Journal of Hydrology, 116 (1990) 101-124 101 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

[3]

I N F L U E N C E O F A C I D A T M O S P H E R I C I N P U T S O N S U R F A C E W A T E R C H E M I S T R Y A N D M I N E R A L F L U X E S I N A D E C L I N I N G S P R U C E S T A N D W I T H I N A S M A L L G R A N I T I C C A T C H M E N T ( V O S G E S M A S S I F , F R A N C E )

A. PROBST 1, E. DAMBRINE 2, D. VIVILLE 3 and B. FRITZ 1

1Centre de Gdochimie de la Surface (CGS), CNRS, 1 rue Blessig, F-67084, Strasbourg Cddex (France) 2Centre de Recherches Forestieres (CRF), INRA Nancy-Amance, Champenoux 54280, Seichamps (France) 3Centre d'Etudes et de Recherches Eco-Gdographiques (CEREG), ULP, 3 rue de l'Argonne, 67083 Strasbourg Cddex (France)

ABSTRACT

Probst, A., Dambrine, E., Viville, D. and Fritz, B., 1990. Influence of acid atmospheric inputs on surface water chemistry and mineral fluxes in a declining spruce stand within a small granitic catchment (Vosges Massif, France). J. Hydrol., 116: 101-124.

The characteristics of surface and soil waters draining an old declining spruce stand, and their chemical behaviour under acid input influence within a small catchment are described using open field precipitations, throughfalls, soil solutions, spring and streamwater data. Element fluxes are calculated in the different ecosystem compartments, and the proton mass balance has been assessed. The results show the importance of acid occult deposits and, more generally, of acid atmospheric inputs, in explaining present soil acidification. This acidification is characterized by the base cation deficiencies observed in spruce thereby showing symptoms of decline. However, stream waters and spring waters are not yet acidified owing to the neutralization of acid inputs by weathering in the regolith. Nevertheless, streamwater chemistry is dominated by strong acid anions which are not characteristic of waters draining granitic bedrock in pristine areas.

INTRODUCTION

I n v i e w of t h e g e n e r a l p o l l u t i o n c l i m a t e in t h e n o r t h e r n h e m i s p h e r e , n u m e r o u s s t u d i e s h a v e b e e n d e v e l o p e d in d i f f e r e n t c o u n t r i e s to a s s e s s t h e i n f l u e n c e o f a c i d i c i n p u t s on t h e f u n c t i o n i n g o f a q u a t i c e c o s y s t e m s . S m a l l c a t c h m e n t s h a v e b e e n c h o s e n as a n a t u r a l l a b o r a t o r y fo r t h e s e i n v e s t i g a t i o n s for a b o u t 20 y e a r s ( L i k e n s e t al . , 1977). T h e s e s t u d i e s h a v e s u c c e s s f u l l y e v a l u a t e d t h e e f fec t s o f l o n g - r a n g e a t m o s p h e r i c p o l l u t i o n on s u r f a c e w a t e r a c i d i f i c a t i o n , p a r t i c u l a r l y in t h e S c a n d i n a v i a n c o u n t r i e s ( W r i g h t a n d J o h a n n e s s e n , 1980; H u l t b e r g , 1985). M o r e r e c e n t l y , s u c h s t u d i e s h a v e b e e n d e v e l o p e d in c e n t r a l a n d s o u t h e r n E u r o p e , to a s s e s s t h e i n f l u e n c e o f a c i d i c p o l l u t i o n on f o r e s t d i e b a c k (Pac~s , 1985; H a u h s a n d W r i g h t , 1986; F e g e r et a l . , 1988). T h i s m o r e

0022-1694/90/$03.50 © 1990 Elsevier Science Publishers B.V.

102 A. P R O B S T E T AL.

recent research has investigated the hypothesis that leaching of base cations from canopies and soils is enhanced by atmospheric pollution (Ulrich, 1984) thereby producing the element deficiencies which characterize the forest dieback phenomenon.

The Strengbach catchment has been selected as an experimental site for studies, within the framework of the French national research programme on forest decline (DEFORPA). The main research in progress in this catchment intends:

(1) to quantify the atmospheric inputs; (2) to assess the influence of such inputs on soil chemical processes and

fertility, and on surface water quality (spring water, stream water); (3) to compare stand-scale and catchment-scale budgets in order to model the

functioning of the whole ecosystem. The first part of this paper deals with the chemical characteristics of atmo-

spheric deposition in the catchment for an established spruce stand. The second part presents the effects of this acid input on soil acidification and its contribution to streamwater chemistry.

E X P E R I M E N T A L SITE

The Strengbach catchment (80 hectares) at Aubure is located at the eastern side of the Vosges mountains (eastern France), 58 km southwest of Strasbourg (Fig. 1). The elevation ranges from 883m at the outlet to 1146m. The basin

STRENGBAC~]028

- - 883

n ,

~ / )~ * air quality s t a! ion , " ~ / 6 meteorological shelter

o oo,,oo, ,, , /k precipitation collector T throughfal and soil solutions site kll / ~ I old spruce stand (80 years) _ T ~ II young spruce stand ( 3 0 years} / i~ spring

o m ~o ~ water level recorder and automatical sampler

Fig. 1. Locat ion map for the studied area.

INFLUENCE OF ACID ATMOSPHERIC INPUTS 103

(I/s) 25O

2O0

150

IO0

5O

0

1986 J F M A M J J A S O N D J F M A M J

1987 r 1988 J A S

4O 80

Fig. 2. Hydrograph of mean daily discharge and corresponding hyetograph.

mainly overlays a granitic bedrock, the Brezouard granite, which is poor in base cations, is a leucogranite characterized by two micas (muscovite and biotite). At the top edge of the catchment, the granite is in contact with a banded gneiss. Stony and acid soils are developed on this base-poor granite; brown acid soils (acid distrocrepts) dominate one half of the catchment, podzolic soils (leptic podzols) the other half. In the valley bottom, a saturated zone with hydromorphic conditions covers ~ 2% of the catchment area.

The climate is temperate oceanic mountainous, with monthly averages of daily mean temperature ranging about - 2 to 14°C. The mean annual precipita- tion is ~1500mm with 25% falling as snow. Highest water flow (Fig. 2)

Needle magnesium content 7

6

%o0 5

D 4

r

Y 3

W

e 2 i g h 1 t

0

Year 4 Year 2 Year I

Needle age

DD @. I Fig. 3. Needle magnesium content in declining (D) and healthy (H) spruce in relation to needle age.

104 A. PROBST ET AL.

generally occurs during snowmelt (spring) and lowest water flow during autumn (September-October).

Spruce of various ages cover two-thirds of the catchment area, mixed beech and fir stands take up the rest of the basin. Forest decline symptons were observed during the 1980s. The mean level of needle loss, which affects fir and spruce, reached 30%. Yellowing of old needles, related to magnesium deficien- cies, (Fig. 3) mainly affects spruce, especially the young stands. However, element concentrations of the needles are high for nitrogen, phosphorus and potassium, and low for calcium.

MATERIALS AND METHODS

Equipment

The Strengbach catchment has been progressively instrumented since August 1985. The main equipment and the different research teams involved in the research programme are listed in Table 1.

Water sampling and measurements

Open field precipitation, throughfall waters and soil solutions were sampled fortnightly. Soil solutions were collected at 5, 10, 30 and 60 cm depths using zero tension lysimeters. Stream waters and spring waters were sampled at least weekly; additional streamwater samples were collected during flood events.

All the samples were collected in polyethylene bottles and filtered in the laboratory (0.45pM Millipore membranes). Alkalinity (obtained by Gran's titration) and pH were measured immediately after filtration. Calcium, magnesium, sodium and potassium were analysed by atomic absorption spec- trometry (open field precipitation, throughfall, spring water and streamwater samples) and by inductively coupled plasma optical emission spectrometry (ICPOES) (soil solution samples). Ammonium, chloride, nitrate, sulphate and silica were measured by automatic colourimetry (Technicon apparatus). Aluminium and manganese were analysed using ICPOES; see Krempp (1988) for analytical methodology.

For an old spruce stand (Fig. 1, Table 1), developed on a brown acid soil, preliminary element fluxes and budgets have been obtained from data covering the period December 1986 to December 1987. All the calculated mean con- centrations are volume weighted.

RESULTS

Characteristics of atmospheric deposition

Open field precipitation characteristics Open field precipitation is acidic (mean pH = 4.5), of low solute concentra-

tion and poorly mineralized (mean total dissolved salts (TDS) = 5.5mg 1 1).

INFLUENCE OF ACID ATMOSPHERIC INPUTS 105

TABLE 1

Characterist ics of the Strengbach basin equipment

Open field input measurements (1,2) quanti ty

quality

7 adding tubes 2 raingauge recorders 2 raingauges (1 pit-gauge type) 5 bulk collectors 2 automatic collectors with

daily sampling (1 bulk col- lector and 1 wet only col- lector)

Spruces

80 years stand 30 years stand

Input measurements under canopy (3) ( throughfall waters)

Soil solution measurements (3)

Climatological parameter measurements (5) and (1,2)

Air pol lutant measurements (4)

Output measurements at the outlet (1,2)

Output measurements at spring tapping (1,2)

9 gutter collectors (1 with daily sampling)

3 rows with 16 throughfall gauges (for quantity)

32 plate tysimeters allocated among 4 levels in 8 pits

2 meteorological schelters (T ° , air moisture, evaporation)

1 raingauge recorder 1 solarimeter 1 anemometer and weather vane

automatic continuous analysers (SO2, NO~, O:~)

1 H-flume type weir with trapezoidal section

2 water level recorders (mechanic and ultrasonic type)

2 automatic water samplers (1 is connected to the level recorder 1 is time dependant)

8 gutter collectors (1 with daily sampling)

1 row with 12 throughfall gauges (for quantity)

8 plate lysimeters allocated among 4 levels in 2 pits

2 water meters

Research teams in charge of the Strengbach catchment. (1) CEREG: Centre d'Etudes et de Recherche Eco-G~ographiques de I 'ULP h Strasbourg. (2) CGS: Centre de G~ochimie de la Surface du CNRS h Strasbourg. (3) CRF: Centre de Recherche Forestibre de I'INRA h Nancy-Champenoux. (4) ASPA: Association pour la Surveillance de la Pollution Atmosphdrique, Strasbourg. (5) SMIRNE: Service Mdt~orologique Inter-R~gional du Nord-Est, Strasbourg-Entzheim.

TA

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(p

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lk p

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(B

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4.47

19

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2.

8 4.

5 11

.9

< 1

< I

33.9

3.

7 T

hro

ug

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ll (

TF

) 3.

94

36.9

46

.4

52.7

17

.8

65.5

2.

3 ,

1 11

4.8

2.5

Flu

xes

(keq

ha

year

) B

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cip

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(B

P)

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rou

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fall

(T

F)

Dif

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nce:

(T

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) D

ry a

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(cal

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T

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(pm

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> ©

INFLUENCE OF ACID ATMOSPHERIC INPUTS 107

The chemistry is dominated by protons, ammonium, sulphate, nitrate and chloride (Table 2). Sulphate and nitrate are mainly supplied directly by atmo- spheric pollution.

Local pollution climate is typical of the eastern part of France. For most of the time, the atmospheric pollution level is low in the Strengbach catchment compared with other European countries (cf. Buldgen, 1984; Ulrich, 1984; Pacbs, 1985; Nys, 1987). However, during winter time, when high pressure, stable anticyclonic atmospheric conditions occur, the Strengbach catchment is subjected to high atmospheric pollution events. These episodes occur when easterly winds, carrying the major inputs of pollutants, are dominant (Fig. 4). These pollutants come from the surrounding countries to the east (Drach and Target, 1987).

Figure 5 shows the variation of concentrations of protons, sulphates and nitrates in bulk precipitation (bp). There is a clear positive relationship between the concentrations of these ions in bulk precipitation and the mean fortnightly levels of pollutants in the air (atm), continuously monitored close to the old spruce stand (Fig. 4). The Pearson's correlation coefficients (0.001 statistical significance level) are: r(H ~ bp, SO2 atm) = 0.74; r(NO3 bp, NO2 atm) = 0.66; r(SO~ bp, SO2 atm) = 0.60.

The highest concentrations of strong acid anions in open field precipitation generally occur during winter and spring (Fig. 5). During these pollution episodes, sulphate, nitrate, ammonium and proton concentrations in bulk precipitation increase with elevation (Fig. 6), in spite of the positive gradient of precipitation amount. For instance, during a 20mm winter snowfall event

Air mean concentrations

9

m 3

1986-1987 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

;1 A .............................................. . . . S 0 2 . . . . . . . . . . . . . . . .

20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15 . . . . . . . . . . . . . . . . .

10_ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

, , .......

D J F M A M J J A S 0 N D

Fig. 4. Mean monthly concentrations of air pollutants (SO2, NO2, NO) in the Strengbach basin (data from ASPA).

108 A. PROBST ET AL.

180-

SO;" 150-

120-

g uJ 90-

60-

0 P

ftlCrl

100

200

30.

120-

N O ; " 100"

8o-

O ~ . 60.

40-

20-

0

105

,_1

75- .%

45 ̧

1 5 -

IID1J IF,M, ~ ~l j , j TA,S,O,N,DI J 'F IMIAIMWJ 'J IA'S 'O 4 'D IJ 'F 'M 'A 'M 'J ' J ' A ' I

I' I J ' ' ' . . . . . . . D 1 ' ' ' ' ' ' . . . . . D IJ ' ' ' ' ' ' ' 'S ' 1986 1987 1988

PH -3.87

-3.98

.4.13

-4.35

• 4.82

Fig. 5. Temporal var ia t ion of pH, n i t ra te and sulphate concent ra t ion in open field bulk precipita- t ion and corresponding hyetograph (P = precipitation).

(Fig. 6), ammonium concentration increased eight times, nitrate and protons four times, and sulphate five times between 760 and l107m altitude. This phenomenon may be due to a pollutant concentration near the cloud strata limit corresponding to a temperature inversion zone (1150 m in this particular case).

For the low pollution episodes, the chemical composition of open field

INFLUENCE OF ACID ATMOSPHERIC INPUTS 109

f.,O LI.I J o

150-

130-

110-

90 -

7 0 -

5O

30

10 700

• H +

• so~-

• NO3" Z~ NH +

I I I I 0 800 900 1000 11 0

ALTITUDE(m)

Fig. 6. Relationships between mean fortnightly concentration of protons, ammonium, nitrate and sulphate in bulk precipitations and the elevation, during a winter pollution episode.

precipitation is rather uniform over the basin; however, the station located on the ridge quite often receives slightly more solute-rich precipitation. The fluxes of ions derived from atmospheric pollutants are maximal during the winter and spring periods (both in relation to the highest concentrations and precipitation amounts). On the basis of their interrelationships (Table 3), four groups of ions can be separated in bulk precipitation: (1) protons, nitrate and sulphate, mainly deriving from atmospheric pollutants; (2) sodium, and chloride, mainly of oceanic origin; (3) calcium and potassium, of continental origin; (4) magnesium, mainly of oceanic origin but also of continental origin. Similar tendencies have been found in other European studies (Buldgen, 1984; Dupraz, 1984; Duysings et al., 1986); this is contrary to results reported by Dethier (1979) who observed no dilution effect. Significant negative relationships are observed between the concentrations of calcium, sulphate and nitrate with the corresponding amount of rainfall.

Inputs of mineral elements to an old spruce stand Bulk deposition and throughfall data were collected in an 80-year-old spruce

stand located in the upper part of the catchment (Fig. 1, Table 2). Base cation open field inputs are rather low compared with the data of numerous other studies in Central Europe (Ulrich, 1983; Pac~s, 1985; Nys, 1987) but very similar to those measured in the Black Forest (Feger et al., 1988). As pointed out by several authors, dry and occult deposition cannot be measured directly but can

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0.85

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0.

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* 0.

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0.7

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0.53

0.

65

0.56

0.

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*

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0.

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0.5

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0.37

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0.45

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0.48

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0.55

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0.93

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0.

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0.96

0.

94

0.95

0.

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0.93

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0.40

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0.49

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INFLUENCE OF ACID ATMOSPHERIC INPUTS 111

be estimated from the ~canopy difference' (Matzner, 1986), i.e. the difference between the chemical composition of open field precipitation and throughfall, if one assumes that no adsorption or leaching takes place within the canopy.

This variation of concentration between precipitation and throughfall results from several processes.

(1) Simple concentration by evaporation of intercepted rain; yearly mean interception has been calculated to be 26% of the nearby rainfall.

(2) Leaching of dry and occult deposition occurring during dry or misty periods. At this elevation, fog events are common, particularly during winter and spring.

(3) Leaching of biological nutrients released by the canopy. (4) Uptake by the canopy. The resulting effect of these processes is a decrease of mean precipitation pH

of ~ 0.6 units and a sharp increase (4 × TDS) in solute concentrations. From the one-year throughfall dataset, three groups of elements have been distin- guished according to their origin.

Internal origin. K and Mn concentrations are, respectively, very low and not detectable in open field precipitation (Table 2); however, they increase very strongly in throughfall water except during winter when the physiological activity of trees is very low. These elements are not present in the needles in structural components of cells and are, therefore, readily leached (cf. Clement, 1989); they have been considered, according to Lovett and Lindberg (1984), as originating mainly (K) or totally (Mn) from the ecosystem itself (Godt et al., 1986).

Mixed origin. Ca and Mg concentrations are rather high in open field precipi- tation (Table 2) but increase in throughfall, even during winter when leaching from needles would be minimal. This suggests an important external supply, such as dry and occult deposits filtered by the spruce canopies. However, except during winter, their concentrations in throughfall are clearly related to Mn concentrations which are of internal origin (Table 3). Finally, as Mg, contrary to Ca, is present in needles essentially in a leachable form, a higher contribution of internal cycling for this element is inferred.

Atmospheric origin. Nitrogen, sulphur, chloride and sodium have been considered to originate from atmospheric deposition, according to Ulrich (1983) and Lindberg and Garten (1988). The reduction of the ammonium to sulphate and nitrate to sulphate ratios, from open field precipitation to throughfall, might result in some nitrogen uptake by the canopy (Tietema and Verstraten, 1988). Accordingly, in order to evaluate dry and occult deposition (Table 2), the following is assumed: 100% of sodium, chloride, nitrogen and sulphur, 80% of calcium, 50% of magnesium and 20% of potassium, in the difference of fluxes between open field precipitation and throughfall were due to dry and occult deposition. The associated proton load has been calculated

TA

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Mai

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oil

char

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r (%

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Tex

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(%

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pH

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(m

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ase

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0.

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0.27

©

INFLUENCE OF ACID ATMOSPHERIC INPUTS 113

from the charge difference between anions and cations. Total deposition was determined by adding wet, dry and occult depositions (Table 2). The resulting total proton input is 1.8keq hectare -] year ]. This value is typical of forest stands from industrialized regions away from any local source of pollution (Grennfelt et al., 1985; Freiesleben et al., 1986; Lindberg et al., 1986).

Effect of atmospheric deposition on soil and surface water characteristics

Evolution of precipitation composition on soil leaching The soil of the spruce stand experimental site is classified as brown acid soil

(according to the French soil classification) or acid distrochrept (according to FAO classification). Fine earth accounts for < 40% of the weight throughout the profile, and the texture is mainly sandy. Therefore, seepage of water is very easy and rapid. The soil is very acid and the base saturation decreases to 5% at 60 cm depth (Table 4). The rooting of the stand is very dense in the upper 10 cm of the soil. Very few roots develop below 60 cm.

Figure 7 shows the annual mean concentrations of major ions in open field precipitation, throughfall, soil solutions, spring waters and stream waters. On passing from the soil surface to the springs or stream, the open field precipita- tion chemistry is successively influenced by three different media.

Organo-mineral soil horizons (O~20cm). These horizons produce a strong decrease of throughfall pH, related to organic acid deprotonation. Indeed, the variation of the mineral counter anion concentrations cannot account for such

BP

TF ..........

F -5 ..........

-10 ..........

$S -30 ....... ~

sp ! SW

/UEQ/L Anions

300 200 100

' ill iii iiii •

Cations l O 0 2O0 30O 4OO

i i i J

0 S042- ~ - Ca 2+ AI3+I

............. NO3- ~ Mg 2+ AIk.+H + ~ H+ K+

Fig. 7. Profile of annua l weighted mean concentrat ion: open field bulk precipi ta t ion (BP), through- fall (TF), soil solut ion (SS) at different depths, spr ing wate r (SP) and s t ream water (SW).

114 A. PROBST ET AL.

a change. Solutions collected at 5 and 10cm depth contain 20-40mg 1-1 DOC, and are yellow to light brown. However, these organically originating protons are mainly reabsorbed by organic molecules which are precipitated and mi- neralized in the B horizon (< 10mg 1 1 DOC at 60cm). Consequently, the influence of organic acids on the cations leaching from the soil profile is low, as lateral flow in surface horizons is negligible. On the other hand, the associated decrease of pH induces additional weathering of mineral phases. Mineralization of the humus layer also produces/releases calcium and nitrate, the concentrations of which increase in the waters from 5 cm; they decrease again at 10 cm depth, probably as a result of root uptake. Sulphate concentra- tions remain approximately constant, corresponding to a low sulphur ac- cumulation in this layer.

Mineral soil horizons (20-60cm). Nitrification is the main proton-producing reaction in these horizons. Indeed, the influence of organic acids is small and the carbonic acid concentrations are negligible. Finally, the sulphate con- centration increases only slightly, probably owing to concentration as a result of soil water uptake by the roots. Leaching of cations at the bottom of the rooting zone is therefore mainly influenced by strong acid anions originating from the atmosphere. Protons are mainly consumed by weathering and cation exchange reactions. However, this proton buffering is only partial. Indeed, the major part of the proton input is buffered by aluminium mobilization, either by ion exchange or dissolution. This limited removal of base cations must be related to the low base saturation, to the extremely low level of calcium in the soil (Table 5) and to the high leaching rate of solutions through this stony and sandy matrix.

These mechanisms lead to low Ca/A1 ratios in the soil solutions (0.5 at 60 cm depth) which might affect base cation uptake by the roots (Rost-Siebert, 1983; Bengtsson et al., 1988).

Regolith zone. The proton input to the regolith derives mainly from the hydroxylation of leached aluminium. Acidity is buffered in this layer essenti- ally by mineral weathering, as shown by the increased concentration of calcium and silica in spring water and stream water compared with soil solutions (Fig. 7; Table 8). This leads to positive alkalinity and the undetectable levels of aluminium in spring waters and stream waters.

The reduction of major ion concentrations (Fig. 7) will be parallelled by a

TABLE 5

Amount of exchangeable and total elements in the upper 60 cm of the soil fine fraction ( < 2 mm)

kg hectare ~ Ca K Mg Na P

Exchangeable 268.5 523.0 61 1024 Total < 1500 100640 11857 11803 2651

INFLUENCE OF ACID ATMOSPHERIC INPUTS 115

consumption of protons. At first sight, root uptake and denitrification, theoret- ically possible in the saturated zone near the stream, might explain the difference between the levels of nitrate concentration at the bottom of the soil, and in the stream. However, this difference remains throughout the year, even during winter, when the biological activity is very low, and is therefore not yet clearly explained. In this basin, the major part of streamwater drains from podzolized soils where the nitrification potential is far lower than in the studied soils (Berthelin et al., 1984). The low nitrate concentration in the streamwater could also result from a dilution effect.

Influence of acid precipitation on soil acidification and base cation leaching The fluxes of inorganic elements through the soil (Table 2) have been

estimated by multiplying the average volume-weighted concentrations measured fortnightly at 60 cm depth by the corresponding seepage water fluxes. These fluxes were calculated by subtracting, from the rainfall amount, the local PENMAN value of evapotranspiration for the summer period. Recent sap-flow measurements (Granier, personal communication) have shown that this last value slightly overestimates the real transpiration of the stand. For the winter period, drainage was assumed to equal the throughfall water amount. These calculations lead to an annual drainage value of 726 mm under the rooting zone (Table 2). Drainage water amounts collected at different depths show that a lateral flow occurs during high rainfall periods at the top of the B horizon (~ 30 cm depth). As concentrations are very similar at this depth to those at

60 cm depth, the differences may not modify the mineral budget estimates significantly.

A proton budget was derived to assess each source and sink of protons (Table 6). The nitrogen cycling contribution to proton sources has been evaluated according to Van Breemen's formula (Van Breemen et al., 1983). Biomass assimilation of cations has been measured by sampling and analysing each biological compartment (needles, twigs, branches and stems) of a 17 tree sub- sample from the spruce stand.

TABLE 6

Soil proton budget

H ~ Sources (keq hectare 1) H + Sinks (keq hectare 1)

Atmospheric deposition 1.760 Cation storage in biomass 0.360

Oxidation of deposited nitrogen 0.630

Oxidation of soil nitrogen 0.070

Total sources 2.820

Sulphate adsorption Weathering and desaturation Ca

total bases cations

Drainage of protons

Total sinks

Mg K Na

AI

0.280 0.320 0.166 0.249 0 0.735 1.640 0.680

3.330

116

TABLE 7

Station fluxes of mineral elements and spruce stand budget

A. PROBST ET AL.

k g h e c t a r e ~ a ~ N C a M g K N a P

A t m o s p h e r i c d e p o s i t i o n (1) > (16.9) 11.6 1.5 5.5 11.1 0

D r a i n a g e a t - 6 0 c m (2) 17.8 10.2 3.0 12.5 10.9 0

B i o m a s s s t o r a g e (3) 5.6 4.97 0.46 2.83 0.4

S t a t i o n b u d g e t (1 - 2 - 3) ( - 6.5) - 3.57 - 1.96 - 9.83 + 0.20 0.4

The results show that the major part of the proton input to the soil is determined by atmospheric deposition (Table 6). About half of this proton load is neutralized by aluminium compound dissolution. The remaining part is accounted for by weathering, cation exchange, and by proton drainage. The difference between sources and sinks of protons may result from an overestima- tion of the proton concentration at 60cm depth owing to an unsuitable pH measurement procedure. In addition, some apparent excess of cations in the soil solution's electrical charge balance may be due to some remaining organic anions not determined and to some underestimation of the chloride content because of insufficient measurements; the annual mean chloride value has been used for missing samples.

The relative contribution of atmospheric inputs, leaching and biomass as- similation to the inorganic budgets in the soil, is presented in Table 7 for several elements. Examination of this table reveals that atmospheric inputs of nitrogen might be underestimated owing to canopy uptake. On the other hand, these inputs are totally leached through the soil towards the stream together with strong acid anions, and for this reason cannot benefit the trees. This means that the spruce stand growth is entirely dependent upon mineralization of humus, cation exchange and weathering.

Comparing the sodium and calcium content of the soil with their annual budget (Table 5) reveals that although soil weathering can provide potassium and magnesium, it cannot account for the present losses of calcium. Conse- quently, this soil is presently losing its exchangeable and, possibly, organic calcium at a rather high rate.

Streamwater characteristics' and response to acid atmospheric inputs As mentioned above, the Strengbach waters are neutral (mean pH -- 6.12).

Their cation chemistry is dominated by calcium (followed by sodium, magnesium and potassium) (Table 8). Sulphate is the dominant anion (Probst et al., 1987), on a mean annual concentration basis, as in all of the catchment surface waters. However, exceptionally, bicarbonate can dominate in periods of low flow (cf. Fig. 8). Alkalinity remains very low, the mean value (41 peq 1-1) representing only 11.4% of the total anionic charge.

The temporal variation of the main chemical components and instantaneous discharge of the stream waters are presented, up to September 1988, in Fig. 8.

TA

BL

E 8

Mea

n c

hem

ical

co

mp

osit

ion

fo

r th

e m

ain

com

pon

ents

in

str

eam

wat

ers

and

spri

ng w

ater

s of

the

Str

engb

ach

ca

tch

men

t

Z

*q

~v

Mea

n c

once

ntr

atio

ns

pH

NH

4

Na

K

Mg

Ca

H'

HC

O 3

C

1 N

O 3

SO

4 H

4SiO

4

TD

S

(peq

1-1

) (p

mol

1

1) (m

g 1

2)

Spri

ng w

ater

s (S

P)

6.02

1.

6 86

.0

22.0

47

.6

179.

0 0.

95

41.6

47

.9

40.3

20

4.5

125.

0 35

.6

Stre

am w

ater

s (S

W)

6.13

1.

3 86

.2

20.6

60

.6

186.

5 1.

0 42

.2

54.5

34

.4

219.

9 11

3.1

35.3

--3

118 A. PROBST ET AL.

7 0

6.5

6.0

5.5

5.0

O

110

30J

150.

100-

50-

0

280

260

240

~ 220

180

140

16o,

120,

100~

0-

H C O 3 "

NO3°

' A' S ' O ' N ' D I J 'F ' M ' A ' M " J ' J ' A ' S ' O ' N ' D I J 'F 'M 'A 'M ' J ' J 'A ' S ' O ' N ' D I J 'F ' M ' A ' M ' J ' J ' A ' S' 1985 1986 1987 1988

Fig. 8. Long-term variat ions of element concentrat ions in Strengbach stream water and corres- ponding ins tananeous discharge hydrograph.

INFLUENCE OF ACID ATMOSPHERIC INPUTS 119

160

120

.~ 100

130- 20 ~r~

0 110 -

70 300

50

180-

80 140.

~ 160

'°1 120

40 100

o , 'A " S 'O 'N 'DI J 'F 'M 'A 'M ' J ' J ' A ' S ' O ' N ' D [ J 'F'M 'A 'M 'J 'J 'A ' S~rg~N,F~F ~ X 'M' J r~ rx r ~

19~ 1986 1987 19B8

F ig . 8. ( c o n t i n u e ( ] ) .

Figure 2 represents the mean daily discharge hydrograph together with the corresponding hyetograph, measured continuously since September 1986. These last two years had very similar hydrological characteristics.

Exceptionally high values for alkalinity have been observed during some summer storm events (Fig. 8), although low values occur with the first increase in autumn runoff: the lowest alkalinity and pH values correspond to high flow values generated by snowmelt (March April).

120 h. PROBST ET AL.

The mean nitrate concentration was rather high (34.4peq 1 1) although ammonium was not detected in the Strengbach waters. This high nitrate value could be an indicator of the state of forest decline in the catchment (cf. Hauhs and Wright, 1986). However, from August 1985 to September 1988, no increase of the nitrate concentration could be detected (Fig. 8), in contrast to the situation in Dicke and Lange Bramke catchments in the Harz mountains, Western Germany (Hauhs and Wright, 1986).

For nitrate and potassium, strong seasonal cycles are observed (Fig. 8). This phenomenon is the result of an intensive recycling of these two elements by the forest ecosystem: potassium concentrations are highest during the first autumn flood events; nitrate concentrations, mainly deriving from atmospheric deposition and mineralization, are high during high water flow in winter, and when vegetation uptake is minimal.

Calcium, magnesium and sodium show less-obvious seasonal cycles (Fig. 8) (influenced by biological activity) which are also observed in other forest streams (cf. Likens et al., 1977; Bartoli, 1986).

Sulphate concentrations are relatively high throughout the hydrological year and are usually highest for the first autumn flood events (particularly after prolonged dry periods, as in the autumn of 1985). This increase of sulphate concentration corresponds to a sudden release of sulphate after an active period of organic matter mineralization (cf. Wright and Johannessen, 1980; Hultberg, 1985).

The chloride variations indicate that this element mainly originates from the atmosphere. This is in agreement with the observations of Feller and Kimmins (1979) in small undisturbed watersheds.

Silicon concentrations are rather high compared with those reported for other granitic basins in mainland Europe (Dupraz, 1984) but similar to values reported from Scotland: active and substantive weathering processes are indicated. Generally, this element is highly diluted in the larger flood events (Fig. 8).

The long-term variations of major ion concentrations (Fig. 8) show more variability since the end of 1986 owing to a more intensive sampling. Nonethe- less, sulphate, magnesium and calcium, exhibit a trend of declining concentra- tion during the studied period (Fig. 8). During the same time, pH seems to increase. Longer-term monitoring is required before the changes can be related to specific reactions; although the hydrological characteristics of 1986 and 1987 were similar, the autumn of 1985 was particularly dry.

The major element concentrations are relatively constant down the stream. The mean dissolved load for the stream water (35 mg 1 1) is similar to that for the spring water (Table 8): seven times higher than open field precipitations; nine times higher for sodium and potassium concentrations; 14 times higher for calcium; 19 times higher for magnesium. In stream water, calcium and magnesium are linearly related (r = 0.9); they have the same lithological origin within the basin. During snowmelt, these cations are highly diluted by meteoric waters (Fig. 8).

INFLUENCE OF ACID ATMOSPHERIC INPUTS 121

Strengbach stream waters are uncharacterist ic of waters draining granitic bedrock; alkalinity usually dominates the anionic charge (Probst et al., 1987). The Strengbach stream chemistry, dominated by strong acid anions, cannot be explained only by the influence of low open field inputs (Tables 2 and 8). However, the importance of dry and occult acid inputs is clear from the throughfall chemistry (Table 2): filtering properties of conifer canopies towards the acid pollutants is now well established (Mayer and Ulrich, 1980). Hultberg (1985) has shown a positive relationship between the proportion of conifer cover and the amount of sulphate in throughfall waters. In small English catchments, Hornung et al. (1987) have clearly indicated the acidifying role of coniferous stands in a zone where 80% of sulphate has a non-maritime origin. According to Lindberg and Garten (1988), the measurement of sulphate con- centration in throughfall is a good assessment of the total sulphur atmospheric deposition. Moreover, the Strengbach stream waters have a high concentra- tion of sulphate which cannot derive from an internal source (e.g. oxidation of pyrite) within the basin (Dahire, 1988). However, these stream waters are not yet acidified, even during snowmelt (lower pH value = 5.5), and hence aluminium is not present. Consequently, until now, the biota have not been in danger.

GENERAL CONCLUSION

Within the framework of research related to forest decline phenomena, the Strengbach catchment provides a valuable addition for developing a better understanding of internal ecosystem functioning. The research combines both the plot and the whole catchment scales on the same site; all the different aspects of the ecosystem are taken into account, from the atmospheric environ- ment to surface waters.

SPECIFIC CONCLUSIONS

(1) Precipitation is characterized by a rather low pH, resulting from a moderate global pollution climate. However, pollution peaks associated with easterly winds cause, during winter and spring, a strong increase of pollutant concentrations in open field precipitation and throughfall waters.

(2) Inputs of protons, sulphur and nitrogen, which originate mainly from dry and occult deposition, are ~ 1.8, 31, and 17 kg hectare ~ year-1, respectively.

(3) Such inputs strongly influence the inorganic mass transfers within the ecosystem. This influence results from the low base content and coarse texture of the soils.

(4) These inputs have led to an acidification of soil solutions, an increase of their aluminium content and a decrease of exchangeable base cations, par- t icularly for calcium; this process might be one of the fundamental causes of the deficiencies which characterize the forest decline phenomenon (cf. Bonneau, 1989).

122 A. PROBSTET AL.

(5) In the regolith, the acid inputs strongly enhance silicate mineral weathering. Even if strong acid anions dominate the total anionic charge, the buffering effect owing to weathering maintains the streamwater pH near neutral values. Consequently, biological life in the Strengbach is not presently in danger, in contrast to other south Vosgian streams where pH is low and aluminium concentration is over the ECC tolerance level (Massabuau et al., 1987). However, the duration of the period during which acidity will still be buffered in the regolith zone is unknown. Complementary studies on hydro- chemical flowpaths are already under development in order to detect the future evolution of this fragile system.

ACKNOWLEDGEMENTS

G~rard Krempp, Yvette Hartmeier and Daniel Million from the CGS (CNRS) are thanked for the chemical analyses. Bernard Pr~vosto and Dominique Gelhaye from the CRF (INRA) are thanked for their contribution to this research programme. Within the framework of the DEFORPA Program, this research has been supported by the ECC-DGXII, the French Ministry of the Environment, the French Ministry of Research and the R~gion Alsace.

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