Adsorbed metal concentrations in the organic soil of a primary succession of Pinus sylvestris forest on poor sandy substrates.

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Adsorbed metal concentrations in the organic soil of a primary

succession of Pinus sylvestris forest on poor sandy substrates.

Melanie Vermeulen

Figure 1. The Hulshorsterzand area. Source: https://upload.wikimedia.org/wikipedia/commons/7/7c/ Wandeling_over_het_Hulshorsterzand_36.jpg.

Supervisor: Dr. Albert Tietema

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verschillende momenten gepland zijn, hebben ze verschillende leeftijden, namelijk 48, 88 en 153 jaar oud. De ontwikkeling van het organische materiaal dat op de bodem ligt, de ontwikkeling van de vegetatie en de metalen concentratie in de bodem zijn onderzocht op de drie verschillende plaatsen. Het onderzoek concludeert dat er meer organisch materiaal op de bodem ligt naarmate de grove dennen ouder worden, maar dat dit proces langzamer gaat na ongeveer honderd jaar. Daarnaast is er gevonden dat de diversiteit in vegetatie toeneemt naarmate de grove dennen ouder worden. Als laatste neemt de concentratie van veel metalen in het bovenste deel van de organische laag toe naarmate de bomen ouder worden. Dit komt omdat er meer organisch materiaal op de bodem ligt en daardoor het bovenste deel hiervan minder gemixt wordt met de minerale bodem, waar de concentratie van deze metalen lager is dan in de plantenresten. In het onderste deel van het organische materiaal en in de minerale bodem zijn weinig verschillen gevonden in metalen concentratie tussen de verschillende leeftijden.

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Key words: cation exchange, Pinus sylvestris L. forest, stand age, metals, humus profile, atmospheric acid deposition

Abstract

In Hulshorsterzand area Pinus sylvestris L. was planted. In this area three stands were selected which had a stand age of 48, 88 and 153 years old in order to examine how stand age of planted Pinus sylvestris L. influences the organic layer, the absorbed metal concentration and the understorey vegetation growth in the Hulshorsterzand area. Sampling in the area was done by means of a transact. In addition, the thickness of the horizons of the humus profile were measured and the vegetation was examined. An extraction with 0.125 M BaCl2 was made in the lab in order to find the

dissolved metal concentration in the samples. The study found that the thickness of the H (Humus) horizon increases with stand age but that after about a century this process decreases considerably. It is also found that diversity in vegetation increases with stand age. The concentration of calcium, magnesium, manganese, potassium and zinc increases in the L (Litter) and F (Fragmentation) horizons with stand age due to a relative decrease in vertical mixing. Ammonium concentration increases with stand age in the latter horizons due to an increase of mineralisation as a result of an increase in organic matter accumulation. A decrease in the aluminium concentration is found in these horizons as a result of vertical mixing and no relationship between stand age and

concentration is found for sodium. The process of cation exchange due to acidification of the soil is not found in this study indicating that the major cause of cation exchange might be atmospheric acid deposition.

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Introduction

Humus profiles are of considerable importance for studies related to ecology and pedogenesis of non-agricultural land (Meyer et al., 1986). Humus profiles may consist of several horizons. The top horizon is typically the L (Litter) horizon and consists of unfragmented particles like leaves and grass foliage. In the following F (Fragmentation) horizon, the particles are fragmentated and partly decomposed, yet particles are still recognizable. The F horizon may be followed by an H (Humus) horizon, in which the material is completely fragmented mostly decomposed (Green et al., 1993; Zanella et al., 2011). The relation between soil and/or humus form and vegetation has been studied severe (e.g. Meyer et al., 1986; Van Berghem et al., 1985; Bernier & Ponge 1994; Emmer, 1995; Descheemaeker et al., 2009).

A relationship between soil and vegetation that is often mentioned is acidification of the soil. According to Pocknee and Sumner (1997) and Walker et al. (2004) organic matter might cause a decrease in soil pH through several processes. In addition, acidic rain might have a considerable effect on soil acidification (Cronan & Reiners, 1983; Walker et al. 2004). A suitable indicator of soil acidification is the concentration of dissolved metals in the soil since soil pH has a considerable effect on the mobility of metals. According to Clemente and Bernal (2006) and Elliott et al. (1986) metals form stable complexes with organic matter; thus, making metals immobile. As a consequence of an excess of H+_{ions, these ions take over the places of other cations in the formed complexes. The latter}

process is called cation exchange and as a result, other cations might be washed away through groundwater and thus the concentration of metals decreases (James and Bartlett, 1983; Naidu et al., 1994).

The latter phenomenon has been studies before in Scots pine forests in the Hulshorsterzand area (e.g. Meyer et al., 1986; Van Berghem et al., 1985; Emmer, 1995). The Hulshorsterzand area is located between Harderwijk and Nunspeet in the Netherlands and is part of the Leuvenhorst and the Leuvenumse National park. Originally, the soil in this area consists of poor sandy substrates since the soil formed after the development of inland drift sands starting in the Middle Ages (Koster et al., 1993; Riksen & Goossens 2007). After the first half of the 19th century, Pinus sylvestris L. (Scots Pine) was planted in this area; as a consequence, the area consists of forest stands of different ages. Meyer at el. (1986) and Van Berghem et al. (1985) used this for their research by selecting 5 stands of the following ages: 15, 30, 55, 91 and 120 years old. The youngest two groups are naturally established while the other three groups are sowed or planted (Emmer, 1995). Due to the fact that the P. sylvestris in the Hulshorsterzand is a primary succession and due to the know origin and age of these stands, it might seem like an appropriate selection for this study since there are relatively little other influences (Emmer, 1995). Thirty-three years after Meyer et al. (1986), and Van Berghem et al. (1985) determined the stand age, the 5 stands can still be found in the Hulshorsterzand area. Thus, the stands are now 48, 63, 88, 124 and 153 years old. This research selects three of these stands; namely the stand of 48, 88 and 153 years old. The research question and the hypothesis are sited below.

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How does stand age of planted Pinus sylvestris L. influences the organic layer, the adsorbed metal concentration and the understorey vegetation growth in the Hulshorsterzand area?

Mehlich (1948) and Mitchell & Soga (2005) found that cation exchange firstly occurs for cations with a positive change of one (e.g. K+_{, Zn}+_{, Na}+_{and NH}

4+). Subsequently, the H+ ions start taking

over the places of cations with a positive charge of two (e.g. Ca2+_{, Mg}2+_{and Mn}2+_{). This process}

takes place for a longer period of time on the older locations than on the younger locations. Therefore, the concentration of cations with a positive charge of one is expected to decrease with stand age. Meanwhile, accumulation of organic matter causes the thickness of the humus horizon to increase. As a consequence, the occurrence of vertical mixing between the L and F horizons and the mineral soil decreases with stand age. Since there is generally an higher concentration of most cations in plant residuals than in the mineral soil, this results in an increase in concentration with stand age for the L and F horizons. However, this only holds true for cations that occur in organic compound of plants like (e.g. calcium, magnesium, potassium, zinc and manganese) (Emmer, 1995; Gower et al., 1996; Kavvadias et al., 2001). Therefore, the hypothesis of this study is that the concentration of cations with a positive charge of one will decrease with stand age, whereas the concentration of cations with a positive charge of two that also occur in organic compounds of plants, will increase with stand age.

This research is extremely relevant since it provides a better understanding of the ecosystem of the Hulshorsterzand area; thus, this information can be used for conservation purposes. In addition, by means of additional, local research, it might be discovered that the processes found in this study also account for other areas worldwide; therefore, this research may contribute to better management strategies of non-agricultural land on a global scale.

Firstly, this paper describes the method used for this section, which consists of an explanation of the field work, lab work and processing of the data. Subsequently, the results of the paper are provided, discussed and a conclusion is drawn.

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Methods

In order to find out more about acidification in the Hulshorsterzand area, field work and lab work were done. In addition, the processing of the data is explained in this section.

Field work

The field work consisted of examination of the thickness of the humus profile horizons, examination of the vegetation and of sampling. Initially, a transact of 20 meters was created, every two meters along the transact a point was marked. This was done for all three locations.

Humus profile horizons and vegetation

The thickness of the humus profiles was measured on the same locations where the samples were taken.

Examination of the vegetation consisted of two parts; namely, examination of forest floor vegetation and examination of the diversity of trees.

The forest floor vegetation was examined within a radius of 70 cm of each point in the transact. For each point, the coverage was estimated of all occurring species within this radius.

The diversity of trees was estimated by means of a transect of 5 by 20 meters. Thus, it was the same transect as the previous one but broader. For every tree the relative distance to the sampling points was registered. In addition, the tree species and diameter at chest height was measured.

Sampling

On ten places along the transect samples were taken of three different horizons for each location. Two sets of samples were taken from the humus form by means of a PVC ring (d = 11.6 cm), which was pushed in the organic layer. The humus form was divided into two parts after the living bits like grass and/or moss were removed. The division was between the L and the F horizons combined, and the H horizon, so that the former sample contains new organic matter and the latter old organic matter. The third set of samples was taken from the first 5 cm of the mineral soil. These samples were taken by means of a PVC pipe (d = 4.2 cm). The samples were taken on a circle with a radius of 70 cm, with the point on the transact as the centre.

Later in this paper the sample of the L and F horizons on the location where the trees are 48 years old will be referred to as 48LF. The humus horizon will be referred to as 48H and the mineral soil as 48M. The same is done for the other locations of 88 and 153 years old.

Lab work

The lab work consisted of two parts, namely determination dry weight and determination of the cation concentration.

Dry weight

For each sample, two iron pots were filled with about 5 gram of the samples and the weight of the pots was determined. Subsequently, the organic matter was put in an oven of 70 °C and the mineral soil in an oven of 105 °C. The samples stayed in the oven for at least 15 hours. Afterwards, the weight

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of the samples was measured again; thus, the weight of the dry sample can be determined by subtracting the weight of the pot of the latter measurement.

Determination of cation concentration

Initially, large roots, twigs and cones were removed from the samples. Secondly, plastic pots were washed with 2 M HCl and ELGA water in order to remove all metals from the pots. In each of the latter pots, four grams of the sample was put. Thirdly, 60 mL of 0.125 M BaCl2 was added to each

sample, so that the ratio sample:BaCl2 was 1:15, and the pots were shaken for two hours. In addition,

a control sample was added each day the experiment took place. The control sample consisted solely of 60 mL BaCl2 and was treated the same as the other samples. Fourthly, the samples were put into a

filter box with a 0.2 µm filter. Fifthly, for each sample 1.0 mL sample, 8.0 mL ELGA water and 1.0 mL acid were combined in a tube. Finally, the tubes were placed in the Inductively Coupled Plasma mass spectrometry (ICP) in order to find the Zn+_{, K}+_{, Na}+_{, Ca}2+_{, Mg}2+_{, Al}+_{, and Mn}2+_{concentrations. In order}

to determine the NH4+_{concentration, 1.0 mL sample, 7.40 mL ELGA water, 0.8 mL salicylate solution}

and 0.8 mL dichloroisocyanurate solution were combined in a tube. In addition, a calibration range was created by means of adding 0, 0.2, 0.4, 0.6, 0.8 and 1.0 mL of an ammonium stock solution (10 mg/L). 1.0, 0.8, 0.6, 0.4, 0.2 and 0 mL BaCl2 respectively was add so that it was 1 mL in total.

Subsequently, the 7.40 mL ELGA water, 0.8 mL salicylate solution and 0.8 mL dichloroisocyanurate solution were added. After an hour, the calibration range and sample solutions were examined in a colorimeter on a wavelength of 670 nm.

Processing of the data

After the lab work was finalised the data was processed. Dry weight

Firstly, the moisture content was calculated by taking the average of the two dry weights measured for each sample and calculating the percentage of this from the total weight. This percentage was subsequently used to determine the absolute dry weight of the samples put into the plastic pots. Ammonium concentration

From the measured extinction values the ammonium concentration was calculated by means of the following formula: NH4 = (E/k)*(60/1)*(1/a). In which E was the measured extinction minus the

measured extinction of the control sample of the concerning day. k was calculated by creating a calibration line from the calibration range and the concerned extinctions. k is the directional coefficient of this calibration line. Finally, a is the absolute dry weight of the concerning sample. E and k differed on each day the samples were measured and a differed for each sample.

All cation concentrations

The ICP measured the Zn+_{, K}+_{, Na}+_{, Ca}2+_{, Mg}2+_{, Al}3+_{, and Mn}2+_{concentration. Initially, the control}

sample values were subtracted from the sample values measured on the concerning day. Secondly, all the concentrations were converted to mmol/kg and the means of the 48LF, 48M, 88LF, 88H, 88M, 153LF, 153H and 153M groups were calculated and visualised in a graph. A parametric test could not be used since there was no normal distribution of the samples and the sample size was not big enough; thus, non-parametric tests were used. A Kruk Kruskall-Wallis test was done in order to find whether there is a significant difference between 48LF, 88LF and 153LF and between 48M, 88M and

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153M for each cation. Subsequently, a Dunn test showed which were significant. Since the H horizon was missing on most of the sample places on the location where the trees had a stand age of 48 years, a Wilcoxon test was performed in order determine whether there was a significant difference between 88H and 153H. The alternative hypothesis was that there is a difference between the different stand ages. The null hypothesis is that there is no difference. If the test is significant, the null hypothesis is rejected, which supports the alternative hypothesis.

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Results

The results for the different horizons and the results for the vegetation are provided in this section.

Sample location characteristics

Figure 2. Average thickness of the LF and H horizons.

No relationship between stand age and the LF horizons was found; however, the H horizon increases with stand age (see figure 2). The latter was confirmed by a Kruskall-Wallis tests. A Dunn test showed that there was significance was between 48 and 153 years and between 48 and 88 years.

Figure 3. Average percentages of ground vegetation occurring in a radius of 70 cm around the marked points on the location with a stand age of 48 years.

0 1 2 3 4 5 6 7 8 Th ickn es s h o rizo n (c m ) 48 88 153 Stand age

Thickness of the LF and H horizons

LF H

Ground coverage at a stand age of 48 years

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.

It might be important to note that on the location of 153 years, vaccinium was found about three meters from the transact (see figure 10). However, since an estimate of the understorey vegetation was only made for a radius of 70 cm around the marked point along the transact, this is not part of the graphs shown above.

Ground coverage at a stand age of 88 years

Grass Moss Vaccinium Fern No coverage

Ground coverage at a stand age of 153 years

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Figure 6. Total surface of the tree species occurring in a transact of 100 m2_.

Diversity in vegetation occurring seems to increase between a stand age of Pinus sylvestris L. of 48 and 88 years. No relationship can be found between a stand age of 88 and 153 years (see figure 3 to 6).

Cation concentrations

The found cation concentrations are provided in this section. Cation concentration in the L and F horizons

Table 1. Mean concentrations of the different groups in the F and L horizons in mmol/kg. Sample group Zn K Ca Na Mg Al Mn NH4 48LF 0.2073 1.7969 14.706 2.0217 8.8561 2.2252 0.5104 2.1341 88LF 0.6781 2.7826 45.130 2.6075 16.138 0.9716 1.4796 12.434 153LF 0.6680 3.3268 56.626 2.7746 20.072 0.5083 2.1830 13.728 0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 Su rf ace (m 2) 48 88 153 Stand age

Surface trees

Scots Pine Birch Douglas fir Oak

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Figure 7. Mean cation concentrations in the F and L horizons in mmol/kg and the concerning standard deviations.

Table 2. Kruskall-Wallis and Dunn tests for the LF horizons.

Cation Do / do not reject the null hypothesis

Significance between groups

P-value

Zinc Reject 48LF and 88LF; 48LF

and 153LF

0.0002

Potassium Reject 48LF and 153LF 0.0303

Calcium Reject 48LF and 88LF; 48LF

and 153LF

5.30532*10-5

Sodium Do not reject 0.2091

Magnesium Reject 48LF and 153LF 0.0003

Aluminium Reject 48LF and 153LF 0.0002

Manganese Reject 48LF and 153LF 0.0002

Ammonium Reject 48LF and 88LF; 48LF

and 153LF 4.86761*10-5 0 10 20 30 40 50 60 70 80 48LF 88LF 153LF Con ce n tra tio n (mm o l/kg) Stand age

Concentrations in the LF horizons

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An increase for the concentrations of zinc, potassium, calcium, sodium, magnesium, manganese and ammonium is found in the L and F horizons. For aluminium a decrease is found with stand age (see table 1 and 2 and figure 7).

Cation concentration in the H horizon

Table 3. Mean concentrations of the different groups in the H horizon in mmol/kg. Mean

Samples Zn K Ca Na Mg Al Mn NH4

88H 0.4311 0.5327 28.926 2.5501 12.579 3.3576 0.3674 3.6044 153H 0.3313 0.2389 21.472 2.3064 11.030 3.3962 0.2870 3.0524

Figure 8. Mean cation concentrations in the H horizon in mmol/kg and the concerning standard deviations. -5 0 5 10 15 20 25 30 35 40 88H 153H Con ce n tra tio n (mm o l/kg) Stand age

Concentrations H horzon

Zn K Ca Na Mg Al Mn NH4

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Table 4. Wilcoxon tests for the H horizon.

Cation Do / do not reject the null hypothesis

P-value

Zinc Do not reject 0.1620

Potassium Do not reject 0.2469

Calcium Reject 0.0376

Magnesium Do not reject 0.3075

Aluminium Do not reject 0.7913

Manganese Do not reject 0.1859

Ammonium Do not reject 0.3847

For all cation concentrations a decreasing trend is visible, except for aluminium, that slightly

increases. However, Wilcoxon test showed a significance decrease only for calcium (see table 3 and 4 and figure 8).

Cation concentrations in the mineral soil

Some of the metal concentrations in the mineral soil were under the detection limit of the ICP; as a consequence, some data is missing.

Table 5. Mean concentrations of the different groups in the mineral soil in mmol/kg. Mean

Samples K Ca Na Mg Al NH4

48M 0.4658 0.0977 0.1947 1.3384 0.2225

88M 0.6517 0.1362 0.2882 1.1839 0.1593

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Figure 9. Mean cation concentrations in the mineral soil in mmol/kg and the concerning standard deviations.

Table 6. Kruskall-Wallis tests and Wilcoxon test for the mineral soil.

Cation Do / do not reject the null hypothesis

Significance between groups

P-value

Calcium Do not reject 0.243

Magnesium Do not reject 0.3845

Aluminium Do not reject 0.2698

Ammonium Do not reject 0.6191

Although a decreasing trend is visible for aluminium and ammonium, no significant relationship is found between stand age and cation concentrations in the mineral soil (see table 5 and 6 and figure 9). -0,2 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 48M 88M 153M Con ce n tra tio n (mm o l/kg) Stand age

Concentrations mineral soil

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Discussion

The sample location characteristics and the absorbed cation concentrations are discussed in this section.

Sample location characteristics

During a succession of a pine forest organic matter accumulation generally increases with stand age. This is due to the fact that in young forests the rate of input of organic matter is faster than the decompensation rate. However, this decompensation rate increases with stand age. As a

consequence, organic matter accumulation decreases considerably after about a century (Emmer, 1995; Gower et al., 1996; Kavvadias et al., 2001). This effect can be seen in this study since there is a significant difference in thickness of the humus horizon between a stand age of 48 and 88 years, but not between a stand age of 88 and 153 years.

Development of a Pinus sylvestris L. forest is known to lead to supporting conditions for other species like dwarf shrubs and eventually other trees like birch (Karjalainen & Kuuluvainen, 2002). Although no clear increase for certain species with stand age is found in this study, it is found that with a stand age of 48 years, only grass, moss and scots pines occur, whereas for a longer stand age an increase in diversity occurs.

Development of a Pinus sylvestris L. forests is especially known for the increasing occurrence of Vaccinium myrtillus L. with stand age (Karjalainen & Kuuluvainen, 2002). Although this development cannot be found in the results of this study since it only estimated the vegetation around the sample points, the shrubs were observed on both the location of 88 stand years and 153 stand years (see figure 10).

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Adsorbed cation concentration

The adsorbed metal concentration was for most cations considerably lower in the mineral soil than in the humus form. The extraction of BaCl2 replaced the cations that formed complexes in the soil; thus,

causing a mobilisation of the cations so that they could be measured by the ICP. The humus form solely consists of organic material, whereas the first five centimetres of the mineral soil contain of mineral soil mixed with organic material (Zanella et al., 2011). The soil in the Hulshorsterzand area consists of sand, which generally has a low cation exchange capacity. Hence, it can be concluded that the cations mostly form complexes with organic material (Chapman, 1965).

Calcium

According to Epstein (1972) calcium is an essential macronutrient for plants. This is in agreement with this study since for calcium the highest overall concentration is found in the humus form; thus, this is the most abundant cation in the organic material. Calcium has a positive charge of 2, meaning that it is not relatively easily replaced by H+_{ions due to acidification (Mehlich, 1948; Mitchell & Soga,}

2005).

In the LF horizons the concentration of calcium increases between a stand age of 48 and 88 years. Due to an increase of thickness of the humus horizon with stand age, less mixing with the mineral soil occurs causing an increase in the concentration (Emmer, 1995; Gower et al., 1996; Kavvadias et al., 2001). This is in agreement with the fact that there is a considerable increase in the thickness of the humus horizon between a stand age of 48 and 88 years.

In the humus horizon the concentration of calcium decreases. The studies of Mehlich (1948) and Mitchell & Soga (2005) in combination with the results found in the LF horizons suggest that the cation exchange of calcium might be neglectable. Therefore, it may be more likely that this decrease in concentration is a result of the uptake by plants. A longer stand age generally causes the pine trees to be larger (Peltola, 2002); thus, uptake by plants may increases with stand age. No trend is found in the mineral soil for calcium, indicating that the difference in above-mentioned processes between the stand ages may be neglectable in the mineral soil.

Magnesium and Manganese

Magnesium and Manganese are both cations with a positive charge of two and both cations can be found in organic compounds of plants. Organic compounds in plants that contain magnesium are more abundant than the once containing manganese (Epstein, 1972). This is in agreement with the results from this study since the overall concentration of magnesium is higher than that of

manganese. Similar to calcium, the concentrations of magnesium and manganese increase with stand age due to a decrease in vertical mixing. No significant changes are found in the H horizon and mineral soil for this cation.

Ammonium

Organic compounds containing nitrogen enter the soil via plant residuals. Subsequently, micro-organisms convert it into ammonium through a process called net mineralisation. Plant residuals generally consist of relatively more carbon than nitrogen. As a consequence, there may be a shortage of nitrogen in the L horizon for micro-organisms, which causes the net mineralisation rate to

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consequence, relatively more nitrogen in comparison to carbon is present in this horizon. Therefore, micro-organisms do not have a shortage of nitrogen and the process of mineralisation increases (Gelfand & Yakir, 2008).

The L and F horizons are relatively shallow in old forests compared to the H horizon (see figure 2); therefore, micro-organisms in the LF horizons can obtain nitrogen from the horizon below and there is no relative shortage of nitrogen in the LF horizons in old forests. However, in younger forests the decomposition rate is much slower and hardly any humus is found (Wang & Alva, 2000) (see figure 2). Therefore, the mineralisation process increases with stand age in the LF horizon and thus the ammonium concentration increases. Similar to calcium, the concentration increases significantly between a stand age of 48 and 153 years but not between a stand age of 88 and 153 years which might be the result of the faster accumulation of organic matter in younger forests (Emmer, 1995; Gower et al., 1996; Kavvadias et al., 2001). No significant changes with stand age were found in the other horizons.

Potassium and zinc

Both potassium and zinc are elements that can be found in organic compounds of plants (Epstein, 1972). Potassium is of considerable importance for photosynthesis and biochemical reactions in the plant (Epstein, 1972; Wang et al., 2013). The concentration of zinc in plants is usually much lower and varies between 0.02-0.04 mg g-1 dry weight (Bowen, 1979). This can be seen by the higher overall concentration of potassium in the LF horizons. Since both cations have a positive charge of one, potassium and zinc are among the first cations to be replaced by H+_{ions due to acidification.}

(Mehlich, 1948; Mitchell & Soga, 2005).

Similar to the previously discussed cations there is an increase of these cations with stand age due to a decrease of vertical mixing with the mineral soil (Emmer, 1995; Gower et al., 1996; Kavvadias et al., 2001). Therefore, it can be concluded that this process is dominant over cation exchange in the LF horizons. Although a decreasing trend is found in the H horizon, it is not significant; thus, the result of cation exchange is not found in this study.

Sodium

No correlation is found between sodium concentration and stand age. According to Epstein (1972) there are no organic compounds in vegetation that are known to contain sodium. Therefore, sodium found in this study may be a result of deposition of salt upon the pine needles (Poikolainen, 1997). Aluminium

Aluminium is another cation that is not found in any organic compounds of plants (Epstein, 1972). In the LF horizons a decrease in aluminium concentration is found with stand age. Aluminium has a positive charge of three; therefore, it is unlikely that cation exchange took place for aluminium (Mehlich, 1948; Mitchell & Soga, 2005). Aluminium is generally found in the mineral soil (Kaiser & Guggenberger, 2003), which can be seen in this study by the relatively high concentration of aluminium in the mineral soil. On the location with a stand age of 48 years, there was usually no humus horizon. Hence, the LF horizons were the only horizons of the humus profile, which is

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LF horizons and the mineral soil. Therefore, it is assumed that the increase of aluminium concentration might be caused by vertical mixing.

Acidic rain

Acidic rain may have been a major cause of cation exchange in the twentieth century (Cronan & Reiners, 1983). However, in the twenty-first century a considerable decrease in the emissions of sulphur dioxide, nitrogen oxides and ammonia caused a considerable decrease in acid deposition (Wettestad, 2018). As a result, little cation exchange may take place in the study area. Since a replacement of cations by H+_{ions may cause a shortage of cations for vegetation, this might have}

positive results for the development of the Scots pine forest in the area (Wood & Bormann, 1977).

For future research

For future research it is recommended to use a more exact way to determine vegetation growth in the area, for instance by using detailed maps in GIS. It is also recommended to use a different soil – solution ratio for the mineral soil than for the humus form, since metal cation concentrations are generally lower in the mineral soil.

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Conclusion

The thickness of the humus profile generally increases with stand age; however, this process decreases considerably after about a century. Meanwhile, diversity in vegetation increases with stand age. Calcium, magnesium, manganese, potassium and zinc are found in organic compounds of plants. Due to a decrease of vertical mixing of the LF horizons with the mineral soil, these

concentrations increase with stand age. Net mineralization of ammonium by micro-organisms in the LF horizons increases with stand age due to a decrease of a relative shortage of nitrogen. As a consequence, the ammonium concentration increases with stand age. Aluminium is a metal that is generally not found in organic compounds of plants but it is generally found in the mineral soil. Therefore, the concentration of aluminium in the LF horizons decreases with stand age due to a decrease in vertical mixing with the mineral soil. The concentration of calcium decreases with stand age in the H horizon due to uptake of plants. Meanwhile, no other significant changes with stand age are found in the H horizon or in the mineral soil. Metals that form complexes with organic matter in the soil can be replaced by H+_{ions due to acidification through a process called cation exchange. In}

the twentieth century the main reason for cation exchange was acidic rain. Thus, a decrease in acidic atmospheric deposition might be an explanation why no considerable decreases in cation

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Acknowledgement

The author wishes to acknowledge the following people for their contribution to this paper: • Albert Tietema as supervisor, for his contribution to the setup of this research, for his

guidance and advises and for commenting on an earlier draft of this paper.

• Jan Sevink for his contribution to the setup of this research, and his guidance and advises concerning the method and field work.

• Rutger van Hall for his guidance in the lab and his advises concerning the processing of the data.

• Remko van Rosmalen for showing the research locations in the Hulshorsterzand area. The author wishes to thank Natuurmonumenten for its permission to do research in the Hulshorsterzand area.

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