STABLE S U L F U R I S O T O P E RATIOS AS A TOOL F O R I N T E R -

P R E T I N G E C O S Y S T E M S U L F U R D Y N A M I C S

R. D. F U L L E R l, M. J. M I T C H E L L 2, H. R. K R O U S E 3, B. J. W Y S K O W S K I 1, and C. T. D R I S C O L L 1

(Received March 22, 1985; revised August 22, 1985)

Abstract. Stable S isotope ratios (b34S) were evaluated in soil solution leachates, soluble plus adsorbed soil SO42-, soil organic S, precipitation, and in stream solutions along an elevational gradient at the Hubbard Brook Experimental Forest in the White Mts. of New Hampshire, U.S.A. The b34S of soil organic S varied with soil horizon and vegetation type, but was generally more negative than adsorbed or soil solution SO42-. In the Bh horizon, b34S of organic S was typically more negative than the Oa horizon or lower mineral soil (Bsl). The patterns suggest a combination of plant and microbial fractionation processes. Stream b34S values decreased with decreasing elevation and were correlated with decreases in SO42- concentration, suggesting an additional S source in higher elevation coniferous sites with a unique 6345.

1. Introduction

Anthropogenically derived 8042- is the predominant anion associated with atmos- pheric deposition as well as in soil and surface water solutions of many forested ecosystems in the northeastern United States. Elevated inputs of a relatively con- servative anion like SO4 a- will facilitate the transport of base cations, and potentially toxic acidic cations (H +, A1 d ÷ ) from soil to surface waters (Christopherson and Wright, 1981; Henrikson, 1979; Likens etal., 1977; Schofield and Trojnar, 1980). Under- standing ecosystem S dynamics is therefore a prerequisite for assessing the role of acidic deposition in acidification of soils and surface waters. Stable S isotope ratios (fi34S), in conjunction with other analytical techniques, represent a rarely used tool for evaluating S cycling in terrestrial ecosystems.

Sulfur isotope ratios have been used with varying degrees of success to fingerprint sources of natural and pollutant-derived S in the atmosphere, aquatic systems and plants (Case and Krouse, 1980; Nriagu and Coker, 1978a; Schwarcz and Cortecci, 1974). Various studies, particularly in aquatic systems, have reported chemical and biological fractionation of S isotopes (Kaplan and Rittenberg, 1964; McCready et aL, 1975; Nriagu, 1974; Winner et al., 1978). However, very few systematic investigations of S isotopes in soil and soil solutions have been conducted. The ability of abiotic and biotic processes in the ecosystem to fractionate stable S isotopes can result in S pools with distinct isotope ratios. If the degree of fractionation in a given S transformation is known, these ratios can potentially be used to evaluate the magnitude of S fluxes in individual soil horizons (Figure 1). The utility of this approach has been demonstrated

i Dept. of Civil Engineering, Syracuse University, Syracuse, NY, 13210, U.S.A. 2 SUNY College of Environmental Science and Forestry, Syracuse, NY, 13210, U.S.A. 3 Physics Dept., The University of Calgary, Calgary, Alberta, T2N 1N4, Canada

Water, Air, and Soil Pollution 28 (1986) 163-171. © 1986 by D. Reidel Publishing Company.

164 R . D . FULLER ET AL.

ORGANIC SULFUR

1 CARBON- BONDED SULFUR

ESTER SULFATE

SULFATE

C

SOLUBLE

SULFATE

ADSORBED

SULFATE

a- ADSORPTION

b. DESORPTION

C- ASSIMILATION. IMMOBILIZATION

d- MINERALIZATION

Fig. 1. Simplified conceptual model of S transformations occuring in a single soil horizon. Boxes refer to S pools, horizontal arrows to in situ S fluxes and vertical arrows to leaching inputs and outputs. Trans-

locations between horizons mediated by plant uptake are not shown.

by studies in western Canada of the transfer and transformation of S from point sources with known b34S (Krouse and Case, 1981; Krouse etal . , 1984; Krouse and Van Everdingen, 1984). Previous studies of terrestrial ecosystems, however, have not utilized study areas in which S inputs were derived from a range of non-point sources of potentially variable b34S.

The objectives of our study were: (1)to determine b34S values in organic and inorganic S fractions within soils, soil solutions and a stream along an elevation/vege- tation gradient and (2) to relate the patterns of ~34S to processes affecting the transfer and transformation of S within the ecosystem (Figure 1).

2. Materials and Methods

At the Hubbard Brook Experimental Forest (HBEF) in the White Mts. of New Hampshire, the "small watershed" approach has been used to evaluate the bio- geochemistry of many elements including S (Likens et al., 1977). Input/output budgets at HBEF have provided important information on integrated ecosystem processes. Recent studies of stream order gradients, however, have suggested that these ecosystems have considerable elevational heterogeneity (Johnson et al., 1981).

Spatial variability in stable S isotope distribution at the HBEF (Figure 2) was evaluated using samples obtained from a previous study of soil and solution chemistry in two adjacent gauged watersheds (WS5 and WS6). The dominant tree species in WS5

STABLE SULFUR ISOTOPE RATIOS AS A TOOL

f

, /

165

7 5 0

i 5 0

500

700

6 5 0 ~"

- - - - - w a t e r s h e d b o u n d a r y

s t r e a m

w e i r

,~ l y s i m e t e r s i te

J / - 4 5 0

550 500 450

Fig. 2. Topographic map of watersheds 5 and 6 (WS5 and WS6) at the HBEF showing locations of lysimeter sites and the approximate distribution of coniferous vegetation (shaded area) within the

watersheds only.

and WS6 are American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis), and sugar maple (Acer saccharum), co-occurring across much of the watersheds (500 to 730 m). Coniferous stands of red spruce (Picea rubens) and balsam fir (Abies balsamea) dominate the highest elevations (> 730 m) and mid elevation ridge crests. Soil samples were obtained from a stratified-random sampling of WS5, in which thirty 0.5 m 2 soil pits were excavated to fragipan immediately above a schist bedrock (Littleton formation). These samples were composited by horizon into three groups representing a low elevation deciduous zone (500 to 650 m), a high elevation deciduous zone (650 to 730 m) and a coniferous spruce-fir zone (Figure 2). Stream samples were collected along an elevational gradient in WS5, and monthly samples representing a complete year were mixed in equal volumes to produce a composite stream sample for each elevation. "Tension free" soil lysimeters (acid-washed quartz sand filled funnels; Turner et al., 1985) in WS5 and west of WS6 were used to collect soil solution samples from beneath the Oa, Bh and Bs2 horizons. Because of the large volumes of water necessary for ~34S

determinations, soil solutions collected in the fall and winter of 1984 were composited by elevation and horizon.

Stable S isotope ratios were determined in S fractions obtained from this sampling regimen. Adsorbed plus soluble S was extracted from soils with 0.016 M NaHzPO4,

166 R. D. FULLER ET AL.

reduced in an HI solution to S 2- and collected in a zinc-acetate solution (Landers et aL,

1983). Since this fraction is largely SO42-, it will be referred to as adsorbed plus soluble SO42- . In a sub-sample of the POn-extracted soil, organic S (largely carbon-bonded S and ester-sulfates) was determined by oxidizing with NaOBr and subsequent reduction with HI. Rock samples from soil pits were powdered and total S extracted in Kiba reagent, a mixture of H3PO 4 and SnCI z (Kiba et al., 1957). Sulfur in bulk precipitation, stream (composited monthly samples from 6/1/82 - 6/1/83) and lysimeter samples (from December, 1983) were processed by HI-reduction. In all samples, S 2- was collected in a zinc-acetate solution, precipitated by addition of AgNO3 and stable S isotope ratios were determined on a mass spectrometer built around a micromass 602 analyser (Krouse and Case, 1981). Sulfur isotope ratios are expressed in the typical ~348

notation:

034S (%0) = (R . . . . ple/Rstd - 1) X 1000

where Rsample and Rstandard refer to the 34S/32S ratio of the sample and the standard Canyon Diablo troilite meteorite, respectively. In soil and stream solutions, S042- concentrations were determined by ion chromatography (Small et aL, 1975).

3. Results and Discussion

Nriagu and Coker (1978b) examined ~348 values in bulk precipitation from 19 sampling sites across the Great Lakes Basin, an area of more than 1000 km in longitudinal extent. All of their sites exhibited a pronounced seasonal trend, with higher ~348 ValUeS in winter (Jan. b34S = +6.39 + 0.98%0) and lower values in summer (Aug. ~345 = + 2.92 + 1-00%o)- Their summer values compare favorably with summer bulk precipitation measurements from HBEF (b34S range = + 2.3 to + 3.0%0) and measure- ments of SO42 - aerosols (Saltzmann et al., 1983) from HBEF (b34S range = + 0.8 to + 3.5%°).

Soil organic S b34S values at the HBEF ranged from - 7 to + 5%0 (Figure 3) and varied with soil horizon. The more negative ~348 of soil organic S relative to atmos-

• pheric S inputs may result from several factors: (1) Pre-acidic deposition S inputs may have had a more negative ~34S value than present day inputs. However, lake sediment profiles from Ontario, Canada suggest the opposite is true (Nriagn and Coker, 1983). (2) Sulfur derived from long-term rock weathering may have a negative b34S. The three predominant rock types at HBEF are Kinsman quartz monzonite (very low concen- trations of S), Camptonite, a fine-grained diabase (63zS = -7.1%o) containing small amounts of pyrite, and the Littleton formation (~345 = + 9.2%o), a schist bedrock (< 1 m soil depth) of SiUimanite grade. Although the Camptonite has 15 times the S concentration of the Littleton formation, it was observed only to a limited extent in soil pits. Weathering of rock S at HBEF appears to be a minor source of SO42- (estimated at 4 ~ of total S input; Likens et al., 1977), so it is doubtful that its effect on b34S of current S042 - fluxes would be large; however, it may have had a significant cumulative effect on the b34S of organic S during soil formation. (3)Biological

STABLE SULFUR ISOTOPE RATIOS AS A TOOL 167

z O N E O

O i e

Oa

E

Bh

Bsl

Bs2

-10

Oie

Oa

E

Bh

Bsl

Bs2

- - ' 0

~ e

Oa

E

Bh

Bsl

Bs2

-1(

i i , ,

' ' ' SPRU'CE-Fml "',A,,,,,, 176° M) I

. , D~c,buous i • . . ~ . . . (TSO .) I

, ,A i

. ~ . ~ o,, (600 M)

I I I I , - 5 0 *5 +10

83,s Fig. 3. b34S in soil horizons at three elevations at HBEF in soil organic S (solid circles), adsorbed plus soluble SO42- (open circles) and soil solution SO4 a- (triangles; sampled Dec. 1983). Missing horizons

either did not contain enough S for ~34S analysis, or did not have lysimeters beneath them.

168 R. D. FULLER ET AL.

fractionation, either through microbial immobilization or plant assimilation, may enrich organic S in 32S and decrease ~34S.

Adsorbed plus soluble SO42- and soil solution SO42- ~34S averaged (+ s.d.) + 4.3 + 3.4%0 and + 5.8 + 1.5~oo across all horizons, respectively (Figure 3). The greater similarity of the b34S of the soil solution SO42- to the adsorbed plus soluble fraction than soil organic S (Figure 3) probably results from rapid equilibration between these two phases and suggests that SOa 2- adsorption does not markedly fractionate S. Greater variability in the adsorbed plus soluble fraction relative to soil solutions may result from sampling bias, since soils were sampled in summer (minimum soil solution flux, maximum biological activity) whereas soil solutions were sampled in fall and winter (rapid soil solution flux, minimum biological activity). The similarity of soil solution and soil adsorbed plus soluble ~34S values to SO42- aerosols and precipitation inputs at HBEF (Saltzman et aL, 1983) suggests that reactive soil SO42 - and soil solution SO42- are derived largely from atmospheric deposition. However more extensive data on seasonal changes in the (~345 of atmospheric inputs are needed to verify this hypothesis.

In all three composite soil profiles (Figure 3) the Bh horizon organic b34S value was at least 5%0 more negative than either the Oa horizon above it, or the Bs below. While there have been very few comparable studies Krouse et al. (1984) found a very similar pattern in surface soil horizons from Alberta, Canada. In these soils, which were classified as degraded eutric brunisols, the surface portion of the A1 horizon had a consistently more negative ~345 than the 02 horizon above.

Organic S in surface mineral horizons (E and Bh) arises largely from three distinct processes: (1) In situ deposition of organic S during root turnover and root exudation, (2) humification of organic matter inputs in forest floor, and resulting mobilization and transfer of a variety of organic S compounds (David et aL, 1984) which are deposited in the mineral soil (particularly the spodic Bh horizon), and (3) microbial immobilization of SO42 - and the formation of ester-sulfate and carbon-bonded S (David et aL, 1984). It is likely that fractionation of S isotopes during one or more of these processes led to the more negative ~34S values observed in HBEF Bh horizons.

Almost all of the evidence on biological fractionation of S isotopes is based on single organism experiments in aquatic systems and is not directly applicable to soils. Frequently, however, biologically mediated transformations select for the lighter isotope, producing a product pool with more negative ~348, and a reactant pool with more positive ~348 (Kaplan and Rittenberg, 1964). The more negative b34S values in the Bh horizon may have arisen from plant S fractionation (Krouse and Case, 1981) during SO4 a- reduction and assimilation (Figure 1) which would contribute to organic S with a more negative ~34S to the Oi (leaves) and Bh (roots), a horizon of root accumulation (Wood, 1980). Mineralization of organic S in the Oie may have produced S in the residual Oa horizon with a more positive ~34S (Figure 3). Finally, microbial immobili- zation (Figure 1) of SO42- may contribute both to the more negative ~348 of Bh organic S and the more positive ~348 of Bh adsorbed plus soluble SO42-. However, the relative contribution of each of these hypothetical processes is unknown until much more information on soil S isotope fractionation processes is available.

STABLE SULFUR ISOTOPE RATIOS AS A TOOL 169

Stable S isotope ratios (Figure 4) in the Hubbard Brook stream (WS5) decreased with elevation from ~348 --- + 14.9%o at the headwaters (748 m) to - 0.2%0 at the lowest elevation site (495 m). Sulfate concentration in the stream also decreased with elevation

750

700 V

~ 6 5 0 m

' < 6oQ > UJ ._.1 LU sso

500

SULFATE ( ,MOLAR) 60 65 70 75

I , l t

~ ~',,- - ~ ' -

t t

/ /

/ /

,v,

i ! - ~ - 0 - z~

! ( ~ 3 4 S , / -4-

I , I I I 0 +5 +10 +15

~34 S

Fig. 4. ~345 in monthly composited (6/1/82-6/1/83) stream samples (solid circles) and mean SO42- concentrations (open triangles) from watershed 5 at HBEF as a function of elevation and vegetation

type.

and was significantly correlated with b348 values (r e = 0.71). The origin of the gradient in ~348 values with stream elevation is not readily apparent from soil and soil solution data. However, the higher concentration of stream SO4 z- at upper elevation sites suggests a source of S with high ~348 in the spruce-fir zone. Some possible sources include: (1) Greater weathering of S from Littleton formation bedrock (b34S = + 9.2%0) at high elevations. Although this rock has relatively low S concentrations, very shallow soils at high elevations (Bormann et aL, 1970) may lead to increased weathering rates.

170 R. D. FULLER ET AL.

(2) Fractionation which accompanies the conversion of SO2 to H S O 3 - can potentially result in increases in ~348 of q- 15-6~oo (Egiazorov e t a L , 1971; Saltzman e t a L , 1983). If coniferous foliage serves as a more efficient reactive surface for capture or conversion of SO 2 aerosols (Lovett et aL, 1982) than deciduous foliage, this may potentially lead to the observed gradient in stream b34S. Alternatively, higher (~345 values in the headwater reaches of the stream may result from SO42- reduction during low flow periods.

4. Conclusions

Stable S isotopes ratios appear to be a valuable tool for interpreting the complex processes affecting SO42- as it moves through and interacts with S pools in each soil horizon. This study indicates that inorganic S in soil and soil solutions of a forested watershed in the northeastern U.S. was probably largely derived from atmospheric deposition. However, biotic transformations would appear to be responsible for the substantial variations in 634S between different S fractions and different soil horizons. In particular the spodic horizon (Bh) appears to be a dominant locus of biotic S transformations.

These patterns in stable S isotope ratios suggest that, although these Spodosols probably do not retain net quantities of SO42-, this ion does not behave in a conservative fashion. Rather, as SO42- moves through the soil profile, it is strongly influenced by both chemical and biological transformations.

Acknowledgments

We are grateful for the assistance of G. B. Lawrence, A. H. Johnson, and M. B. David in sample collection and analysis and Y. DeGoufffor manuscript preparation. Financial support was provided by the National Science Foundation (DEB-8206980) and the Syracuse University Institute for Energy Research. The Hubbard Brook Experimental Forest is operated by the USDA Forest Service, Broomall, Pennsylvania.

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STABLE SULFUR ISOTOPE RATIOS AS A TOOL 171

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