et al., 1989), and 3.5 to 5 in the forested Bickford watershed, MA (Eshleman and Hemond, 1985). The total acidity of DOC can be estimated from the content of different types of acid groups.
The objectives of this study are to investigate the consequences of planting sitka spruce on former heathland on (i) soil-water chemistry and on (ii) soil profile development of podzolized soils. The planting of spruce on part of a former heathland provides a unique opportunity to study the influence of vegetation change on soil forming processes because the development under heath continues undisturbed within 500 m from the sitka plantation. The only active variable is therefore the vegetation and the differences it provides such as changes in deposition, evapotranspiration, litter composition, and organic acids.
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MATERIALS AND METHODS
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The Field Site
The study area, Hjelm Hede, is an inland heath in the northwestern part of Jutland, Denmark (Fig. 1)
. The parent material is meltwater deposit from late Weichsel about 16000 BP (Kronborg, 1995). The plain is 
18 m above sea level and the sediment consists of up to 70 m of fluvioglacial sand (Danish Geological Survey, 1989). The proximity to the North Sea and the long open fetch in the dominating westerly wind direction create very different deposition rates between the low heather and the tall (12 m) spruce system.
The dominant heathland vegetation is heather and crowberry. Pollen analyses indicate that the area was increasingly influenced by heather in the period 3000 BP–1800 BP. Thereafter, the heath was managed to prevent natural succession of the vegetation, e.g., oak shrubs (Odgaard, 1994). A large part of the heath was protected by statute in 1935 and 1967. In 1933, 10 ha of the heath were planted with sitka spruce (Fig. 2)
. The upper 15 cm of the soil was plowed before planting. No fertilizers have been added to the system.
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Fig. 2. The study site with location of plots and vegetation distribution. The white spruce area is 10 ha.
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The mean annual temperature is 7.5°C (1961–1990). During the same period, the mean monthly temperature ranged from 0.7°C in January and February to 16.1°C in August. The mean annual precipitation is 875 mm (1961–1990) with the highest intensity from September to December (Danish Meteological Insititute, 1997). The climate is humid temperate. The soil temperature regime is mesic close to frigid (Soil Survey Staff, 1998). The water table is at least 15 m below the surface.
Previous investigations of soil profiles at Hjelm Hede classify the soils under heather and spruce as Typic Haplorthods, on the basis of the presence of a spodic horizon, compare Soil Survey Staff, 1990 (Madsen and Nørnberg, 1995; Nørnberg et al., 1993). Pedon descriptions from the two vegetation types are presented in Table 1.
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Table 1. Pedon descriptions of Heath [Typic Haplorthod with vegetation of mainly heather (Calluna vulgaris) and crowberry (Empetrum nigrum)] and Spruce [Typic Haplorthod with vegetation of sitka spruce (Picea sitchensis) there is no ground vegetation].
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Field Methods
Precipitation, throughfall and soil water were collected monthly in the two ecosystems from January 1998 to May 1999. All samples were collected in polypropylene or polyethylene samplers. Precipitation was collected with two replicates, throughfall with four to six replicates, and soil water with two replicates. Soil water was collected by gravitation lysimeters of funnel type installed from a tunnel in a well without disturbing the above lying soil horizons. The lysimeters were installed to collect water from the bottom of the A (10cm), E (20cm), Bh (30cm), and Bs (60cm) horizons (Nielsen and Nørnberg, 1995). The chemical flux for all the observed ions was calculated from a calibrated hydrological model (Mossin and Ladekarl, unpublished data, 1998–1999) by multiplying the monthly percolation with the concentration of each ion in each sample.
Soil samples were collected from each horizon in the investigated profiles. The samples were collected to represent the entire horizon in a 120-cm-wide soil profile. The bulk densities were determined by squeezing a steel cylinder of known volume horizontally into the soil horizon. The bulk density was used for estimating total amount of accumulated Fe and Al in the profile.
Laboratory Analyses
Acidity, titration to pH 4.5 (NaOH/HCl), and pH were measured in the laboratory within 5 h after sampling. Immediately thereafter, the samples were filtered through a 0.45-µm cellulose nitrate filter and stored in the dark at 4°C. Silicon, Fe, Al, Ca, Mg, K, and Na were analyzed with Atomic Absorption Spectroscopy (AAS) on a Perkin Elmer 5100 PC (Perkin Elmer, Denmark) using MERCK 1000 ± 2 ppm standards (Merck, Denmark). Chloride, NO3, and SO4 were analyzed with reverse phase HPLC on Perkin Elmer Series 10 Liquid Chromatograph (Perkin Elmer, Denmark) with UV-detection. An eluent of tetrabutylammoniumhydroxide (TBAH) at pH 7.8 and a 3-cm C18 column was used. Chloride was determined as well by titration with AgNO-3. Ammonium was analyzed on an ISMATEC ASIA-FIA flow-injection analyzer (ISMATEC, Zurich, Switzerland). All cations, except Si, and anions were used to test charge balance in each soil water sample. Dissolved organic carbon was measured by evaporating to increase concentration, combustion, and gravimetric analysis of the produced CO2. All glass articles used were soaked in 7% HCl (v/v) for at least 1 h. All chemicals were of analytical grade. Distilled water was used for all analysis.
Iron and Al in the solid material were determined in Na–PYR, pH 10 (Bascomb, 1968) and Dithionite Citrate Bicarbonate (DCB) (Mehra and Jackson, 1960) extractions. Iron and Al were analyzed by AAS.
All collected samples were analyzed separately without bulking replicates. To make a common value, mean values were used. The uncertainty on the mean values is given by the standard deviation (SD).
 | (1) |
To test if the pH of the soil water is different in the two ecosystems it was first tested whether their variances were in agreement (
21 = 22). There was a high probability that they were different. Therefore, to test the agreement between mean values of the pH in the two ecosystems, a Sattertwhaite t-test was used (Sattertwhaite, 1946).
The hydrological model used for calculation of the chemical fluxes is an evapotranspiration model (Thomsen and Bille-Hansen, 1998). Input to the evapotranspiration model was daily precipitation and potential evapotranspiration. Other essential parameters were wilting point, field capacity, maximum root depth, root distribution, and leaf area index. Soil moisture was measured manually by time domain reflectometry once a month. The percolation estimated by the model was compared with percolation determined from measured Cl concentrations since Cl is assumed to be conservative in ecosystems.
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RESULTS
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The chemical-water fluxes estimated from concentration mean values and percolation estimated by the hydrological model are shown in Table 2. Iron is immobilised in the B horizon under both spruce and heather. The Fe flux drops off higher in the profile under spruce than under heather. Dissolved organic carbon transport decreases with depth in the profile, with the most marked drop from below E to below Bh. Under heather, the DOC transport is very low in the lower Bs horizon whereas under spruce, the transport has not terminated in the lower Bs-horizon, like the most di- and trivalent cations. Under heather, the Al transport continues but decreases towards the lowest lysimeter (Fig. 3)
.
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Table 2. The chemical fluxes in the spruce system and the heather system. The annual fluxes are estimated from monthly mean concentration values and the percolation used is monthly data from a calibrated hydrological model.
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Fig. 3. Transport of Al, Fe, and dissolved organic C (DOC) under the two ecosystems. Notice the downward increasing Al transport under spruce.
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The fluxes in Table 2 show that salt concentrations, especially Na, are 4.5 times higher under spruce compared with heather. Chloride flux is also higher under spruce compared with heather. The concentration differences are even bigger than the flux differences because there is a larger up concentration under spruce because of a higher evapotranspiration loss.
The total content of extractable Fe and Al in the soil profiles down to 160 cm was calculated from bulk densities and extractions from each soil horizon. Figure 4
shows that the total content of PYR-extractable Al and Fe is significantly lower under spruce than under heather. Imogolite was never found in the two ecosystems (Per Nørnberg, personal communication).
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Fig. 4. Comparison of the dithionite citrate bicarbonate- (DCB) and pyrophosphate- (PYR)extractable Al and Fe under heather and spruce. The content is the total in the upper 160-cm of the soil. The bars mark the standard deviation. For spruce n = 5 and for heather n = 14.
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The chemical fluxes and the charge balances in the soil water were calculated from individual samples. In Fig. 5
, the deficit of anions for the two ecosystems is shown as a function of DOC. Both the anion deficit and DOC are lower under heather than under spruce. Under heather the relation between DOC and anionic deficit is almost linear (R2 = 0.809), whereas under spruce the distribution is more random (R2 = 0.322). The lower pH in the soil water under spruce (3.0–4.1, mean = 3.64) compared with heather (3.7–4.2, mean = 3.95) implies that most acid groups are protonated unless much stronger acids exist under spruce (Table 3). The difference in acidity in the two ecosystems was clearly established by the Sattertwhaite t-test. The test-probability (
) for identical pH values in the two ecosystems was 5.2 x 10-6. There is a depth-dependent difference in DOC between the samples under spruce. The samples from 30 cm and 60 cm (B horizon) have a relatively higher anion deficit/DOC ratio than samples higher in the profile. If the anion deficit is ascribed to the DOC, the negative charge under spruce is 5.6 to 31.9 mmol g-1 DOC with a mean of 13.9 mmol g-1 DOC, and under heather it is 4.5 to 13.8 mmol g-1 DOC with a mean of 7.1 mmol g-1 DOC.
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Fig. 5. The relationship between dissoved organic C (DOC) and anion deficit in the soil water measured in individual samples. There is a clear linear relationship between DOC and anion deficit (a) under heather, whereas (b) under spruce the distribution is more random and divided into areas according to depth. Results from 60 cm below heather are missing because of too little water to make all the analysis necessary to make a charge balance.
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DISCUSSION
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Aluminum, Fe, and DOC are the most relevant parameters in relation to the podzolization processes because of their significance in forming the diagnostic horizons (Fig. 3). However, the connection between Si and Al transport in the two ecosystems is also noteworthy if a discussion of imogolite formation was relevant (pH > 5) (Table 2). The data clearly show that Al is transported out of the B horizon under spruce whereas Fe and most DOC are immobilized in the B horizon under both heather and spruce (Fig. 3). The Si transport pattern (Table 2) is identical under the two ecosystems. This clearly indicates that weathering of silicates and clay minerals cannot be the only sources of Al in the transport under spruce, because it would require the same Al/Si ratio as under heather. The weathering is most intense in the upper horizons (Madsen and Nørnberg, 1995) but the most intense Al transport occurs in the B horizon and the source of Al must, therefore, be material in the spodic horizon. The Al transport under spruce does not follow the expected pattern. The high transport of Al above the B horizon is expected, but the continuing increase in the B horizon is unusual (Fig. 3.)
Analyses of numerous soil profiles from Hjelm Hede, Jutland, Denmark show that the soil under spruce has less PYR-extractable Fe and Al compared with the soil under heather. The high acidity naturally produced in the spruce system because of a very acid litter layer and the high seasalt deposition resulting from the high filtering effect of the tall spruce vegetation and the proximity of the North Sea are augmented by a high evapotranspiration from the spruce system. Percolation out of the uppermost 1 m is only 21% of incoming precipitation (Mossin and Ladekarl, unpublished data, 1998–1999).
The lower content of PYR-extractable Fe and Al in the spruce soil might indicate that formerly deposited Fe and Al are now dissolved. The depletion of Fe from the solid profile is not seen in the fluxes. This is because Fe might be reprecipitated as an inorganic compound, e.g., ferrihydrite (Kodama and Wang, 1989) or other amorphous phases. The low pH approaches the Fe buffer (pH < 3.5) (Borggaard, 1998) in the lower horizons so if the acidification continues, the Fe will be leached as well. Iron will not exist in a reduced form because oxidizing conditions predominate in the soil system.
There is no sign of any significant changes in the DCB-extractable Al and Fe pools. In any case, the low pH favors the dissolution and transport of inorganic Al. Dissolution can occur even though no changes in the DCB-extractable pools are visible. The very low pH will increase weathering in the upper soil horizon and it will increase the Al and Fe supply to the system. This is also indicated by Al and Fe fluxes in A and E horizons, which are two to three times higher in spruce than in heather. Therefore, loss of the illuvial inorganic Al and Fe in the B horizon may be compensated by increased supply from weathering.
This investigation suggests two things; (i) Decomposition of illuvial organo-metal complexes takes place under spruce and (ii) Al is no longer immobilized in the B horizon and is thus not participating in the podzolization process as a sorptive agent for organic compounds nor as a constituent of allophane or imogolite.
Disturbance of the podzolization processes was noticed by Lundström and Giesler, (1995) in a very acid (pHsoil wates0 = 3.3–3.8) environment in the Czech Republic. They observed an increase in the ratio of monomeric Al/total Al in soil water and a significant drop in organically complexed Al, when the acidity of the soils increased. This was interpreted as the result of a destruction of the complexing abilities of the DOC because of protonation. The consequence of strong acidification is an increase in the protonated part of DOC and, therefore, a reduction in the complexing ability. On this basis, it is not straightforward to explain the observed anionic deficit in the sitka spruce system.
From Fig. 5, it is obvious that the anion deficit under spruce cannot be ascribed only to dissociated DOC. The COO-/C ratio can be estimated from the slopes in Fig. 5, if the whole deficit of anions is ascribed to the DOC. The existence of substituted benzoic acids in the two ecosystems is known from earlier investigations (Nørnberg et al., 1993) e.g., gallic acid (pKa1 = 11.5; pKa2 = 8.70; pKa3 = 4.26), protocatechuic acid (pKa1 = 8.84; pKa2 = 4.35), p-hydroxybenzoic acid (pKa1 = 9.46, pKa2 = 4.58). Based on the literature, DOC is assumed to be composed of aliphatic mono-carboxylic acids (pKa = 4.6), formic acid (pKa = 3.5), hydroxy acids (pKa1 = 3.9; pKa2 = 2.8), aromatic dicarboxylic acids (pKa1 = 4.9; pKa2 = 2.7) and aliphatic dicarboxylic acids (pKa1 = 3.8; pKa2 = 1.2) (Huang and Schnitzer, 1986). The distribution of different kinds of acid groups on DOC is very important for the complexing capacities of the DOC and the contribution to the anionic deficit. The slightly higher pH under heather favours a higher deprotonation and facilitates the formation of complexes. Only acids with very low pKa are dissociated at the pH found in the soil water. With the existing DOC concentrations and low pH, not many dissociated acid groups can be present and those existing might be very weak complexes, except for oxalic acid. For heather, an estimated C/COOH is about 4.5 to 5, and for spruce, the ratio is as low as 2, if dissociated acids must account for the entire anion deficit. These ratios are low compared with the literature values of 4.4 to 12 (Cronan and Aiken, 1985). Our results indicate, that at least for spruce, the negative charge on the DOC cannot be the only explanation of the high anionic deficit. Therefore, especially in view of the high Al concentration, the deficit must be split into three contributors.
 | (2) |
where R is part of DOC, Al can complex with DOC (n = 3, n = 2, n = 1), and Al(OH)(3-m)+m the pH-dependent charge of the inorganic Al species (3 
m 0).
With the laboratory methods used in Al determination as total Al (AAS), Al speciation was not taken into consideration in the charge balance calculations. Al as Al(OH)2+ and Al
+2 reduce the positive part in the charge balance and thus lower the anion deficit. Thus, the possible reduction in Al charge from OH- coordination can account for a minor part of the deficit (0.1 mmol). In samples with a pH above 4.5, a part of the Al coordinated OH- is included in the alkalinity.
The remaining part of the deficit might be ascribed to a reduction in the positive charge because of the presence of organo-metal complexes. The existence of organo-metal complexes does not suppress the measured absolute cation concentrations since they are measured with atomic absorption, but their positive contribution to the charge balance is reduced because of positively charged complexes with DOC (e.g., R-(COO)2Al+). If an acid forms a complex with a metal ion, the metal can replace H+. It does not influence the charge balance since the H+ is already, via pH, included. The presence of organo-metal complexes is not obvious since the low pH does not in any way favor the formation of complexes against a normal protonation, except if the acids are strong complexants, e.g., oxalic acid. Oxalic acid is a very strong complexing agent with a high affinity for elements such as Al and Ca. Such complexes may also have formed before they entered the profile, i.e., they could have come from biocycling of Al (David and Driscoll, 1984). It is therefore necessary to include the existence of the complexes to account for the high anion deficit and their existence is generally accepted in connection with podzol formation (Lundström et al., 2000).
The anion deficit and DOC relation under spruce splits up into two areas. In Fig. 5a, the graph slope is not identical in the upper and the lower horizons. The acids in the B horizon generally have a higher anion deficit per milligram of C. Transport mechanisms can also differ according to the position in the profile. The weaker acids can be expected to stay in solution longer than stronger acids because the stronger acids are, because of their negative charge, more strongly sorbed (Guggenberger and Kaiser, 1998). The continued transport of DOC deeper in the spruce system (Fig. 3) may also indicate a continuous existence of organo-metal complexes. This indicates that although the ionic strength is much higher under spruce compared with heather, the organo-metal complexes are not saturated and still charged so the complexes are kept in solution contrary to what is observed under heather.
A similar division of the anion deficit is not noticed in the heather system and the expected linear correlation between DOC and anion deficit can be observed for the heather system (Fig. 5b). However, there will be an inorganic species distribution of Al. The metal-organo complexes exist because of the low Al concentration in the ecosystem, however, the influence is negligible.
Under spruce, there is a large variation of the anion deficit in samples with the same level of DOC. The relative STD between the samples is 13% for 10-cm samples, 32% for 20-cm samples, 37% for 30-cm samples, and 39% for 60-cm samples. There is no sign of any particular annual variation or correlation with Al content in the samples. Hence, the variation cannot be correlated directly to any other data.
The annual leaching of Al is known from flux analyses. The total loss of Al from the profile compared with present day conditions under heather is also known. Using the present day leaching rate, it would take 107 yr to achieve the measured depletion in Al from the spruce system. If the standard deviation on the extractable Al is included, the uncertainty in the time factor is quite large. However, the increasing Al transport under spruce has hardly been the same at all times since the planting. As a result of human activity, acidity has probably been increasing during this century. Yet, the calculation provides an idea of the time that it would take to remove former deposited material with present day transport rates. These rates might, however, be decreasing as atmospheric acidity has been reduced during the last two decades (Hovmand and Kemp, 1996).
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CONCLUSION
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The change in vegetation from heather to spruce causes a more acid environment. The increased acidity is because of a more acid litter in the spruce system and a higher deposition of seasalt because of the high atmospheric filtering effect of the taller canopy. Both effects are increased by a high evapotranspiration in the spruce system. It cannot be determined whether the main reason for the acidification is incoming acidity or salts, or the fact that an old spruce system forms its own strong acidity. It is likely that over time the litter acidity will be the most important. However, it is clear that the high interception and transpiration loss of water is critical since it concentrates the already high deposition and the internal acidity in the system.
The investigations of the chemistry of the solid profile and analysis of soil water from four depths below the surface clearly show that the podzolization process is disturbed in the acid environment under spruce and a dissolution of the organic complexed metals results in leaching of Al and an inorganic reprecipitation of Fe. It is also clear that Al concentrations calculated as Al3+ when Al occurs in high concentrations, make a linear correlation between DOC and anion deficit under spruce, as seen under heather and in many earlier investigations, impossible. The deficit of anions under spruce must be ascribed to both negative charge on DOC, the inorganic speciation of Al and existence of organo-metal complexes.
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ACKNOWLEDGMENTS
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We are grateful to Bo Vangsø Iversen, Danish Institute of Agricultural Sciences, for allowing us to use his data on three soil profiles under spruce. Conrad Aub-Robinson is thanked for critical reading of the manuscript and improving the English. Thanks are also due to laboratory technician Ann Berith Jensen and to Ruth Nielsen for help with the drawings.
Received for publication April 3, 2000.
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