tree species (picea abies and fagus sylvatica) effects on soil water acidification and aluminium...

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Tree species (Picea abies and Fagus sylvatica) effects on soil water acidification and aluminium chemistry at sites subjected to long-term acidification in the Ore Mts., Czech Republic Filip Oulehle a,b, * , Jakub Hrus ˇka a a Czech Geological Survey, Department of Environmental Geochemistry and Biogeochemistry, Klarov 3, 118 21 Prague, Czech Republic b Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague, Czech Republic Received 29 March 2005; received in revised form 2 June 2005; accepted 14 June 2005 Available online 28 July 2005 Abstract The effect of European beech (Fagus sylvatica) and Norway spruce (Picea abies) on acid deposition and soil water chemistry was studied at a site in the Ore Mts., Czech Republic, that has been subjected to decades of elevated acidic deposition. Dry deposition onto the spruce canopy significantly increased acid input to the soil in comparison to the beech canopy. As a result soil waters were more acidic; Al, SO 2 4 , and NO 3 concentrations were significantly higher; and Ca and K concentrations were lower in the spruce stand than in the beech stand. The concentrations of potentially toxic inorganic aluminium (Al in ) were, on average, three times higher in the spruce stand than in the beech stand. Thus, Al played a major role in neutralizing acid inputs to mineral soils in the spruce stand. Despite the higher dissolved organic carbon (DOC) concentrations in spruce organic soil solutions, organic Al (Al org ) accounted for only 30% of total Al (Al tot ), whereas in beech organic soil solutions Al org was 60% of Al tot . Soil waters in the beech stand exhibited Al in concentrations close to solubility with jurbanite (Al(SO 4 )OH 5H 2 O). The more acidic soil waters in the spruce stand were oversaturated with respect to jurbanite. The Bc/Al in ratio (Bc = Ca + Mg + K) in O horizon leachate was 4.6 and 70 in spruce and beech stands, respectively. In beech mineral soil solutions, the Bc/Al in ratio declined significantly to about 2. In the spruce stand, mineral soil solutions had Bc/Al in values below the critical value of 1. The observed Bc/Al in value of 0.4 at 30 cm depth in the spruce stand suggests significant stress for spruce rooting systems. A more favourable value of 31 was observed for the same depth in the beech stand. The efficiency of the spruce canopy in capturing acidic aerosols, particulates, and cloud water has resulted in the long-term degradation of underlying soils as a medium for sustainable forest growth. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Acid deposition; Norway spruce; European beech; Aluminium; Soil solution 1. Introduction Elevated concentrations of Al are evident in soil solu- tions and surface waters draining forest soils impacted by acid deposition. An understanding of the biogeo- chemistry of Al is critical because of its toxicity to veg- etation and aquatic organisms as well as its role in the buffering of acid waters and linkages to other elements. The mountain regions of the Czech Republic are highly impacted by acidic deposition and forest damage is his- torically well documented [1–3]. The most damaged areas lie in the Ore Mountains (Erzgebirge in German, Krus ˇne ´ hory in Czech; Fig. 1). Until the 1990s about 25 000 ha of dead or severely damaged Norway spruce (Picea abies) forests were clear-cut [4], representing more than 50% of the forest area in this region. This area was subjected to extremely high levels of atmospheric pollu- tion due to the intensive burning of locally mined lignite, 0162-0134/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2005.06.008 * Corresponding author. Tel.: +42 025 108 5431; fax: +42 025 181 8748. E-mail address: [email protected] (F. Oulehle). www.elsevier.com/locate/jinorgbio Journal of Inorganic Biochemistry 99 (2005) 1822–1829 JOURNAL OF Inorganic Biochemistry

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JOURNAL OF

www.elsevier.com/locate/jinorgbio

Journal of Inorganic Biochemistry 99 (2005) 1822–1829

InorganicBiochemistry

Tree species (Picea abies and Fagus sylvatica) effects on soilwater acidification and aluminium chemistry at sites subjectedto long-term acidification in the Ore Mts., Czech Republic

Filip Oulehle a,b,*, Jakub Hruska a

a Czech Geological Survey, Department of Environmental Geochemistry and Biogeochemistry, Klarov 3, 118 21 Prague, Czech Republicb Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague, Czech Republic

Received 29 March 2005; received in revised form 2 June 2005; accepted 14 June 2005Available online 28 July 2005

Abstract

The effect of European beech (Fagus sylvatica) and Norway spruce (Picea abies) on acid deposition and soil water chemistry wasstudied at a site in the Ore Mts., Czech Republic, that has been subjected to decades of elevated acidic deposition. Dry depositiononto the spruce canopy significantly increased acid input to the soil in comparison to the beech canopy. As a result soil waters weremore acidic; Al, SO2�

4 , and NO�3 concentrations were significantly higher; and Ca and K concentrations were lower in the spruce

stand than in the beech stand. The concentrations of potentially toxic inorganic aluminium (Alin) were, on average, three timeshigher in the spruce stand than in the beech stand. Thus, Al played a major role in neutralizing acid inputs to mineral soils inthe spruce stand. Despite the higher dissolved organic carbon (DOC) concentrations in spruce organic soil solutions, organic Al(Alorg) accounted for only 30% of total Al (Altot), whereas in beech organic soil solutions Alorg was 60% of Altot. Soil waters inthe beech stand exhibited Alin concentrations close to solubility with jurbanite (Al(SO4)OH Æ 5H2O). The more acidic soil watersin the spruce stand were oversaturated with respect to jurbanite. The Bc/Alin ratio (Bc = Ca + Mg + K) in O horizon leachatewas 4.6 and 70 in spruce and beech stands, respectively. In beech mineral soil solutions, the Bc/Alin ratio declined significantlyto about 2. In the spruce stand, mineral soil solutions had Bc/Alin values below the critical value of 1. The observed Bc/Alin valueof 0.4 at 30 cm depth in the spruce stand suggests significant stress for spruce rooting systems. A more favourable value of 31 wasobserved for the same depth in the beech stand. The efficiency of the spruce canopy in capturing acidic aerosols, particulates, andcloud water has resulted in the long-term degradation of underlying soils as a medium for sustainable forest growth.� 2005 Elsevier Inc. All rights reserved.

Keywords: Acid deposition; Norway spruce; European beech; Aluminium; Soil solution

1. Introduction

Elevated concentrations of Al are evident in soil solu-tions and surface waters draining forest soils impactedby acid deposition. An understanding of the biogeo-chemistry of Al is critical because of its toxicity to veg-etation and aquatic organisms as well as its role in the

0162-0134/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.jinorgbio.2005.06.008

* Corresponding author. Tel.: +42 025 108 5431; fax: +42 025 1818748.

E-mail address: [email protected] (F. Oulehle).

buffering of acid waters and linkages to other elements.The mountain regions of the Czech Republic are highlyimpacted by acidic deposition and forest damage is his-torically well documented [1–3]. The most damagedareas lie in the Ore Mountains (Erzgebirge in German,Krusne hory in Czech; Fig. 1). Until the 1990s about25 000 ha of dead or severely damaged Norway spruce(Picea abies) forests were clear-cut [4], representing morethan 50% of the forest area in this region. This area wassubjected to extremely high levels of atmospheric pollu-tion due to the intensive burning of locally mined lignite,

Fig. 1. Site locations in the Ore Mountains, Czech Republic (grey – nature reserve).

F. Oulehle, J. Hruska / Journal of Inorganic Biochemistry 99 (2005) 1822–1829 1823

or ‘‘brown coal’’. In 1992–1998 the power plants weredesulphurized and sulphur emissions declined. Emis-sions of SO2 peaked in the 1980s at 800 000 tons yr�1

and decreased to 50 000 tons in 2003. Despite this sub-stantial decrease, forest health has not improved signif-icantly [5].

Soil solution chemistry provides insight into elementcycling, nutrient uptake and availability, mineral trans-formation, and pollution transport processes withinthe subsurface environment [6]. Aluminium toxicity re-lated to high soil solution concentrations is consideredan important factor in forest decline in Central Europe[7]. Chronically high atmospheric inputs of H+ and acidanions to the soil may exhaust the buffer capacity pro-vided by base cations, resulting in aluminium buffering.However, the mechanisms controlling the solubility ofdifferent species within the solution and how they influ-ence soil acidification are frequently site-specific and re-main a topic of debate. A better understanding ofaluminium behaviour in soil solution is important forsimulating the effects of acidic deposition on soil andwater systems by models [8,9].

Many investigators have focused on the role of differ-ent lithologies [10,11], different levels of acid deposition[12], and the influence of tree species [13,14] on the bio-geochemistry of forest ecosystems. Tree species maygreatly influence the biogeochemistry of forest ecosys-tems [15]. Throughfall deposition and soil leaching ofsulphate, nitrogen compounds and acidity is signifi-cantly higher in spruce compared to beech stands. Thedifference between the two species increases with thedeposition load and is negligible in remote, relativelyunpolluted areas [16]. However, the filtering surface ofthe leaves (dry deposition) is about 3 times higher inspruce compared to beech due to a higher leaf area indexand leaf retention period. The aim of this paper is todemonstrate the effect of tree species (Norway spruceand European beech) on aluminium forms and concen-trations in soil solutions in stands affected by long-termacid deposition.

2. Site description

We studied spruce and beech stands located near theborder between the Czech Republic and Germany in theOre Mountains, close to the villages of Kienhaide (Ger-many) and Nacetın (Czech Republic), Fig. 1. The dis-tance between the stands is only 700 m. Thus, thestands have experienced similar climatic and pollutionconditions in the past. Average annual temperature is6.3 �C (1991–2004), average annual precipitation is842 mm (1991–2004), and the slope is to the north-west.Paragneiss underlies both stands. The dominant soils aredystric cambisols [17]. The spruce stand (50�35 02600N,13�15 01400E) lies at an elevation of 784 m a.s.l. and iscovered by 68 year old Norway spruce (Picea abies).The spruce forest at this site is one of the very fewremaining mature spruce stands in the Ore Mts. thathas never been limed. The beech stand (50�35 02200N,13�16 00700E) lies at an elevation of 823 m a.s.l. The siteis a part of nature reserve. This reserve is predominantlycovered by beech (Fagus sylvatica) that are older than120 years. The spruce site at Nacetın has been studiedsince the late 1980s [18], very intensively in 1994–1999[19].

3. Methods

Throughfall below the spruce and beech canopy wascollected monthly (November 2003–October 2004) withnine collectors in each of two plots (15 · 15 m). Twobulk deposition collectors were also sampled monthlybetween the stands at an open area. The precipitationwas sampled by polyethylene funnels (area of122 cm2) replaced in winter by plastic vessels (area of167 cm2).

Soil water has been collected in the spruce plot sincethe 1990s using PRENART� suction lysimeters atdepths of 30, 60 and 90 cm in the mineral soil (7 lysime-ters at 90 cm, and 4 lysimeters at 30 and 60 cm). Both

1824 F. Oulehle, J. Hruska / Journal of Inorganic Biochemistry 99 (2005) 1822–1829

stands were also equipped with three zero-tension lysi-meters below the organic (O) horizon in each plot inspring 2003. At the same time PRENART� suction lysi-meters were installed 30, 60 and 90 cm below the top ofmineral horizon in the beech plot (6 lysimeters at eachdepth). All lysimeters were sampled monthly and bulkedat the end of the month to create one sample from eachdepth at each plot for each month.

Solution pH was measured using a Radiometer TTT-85 pH meter with a combination electrode (Radiometermodel GK-2401C). Cl�, SO2�

4 ; NO�3 were measured by

ion exchange chromatography, and F� by potentiomet-ric ion selective electrode after TISAB addition. Ca, Mg,Na, K, and Al were determined by flame atomic absorp-tion spectrometry (FAAS), and NHþ

4 by indophenolblue colorimetry. Ionic concentrations of Ca2+, Mg2+,Na+, and K+ were assumed to equal the total concentra-tions. Dissolved organic carbon (DOC) was determinedby oxidation and detection as CO2 (Apollo 9000, Tek-mar-Dohrmann). Alkalinity (ANC) was measured bystrong acid (0.1 M HCl) titration with Gran plotanalysis.

Total monomeric Al (Alm) and Alorg were measuredusing the pyrocatechol violet method described by Rø-geberg and Henriksen [20] and modified by LaZerte[21]. Organic monomeric Al (Alorg) was separated usinga strong cation exchange resin (Dowex 50 W) in mixedH+ (98%) and Na+ (2%) form [22]. Inorganic mono-meric Al (Alin) was calculated as the difference betweenAlm and Alorg. Acid soluble Al (Alas), assumed to bemainly in particulate or microcrystalline form, was cal-culated as the difference between Altot(FAAS) and Alm[23].

Table 1Mean annual ion concentrations (volume-weighted by precipitation) for the

Parameter (leq L�1) Spruce (Picea abies)

Precipitation Soil depth

Bulk Throughfall O horizon �30 �60

pHa 4.68 4.35 3.66 4.31 4.0Na+ 14 41 56 91 115K+ 4.8 85 48 9.9 23Mg2+ 6.2 33 106 83 98Ca2+ 13 73 122 38 50NHþ

4 43 91 8.3 0.87 7.4Aln+b 0.46 2.6 65 197 169F� 0.53 3.5 10 19 17Cl� 17 56 94 91 124NO�

3 30 108 230 206 60SO2�

4 35 149 259 433 512DOCb 144 111 2994 358 889ANC �28 �42 �256 �36 �81Bc/Alc – – 4.6 0.4 0.7

a Units.b lmol L�1.c mol/mol.

4. Results and discussion

4.1. Atmospheric deposition

The annual precipitation and throughfall quantitiesfor the 2004 hydrological year (Nov. 2003 to Oct.2004) were 958 mm of bulk precipitation and 572 and560 mm of throughfall in the spruce and beech plots,respectively. The concentration of all solutes increasedbelow the canopy in both stands except for H+ in beechthroughfall (Table 1). The pH in beech throughfall was4.90 compared to 4.68 in bulk precipitation and 4.35 inspruce throughfall. The majority of the solute concen-trations were enriched more in spruce throughfall thanin beech throughfall (Table 1). This differential enrich-ment was most profound for SO2�

4 and NO�3 . For Mg,

K, NH4 and DOC, similar enrichment was observed inspruce and beech stands.

Ionic fluxes (Table 2) increased below the canopy forall elements with the exception of H+ in the beech stand(Table 2) and NO�

3 –N (not shown). However, DINfluxes increased in the canopies of both stands. Therewere significant differences in anion fluxes in beechand spruce forests. The deposition of sulphur inthroughfall was 100% higher in the spruce stand com-pared to the beech stand (14 vs. 7 kg ha�1 yr�1). TheDIN and Cl fluxes were also significantly higher underthe spruce canopy. The cation fluxes were similar forK and Mg (19 and 2.5 kg ha�1 yr�1, respectively) butfor Ca, Na and Al were higher in the spruce stand (Table2). As a result the H+ flux was significantly higher underthe spruce canopy (0.26 vs. 0.07 kg ha�1 yr�1). Drydeposition onto the canopy was a significant source of

water year 2004

Beech (Fagus sylvatica)

Soil depth

�90 Throughfall O horizon �30 �60 �90

8 4.38 4.90 4.45 4.70 4.43 4.5464 29 34 54 53 649.6 89 80 14 5.4 1254 35 134 128 109 14051 49 129 120 39 580.57 93 30 6.1 0.56 2.2

135 1.1 19 27 54 5713 1.8 3.2 4.2 7.9 8.947 41 52 56 63 6018 48 157 51 28 22518 80 11 144 259 315210 818 2014 1953 323 176�31 64 �26 30 �31 �25

0.5 – 71 31 2 2.1

Tab

le2

Meanan

nual

massfluxesforthewater

year

2004

ðDIN

¼N–NO

� 3þN–NH

þ 4Þin

bulk

andthrough

fallprecipitation

Na

(kgha�

1yr

�1)

K (kgha�

1yr

�1)

Mg

(kgha�

1yr

�1)

Ca

(kgha�

1yr

�1)

Aln+

(kgha�

1yr

�1)

H (kgha�

1yr

�1)

F (kgha�

1yr

�1)

S–SO

2 4�

(kgha�

1yr

�1)

Cl

(kgha�

1yr

�1)

DIN

(kgha�

1yr

�1)

DOC

(kgha�

1yr

�1)

Bulk

3.2

1.8

0.72

2.5

0.12

0.20

0.10

5.4

5.7

9.7

15THFbeech

3.8

202.4

5.6

0.16

0.07

0.19

7.2

8.2

1153

THFspruce

5.4

192.3

8.3

0.39

0.26

0.38

1411

1655

F. Oulehle, J. Hruska / Journal of Inorganic Biochemistry 99 (2005) 1822–1829 1825

acidic deposition in the spruce stand, as indicated bySO2�

4 throughfall/bulk precipitation flux ratios of 2.5and 1.3 in spruce and beech stands, respectively (Table2).

As a result of SO2 emission reductions during the1990s [24], the present SO2�

4 concentrations in sprucethroughfall are one-third of concentrations measuredin 1992 [17]. Similarly, present-day concentrations ofCa, Al, and H+ are 1/2, 1/7, and 1/5 of 1992concentrations.

4.2. Soil solution

4.2.1. pH and anion concentrationsThe pH measured in organic soil solution was

much lower in the spruce stand (3.66) than in thebeech stand (4.45). In the mineral soil (60 cm depth)the pH increased at the spruce stand (4.08) and re-mained stable in the beech stand (4.43). At 90 cmdepth, pH rose in both stands but remained lowerfor the spruce stand (pH = 4.38) compared to thebeech stand (pH = 4.54).

Soil solutions draining the organic horizon of thespruce stand exhibited considerably higher concentra-tions of SO2�

4 (259 leq L�1) than in the beech stand(111 leq L�1). The SO2�

4 concentration increased withsoil depth (60 cm) in both stands (512 leq L�1 in sprucevs. 259 leq L�1 in beech, Table 1). The higher SO2�

4 con-centrations in spruce soil water is the result of higherdeposition (Table 2) as well as elevated soil sulphurpools that were the result of historically high acid depo-sition. The spruce soil S pool is about 1500 kg ha�1 andit is enough to supply the current seepage flux forroughly 60 years [25].

The NO�3 concentrations in shallow soil water (30 cm

depth) were higher in the spruce stand (206 vs.51 leq L�1), but in deep soil solutions (90 cm) therewas no difference between the sites (18 vs. 22 leq L�1).

The concentrations of fluoride and chloride were alsosignificantly higher within the whole soil profile of thespruce stand (Table 1).

4.2.2. Base cation concentrations

The sum of the base cation concentrations (Ca, Mg,Na, K) in organic soil solutions were not very differentbetween the two stands (330 vs. 380 leq L�1). The con-centrations decreased in the mineral soil, with theexception of Na. Significant differences were noted be-tween the stands in the concentrations of Ca and Mg insoil water from mineral horizons. At 30-cm depth, theconcentration of Ca was three times higher in thebeech soil water (120 vs. 38 leq L�1), but this differ-ence diminished in deeper soil solutions (Table 1).The Mg concentration was higher in the whole soilprofile in the beech stand (109–140 vs. 54–106 leq L�1;Table 1).

0 50 100 150 200

So

il D

epth

Al (µmol L-1)

Organic Monomeric Al

Inorganic Monomeric Al

Acid Soluble Al

O horizon

-90cm

-60cm

-30cm

pH = 3.66

pH = 4.54

pH = 4.08

pH = 4.43

pH = 4.31pH = 4.7

pH = 4.45

pH = 4.38

Fig. 2. Mean pH and concentrations of Al fractions in soil water (upper column – beech, lower column – spruce).

1826 F. Oulehle, J. Hruska / Journal of Inorganic Biochemistry 99 (2005) 1822–1829

4.2.3. Aluminium and DOC

The concentration of Altot in water draining the or-ganic horizon was higher in the spruce stand(65 lmol L�1) than in the beech stand (20 lmol L�1).The Altot in mineral soil solutions was much higher inthe spruce forest (140 to 200 lmol L�1) than in thebeech forest (30–60 lmol L�1; Table 1).

Soil solutions draining the organic horizon exhibitedhigh concentrations of DOC, approximately 3000 and2000 lmol L�1 in spruce and beech stands, respectively.Despite the high DOC concentrations, about 50% ofAltot in spruce O horizon leachate was in the potentiallytoxic inorganic monomeric fraction (Alin). In contrast,the beech organic soil solution consisted of 60% oforganically-bound Al (Alorg) and the total concentrationof Al (Altot) was one-third of the concentration in themore acidic spruce stand (Fig. 2). The highest concen-

0

5

10

15

20

25

30

-4.0 -3.8 -3.6 -3.4 -3.2log DOC

Al o

rg (

µmo

l L-1

)

beech organic spruce orga

beech mineral spruce mine

Fig. 3. The concentrations of organic monomeric Al (Alorg) as a function

tration of Altot (197 lmol L�1) was observed at the edgeof the rooting zone (30 cm depth) of the spruce stand,where more than 80% was in the Alin form. Deeper inthe mineral soil, Altot decreased only slightly and Alin re-mained the dominant fraction (Fig. 2).

In the upper mineral soils of the beech stand (30 cm)the concentration of DOC remained high (1953 lmolL�1) and Alorg was still the dominant form of Al. Inthe beech stand, Altot was only about 15% of the levelsobserved in the spruce stand at 30 cm depth. In the dee-per mineral soil solutions of the beech stand, Alin dom-inated, but the concentrations were one-third of those inthe spruce stand in the same depth (Fig. 2).

Despite the different concentrations of total Al, DOCand pH in the two stands, there was a strong relation-ship (Fig. 3) between Alorg and DOC concentration(R2 = 0.74, p < 0.001) for all horizons in both stands.

-3.0 -2.8 -2.6 -2.4 -2.2(mol L-1)

nic

ral

of DOC (log transformed) for different horizons (linear regression).

F. Oulehle, J. Hruska / Journal of Inorganic Biochemistry 99 (2005) 1822–1829 1827

Thus, it appears that DOC at both stands exhibits sim-ilar Al-binding properties.

Aluminium in throughfall precipitation contributesonly slightly to the cation charge balance (2–3%),whereas base cations contribute 80–90% of the cationcharge in throughfall in both stands. Similar patternswere observed for soil solutions below the organic hori-zon and at 30 cm depth in the beech stand, where themajority of acidity was neutralized by base cations(computed as the charge balance between the cationsand anions). In deeper soils of the beech stand, abouttwo-thirds of inorganic acidity was neutralized by basecations and about one-third by aluminium. In contrast,aluminium neutralized 16% of the acidity in solutionsdraining the spruce organic horizon, and up to 60% ofthe acidity in the mineral soil. These data suggest thataluminium mobilization is the main buffering processfor acidity in the more acidic spruce soil.

The solubility of Al with respect to selected mineralphase equilibria was evaluated using thermodynamicequilibrium constants at standard temperature and pres-sure. The pH of organic horizon soil solutions was lowerthan the bulk precipitation, especially in spruce solu-tions, due to the mobilisation of DOC and associatedorganic acidity [26] and the capture of strong mineralacids by the forest canopy. Thus the organic horizon soilsolutions were undersaturated with respect to all of thesolid phases (Fig. 4) that could be responsible for con-trolling aluminium concentrations in the soil water(gibbsite, jurbanite, alunite). In organic soil layers, thereis a lack of aluminium due to low mineral content andlong-term leaching. Hence, one would not expect to findAl(OH)3 or other minerals capable of controlling alu-minium release. Therefore, solution aluminium concen-tration in organic soil horizons is considered to be

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

3.5 4.0 4.5p

log

(A

l in)

Al(OH)3 (amorphoAl(OH)3 (gibbsite)

Fig. 4. Concentrations of inorganic monomeric Al (log transformed) as a fun(full) and 9.66 for the amorphous Al(OH)3 (dotted) was used for the chemical�3.8 and SO2�

4 concentration of 126 lmol L�1 (average solution concentrati

mainly a result of exchange reactions between an organ-ic exchanger and the solution [27] and/or the release ofAl from decomposing organic matter [28]. The undersat-uration of soil solutions was also observed for otheracidified soil solutions in the Czech Republic [23,29].

Waters from the mineral horizons at both stands wereoversaturated with respect to gibbsite (log K = 8.04;Fig. 4). At the spruce site, the very acidic water fromthe organic horizon (pH = 3.66) probably promotes ra-pid weathering of aluminosilicates in the mineral soil,leading to consumption of H+ and release of Al3+. Asa result Al3+ concentrations approach equilibrium withgibbsite and amorphous Al(OH)3 (log K = 9.66; Fig 4).

Organic soil solutions entering the mineral soil at thebeech site were much less acidic (pH = 4.45) than in thespruce stand. Thus, there is only a slight pH increaseand small release of Al3+ in the upper 30 cm of mineralsoil. While mineral soil solutions also lie between gibb-site and amorphous Al(OH)3 equilibria, there is a veryclose relationship with jurbanite equilibrium (Fig. 4).While numerous authors have found that the Al3+ activ-ity in soil waters appears to be controlled by the solubil-ity of gibbsite-like minerals [30–32], the apparentequilibrium with jurbanite in beech mineral soil solu-tions is quite unexpected. Jurbanite is reported to be sta-ble at low pH and high SO2�

4 activities [33], but thebeech stand at Nacetın is relatively low in SO4 andH+, at least in comparison to the spruce stand, whereequilibria is closer to Al(OH)3 control (Table 1, Fig 3).Nevertheless, many authors have reported that theactivity of Al3+ in acidic groundwaters [34] and soilsolutions [6,35,29,36] appears to be regulated by the sol-ubility of jurbanite.

Assessing the geochemical controls on Al concen-trations in drainage waters involves considerable

5.0 5.5 6.0H

Spruce Mineral Beech MineralSpruce O rganic Beech OrganicSpruce Jurbanite Beech Jurbanite

us)

Al(SO)4(OH).5H2O (jurbanite)

ction of pH for different soil waters. A log (K) of 8.04 for the gibbsiteequilibrium calculations assuming a temperature of 25 �C. A log (K) ofon in the beech mineral soilwater) was used for the jurbanit equlibria.

1828 F. Oulehle, J. Hruska / Journal of Inorganic Biochemistry 99 (2005) 1822–1829

uncertainty. There is a high variability of apparentsolubility constants [37] and inconsistencies in mea-surement procedures [38]. Our results suggest thathigh H+ inputs to the mineral soil (spruce stand) re-sult in Al3+ concentrations in equilibrium with gibb-site-like minerals. At lower H+ concentrations (beechstand), the equilibrium is close to the hydroxysulfatemineral, jurbanite.

High concentrations of aluminium, or low base cat-ion to aluminium ratios (Bc/Al, where Bc = K +Ca + Mg), in soil solution can cause physiological stressfor the spruce root system [7]. In particular, Ca/Al [37]or Bc/Al [39,40] ratios below 1.0 have been proposedas threshold values, below which there is risk of signifi-cant damage of plants. Al concentration (or Ca/Al ra-tio) and acid deposition-induced Mg (and possibly Ca)deficiency are important factors influencing root growthand distribution in acidic forest soils [41]. The Bc/Alinratio in O horizon leachate was 4.6 and 70 in spruceand beech stands, respectively. These relatively high ra-tios suggest that the rooting environment in organichorizons is healthy. The Bc/Alin ratio declined signifi-cantly in the mineral soil at both sites, and fell belowthe critical value in spruce soil waters in the mineral soilhorizons. The Bc/Alin value of 0.4 at 30 cm depth in thespruce stand (Table 1) suggests significant stress for thespruce rooting system. In comparison, a more favour-able value of 31 was observed for the same depth inthe beech stand as a result of lower acidity, lower Alin,and higher base cation concentrations.

5. Conclusions

Atmospheric deposition of sulphur and nitrogen, asmeasured by throughfall fluxes, were 94% and 45%greater under spruce than in the beech forest as a resultof enhanced dry deposition onto the spruce canopy.Thus the acidity of soil solutions was significantly higherin the spruce stand. Elevated acidity in organic soil solu-tions resulted in high concentrations of aluminium inmineral soil solutions due to aluminosilicate weathering.Despite the higher DOC concentration in spruce organicsoil solution, the Alorg concentration contributed only30% of Altot, whereas in the beech stand organic soilsolution the value was 60%. The concentrations ofpotentially toxic inorganic aluminium (Alin) was onaverage three times higher in the spruce stand comparedto the beech stand. Aluminium plays a major role inneutralizing acidity in mineral soils of the spruce stand.Our results indicate that the aluminium chemistry of soilsolutions was significantly affected by tree species. Dur-ing long-term acidification, the beech stand was able topartially mitigate incoming acidity and keep the soilenvironment more favourable for forest growth thanthe spruce stand.

Acknowledgements

This study was supported by the Grant Agency of theCzech Republic (No. 526/03/0058). The comments onthe English language of Chris E. Johnson, Syracuse Uni-versity, NY, USA was greatly appreciated.

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