effects of forest age on surface drainage water and soil solution aluminium chemistry in...

EFFECTS OF FOREST AGE ON SURFACE DRAINAGE WATER AND

SOIL SOLUTION ALUMINIUM CHEMISTRY IN STAGNOPODZOLS IN

WALES

S. HUGHES, D. A. NORRIS, E A. STEVENS, B. REYNOLDS and T. G. WILLIAMS Institute of Terrestrial Ecology, Bangor Research Unit, University College of North Wales,

Deiniol Road, Bangor, Gwynedd. LL57 2UP. UK

and

C. WOODS Institute of Terrestrial Ecology, Merlewood Research Station,

Grange-over-Sands, Cumbria, LAI1 6JU. UK

(Received 19 November, 1992; accepted in final form 3 October, 1993)

Abstract. The influence of forest development on soil solution and surface drainage water aluminium chemistry was investigated in Sitka spruce (Picea sitchensis) plantations in Wales. Comparisons with semi-natural grassland and moorland sites are described. A highly significant positive relationship was shown between increasing forest age and soilwater aluminium concentrations in the B horizons. Short- term/episodic peaks in A1 concentrations were strongly related to incidences of high concentrations of neutral, marine-derived, salts in the soilwater. Nitrification may be an important factor in soil acidification and the mobilization of A1 in soilwaters beneath the older mature-forest plantations in Wales. Labile monomeric A1 concentrations were largest in surface waters draining the oldest forestry plantations compared with younger forest catchments and moorland, although response to discharge of soilwater acidity to the surface waters at individual sites was dependent on the acid neutralizing capacity of the groundwater component of the surface waters.

1. Introduction

In the past two decades, concern over the effects of acid precipitation on terrestrial and aquatic ecosystems has generated much interest in soil and surface water acidification. In Britain, there is continuing debate regarding the effect of conifer plantations on surface water acidification in base-poor areas which receive acid deposition (Ormerod et al., 1989). Previous studies have shown that soil solutions under plantation forestry, and surface waters draining forested catchments, are more acid and contain larger concentrations of dissolved aluminium than their semi-natural moorlands counterparts (Reynolds et al., 1986; Neal et al., 1989). In a survey of 113 Welsh catchments of contrasting land use, Ormerod et al. (1989) reported that, within each of three different ranges of acid sensitivity (< 10, 10-15, 15-25 mg CaCO3 dm -3 total hardness), streamwater pH declined and aluminium concentrations increased significantly with increasing percentage forest cover. Whereas these studies implicate plantation forestry in soil and surface water acidification, there is continuing uncertainty on the relative roles of acid deposition to the forest canopy and tree physiology (e.g. base cation uptake in the biomass, organic acid production in the forest litter) in soil acidification (Hornung, 1985).

Water, Air and Soil Pollution 77:115-139, 1994. (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands.

116 s. HUGHES ET AL.

It is well established that organic acids liberated from decomposition of litter in the forest floor can initiate weathering reactions and provide a means of dissolution and transport of aluminium in the solum (Driscoll et al., 1985; Hughes et al., 1990; Tam and McColl, 1991; Ugolini and Sletten, 1991). Neal et al. (1989) stressed the roles of weathering and ion exchange in determining soil solution A1 chemistry in the lower horizons of forested podzols in Wales. They argued that ion exchange processes, which occur rapidly, are the proximal mechanisms controlling short-term/episodic fluctuations in A1 chemistry in the soilwater; mineral weathering being more important as a longer-term control. These contrasting control mechanisms can influence not only solubility and mobilization of A1 in soil but also its speciation, which is widely regarded as being important in determining the possible toxic effects of aluminium to vascular plants and aquatic biota (Driscoll et al., 1980; Hue et al., 1986; Andersson, 1988).

Much of the earlier research on the effects of plantation forestry concerned spatial comparisons between only a limited number of mature forest sites and adjacent moorland (Harriman and Morrison, 1982; Stoner et al., 1984; Reynolds et al., 1986). There is also a need, identified by Ormerod et al. (1989), for the collection of temporal data on soil solution and surface waters from forests of different ages at a range of sites so that changes in chemistry can be assessed during forest development.

This paper presents data on soil solution and surface drainage water aluminium chemistry collected in a survey of Sitka spruce plantations of differing ages in five forests in Wales. Comparisons with semi-natural grassland or moorland sites are described.

2. Survey

Twenty five sites in north and central Wales were chosen for the survey (Figure 1). Five of the sites were on agriculturally unimproved grassland or moorland, all other sites were located in Sitka spruce (Picea sitchensis (Bong.) Carr) plantations of various ages in 5 major forest areas (Beddgelert, Dyfi, Dyfnant, Hafren and Tywi). In planning the investigation it was intended that each forest area would provide a moorland site and a forest site in each of 4 age classes; age classes covering 15 years of growth. This proved impracticable and the actual age classes used are as shown in Figure 1. All forest sites were first rotation (except for BT2, which was later found to be second rotation) and originally planted on agriculturally unimproved grassland or moorland. Moorland sites BT0, DI0 and HN0 were dominated by Nardus stricta - Festuca ovina grassland, TY0 by Molinia caerulea grassland and DN0 by heather moorland composed primarily of Calluna vulgaris and Erica cinerea. All moorland catchments were grazed by sheep. Catchments were required for our study as we intended sampling streamwater, in addition to soilwater and throughfall. Great difficulty was experienced finding suitable forested catchments occupied entirely by trees of the required age. In some cases, catchments required to

EFFECTS OF FOREST AGE ON SURFACE DRAINAGE WATER ] 17

C

I

CARDIF

? - / I km

Fig. 1. Location of the 25 sites used in the survey in Wales.

have mature trees also contained small areas of young trees. However , we avoided those ca tchments where the requirement was for young trees, but where older trees were also present. Areas of felling or extensive windblow in any catchment also resulted in rejection of that catchment. Several forested catchments contained areas


of moorland. Summary data of characteristics of the forest and moorland sites are presented in Table I.

The soils in the survey were predominantly stagnopodzols (Aquods in the USDA soil classification) (18 sites) with brown podzolic (4 sites) and stagnopodzol/stagnoley intergrade soils (3 sites) comprising the remainder (Table I). Each site was underlain by base-poor, slowly weathering Silurian or Ordovician mudstones, slates or shales. Detailed profile descriptions of typical unimproved grassland and afforested stagnopodzols from the Hafren and Beddgelert area have been reported previously (Reynolds et al., 1988; Stevens and Homung, 1988), so only a brief description of the soils will be given here.

Stagnopodzols in upland Wales are characteristically acid (pH 3-4 in 0.01 M CaC12) with a low base saturation (~ t0%). They usually possess a peaty/organic surface horizon of between 5-25 cm thickness overlying a leached, grey anaerobic E horizon, and strong brown aerobic Bs horizon. Overlying the surface organic horizon in the forest sites, there is a variable depth of forest floor from 0 cm in some of the youngest stands to 14 cm in an intermediate aged stand. This horizon comprises of fresh and decomposing spruce needles plus some fermented material from previous vegetation. Most of the fine roots are located in this and/or the lower pre-existing organic horizon. Brown podzolics differ by having a slightly higher base saturation (~15%), the absence of a peaty surface horizon, and they generally possess less distinctive mineral soil horizons than stagnopodzols. Brown podzolics are also more freely drained and the soil is aerobic throughout the profile. Stagnopodzol/stagnogley intergrade soils represent a gradational soil type between Ferric stagnopodzols and stagnogley soils. Detailed classification of these soils are described by Avery (1980).

3. Materials and Methods

Throughfall, soil solution and surface drainage water at each site were sampled at 4 weekly intervals over a one year period (Nov. 90-Oct. 91). Twelve polyethylene funnel-type throughfall collectors were installed along a transect at each forest site (3 trough-type collectors in the moorland sites) and the samples bulked (by equal volumes) before analysis. Likewise, triplicate soil solution samples from each of the O and B horizons were taken and bulked before chemical analysis. Tensionless PVC tray lysimeters were inserted beneath the O horizons; conventional porous ceramic cup samplers (acid-washed) were installed in the B horizons (50-80 cm depth) and field-equilibrated for two months before commencement of the investigation (Hughes and Reynolds, 1988). Ceramic cup samplers were evacuated to 50 KPa for soilwater sampling. Laboratory studies, investigating the possible A1 contamination of acidic solutions isolated by these suction cups, have shown that negligible A1 contamination occurs by degradation of the ceramic material in waters with pH > 4 (Hughes and Reynolds, 1990). In a field study comparing A1 speciation of soilwaters isolated by polyethylene/polyester tensionless lysimeters and ceramic

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to

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),

EFFECTS OF FOREST AGE ON SURFACE DRAINAGE WATER 121

cup samplers, Hendershot and Courchesne (1991) reported that no systematic differences existed in the chemistry of soilwaters isolated by the two methods.

Following filtration through 0.45 #m membrane, samples were analysed for Na, K, Ca and Mg by Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES). Chloride, NO3 and SO4 were analysed by Ion Chromatography, and Dissolved Organic Carbon (DOC) by autoanalyser following a UV initiated radical- chain digestion procedure. Silicon was determined by an autoanalyser technique based on the Si-Mo blue method (Alien et al., 1974), and pH was measured potentiometrically on unfiltered sub-samples.

Total A1, Total monomeric A1 and Non-labile monomeric A1 were determined following the fractionation procedure of Driscoll (1984) which speciates A1 according to the equation: A1 total = A1 +++ inorg + AI org + A1 poly. Thus, total A1 was determined by Electrothermal Atomic Absorption Spectrometry (EAAS) and con- firmed by ICP-OES. Total monomeric A1 was analysed colorimetrically using catechol violet as the chromogenic reagent (Dougan and Wilson, 1974). Non- labile monomeric (organic) A1 was determined by fractionation through a cation exchange column (Amberlite IR-120 Na) followed by analysis for monomeric A1, and labile monomeric (inorganic) A1 by difference between total monomeric A1 and non-labile monomeric A1. Colloidal and organically occluded A1 (herein col- lectively referred to as polymeric A1) were determined by difference between total A1 and total monomeric A1. A common set of analytical standards were used to determine the forms of monomeric A1. A different set of standards were used for determination of total A1 (by EAAS) and cross-referenced to standards used for monomeric A1. Analytical errors were 4- 5% for Total A1 and Total monomeric A1 determinations, and 4- 10% for Non-labile monomeric A1.

3.1. STATISTICAL ANALYSIS

The sampling programme, comprising 13 samples at 4 weekly intervals for each site, incurred occasional loss of samples because of contamination, equipment failure, spillages etc. Data of this type are subject to seasonal cycles with peaks and troughs occurring at different times of the year. There are advantages in calculating annual means by the method of least squares, since these make adjustment for the missing values according to any seasonality present in the data. Our objectives were to examine and estimate the effects of tree ageing on selected variables. Analysis of Covariance (ANCOVA) was chosen as a suitable analysis for these objectives. Annual least square means of selected variables were analysed using ANCOVA incorporating forest group as a factor and age as a covariate (i.e. individual forest age not age class). Semi-natural moorland sites were included in ANCOVA and treated as being age zero. Results of ANCOVA are presented in Table IV.

122 S. HUGHES ET AL.

4. Results and Discussion

Arithmetic mean data on throughfall, soilwaters and streamwater chemistry for each of the 4 forestry plantation age classes and moorland sites are presented in Tables II and III. Waters from the O horizons are more acid than the throughfall input, but both of these increase significantly (p < 0.001) in acidity with increasing forest age (Tables II and IV). Indeed, ANCOVA (using annual least square means) indicates a decline in pH of 0.09 units for the O horizon leachates per decade of tree growth (Table IV; column a).

Sodium is the dominant cation in the soilwaters, and C1- the dominant inorganic anion, accounting for about 65% of the total base cation and 75% of the inorganic anion charge respectively. This reflects the dominance of marine-derived ions in the precipitation inputs in Wales (Reynolds et al., 1984; Donald and Stoner, 1989). Moreover, there is a highly significant (p < 0.001) increase in chloride concentrations in the soilwaters with increasing forest age (Tables III and IV). This results partly from enhanced capture of marine-derived ions in the atmosphere by the tree canopies. Atmospheric inputs as particulate, aerosol and occult deposition are greater onto the rougher conifer tree canopies than onto the smoother grass canopies (Fowler, 1984; Fowler et al., 1989). Evapotranspiration is also greater from the tree canopies than the grass vegetation, and accounts for approximately 20-30% of incident precipitation (Hornung et al., 1990). These combined processes, acting on anthropogenic pollutant ions in the atmosphere, also account for the increased acidity (and sulphate concentrations) of the throughfall inputs in forest compared with grassland.

Potassium concentrations in the soilwaters are much smaller than in throughfall (Table III), indicating rapid plant uptake and cycling of this element in these soils and limited release to the soilwater by mineral weathering in the soil. Calcium concentrations are also generally smaller in the soilwaters than in throughfall and this may partly be attributable to immobilisation of Ca by organic matter complex- ation in the O horizon (Thurman, 1985). Concentrations of DOC in the soilwaters are very large in the O horizons but decrease with depth in the soil (Table II). Humic and fulvic acids constitute the major fractions of DOC in soilwaters (Ugoli- ni and Sletten, 1991) and ionisation of phenolic and carboxylic acid functional groups on these acids accounts for much of the increased acidity in the O horizon soilwaters compared with the throughfall inputs. However, another strongly acidifying process, nitrification, may be responsible for significantly increasing the acidity of O horizon soilwaters beneath the older mature-forest plantations. Nitrate concentrations increase substantially in the soilwaters beneath these older forestry plantations compared with the younger forest sites and grassland (Tables III and IV). Indeed, there is evidence that these sites may be suffering from 'nitrogen saturation' (Stevens et al., 1994). Detailed information on processes responsible for controlling inorganic-N concentrations in throughfall, soilwaters and streamwaters at these sites are provided by Stevens et al. (1994). They also report that ammoni-

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• 44

34-1

9 5.

084-

0.08

62

.14-

3.9

+ 13

- 16

- 50

+ 71

- 12

+ 1

- 26

+ 48

+ 9

- 27

- 61

+ 40

- 26

-10

8

-10

6

+ 16

�9

-1

> �9

Z

T O

B D

4874

-46

N.D

. N

.D.

N.D

. 65

04-8

1 67

64-8

6 4.

334-

0.07

0.

34-0

.1

1566

4-13

9 1.

8-4-

0.2

2.54

-0.4

3.

04-0

.4

5874

-57

7214

-78

3.90

-4-0

.04

14.1

-4-2

.0

3014

-17

2.84

-0.4

62

.14-

4.0

2.54

-0.7

54

54-2

0 70

64-2

6 4.

424-

0.02

73

.44-

3.6

1014

-11

1.24

-0.2

12

.54-

1.2

0.34

-0.1

51

64-1

2 49

54-1

3 4.

94-4

-0.0

6 52

.34-

2.0

- 26

-13

4

-161

+ 21

t,

J

TA

BL

E I

II

Ari

thm

etic

mea

n co

ncen

trat

ions

(4-

S.E

.) o

f ba

se c

atio

ns a

nd i

norg

anic

ani

ons

in T

hrou

ghfa

ll (

T),

O h

oriz

on s

oilw

ater

s (O

),

B h

oriz

on s

oilw

ater

s (B

) an

d su

rfac

e dr

aina

ge w

ater

s (D

) fo

r ea

ch a

ge c

lass

to

4~

Age

S

ampl

e N

a K

C

a M

g C

1 N

O3

504

Cla

ss

Typ

e /z

e dm

-3

#e

dm -3

/z

e dm

-3

#e

dm -3

/~

e dm

-3

#e

dm -3

/~

e dm

-3

0 T

15

24-1

4 72

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8.4

34.3

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6 59

.74-

4.1

2284

-18

18.1

4-2.

4 59

.44-

4.8

O

1724

-13

18.5

4-2.

5 18

.94-

1.3

49.9

4-4.

2 21

24-2

3 12

.74-

2.1

50.7

4-3.

1

B

1874

-7

6.54

-0.5

15

.54-

0.8

42.4

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0 23

94-2

6 13

.04-

1.3

49.5

4-1.

7

D

1794

-3

4.74

-0.2

73

.44-

7.1

82.3

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8 21

94-6

7.

84-0

.9

42.4

4-1.

0

1 T

2094

-29

41.8

4-5.

7 32

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3.3

62.0

4-7.

6 26

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6 29

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4.7

64.8

4-6.

9

O

2074

-15

21.8

4-2.

9 39

.04-

4.6

61.8

4-4.

4 24

04-2

6 14

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2.7

74.9

4-4.

5

B

2494

-8

4.44

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1

4.6

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48.2

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6 27

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04-0

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68.6

4-1.

5

D

2104

-6

11.2

4-4.

3 54

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9 25

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04-1

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45.0

4-2.

4

2 T

32

44-5

1 85

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8.5

67.6

4-9.

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9.74

-18.

0 40

44-6

7 53

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9.5

126.

94-1

9.7

O

3164

-26

40.7

4-8.

2 49

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82.1

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1 37

74-4

2 37

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5.3

99.7

4-6.

9

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3514

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9.54

-1.3

31

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3.3

78.0

4-4.

1 40

64-2

1 37

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6.9

86.8

4-3.

5

D

2554

-8

4.54

-0.4

78

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3.9

78.2

4-4.

1 31

34-1

2 9.

44-0

.8

55.2

4-2.

3

3 T

31

64-4

5 74

.34-

8.4

60.1

4-7.

1 11

1.04

-15.

0 39

74-5

8 68

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10.1

12

1.34

-16.

5

O

3894

-36

30.5

4-2.

5 29

.64-

2.7

110.

74-1

2.6

456-

t-55

88

.14-

11.0

12

1.44

-12.

2

B

4294

-24

12.2

4-1.

3 19

.64-

1.5

84.6

4-3.

9 47

24-2

8 72

.34-

7.3

108.

34-3

.7

D

2904

-11

5.14

-0.2

68

.34-

3.1

96.0

4-2.

3 34

54-1

5 33

.04-

3.5

65.0

4-1.

1

4 T

39

54-6

0 67

.34-

6.7

63.7

4-7.

0 12

3.94

-16.

0 49

04-8

0 64

.44-

8.6

121.

84-1

4.4

O

4064

-40

30.4

4-2.

9 41

.24-

4.9

109.

24-1

2.1

5204

-70

82.1

4-9.

8 11

9.14

-11.

4

B

4004

-16

11.2

4-0.

4 33

.54-

3.3

100.

94-3

.7

4584

-26

136.

44-9

.5

111.

14-3

.4

D

3124

-7

6.04

-0.1

88

.74-

4.8

109.

44-3

.6

3794

-11

40.1

4-2.

6 76

.74-

1.4

t-n

,..]


TABLE IV

Results of ANCOVA on concentrations of selected variables with tree crop age. The regression coefficients (:t:S.D.) for age are shown in column a, F tests of the significance of age effects are shown in column F

Constant a R 2 F d.f. p

Concentrations Stream inorganic-A1 0.844-2.44 0.2281• 0.289 8.88 1,19 0.01 Bshorizon inorganic-A1 19.35• 3.0707• 0.580 33.46 1,19 <0.001 B s h o r i z o n NO3 -10.88• 2.4034-t-0.4970 0.553 23.39 1,19 <0.001 Bs horizon C1 243.12• 4.772 • 0.442 18.94 1,19 < 0.001 O horizon pH 4.324-0.06 -0.0092• 0.351 28.02 1,19 < 0.001 Throughfall pH 5.18• -0.0182• 0.592 44.98 1,19 <0.001

TABLE V

Correlation coefficients between selected properties of the O horizon soil waters (O), B horizon soilwaters (B) and surface waters (D) for all 5 sites at Beddgelert; n = 65

Sample type

Properties O B D

pH and E Anions -0.269 ~ -0.438 c -0.345 b NaJC1 and CI-~ -0.596 c -0.498 c -0.355 b Organic -A1 and DOC 0.454 c 0.418 c 0.515 c

Inorganic-A1 and pH -0.399 b -0.600 c -0.488 ~ Inorganic -A1 and NO~- 0.444 ~ 0.701 ~ 0.919 ~ Inorganic -A1 and SOl- 0.403 ~ 0.480 ~ 0.571 ~ Inorganic -A1 and C1- 0.782 c 0.626 c 0.881 ~

a Significant at the 5% probability level. b Significant at the i% probability level. c Significant at the0.1% probability level.

um, w h i l s t a b s e n t or e l se p r e s e n t on ly in n e g l i g i b l e quant i t i es in B h o r i z o n s o i l w a t e r

and s t r e a m w a t e r , c o m p r i s e s ~ 5 0 % o f the i n o r g a n i c - N in t h rough fa l l and ~ 3 0 %

o f the i n o r g a n i c - N in O h o r i z o n so i lwa t e r s r e spec t ive ly , at these sites.

A l u m i n i u m , c h e m i s t r y s h o w s m a r k e d ver t i ca l va r i a t ions in the s o i l w a t e r in

b o t h the fo res t a n d m o o r l a n d soi ls . O r g a n i c a l l y c o m p l e x e d m o n o m e r i c A1 and

o r g a n i c a l l y o c c l u d e d ( p o l y m e r i c ) A1 fo rms are impor tan t , and m a y d o m i n a t e , in

the O h o r i z o n so i lwa te r s , w h e r e a s i no rgan ic A1 fo rms d o m i n a t e in the l o w e r B

h o r i z o n s o i l w a t e r s ( F i g u r e s 2a and 2b; Tab les II and V). T h e s e da t a suppo r t the


r I

E "O

O E :::1.

<

100-

90-

80-

70-

60-

50-

40-

30-

20-

10-

of' I I I I I I ! I I t I I I I I I I I I I I I 1 I

0 0 0 0 0 1014141516192427282930323740445153535355

0 I 1 I AGE I I 2 3 4

POLY AI

ORG AI

INORG AI

Fig. 2a.

Figs. 2(a)-(c). Stack plots of mean aluminium species concentrations against forest age in (a) O horizon soilwaters, (b) B horizon soilwaters and (c) surface drainage waters.

view of organic controls over A1 chemistry in the surface horizons of podzols (Driscoll et al., 1985; Hughes et al., 1990). There are marked increases in inorganic A1 concentrations in the B horizon soilwaters within each forest age class and grassland compared with the O horizon soilwaters, but the concentrations of this biologically more toxic A1 species increase significantly in soilwaters beneath the older forest plantations (Tables II and IV; Figure 2b). It should be noted that whilst a negative relationship exists between A1 (inorg) and pH in the soilwaters within each soil horizon (Table V), protons are consumed with depth down the profile to be replaced by A1 (inorg) and Si (Table II). Such a change is characteristic of a weathering reaction whereby protons generated in the upper soil are consumed in the lower soil by breakdown of aluminosilicate and oxide/hydroxide phases (Neal et al., 1989). Soilwater Si concentrations may also, however, be buffered by SiO2 solubility controls in these soils.

Whereas mineral weathering is the ultimate source of A1 dissolution in the soil, it has been argued that ion exchange processes are the proximal mechanisms con-


co I

E "O

O E

POLY AI

ORG AI

INORG AI

0 0 0 0 0 1014141516192427282930323740445153535355

0 I 1 I AGE{ I 2 3 4 Fig. 2b.

trolling short term/episodic fluctuations in A1 chemistry in B horizon soilwaters in stagnopodzols in Wales (Neal et al., 1989; Adams et al., 1990). Moreover, simple mineral solubility reactions (e.g. Gibbsite) were not responsible for the observed fluctuations in A1 chemistry in the soilwaters (Neal et al., 1989; Adams et al., 1990). Evidence in favour of ion-exchange controls for A1 in soilwaters is inferred from relationships between labile monomeric A1 and total inorganic anions (i.e. salt content) in the soilwater (Neal et al., 1989). Figure 3 presents data of labile monomeric (inorg) A1 against sum of inorganic anions (i.e. C1 + NO3 + SO4) in the B horizon soilwater for each collection date and for all 5 forest groups - data is presented for the 18 stagnopodzol sites only, i.e. excluding the 7 sites with brown podzolic or stagnopodzol/stagnogley soils. Our aim was to examine the relationships between these variables, across sites, for the predominant soil type in this survey. Figure 3 shows that a positive linear relationship exists between these variables within each forestgroup and, indeed, (although to a lesser extent) within many individual forest sites. The gradient of linear regression between these variables differs, however, between the 5 forest groups (Figure 3). These data indicate that ion exchange processes are responsible, to varying extents, for controlling soil-


t~9 I

E -O

O E

<~

100-

90-

80-

70-

60-

50-

40-

30- 20-

10 t Or I I I I I I I I I I I I I I I I I I I I I t I I

0 0 0 0 0 1014141516192427282930323740445153535355

0 I 1 I AGE I I 2 3 4 Fig. 2c.

POLY AI

ORG AI

INORG AI

water A1 (inorg) concentrations in these soils. The differences between the 5 forest groups, however, indicate that the overall picture is more complex and that other factors (e.g. mineral weathering rates) are also involved in controlling soilwater A1 concentrations.

Mineral solubility controls (e.g. Gibbsite) are also widely held to be important in determining soilwater and streamwater A1 (inorg) concentrations (Van Breemen, 1973; Eriksson, 1981). Figure 4 presents range-data plots of inorg A1 against H + ions in the B horizon soilwater for each forest age class and moorland; solubility curves for Microcrystalline and Amorphous Gibbsite at a range of temperatures are also shown. For the reaction AI(OH)3 + 3H + ~- A13+ + 3H20 :- Equilibrium constants (K) for Microcrystalline and Amorphous Gibbsite were derived from the equation:

log I(1 = AG/2.303RT,

(where AG e =-11.04 Kcal tool -1 (May et al., 1979) and -15.14 Kcal mo1-1 (Krauskopf, 1985) for Microcrystalline and Amorphous Gibbsite respectively, R = Gas constant, T = temperature (K)).


r I E

ID

< (D

500

450

400-

350-

300-

250-

200-

150-

100-

50-

0 0

BT

y = -22.9 + 0.146 x R = 0.939 N = 26

2I 4 4 4

Oo ~ 200 4()0 660

Sum of Anions

4 4 4

860 1600 i 2 0 0 1400 juo dm -3

Fig. 3a.

Figs. 3(a)-(e). Plots of labile monomeric A1 concentrations against Einorganic anions in stagnopodzol B horizon soilwaters for each collection date and for each forest group and individual sites. Linear regression equations and correlation coefficients (R) are shown in the top left-hand comer of each plot.

Derived log K1 values (298.15 K) were 8.09 for Microcrystalline Gibbsite and 11.1 for Amorphous Gibbsite; log K values at temperatures 0, 5 and 10 ~ were derived from the Van't Hoff equation:

AHe(T2 - T1) log K2 - log Kt = 2.303R(T2 T1)

(using values of A H e (Kcal mo1-1) of -28.8 (May et al., 1979) and -27.2 (Krauskopf, 1985) for Microcrystalline and Amorphous Gibbsite respectively). Solubility curves of A1 (inorg) vs H + for Microcrystalline and Amorphous Gibb- site were then obtained from the equation:

A13+ = 100ogK) • (H+) 3 x (1 • l05) #tool dm -3

(this assumes that all A1 (inorg) is present as A13+). It can be seen in Figure 4 that, for each age class, there are ranges where soilwater

A1 (inorg) concentrations are greatly undersaturated with respect to Amorphous

130

Co I E

-o

:::k

,<

(D ..Q

500

450

400-

350

300-

250-

200-

150-

l oo ~

y = - 7 7 . B + 0 . 4 3 7 x

S. HUGHES ET AL.

DI

R=0.879 N=30

71 4 4

4 4 4 ~---4

4 4

~ ]

b obo 860 I oo i oo 14oo Sum of Anions 12e dm - 3

1

i / .... i

200 400

Fig. 3b.

and Microcrystalline Gibbsite. This is especially evident in Age Class-3 forest sites which have the most acid soilwaters. There is perhaps a trend towards Gibbsite solubility controls in Age Class-4 forest sites, although it should be noted that it is these sites which also show the most marked response of A1 to salt content (Figure 3).

Large concentrations of inorganic AI are associated with high concentrations of chloride in the soilwater (Figure 5 and Table V). It can be seen in Figure 5 that the largest concentrations of A1 (inorg) in the B horizon soilwater at sites HN4 and DN4 occurred during the months of February and March 1991. Moreover, it is notable that this period is marked by substantial increases in chloride concentrations (Peak values > 1000 #e dm -3) and a significant reduction in the acid nuetralising capacity (ANC) in the soilwater - sulphate and NO3 concentrations, in contrast, remained relatively steady or, in the case of SO4, actually decreased during this period. Such so called 'salt events' have been reported and implicated in episodic surface water acidification (Skartveit, 1981; Heath et al., 1992). Furthermore, soil column studies by Njos (1978) indicate that soil humus has a large capacity to exchange H + for Na +. It is probable that during instances of high-salt events, ion exchange of exchangeable soil acidity in the surface soil horizons, especially in the older forest plantations, is responsible for the movement of the strong mineral acid

O~ I E

: : t

25 t~

._.1

500

450

400-

350-

300-

250-

200-

150-

1 0 0 -

5 0 . . . . . . . . . .

0 0

EFFECTS OF FOREST AGE ON SURFACE DRAINAGE WATER

DN

y = -41.5 + 0.345 x R = 0.827 N = 52

131

2 3 �84

2 2 3 -.3- . . . . . 3

0 0 1 ,

200 400 660 Sum of Anions

Fig. 3c.

860 1600 12'o0 1400 jJe dm - 3

HC1 in the solum. Subsequent consumption of protons in the lower soil horizons by mineral weathering and/or ion exchange of A13+ for H + on the soil surfaces results in increased concentrations of labile monomeric A1 in the soilwater. Aluminium may also be mobilized directly by ion exchange with Na or Mg in the soil solution. Large neutral salt concentrations are necessary for the salt effect to be observed because of the unfavourable thermodynamics of trivalent A1 and proton exchange for Na + on the soil surfaces (Heath et al., 1992).

An interesting feature of the salt-events, shown in Figure 5, is that SO4 concentrations in the soilwater decreased during this period. Although these relatively small changes could be attributed to seasonal fluctuations, it is possible that SO4 adsorption on the soil increased during this period in response to the reduction in ANC (i.e., increased acidity) caused by the salt-effect. These observations are con- sistent with those of Curtin and Syers (1990) who observed, during experimental manipulations, that SO4 adsorption in net-negatively charged soils was increased by additions of C1 or NO3 (as the K salt) of up to 0.05-0.1 M. They attributed this to the effect of the added electrolyte on soil pH - the resulting depression in pH favouring SO4 adsorption.

Empirical evidence for the salt-effect is the depression in the Na:C1 molar ratio in the soilwaters (Heath et al., 1992). The ratio in precipitation inputs in Wales

132

500-

S. HUGHES ET AL.

HN

e~ I

E

,_J

450-

400- - -

3 5 0

300-

250-

200-

150-

100-

50-

0 0

y = -51.8 + 0.312 x R = 0.897 N = 63

4

4

3

4 44 ~B 3_ 3

441 433 3 ....

~4 33 4

. . . . . . . . . . . .

200 400 600 Sum of Anions

2 2

86o 1 oo 12'oo 14oo ~e dm-3

Fig. 3d.

average 0.86 (Reynolds et al., 1984), which is identical to that in seawater. Under low salt conditions the ratio in the soilwater can exceed unity (Figure 6) due to Na release from weathering in the soil. Depressions in the Na:C1 ratio below 0.86 are indicative of salt effect acidifications and ratios as low as 0.72 are evident in B horizon soilwaters during large salt events; as shown for example by the two oldest plantations in Beddgelert (Figure 6).

Ion exchange of exchangeable soil acidity initiated by neutral salts (predominantly marine-derived NaC1 and MgC12) in the soil solution can be regarded as an acidifying influence to the soil solution and surface waters. Depletion of the exchangeable acidity on the cation exchanges sites of the soil will be replenished, in the longer term, by weathering and exchange reactions initiated by proton donors in the soilwater (Neal et al., 1989). Acidification of soil solution and surface waters by neutral salts should therefore be regarded as a water quality problem and not, mistakenly, linked to soil acidification (Hendershot et al., 1991).

Between episodic salt events, natural and anthropogenic sources of acidity are primarily responsible for the increased A1 concentrations in the forest soilwater; the natural processes being organic acid production in the litter, base cation immobilisation in the biomass, respiration and nitrification. The latter process, which

500

EFFECTS OF FOREST AGE ON SURFACE DRAINAGE WATER

TY

133

r I E

.Q

_J

450-

400

350-

300

250-

200-

150

1 0 0 -

50-

y = -66 .3 + 0 . 3 9 8 x R = 0 . 8 9 7 N = 44

4

4

4 4 4

4 4

44 4

4

0 0 2()0 4()0 6()0 8()0 1 ()00 12'00 1400

Sum of Anions pe dm -3

Fig. 3e.

provides protons and a mobile anion, may be of great importance in influencing A1 mobilization in soilwaters beneath the older forest plantations.

High concentrations of A1 and H + in the soilwaters beneath forestry plantations do not necessarily lead to increased levels of these elements in the surface drainage waters. Figure 2c presents data on mean A1 chemistry in surface waters draining the various ages of plantation forests. Although ANCOVA, on mean surface water A1 concentrations, reveals a significant positive relationship with forest age (Table IV), not all surface waters draining forestry plantations possess larger mean A1 concentrations compared with waters draining moorland sites, in spite of increased A1 concentrations in the soilwaters. Mixing and chemical reactions with calcium beating groundwater inputs to these surface waters are responsible for these observations (Neal et al., 1989). Table II and III show that the surface waters draining forestry plantations have higher pH values and larger calcium concentrations than the soilwaters. Table II also shows that the forest and moorland soilwaters have zero or negative ANC and that this decreased markedly (i.e., becomes more negative) with increasing age class. This is accompanied by a sympathetic decline in the (positive) ANC in the surface drainage water. This indicates that the ANC of the groundwater components at these base-poor sites is very limited and there are examples of episodic streamwater acidification evident at many of these forest


160

140

~o 120 I

E "to

"6 100 E :3.

~: 80 .o_ t " t~

60 o r

40

20

Fig. 4.

0 t TM - - - r - - , ~ - ~ - , ~ - I ~ : - 0 lO 20 30 40 5'0 do

H+ions IJmol dm -3

Age C l a s s

o l

2 ~

3 ~

4

9'o 16o l io 12o

Range-data plots of inorganic AI against H + ions in the B horizon soilwater for each forest age class and moorland.

sites; as shown for example by stream Hafren 4 (Figure 7). These events involve the transfer of acidity from the soils to surface waters (e.g., during salt events) in amounts exceeding the ANC of the groundwater component of the drainage water. Alternatively, large rainfall events may also result in the transfer of acidity from the soils to surface waters along different hydrological pathways preventing direct or prolonged contact with weatherable, calcium-bearing, parent material (Chappell et aI., 1990; Soulsby and Reynolds, 1993). Clearly, instances whereby acidity in the discharge waters from the soil exceeds the ANC of the groundwater component of the surface waters are likely to be more frequent amongst the older forestry plantations for reasons given above.

5. Conclusions

These results suggest that, as Sitka spruce plantations age, they significantly influence soil solution aluminium chemistry compared with semi-natural moorland vegetation on similar soils. Inorganic A1 concentrations in the B horizon soilwaters increase progressively with increasing forest age. Short-term/episodic peaks in aluminium concentrations in the forest soilwaters are closely associated with incidences of high concentrations of neutral, marine derived, inorganic salts. Nitri-


co I

E

::L

1200

1000-

8oo_\ / . . . . - - \ . 6oo . . . . ~ . . . . . . . . \ _ _ . - - - . - ~ _

4 O O l . 2 - - - . . . . . ~ o o 1 - ~ ~ - - ~ ~ ~ - -

0 / +-------+'---"--4 . ~--------+ ~ ~ --..+

-600- I I I I t I I I I 1 I I

1 -Nov-g021-Jan-91 18-Mar-9113-May-91 08-Jul-91 02-Sep-91 31-O'ct-91 02-Jan-91 19-Feb-91 15-Apr-91 10-Jun-9105-Aug-91 03-Oct-91

CI t N O 3 >K S 0 4

[] AI >< A N C

Fig. 5a.

Figs. 5(a)-(b). Plots of inorganic A1, CI-, NO 3, SO~-, and ANC against time in the B horizon soilwater at sites (a) Hafren 4 and (b) Dyfnant 4.

fication may be a major factor involved in soil acidification and the mobilization of aluminium in soilwaters beneath the older mature-forest plantations in Wales. Inorganic A1 concentrations, in the surface drainage waters, are significantly higher in waters draining the oldest forestry plantations, although response to discharge of soilwater acidity to the surface waters at individual sites is dependent on the ANC of the groundwater component of the surface waters.

Acknowledgements

The authors thank the Forestry Commission for their assistance during this study, especially for the provision of sites. We also thank Mrs S. A. Brittain and J. Ingrams


1200-

I

E " O

::L

1 0 0 0

8 0 0 -

6 0 0 -

4 0 0 - - - ~ - - -

200- ~ . ~ " " ~ - " - ~

0

-400- , , , , , , , , , , , , 1 1 - N o v - 9 0 2 1 - J a n - 9 1 1 8 - M a r - 9 1 1 3 - M a y - 9 1 08 -Ju l -91 0 2 - S e p - 9 1 3 1 - O c t - 9 1

0 2 - J a n - 9 1 1 9 - F e b - 9 1 1 5 - A p r - 9 1 1 0 - J u n - 9 1 0 5 - A u g - 9 1 0 3 - O c t - 9 1

- u - C l i N O 3 z S O 4

[] AI >< A N C

Fig. 5b.

for help with fieldwork, T. H. Sparks for statistical advice and Mrs E. Peters for assistance in speciating aluminium. We thank colleagues of the Institute of Terres- trial Ecology, Chemistry Service, Merlewood for analysing major cations, anions, silicon and DOC. This work was partly funded by National Power/Powergen Joint Environmental Programme and the UK Department of the Environment under contract numbers LC/5/0004 and PECD/7/12/110.


1.1

o

r r

o

o �9 ~

Z

0.9-

0.8-

4 ~ 4

3

4 3 3 4

4 8 3 3 ~1-- Seawate~

4 4 4 3

-3

3 4

0.7 2oo 46o e6o 86o looo

C! pe dm -3

Fig. 6. Plot of Na:C1 molar ratio against CI- concentration in the B horizon soilwaters in age class-3 and 4 forest sites at Beddgelert.

References

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13 8 s. HUGHES ET AL.

20- -6.5

'E "0 0 E

< ._J < I-- 0 I--

16- -6

12-

_

-5.5

4 - + - -

0 I i I

l 1-Nov 21-~Jan ' 18-Mar 02-Jan

-5

13-May 08SJul 02-Sep 31-Oct 19-Feb 15-Apt 10-Jun 05-Aug 03-Oct

DATE

W 0 . .

I + T o t a l A I i pH

Fig. 7. Plots of total A1 and pH against time in stream Hafren 4.

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effects of forest age on surface drainage water and soil solution aluminium chemistry in...

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