Steven E. Lindberg - Coweeta LTERcoweeta.uga.edu/publications/658.pdf · Ono \ Multidisciplinary...

$: Steven E. Lindberg - Coweeta LTERcoweeta.uga.edu/publications/658.pdf · Ono \ Multidisciplinary Study TJ.CKuiperandM. Bos ... itrient Cycling D.W. Johnson and Dale W. Johnson Steven$
of Florida (1990)!_I. Livingston

Tropical Biota: Ecosystemnd Global Challenges (1990).G. Goldammer

:-Cycle Concept of Ecosystems

i. Remmcrt

Heterogeneity (1991)[. Kolasa and S.T.A. Pickett

I Grasses: The NutritionalEquids, and Their Impact ongue (1991)can

and El Nino: Responses to;ntal Stress (1991)F. Trillmich and ICA. Ono

\ Multidisciplinary Study

TJ.CKuiperandM. Bos

mistry of a Subalpinee Loch Vale Watershed (1992)Jill Baron

ric Deposition anditrient CyclingD.W. Johnson and

Dale W. Johnson

Steven E. LindbergEditors

Atmospheric Deposition and

Forest Nutrient Cycling

A Synthesis of

the Integrated Forest Study

With 236 Illustrations

Springer-VerlagNew York Berlin Heidelberg London Paris

Tokyo Hong Kong Barcelona Budapest

Ecological Studies

Analysis and Synthesis

Edited by

W.D. Billings, Durham (USA) F. Golley, Athens (USA)

O.L. Lange, Wurzburg (FRG) J.S. Olson, Oak Ridge (USA)

H. Remmert, Marburg (FRG)

Volume 91

90 Atmospheric Deposition and Forest Nutrient Cycling

3500Total Sulfate Deposition (eq ha"1/')

<— Regression Line ( r 2 • 0.97)

500 1000 1500 2000 2500 3000 3500Flux of Sulfate in TF+SF (eq ha"1/1)

'- Annual IPS Data Regr.(y-0.84x + 170)

Figure 5.12. Estimated total annual atmospheric deposition of SO42 at each IPS

site as function of measured flux of SO4:~ in throughfall plus stemflow. Data are

plotted for each individual IPS sampling year for 1986-1989; linear regression lineis shown (see text).

relative difference between the two fluxes at any site (NCE/total deposition,see Figure 5.11) is well within the typical error of total deposition estimates(Hicks et al. 1986). These results have important implications in many areas.Given the overall uncertainty in all types of deposition estimates, measure-ments of fluxes of SO4

2~ in throughfall beneath forests can provide usefulestimates of spatial and temporal trends in the atmospheric deposition of S,particularly in industrialized areas. This application of TF + SF measure-ments may be especially important in forests in complex terrain where stan-dard micrometeorological methods cannot be routinely used to estimate drydeposition (Hicks et al. 1986). The method also has important applicationsto deposition model development and evaluation and to prediction of soiland water acidification, and can provide the field data necessary for scalingup from point measurements of atmospheric fluxes to watershed scales. Arecent European study has adopted this method to test the results of long-range transport and deposition models (Ivens et al. 1990).

Sulfur Distribution and Cycling in Forest EcosystemsM.J. Mitchell, R.B. Harrison, J.W. Fitzgerald, D.W. Johnson,

S.E. Lindberg, Y. Zhang, and A. Autry

The distribution and cycling of sulfur in forest ecosystems have been thefocus of a number of studies because of the linkage of sulfur with acidic

:nt Cycling 5. Sulfur Cycles 91

<— 1:1 Line

sion Line (r* • 0.97)

deposition. Previous work has been reviewed by Fitzgerald and Johnson(1982), David et al. (1984), Johnson (1984), and Mitchell et al. (1991). Inthis section, we focus on the IFS sites but also include information for otherselected sites for which detailed information on sulfur is available (Table5.1). Within this section, information in figures will be presented with sitesranked from high to low atmospheric inputs (Figure 5.13) as indexed byeither total deposition (i.e. IFS sites) or bulk throughfall plus stemflow (mostother sites) since total deposition is generally not available for most forestecosystems. This latter measurement, however, gives a close estimate oftotal sulfur deposition as discussed in the previous section.

500V1)

3000 3500

34x * 170)

on of SOr~ at each IFSplus stemflow. Data are

>89: linear regression line

(NCE/total deposition,:al deposition estimatesications in many areas,on estimates, measure-:sts can provide usefulpheric deposition of S,of TF -i- SF measure-

ilex terrain where stan-ly used to estimate dryimportant applicationsd to prediction of soila necessary for scalingo watershed scales. A:st the results of long-990).

st Ecosystems. D.W. Johnson,Vutry

ystems have been theof sulfur with acidic

Sulfur Content and Constituents

M.J. Mitchell and Y. Zhang

Vegetation

For most sites, vegetation represents a small (<12%) component of the totalsulfur content of the ecosystem (Figure 5.14). Similar contributions of veg-etation to ecosystem pools are found for other elements including nitrogen(see Chapter 6). In contrast, vegetation in the Coweeta Pine site (CP) andDouglas fir site in Australia (AD) constitutes a larger fraction of the totalsulfur mass (17% and 37%, respectively). For the latter site, this high valueis attributable to the very small total sulfur pool in the soil and the absenceof major anthropogenic sources of sulfur, which results in very low atmo-spheric sulfur inputs. For the CP site, this higher value also reflects thelower sulfur content in the soil, but this is because its very high coarsefragment (>2 mm) content of the soil mineral horizons (22%-37%) reducesthe soil that is biogeochemically active. In contrast, the adjacent hardwoodsite (CH) has a coarse fragment content that is substantially lower withinmineral horizons (9%-12%), resulting in higher sulfur content [see the Ap-pendix of site data for further details]. In examining the nitrogen pools forthe IFS sites, it has also been found that the CH site had proportionally morenitrogen in the vegetation (see Chapter 6).

In examining all the sites along the sulfur depositional gradient, includingboth IFS and non-IFS sites, it is clear that there is no relationship betweenvegetation content and sulfur inputs (p > .05, r — .10, n = 20). With thepossible exception of the low input site in Australia, the sulfur content ofthe vegetation of these systems shows little variation, especially comparedto an element such as nitrogen that more typically may limit forest growth(see Chapter 6). These results concur with previous studies, which also haveshown that there is no linear relationship between vegetation content andatmospheric input of sulfur for forest ecosystems (Johnson 1984; Mitchellet al. 1991). The absence of this relationship is a function of sulfur avail-ability, which is in excess of nutritional needs for most forests.


O ~" """ ^^00 3O 00 OSo So — ~ CT- —x ̂so so •-' *— \o ~"22 5 = IS§r~-l6slUS=5co a

'-> -= = -^ •-

SL

soC3

Loca

s -s ~s

<5<C

II .§ 5.d =_*> •? -a ^ c- K

k, 5; c5iO

1 111

003

az

g g §-

o.S.S^ ||

5. Sulfur Cycles 93

03.C

^ 13

0) 2

3 1§*

p

PO GS ST SF HB DL SB TL CH HF FL DFAS AO SS FT WF LP GL NS CP FS RA AD

M TOTAL DEPOSITION* Non-IFSSite

SITE

HTF+SF

Figure 5.13. Ranking of sites by sulfate inputs using either total deposition or bulkthroughfall + stemflow. Codes for IPS sites are explained in text; (*, non-lFS site).

It has been generally assumed that there is a tight linkage between thenitrogen and sulfur concentrations, because the ratios of nitrogen- and sul-fur-bearing amino acids are relatively constant and account for almost allthe organic nitrogen and sulfur constituents of forest foliage (Turner et al.1977; Johnson 1984). The ratio between organic nitrogen and organic sulfuris generally presumed to be about 34 on a molar basis or 15 on a mass basis(Kelly and Lambert 1972; Turner and Lambert 1980). This ratio may vary,however, especially from nitrogen fertilization, which may increase the ratioby the formation of compounds such as arginine (Lambert et al. 1976; Hom-ann, personal communication). Lower values would indicate excess sulfuruptake and subsequent storage of SO4

2~ in foliage. For additional discussionon elemental ratios in vegetation also see Chapter 7.

An examination of N:S ratios (Figure 5.15) shows that all sites are below34, except for CH in which the ratio is approximately 35 and the RA sitein which it is exceeded (ratio = 40). This latter site has very high nitrogenavailability because of autotrophic nitrogen fixation and low sulfur inputs,and thus sulfur may be limiting. In addition, sulfur limitations have beenshown for other forests in the northwest region of the United States wherethe RA site is located (Turner et al. 1977).


2500

2000

m 1500

CO

i?1000

500

5*3

PO GS ST S3 SF FT HB WF DL LP SB GL TL CH CP HF FS FL RA DF AD

SITE

^MINERAL SOIL HFOREST FLOOR

S VEGETATION*Non-IFSSite

Figure 5.14. Sulfur content in vegetation, forest floor, and mineral soil (*, non-IFSsite).

For six selected sites (Table 5.2), additional analysis of foliage was un-dertaken to obtain more detailed information on total sulfur, inorganic sul-fur, total nitrogen, and total carbon. Total carbon was analyzed using a Per-kin-Elmer 2400-CHN Analyzer, total sulfur by a Leco SC-132 SulfurDeterminator (David et al. 1989), and inorganic sulfur (assumed to be sul-fate) by HI (hydriodic acid) reduction (Landers et al. 1983). Nitrogen con-centrations were substantially higher and thus C:N lower in the Americanbeech and maple from HF than in the conifers from the other sites (Figure5.16). The mean value of sulfate in foliage was approximately 6% of totalsulfur (cr = 2.5, n = 60) with a range from 2.2 to 13.2. Of the sites ana-lyzed, loblolly pine had the highest sulfate concentration, 10.4% of totalsulfur, and this may be attributed to the very high SO4

:~ concentrationsfound in soil solution from this site (i.e., Bt2 horizon = 312 jueq L~').

When the inorganic sulfur fraction (HI-S) was subtracted from total S,there was a significant correlation between organic sulfur and total nitrogenfor hardwoods (r = .61, p < .01) and conifers (r - .46, p < .01), but forboth vegetation types there was considerable variation in the mean N:S mo-lar ratios (23.2 hardwoods; 21.9 conifers; on a mass basis, 10.1 and 9.5,respectively). These ratios are lower than typically reported for some forestecosystems, as was discussed previously. There was no apparent discernible


CPHFFS FLRADFAD

' FLOOR

1 mineral soil (*, non-IFS

45

40

35_O

30

25

20

15

RA

CH

DF FtHF

SB

TL

DLST

CP LP

10 15 20 25 30 35

kg S/(ha yr) [TF+SF]

Figure 5.15. N:S ratios in foliage versus inputs in tliroughfall + stemflow.

/sis of foliage was un-1 sulfur, inorganic sul-5 analyzed using a Per-

Leco SC-132 Sulfurur (assumed to be sul-. 1983). Nitrogen con-ower in the Americanthe other sites (Figureroximately 6% of total[3.2. Of the sites ana-ration. 10.4% of total

SO.f~ concentrationsn = 312 neq L~').btracted from total S,ilfur and total nitrogen.46. p < .01), but fori in the mean N:S mo-; basis. 10.1 and 9.5,ported for some foresto apparent discernible

pattern in the relationship between organic sulfur and total nitrogen amongsites or subsamples on the basis of either solution chemistry or vegetationrequirements (Figures 5.17 and 5.18). This lack of pattern may be attributedto the absence of sites in this limited sample showing either low sulfur avail-ability or nitrogen limitations.

Forest Floor and Mineral Soil

The dominant sulfur pool in all sites (Figure 5.19) was the mineral soil(71%-97% of the total sulfur in the ecosystem), with the exception beingthe Douglas fir site in Australia (AD), which had only 47% of total sulfurin soil. The forest floor contained a similar amount of sulfur as that in thevegetation, and contributed from 2% to 17% of total sulfur in each site.Unlike vegetation, however, there was a significant correlation between totalsulfur content in the forest floor (p < .05, r = .57, n = 17) and mineralsoil (p < .05, r = .54, n = 21) of these forest ecosystems including bothIPS and non-IFS sites and the input of S, but this single factor explainedless than 34% and 30%, respectively, of the variation in sulfur content ofthese ecosystem strata. The correlation of sulfur inputs with the sulfur con-tent of the forest floor and mineral soil is likely not directly linked to sulfurinputs; this correlation is probably more the result of differences in the or-

'96 Atmospheric Deposition and Forest Nutrient Cycling

Table 5.2. Vegetation Samples Analyzed for Total Sulfur, HI Sulfur, Total Nitro-gen, and Total Carbon

SiteDLDLDLSTSTSSSSMSMSMSNS

NSNSNSHFHFHFHF

ReplicateVegetation Samples Sample Description

Pinus taedaPinus taedaPinus taedaPices rubensPices rubensPices rubensPices rubensPices rubensAbies balsameaPinus strobusPicea abies

Picea abiesPicea abiesPicea abiesFagus grandifoliaFagus grandifoliaAcer saccharumAcer saccharum

44255551112

233'4444

Current flush, four treesSecond flush, four treesThird flush, four treeNew foliageOld foliageNew foliageOld foliageNew foliage composited from five samplesNew foliage composited from two samplesComposited from three samplesComposites from six trees, pH 6 treatmentComposites from eight trees, pH 2.5

treatmentComposites from five trees, A4-R1Composites from eight trees, A4-R2Foliage from individual trees in 1986Foliage from individual trees in 1987Foliage from individual trees in 1986Foliage from individual trees in 1987

ganic content of soil. For example, the high-elevation sites, which generallyhave higher sulfur inputs, are characterized by large pools of organic sulfurthat correlate closely with carbon content (Mitchell et al. 1991) of these soils(see pp. 119-121) for further details). However, David et al. (1988) alsofound that forest floor and soil sulfur contents were correlated with atmo-spheric deposition of sulfur over a broad geographical range in the north-central United States.

As has been established from previous studies, the dominant form of sul-fur in the soil is generally organic (>75% of total S), especially for Dur-ochrepts (DF and RA), Cryandepts (FL), Inceptisols (WF, SS and SB), andSpodosols (HF and TL) (see Figure 5.19). This organic sulfur in soils iscomposed of two major fractions (Freney 1967; Mitchell et al. 1991): car-bon-bonded sulfur and ester sulfates, the composition of which has beendetermined for selected soil horizons at IPS sites (Autry et al., 1990).

For Hapludults (CH, CP, DL, GL), sulfate is more predominate (see Fig-ure 5.19) and constitutes from 43% to 70% of the total sulfur in the mineralsoil. These sites in the southeastern United States are south of the mostrecent continental glaciation and have soils more highly weathered than thoseof the northern sites. Thus these Hapludults have higher sulfate adsorptionpotential and have accumulated sulfate. The importance of weathering aswell as sulfate adsorption potential has been previously established for otherregional comparisons of soil sulfate retention capacities (Johnson et al. 1980;Rochelle et al. 1987).

nt Cycling 5. Sulfur Cycles 97

:. HI Sulfur, Total Nitro-

j Descriptiontreestreesse

sited from five samplessited from two samplesiree samplesx trees. pH 6 treatmentght trees, pH 2.5

/e trees. A4-R1ght trees. A4-R2hial trees in 1986lual trees in 1987lual trees in 1986lual trees in 1987

sites, which generallytools of organic sulfuril. 1991) of these soilsvid et al. (1988) alsocorrelated with atmo-il range in the north-

dominant form of sul-), especially for Dur-WF. SS and SB), andtnic sulfur in soils isicll et al. 1991): car-n of which has been:ry et al., 1990).jredominate (see Fig-sulfur in the mineral

•e south of the mostweathered than thoseicr sulfate adsorptionice of weathering asestablished for other(Johnson etal. 1980;

HF DL NS

SITE

BiC:N SN:SLetters above bars indicate vegetation type

MS ST SS

i'N:OrganicS

Figure 5.16. Foliage analyses of selected IFS sites (LP, Loblolly pine; RS, red spruce;NS, Norway spruce; WP, white pine; BF, balsam fir; AB, American beech; SM,sugar maple).

Fluxes and Regulating Factors

M.J. Mitchell, D.W. Johnson, and S.E. Lindberg

Throughfall, Stem/low, and Foliar Leaching

The throughfall plus stemflow (TF + SF) fluxes of SO42~ at the IFS sites

cover a wide range, from about 300 to 2500 eq ha~' yr~' (see Figure 5.7).The sites fall into roughly the same three categories discussed earlier inChapter 5 for total deposition (highest at the mountain sites and at thosewith the highest airborne sulfur concentrations, and lowest in Washington).The sources of SO4

2~ in TF + SF may be external or internal to the plantand include SO4

2~ in precipitation and intercepted cloud water, washoff ofdry-deposited aerosols and particles, and leaching of internal plant SO4

2~.For the IFS sites with complete data sets on wet-only TF + SF fluxes andsulfur content in vegetation and soils (n = 9), there was no significant re-lationship between SO4

2~ flux in TF + SF and soil content of sulfur (r = .41),and only a marginally significant relationship (p < 0.10) between TF + SFand the vegetation content of sulfur (r = .65). However, across all IFS sites,


50

45

C/5 40O'c03 35O)6o 3 0

D 25

20

LPLP

WPRS

LR.P RS RS RS

400 500 600 700 800

umol Nitrogen900 1000

LP=Loblolly Pine; RS=Red Spruce; NS=Norway Spruce; WP= White Pine: SF=Balsam FirPredicted Organic S

Organic S = 0.0175 x N + 19.48 (r = 0.46, n=42)

Figure 5.17. Relationship between organic S and total N of conifer foliage.

the fluxes in deposition and TF + SF are strongly correlated (r = .99) (seeFigure 5.8). At five IPS sites, the fluxes are essentially the same above andbelow the canopy, and at the remaining sites the differences are generallysmall. The IPS and other recent data (Garten et al. 1988; Lindberg and Gar-ten 1988; Garten 1990) suggest that SO4

2~ in TF + SF is largely controlledby atmospheric deposition and that foliar leaching is minimal.

Litterfall, Translocation, and Root Turnover

Litterfall inputs of sulfur show little variation among the forest sites used inthe present comparison (mean, 3.8 kg ha"1 yr~'; cr = 1.8, « = 19). Similarlirterfall inputs combined with low variation in sulfur concentration of litterresult in the similarities in sulfur litter flux among sites. For those sites withmore than 10 kg S ha~' yr~', litter inputs are less than total atmosphericdeposition, which suggests that for these sites the geochemical cycling ofsulfur may be more important than its biological cycling. The relative im-portance of biological cycling among sites can be indexed by calculatingwhether atmospheric inputs exceed requirements (Figure 5.20). We wouldhypothesize that for those sites in which requirements are greater than at-mospheric inputs, the cycling of sulfur through the vegetation would be es-

nt Cycling 5. Sulfur Cycles 99

WP

900 1000

:e Pine 3F=3alsam Fir

N of conifer foliage.

100

h; 90

"5CO 80o

03 70O)O~o

60

50

40

SM

ABAB

SM

700 900 1100 1300 1500 1700 1900800 1000 1200 1400 1600 1800

umol Nitrogen

SM=sugar maple; AB=American Beech

Predicted Organic SORGANIC S = 0.0261 x N + 22.85 (r = 0.61, n = 15)

Figure 5.18. Relationship between organic S and total N of hardwood foliage.

rrelated (r = .99) (seely the same above andTerences are generally88; Lindberg and Gar-F is largely controlledminimal.

mover

:he forest sites used in1.8. n = 19). Similarconcentration of litters. For those sites withtian total atmosphericlochemical cycling of;ing. The relative im-idexed by calculatingure 5.20). We woulds are greater than at-getation would be es-

pecially important in regulating sulfur flux. For two sites (CP and RA), thiscriterion is met and thus biological cycling of sulfur through the vegetationwould be required to meet nutritional demands. Even for those sites (DF,HF, TL) in which inputs exceed requirements by less than 4 kg S ha""1 yr~' ,biological cycling of sulfur through the vegetation would also be important.Because inputs are calculated on an annual basis while requirements will bemaximal during the growing season, some of the atmospherically derivedsulfur may be leached from the soil during dormant periods and thus not beavailable for plant uptake. Thus, internal recycling of sulfur may be neededto meet plant requirements.

In considering the ways in which sulfur might be retained within the for-ests of the IPS network, we have made the broad distinction between organicand inorganic processes. Within the category of organic processes, the po-tential for sulfur incorporation by microbial processes and the inorganic pro-cesses are discussed in detail in the third part of this chapter. Neither ofthese sections addresses a third way in which sulfur could be retained withinforest ecosystems, however, which is by tree uptake, return by litterfall, andaccumulation within the forest floor. This process cannot be addressed di-rectly because with lack of data on the rate of forest floor accumulation onthe IFS sites, but we can perform an approximate analysis of this potential


2500

2000

03.C

1500

1000

500 -

ST SS WF DL LP SB GL TL NS CH CP HF FS FL RA DFSite

iH Total S H Sulfate-S

Figure 5.19. Total sulfur and sulfate content of mineral soil.

contribution by comparing litterfall rates with net ecosystem sulfur accu-mulation rates to see if there is any relationship.

As the data in Figure 5.21 show, litterfall sulfur equals a substantial frac-tion of net ecosystem sulfur retention in many of the IPS sites. However,only a fraction (if any) of this litterfall sulfur return would be accumulatingin the forest floor. For estimating the maximum contribution of this litterinput, we can calculate that if all the litterfall accumlated in the forest floor,it could account for half or more of the net ecosystem sulfur accumulationin all sites except those in sites in the Smoky Mountains (SS, ST, SB). Ifat little as one-tenth of litterfall sulfur accumulated in the forest floor an-nually, this accumulation would be minor for all sites. Forest floor accu-mulations are most likely to be occurring at the colder, high-elevation conif-erous sites (SS, ST, FL). However, in each of these cases, the potential forsulfur accumulation via this mechanism is not high, given (1) the lack ofsulfur retention now occurring or (2) the relatively small amount of litterfallsulfur as compared to ecosystem sulfur accumulation.

Solute Concentrations and Flux

The fluxes of SO42~ through the various ecosystem strata are given in Figure

5.22. A comparison of total sulfur inputs via atmospheric deposition versusSO4

2~ leached from beneath the B horizon is shown in Figure 5.23; in gen-

Cycling 5. Sulfur Cycles 101

IF FS FL RA DF

Mineral soil.

40

CO 30=E-COO)

XL4-. 20CDECD

"I 1°CDcc

ST

5 10 15 20 25 30 35Input kg S/(ha yr) [total deposition]

_ INPUT = REQUIREMENT

Figure 5.20. Vegetation requirements versus inputs of sulfur.

40

>system sulfur accu-

ils a substantial frac-IFS sites. However,>uld be accumulating•ibution of this litter:d in the forest floor,sulfur accumulation

ins (SS, ST, SB). Ifthe forest floor an-

;. Forest floor accu-ligh-elevation conif-ses. the potential for;iven (1) the lack of.1 amount of litterfall

a are given in Figureric deposition versusFigure 5.23; in gen-

eral, SO42 is behaving as a conservative ion with most of the SO4"" entering

the system being leached from the mineral soil (p < 0.01, r~ = .94, n = 15,slope = 1.4, constant = 0.37 keq SO4

2~ ha"1 yr~' leached). Similar resultshave been reported for other sites, especially where sulfate adsorption po-tentials are low (Rochelle et al. 1987; Mitchell et al. 1991). Note, however,for some sites there is marked deviation from this relationship (Figure 5.24).The deviations from conservative behavior can result from processes thatdecrease sulfate retention (e.g., sulfate desorption or sulfur mineralization)not being in steady state with processes which increase sulfate retention (e.g.,sulfate adsorption or sulfur immobilization). The relative roles of these pro-cesses are evaluated in later sections. In addition, any errors in calculatinginputs and outputs may contribute to such differences. The importance ofvarious assumptions in determining sulfur input-output budgets, especiallythe importance of accurately determining dry deposition, was discussed ear-lier in Chapter 5 and has been detailed elsewhere for the HF sue (Shepardet al. 1989).

For the IPS sites, there is no evidence that either chemical weathering orthe formation of secondary sulfate minerals play significant roles (see Chap-ter 10). These processes, however, cannot be ignored for other forest eco-systems. For example, in forest ecosystems underlain by sulfar-rich sedi-mentary rocks, weathering inputs can exceed atmospheric inputs (Gibson et

Net Ecosystem S Retention and U'tterfall S Return

20

II

-10

NET RETENTIONLITTERFALL

NS1 NS2 TL CF RA FL S3 ST1 ST2 S81 SB2 LP1 LP2 IF GL FS DL CP CH

SITE

Figure 5.21. Litterfall versus sulfur retention.

•ao

S?•§

o3§a

Io3

(TO

it Cycling 5. Sulfur Cycles 103

£2

d

CO03

CD<n LUHCO

03

3000

2500 -

2000 -

3COcr0)

1500 -

1000 -

500

ST SS WF DL LP SB GL TL NS CH CP HF FS FL RA DFSITE

I DEPOSITIONi FOREST FLOOR LEACHATE

I THROUGHFALL + STEMFLOWI LEACHATE BENEATH B HORIZON

Figure 5.22. Flux of sulfate through ecosystem strata.

3CO

al. 1983; Mitchell et al. 1986). In Europe, with substantially higher atmo-spheric inputs (see Figure 5.13) and concentrations of SO4

2~ in the soil so-lution, the formation of sulfur minerals may also be important (Van Breeman1973; Adams and Hajek 1978) and may be related to the very high SO4

2~concentrations in certain non-IFS sites (GS and SF).

Within the IPS sites for which organic sulfur in solution was measured(Homann et al. 1990), it contributes to solute flux through the various strataof all ecosystems (Figure 5.25). The importance of organic sulfur as a con-tributor to sulfur flux through solution decreases with increasing SO4

2~ con-centration, because the amount of organic sulfur in solution shows a smallerrange than that of SO4

2~. The amount of organic sulfur in precipitation ap-pears negligible, but it increases as rain passes through the canopy and theforest floor, the latter of which has the highest organic sulfur concentrations.The concentration of organic sulfur generally shows a marked decrease afterpassage through the mineral soil. This decrease could be caused by the com-bined effects of catabolism and chemical precipitation of dissolved organicsulfur in the mineral soil. Although some of the soluble organic suifur formedby microbial processes may be rapidly catabolized, it has aiso been sug-gested that the transport of soluble organic sulfur is an important mechanismfor organic sulfur accumulation in soil (Schoenau and Bettany 1987; Mitch-


CT3

CD

IoXDQEO

a•+-•

O

in•a8,o FT GS

ThousandsInput of TF+SF eq/(ha yr)

Input = Output

Figure 5.23. Total sulfur deposition versus leaching from B horizon.

ell et al. 1989) as has been shown for organic carbon (McDowell and Likens1988). Because dissolved organic sulfur that is deposited in the mineral soilis apparently recalcitrant to decomposition, it may contribute to the largeorganic sulfur pool, most of which shows low biogeochemical activity(McLaren et al. 1985; Schindler and Mitchell 1987). For further details onorganic sulfur in solutions for the IPS sites, with emphasis on the RA andDF sites in Washington, Homann et al. (1990) should be consulted.

Inorganic Sulfate Dynamics

R.B. Harrison and D.W. Johnson

Role of Sulfate Adsorption in Regulating Sulfate Flux

The ability of many soils to retain significant quantities of SO42~ by inor-

ganic adsorption mechanisms has long been recognized (Ensminger 1954;Kamprath et al. 1956; Tikhova 1958; Chao et al. 1962). Because one of thepredominant inputs in acidic precipitation is the SO4

2~ anion, the role ofadsorption in regulating the flux of sulfate and associated cations throughsoil has been considered to be a factor of great importance (Cole and John-son 1977; Johnson et al. 1983; Gaston et al. 1986). The mobility of SO4

2~has received considerable attention because net SO4

2~ retention by soils can

:nt Cycling

yr)

is from B horizon.

(McDowell and Likenssited in the mineral soilcontribute to the largeiogeochemical activity. For further details onnphasis on the RA andId be consulted.

Sulfate Flux

ities of SO42~ by inor-

ized (Ensminger 1954;>2). Because one of the)4~~ anion, the role oficiated cations through.rtance (Cole and John-The mobility of SO4

2~" retention by soils can

5. Sulfur Cycles 105

2500

ST WF DL LP GL TL NS CP HF FS RA DF

I Deposition S3 Output from B Horizon

• Net Retention or Loss

Figure 5.24. Net retention or loss of sulfur from forest ecosystems.

PwTFSF

OA

16RA

PTFSFO

10 20

TL

1222

TFSF0E

Bs10

NS

20

HF

16TFSFO

a 10

14

10 20

CH^SfSSSSSSKf^iSWa 54 50

J43

10 20 30

0 10 20 0 10 20

ORGANIC S (/jmol L'1)

Figure 5.25. Organic S concentrations in various strata of selected IPS sites (P, bulkprecipitation; Pw, wet-only precipitation; TF, throughfall; SF, stemflow; others, soilhorizon solutions). Values following bars are organic S as percentage of total S.(From Homann et al. 1990).


result in reductions in cation leaching. Where an equivalent displacement ofOH~ or other anion does not occur during the SO4

2~ retention process, ca-tions are coadsorbed with SO4

2~ (Johnson and Cole 1977; Cronan et al.1978; Singh et al. 1980).

Most research, including that completed as part of the U. S. Environ-mental Protection Agency Direct/Delayed Response Program (Church et al.1989), has centered on sulfate adsorption potential as a result of increasingdeposition levels only. The effect of SO4

2~ on ecosystem processes is con-sidered primarily from the amount of SO4

2~ adsorption that will preventsulfate from leaching through a forest ecosystem as well as the amounts ofbase cations or aluminum retained within the soil profile and not releasedto the aquatic environment. However, air pollution reductions in many re-gions have resulted in a decrease in SO4

2~ inputs, and the effect of decreas-ing SO4

2~ deposition levels on soil solution chemistry is equally important.The degree to which SO4

2~ adsorption is reversible is an important consid-eration in projecting recovery of acidified surface waters and acidified soilsto preacidified levels following decreases in atmospheric deposition (Gal-loway et al. 1983; Reuss and Johnson 1986). Reductions in ambient SO4

2~concentrations of solutions in forest ecosystems would be prolonged con-siderably if SO4

2~ (along with an equivalent quantity of acidic and basiccations) desorbs from acidified soils when atmospheric inputs decrease,whereas large decreases in SO4

2~ concentrations could be virtually instan-taneous if SO4

2~ does not desorb or desorbs very slowly. The potential ef-fects of varying degrees of reversibility of SO4

2~ adsorption on soil solutionchemistry following a reduction in SO4

2~ inputs are shown in Figure 5.26.The possibilities for SO4

2~ reversibility are (1) more release of SO42~ than

d-COo

oU)

no adsorption inputs = outputs

"reversible"

slowly reversibleX

inputincrease

inputdecrease

Time-

Figure 5.26. Paths of soil solution sulfate concentration with and without adsorptionfollowing deposition increases show variety of sulfate reversibility responses fol-lowing decreases in deposition.

nt Cycling

livalent displacement ofretention process, ca-

le 1977; Cronan et al.

of the U. S. Environ-Program (Church et al.is a result of increasing/stem processes is con-ption that will preventwell as the amounts ofirofile and not releasedreductions in many re-id the effect of decreas-•y is equally important,is an important consid-aters and acidified soilspheric deposition (Gal-tions in ambient SO4

2~>uld be prolonged con-ity of acidic and basic>heric inputs decrease,uld be virtually instan-Dwly. The potential ef-orption on soil solutionshown in Figure 5.26.; release of SO4

2~ than

"reversible"

slowly reversibleX

1 irreversible

th and without adsorptionversibility responses fol-


was absorbed (mobilization of other S pools); (2) complete or near-completereversibility (all adsorbed SO4

:~ released); (3) partial reversibility of ad-sorbed SO4

2~; and (4) little or no reversibility of SO42~ adsorption.

Researchers have not often reported observing more SO42"~ released by

desorption than was retained during previous adsorption studies, which mayresult from the nature of the soils for which results are published. Researchhas centered on soils that show a high degree of SO4

2~ adsorption. Thus,the soils generally would show more retention than release. However, soilsthat have been subject to previous high S loading and have a relatively smallSO4

:~ retention capacity may easily show more release than adsorption. Thisobservation could also be seen in a situation in which an extractant thatremoved previously insoluble SO4

2~ was used to determine SO42~ adsorption

reversibility or because organic S mineralization could release soluble SO42~.

For instance, Couto et al. (1979) speculated that a high percentage ofadsorption reversibility observed in several samples they studied (more than100% in the case of an Acrohumox B2 horizon) might be caused by mo-bilization of a high native amount of SO4

2~ in their samples, which wouldalso explain a weak retention of SO4

:" initially. Equally as possible wouldbe the mineralization of organic S to soluble SO4

2~, but no determinationsof organic S pools were made in their study.

Chao et al. (1962) found SO42~ adsorbed by 11 of 15 Oregon soils was

readily desorbed by water. Four of the 15 horizons studied demonstratedrelatively irreversible SO4

2~ adsorption. The previously adsorbed SO42" was

much more readily extracted with KH2PO4 solution, as would be expectedby the potential SO4"~ retention mechanisms outlined here, because phos-phate could effectively displace inner- and outer-sphere SO4

2~ simply by itshigher negative valence and associated greater charge density. This supportsthe inner-sphere concept of SO4

2~ adsorption as providing for adsorptionirreversibility. Unfortunately, the concentrations of the inputs were not spec-ified in this study, and the evaluation of relative nonadsorbance was basedon movement of SO4

2~ down into a column of soil, so little can be said ofthese results in the sense of comparing soil to soil.

In the study mentioned previously (Chao et al. 1962), researchers foundthat 4 of 15 Oregon soils showed an appreciable ability to hold an unspec-ified concentration of 35S sulfate ions against leaching. The interpretation of"appreciable" was that little of the SO4

2~ added was leached below a 4-cmdepth in the soil column with an application of 20 cm of water, and thatthat soil exhibited a considerable degree of SO4

2~ adsorption.Further studies of two of the sulfate-adsorbing soils (Chao et al. 1962)

showed that SO42~ adsorption was mostly reversible (>70%) after four ex-

tractions with deionized water (samples were desorbed with 2-hour extrac-tions successively (soil to solution ratio, 1:5)). Thus, the observation thatadsorption reversibility depends on the time period of desorption applies inthese studies. In a study of the SO4

2~ adsorption reversibility by miscibledisplacement (Hodges and Johnson 1987), it was estimated that completely


desorbing previously adsorbed SO42 would require a time period 10 to 20

times greater than the initial adsorption time period, although researchersdid not actually follow desorption through its completion.

In a study of SO42~ adsorption on kaolinite (Aylmore et al. 1967), SO4

2~adsorption was mostly reversible (>50%), but SO4

2~ adsorbed onto Fe andAl oxides was essentially irreversibly adsorbed (<^50%). It would be ex-pected that SO4

2~ adsorbed onto amorphous Al and Fe oxides might showa higher degree of irreversibility compared to kaolinite, because there wouldbe a higher potential for the SO4—ion-to-en-ter-t-he inner sphere of the amor-phous surface than the crystalline structure of kaolinite. In another study,one of four soil horizons (10-20 cm; sampling down to 30 cm) showedirreversible SO4

2~ adsorption (Weaver et al. 1985). The particular horizonshowing irreversible adsorption had the lowest pH measured and a lowernative SO4

2~-S concentration than soil horizons above it. This would beexpected because soil properties that lead to higher initial SO4

2~ adsorptionappear to decrease adsorption reversibility.

In a study of SO42~ adsorption reversibility, 10%-30% of the total of

previously adsorbed SO42~ was extracted with dilute HC1 (pH 3.5), 20% to

40% with water (pH 5.6), 50% to 60% with 0.01 M Ca(NO3)2, and nearly100% of adsorbed SO4

2~ with 0.01 M Ca(H2PO4)2 by repeated extractionsof several soils (Singh 1984). Soil samples included iron-podzols (highestSO4

2~ adsorption) and brown earths (lowest SO42~ adsorption). In another

study, 33% to 76% of adsorbed SO42~ was desorbed with four KH2PO4 (16

mM P) extractions from clay and sandy loam soils from the West Indies(Haque and Walmsley 1973).

Other studies (Nodvin et al. 1986; Fuller et al. 1987) noted that the sub-surface horizons of some Northeastern spodosols adsorbed up to 0.5 mmolSO4

2~ kg"1 soil (final solution concentration of 1 mM Na2SO4). Little of theSO4

2~ retained in these horizons was desorbed by water, but desorption washigher with phosphate extraction, ranging from 0.3 to 2.1 mmol kg"1.

Bornhemisza and Llanos (1967) observed leaching of 35S-SO42~ [applied

as CaSO4, K2SO4, and (NH4)2SO4)] by water in columns of three Costa Ri-can soils, including the A and B horizons of a regosol, a pumice-originatedalluvial soil, and a latosol. They found that 35S did not move to any largedegree in the latosol and the alluvial soil, but that the regosol showed littleretention. Where orthophosphate was intermixed with SO4

2~ applications[Ca(H2PO4)2 or triple superphosphate], the added SO4

2~ was more mobilebecause of displacement of SO4

2~ by phosphate; however, in the subsoil ofthe latosol, SO4

2~ was retained despite the additions of phosphate.As mentioned previously, Aylmore et al. (1967) found SO4

2~ adsorptionon kaolinite to be mostly reversible, but SO4

2~ adsorbed onto Fe and Aloxides was essentially irreversible, indicating that soils with high Fe and Aloxide content show high adsorption and low adsorption reversibility. Khannaand Beese (1978) found that leaching of chloride and nitrate salts througha podzolic acid brown soil released little previously adsorbed SO4

2~. Chlo-


: a time period 10 to 20d. although researchersletion.lore et al. 1967), SO4

2~adsorbed onto Fe and

50%). It would be ex-!. Fe oxides might showte. because there wouldiner sphere of the amor-inite. In another study,•wn to 30 cm) showedThe particular horizonmeasured and a lower

iove it. This would be.nitial SO4

2~ adsorption

£-30% of the total ofHC1 (pH 3.5), 20% to

f CaiNO3)2, and nearlyDV repeated extractionsI iron-podzols (highestidsorption). In anotherwith four KH2PO4(16from the West Indies

87) noted that the sub-;orbed up to 0.5 mmol'Na:SO4). Little of theter, but desorption was:o 2.1 mmol kg"'.: of 35S-SO4

2~ [appliedTins of three Costa Ri->1. a pumice-originatednot move to any large; regosol showed little.th SO4"~ applications)4"~ was more mobileever, in the subsoil ofof phosphate.>und SO4

2~ adsorption)rbed onto Fe and Als with high Fe and Ali reversibility. Khannai nitrate salts throughdsorbed SO4

2~. Chlo-

ride and NO3~ would not be expected to compete to a high degree withSO4

2~ for retention sites because of their lower charge density; however, thehigh concentrations of these monovalent ions used in the studies of Khannaand Beese should have displaced SO4

2~ retained in the outer sphere of theretention surface by mass action. This would be analogous to displacingexchangeable Ca2+ by adding high concentrations of KC1 to a soil, thusdisplacing the Ca2+ by mass action. These observations tend to support in-ner-sphere retention of SO4'~sorption.

as the primary mechanism for irreversible ad-

Adsorption/'Desorption/Reversibility Results of IFS Studies

Studies of SO42~ adsorption potential and the reversibility of SO4

2~ adsorp-tion in the IFS soils were undertaken to determine the impact that SO4

2~adsorption properties of soils have on regulating the flux of sulfate and as-sociated cations through the forest ecosystems studies as part of the IFS.Soil samples collected from IFS sites of differing soil and forest type, at-mospheric deposition histories, and physiographic locations were analyzedfor soil properties, including pH (in water and 0.01 M CaCl:), total S, esterSO4

2~, phosphate, and water-extractable SO4:~, Bray P, total C, extractable

Al and Fe (dithionate/citrate, oxalate, and pyrophosphate), cation exchangecapacity (CEC), exchangeable Ca, Mg, K, Na, and Al. SO4

2~ adsorptioncapacity was determined by sequential equilibration with a percolating so-lution (0.25 mM CaSO4) and desorption reversibility by leaching with deion-ized water. Two sequences of adsorption, desorption, and extraction werefollowed to estimate five sulfate pools as follows (procedure followed andsulfate fraction measured):

l.a. Samples are leached with deionized water until the effluent SO42~ con-

centration is less than 0.25 mM. This gives a measure of native water-soluble SO4

2~.l.b. Following procedure l.a., samples were extracted with 1000 ppm P as

NaH2PO4 until the SO42~ concentration dropped below the detectable

limit for this matrix. This gives a measure of the native insoluble SO4.2.a. Samples are leached with 0.25 mM CaSO4 solution until the concen-

tration of the percolate reaches 0.25 mM. This gives SO42~ adsorption

from a 0.25 mM solution.2.b. Following procedure 2.a., samples were leached with deionized water

as in l.a. The value "la + 2a — 2b" gives a measure of reversibleSO4

2~ adsorption.2.c. Samples from 2.b. were extracted with phosphate as in l.b. The results

of "2c — Ib" give a measure of irreversibly adsorbed SO42~ from SO4

2~adsorption sequence in 2a.

The soils vary widely in the quantities of SO42~ adsorbed, desorbed, and in

phosphate-soluble pools. All but one subsurface soil horizon showed net


SO4; adsorption (up to 4.5 mmol SO.,2 kg ' soil), indicating many of these

soils were not yet saturated with respect to an input concentration of 0.25mM SO4

2~. Most subsurface soil horizons also showed irreversible SO42~

adsorption as evidenced both by changes in phosphate-extractable SO42~ pools

and by a comparison of desorption of SO42~ before and after saturation by

the 0.25 mM SO42~ solution.

The characteristics all of these pools for each soil are important in de-termining potential SO4

2~ adsorption capacity. For instance, the sequenceof adsorption, desorption, and extraction for the Duke Btl and Findley LakeB2ir soil horizons are shown in Figure 5.27. Note that the major differencebetween the Findley Lake and Duke soils is the amount of phosphate-ex-tractable SO4

2" and the solution concentration at which the soil begins toadsorb SO4

2~. The Findley Lake soil represents a relatively young soil in alow-deposition environment capable of retaining a relatively high amount ofadditional SO4

2~ whereas the Duke soil represents a more developed soilwith a high SO4

2~ adsorption capacity that is nearly filled.The observations of SO4

2~ adsorption, desorption, and extractable poolsfor several other soils are summarized in Table 5.3. The relative proportionof sizes of the five operational pools varied considerably with site and soilhorizon. For instance, the Beech site BC horizon had a large pool of phos-phate extractable SO4

2~ (about 13 mmol kg"1), and this is probably the pri-mary reason it showed relatively low levels of net SO4

2~ retention from a0.25 mM CaSO4

2~ solution. Most of the soils showed a relatively close cor-

Solution concentration (mM sulfate)

inputconcentration

0.25 mM sulfate

Figure 5.27. Sulfate adsorption, desorption, and extraction sequences for FindleyLake B2ir and Duke Btl soils.

5. Sulfur Cycles

relation between SO4" irreversibly adsorbed and increases in the phosphate-extractable pools. However, during the time period of the extraction pro-cedure several soils appeared to mineralize organic S to SO4

:~, and one soilsample may have incorporated SO4

3~ organically, probably as a result ofmicrobial incorporation.

A measure of the reversibility of SO42~ adsorption is calculated by mak-

ing two estimates of irreversibly adsorbed SO42~: (1) by difference in phos-

phate-extractable fractions, and (2) by comparing the desorption of SO42~

following the adsorption treatment with SO42~ with desorption from the un-

treated soil. Comparisons are as follows:

Irreversibly adsorbed SO42 = "2c" — "Ib" [5.3]

Irreversibly adsorbed SO4 "2a" - ("2b" - "la") [5.4]

Evaluation of the reversibility of SO4: adsorption is very important be-

cause SO42~ that is irreversibly adsorbed represents a pool removed from

inorganic leaching. In the present study there are two measures of SO42"

adsorption reversibility. One is the increase in phosphate-extractable SO42~

(Eq. [5.3]) brought about by SO42~ adsorption from solution during equi-

libration "2a." It would be expected that irreversibly or slowly reversibleadsorbed SO4

2~ would increase in an insoluble inorganic pool. The secondmeasure of irreversibly adsorbed SO4

2~ is the difference between native water-soluble SO4

2~, SO42~ adsorbed from CaSO4

:~ inputs, and SO4:~ desorbed

after CaSO42~ treatment (Eq. [5.4]). If these two measures of the irrever-

sibility of SO42~ adsorption are equivalent, there would be a strong indi-

cation that organic S incorporation or mineralization processes are not im-portant relative to the inorganic SO4

2~ adsorption/desorption processes.A total of 29 of 36 subsoils showed irreversible adsorption based on in-

creases in phosphate-extractable SO42~ (Eq. [5.3]) while 28 of 36 subsoils

showed irreversible adsorption based on the adsorption/desorption criteriaof Eq. [5.4]. Despite air-drying and the potential perturbations associatedwith laboratory procedures on S incorporation and mineralization processes,most soil samples showed conservation of added sulfate. In other words,SO4

2~ adsorbed from solution was recovered quantitatively by water or phos-phate extraction, or:

"2c" - "Ib" = "2a" - ("2b" - "Ib") [5.5]

A 1:1 graph of these two measures of irreversibly adsorbed SO42~ is shown

in Figure 5.28. Although two soil samples, the Oak Ridge (loblolly pine)B horizon and the Cedar River (Douglas fir) B22 horizon, showed widedeviations in these comparisons, most of the other soil samples showed astrong 1:1 relationship. These observations suggest that inorganic processesare dominant in these air-dried soils, and that inorganic adsorption is likely


Table 5.3. Selected Soil Properties and Sulfate Adsorption Data for Sites

SiteOak RidgeOak RidgeOak RidgeOak RidgeOak RidgeCamp BranchCamp BranchCamp BranchSmoky Mts.Smoky Mts.Smoky Mts.Smoky Mts.Smoky Mts.Smoky Mts.CoweetaCoweetaCoweetaDuke ForestDuke ForestB.F. GrantB.F. GrantHondaFloridaMaineCedar RiverCedar RiverCedar RiverCedar RiverFindley LakeWhiteface Mt.Huntington ForestTurkey LakesTurkey LakesTurkey LakesNorwayNorwayNorway

VegetationLoblolly pineChestnut oakChestnut oakYellow poplarYellow poplarWhite oakWhite oakWhite oakRed spruce 1Red spruce 1Red spruce 2American beechAmerican beechAmerican beechOak-hickoryWhite pineWhite pineLoblolly pineLoblolly pineLoblolly pineLoblolly pineSlash pineSlash pineRed spruceRed alderRed alderDouglas firDouglas firP. silver firRed spruceSugar mapleSugar mapleSugar mapleSugar mapleNorway spruceNorway spruceNorway spruce

SoilHorizon

BBlB2BlB2B

ClC2BCBlAB

BCBtBtBCBtlCBtBCBtCBs

B21B22B21B22Bs

BupBsl

BfLlBfh2

BfBsBCC

SoilPH5.24.74.8554.74.954.14.74.344.64.85.35.65.84.84.85.25.24.64.74.25.15.25.65.54.64.84.94.64.94.94.855.2

TotalC182

13132323

———3017407

2853529021901404

—375167214114263

75180102

06524403321991191540254362787429132812245795

78160

OxalateAl6736374141594867

170145182133170245

375941636747515132

803778826730693

101266369329649340012812370

TotalS3.13.15.53.93.63.23.34.49.62.79.6

12.713.815.75.75.33.1

14.314.93.33.91.60.83.1

10.99.46.67.9

13.014.42.2

11.27.46.32.22.41.6

to be the most important factor in controlling SO4 concentrations in watersequilibrating with these soils. For example, the Bs horizon in the FindleyLake (Pacific Silver fir) site had no native water-soluble SO4

2~ and only 0.6mmol kg"1 of native insoluble adsorbed SO4

2~ (see Table 5.3). The soilretained 5.0 mmol SO4

2~ kg"1 on treatment with 0.25 mM SO42~ solution,

far more than the native pools combined. A water leaching resulted in theremoval of 3.2 mmol kg"1 of this retained SO4

2~, and an additional phos-phate extraction removed 2.7 mmol SO4

2~ kg"1. These results show con-servation of added SO4

2~. Applying Eq. [5.5], we see that the net balanceis 0.2 mmol SO4

2~ kg"1, a relatively small amount.

nt Cycling

>n Data for Sites

TotalC182!3132323—

——3017407285352902IW14(14

—3751672:41 142?>375ISO1020

55241033>I9911915402>436!7S7129 11281'.24579578160

OxalateAl6736374141594867170145182133170245375941636747515132803778826730693101266369329649340012812370

TotalS3.13.15.53.93.63.23.34.49.62.79.612.713.815.75.75.33.114.314.93.33.91.60.83.110.99.46.67.913.014.42.211.27.46.32.22.41.6


Extraction Series

EsterSulfate

1.62.431.81.91.52.23.75.3->5.6

12.310.412.74.44.52.7

12.614.2n.d.n.d.n.d.n.d.n.d.4.13.44.46.44.85.83.73.732.7

n.d.n.d.n.d.

2a0.420.370.75

-0.150.030.501.387.220.310.470.03

-0.760.280.841.342.673.891.752.783.096.391.000.931.052.311.933.182.424.970.641.541.240.820.75

-0.41-0.28-0.04

2b2.461.451.260.400.621.161.345.080.880.972.240.861.213.301.902.562.333.795.312.224.650.800.700.772.191.652.403.533.171.161.891.581.340.931.310.940.48

2c0.351.091.820.080.920.541.825.100.320.710.980.370.545.951.922.703.009.29

11.903.985.590.770.572.220.560.673.312.682.680.491.230.620.410.390.861.780.40

la0.961.180.900.490.920.790.670.101.080.492.601.621.202.261.350.560.103.583.620.000.000.370.000.000.290.440.870.790.000.620.630.630.490.231.931.320.65

Ib0.070.951.940.110.420.301.143.020.200.150.890.270.445.901.071.911.467.62

10.303.003.690.580.342.150.140.391.471.790.640.430.700.290.330.370.831.660.28

IrreversibleAdsorbed

Sulfate2c-lb

0.280.14

-0.12-0.03

0.500.240.692.080.120.560.090.100.090.050.850.791.541.671.600.971.890.200.230.070.420.281.840.892.040.070.530.320.080.020.030.110.12

IrreversibleAdsorbedSulfate

2a-2b-la-1.08

0.090.39

-0.060.330.130.712.230.51

-0.010.39

-0.010.27

-0.210.790.671.651.551.100.871.740.560.230.280.410.711.65

-0.321.800.100.280.29

-0.030.060.220.100.13

ncentrations in watersorizon in the Findleyle SO4

:~ and only 0.6Table 5.3). The soil

; m/V/ SO42~ solution,

iching resulted in thed an additional phos-:se results show con-'; that the net balance

In the sequence of 2a-2b-2c for the Findley Lake Bs, 1.8 mraol SO42

kg"1 soil or 36% of added SO42~ was retained irreversibly by the phosphate-

extractable SO42~ pool. In general, soils that adsorbed higher amounts of

SO42~ also showed higher amounts of irreversibly adsorbed SO4'~ (Figure

5.29). On average, approximately 36% of adsorbed SO42~ was irreversibly

adsorbed for all soils, although the data were variable.The effects of tune on the permanence of this retention are questionable

because they were incompletely evaluated in this study. Researchers havenoted that time has a positive influence on SO4

2~ adsorption (Barrow andShaw 1977), but Singh (1984) saw no correlation between SO4

2~ desorption


3.0

12.01

Q Spodosols and Inceptisols• Ultisols

-1.0

1:1 line

O Cedar River Douglas-fir "B22" horizon

Oak Ridge Loblolly pine "B" horizon

0.0 1.0 2.0 3.0

sulfate irreversibly adsorbedmmol/kg [2a - (2b - la)]

Figure 5.28. Comparisons of two methods for estimation of irreversibly adsorbedSO4

:~ for subsoils from IFS sites.

•0 =a.

-2

Q Spodosols and Inceptisols• Ultisols

Irreversibly adsorbed sulfate =- 0.01 + 0.36 sulfate adsorption

r2 =0.75

- 2 0 2 4 6 8sulfate adsorption (mmol/kg)

Figure 5.29. Irreversibly adsorbed SO42~ versus total SO42~ adsorption from 0.25

mM CaSO4 solution for subsoils from IFS sites.


i:l line

3ouelas-iir "822" horizon

e "B" horizon

2.0 3.0

i of irreversibly adsorbed

I sulfaie =le adsorption

•" adsorption from 0.25

and time. Irreversible SO42"

of many of these soils.adsorption appears to be an important property

Soil Properties Used in Predicting SO4~ Adsorption

The exact nature of SO42~ adsorption in soils is chemically complex. How-

ever, SO42~ adsorption is generally considered to take place by means of

two mechanisms, one of which results in an equivalent release of anotheranion, typically OH~. The other mechanism results in neutral adsorption ofSO4

:~ with accompanying cations, which is often called "selective" or "salt"adsorption. The multiple negative charge and the bridging that can poten-tially occur as a result of the divalent nature of the SO4

2~ ion are importantfor coadsorprion of SO4

2~ with cations.Several soil factors have been observed by a variety of researchers to

influence SO42~ adsorption by soils. Adsorption typically increases as (1)

the quantity of Al and Fe oxides increases; (2) soil pH decreases; (3) organicmatter decreases; and (4) the concentration of similarly adsorbed anions de-creases. The relative impact of each of these soil factors varies greatly inits contribution to SO4

2~ adsorption, and researchers have sometimes con-centrated on one factor in making an evaluation of the relative effect onSO4

2~ adsorption. For instance, soils of a finer texture and higher Fe andAl oxide content often have higher levels of organic matter than similar soilsof a coarser texture. If one were looking specifically at the effects of organicmatter on SO4

2~ adsorption, higher organic matter levels might then appearin some studies to enhance SO4

2~ adsorption. Procedures that alter a partic-ular soil property experimentally may also change other soil properties, andunless all changes in soil properties are carefully evaluated, their actual ef-fect on SO4

2~ adsorption might not be noted.

Effect of Fe and Al Oxides on Adsorption

In general, the relative SO42~ adsorpion characteristics of soils have been

most closely related to Al and Fe hydrous oxide content (Chao et al 1962;Adams and Hajek 1978; Rajan 1978; Couto et al. 1979; Johnson and Hen-derson 1979; Neary et al. 1987), although the relationship between SO4

2~adsorption and extractable Al and Fe is often quite variable. Such surfaceshave a variety of mechanisms for SO4

2~ retention, and potential adsorptionis altered by pH in the same way as empirical observations occur. Recentstudies (e.g., Nodvin et al. 1986) have shown a pH level at which the ad-sorption reaches a maximum for several soils, and lower pH levels typicallyresult in solubilization of surface coatings of Fe and Al.

Thus, the lower pH levels may destroy the SO42~ adsorption surfaces and

reduce potential SO42~ retention sites unless formation of new surfaces takes

place. One factor typically cannot explain observations fully, because SO4:~

adsorption is considered to be a complex factor of interaction of soil SO42~


with organic matter, pH, and extractable Al and Fe (Parfitt and Smart 1978;Johnson and Todd 1983; Dethier et al. 1988).

Effects of pH on SO4~~ Adsorption

Sulfate adsorption typically increases with decreasing soil pH (Kamprath etal. 1956; Elkins and Ensminger 1971; Gebhardt and Coleman 1974; Coutoet al. 1979; Singh 1984; Bergseth 1985; Zhang et al. 1987). Thus, if soilsacidify by natural soil formation or man-caused processes, they may retainadditional SO4

2~ and support a reduced SO42~ concentration in solution for

a given level in soil. Some studies in which soils were artificially acidifiedindicate that there is a pH level below which SO4

2~ adsorption begins de-creasing (Nodvin et al. 1986). This point at which SO4

2~ adsorption begandecreasing was associated with the release of soluble aluminum, indicatingthat the precipitation and surface adsorption of Fe and Al hydroxy-sulfates(Adams and Rawajfih 1977; Rajan 1978; Parfitt and Smart 1978; Singh 1984),which are driven by lower pH, were probably not the primary mechanismfor SO4

:~ retention in these studies. Rather, retention was associated withsurface coatings of Fe and Al oxides, which dissolve at lower pH and releasesoluble Fe and Al.

As noted, the possible observations of the effects of pH on SO42~ ad-

sorption are likely to depend on the specific mechanism responsible for SO42~

adsorption, and these may vary from soil to soil. Most retention mechanismswe can consider, however, would show an increase in adsorption with de-creasing pH. Considering the simplest retention mechanism (simple anionadsorption by anion exchange), for instance, the adsorption of SO4

2~ wouldbe increased by adding H"1" to the system. Several researchers found SO4

2~adsorption to be correlated to measurements of surface positive charge (VanRaij and Peech 1972; Marsh et al. 1987). Based on the observations of thesestudies, positive and negative adsorption sites are considered to be separatedin space, and the amphoteric nature of the anion exchange capacity gave anincreased SO4

2~ adsorption with decreasing pH.As H+ is forced onto pH-dependent sites, the mineral surface gains pos-

itive charge and thus can retain more anions. While supporting a net negativecharge at higher pH levels, the nature of the retention sites is changed fromnegatively charged to neutral to positively charged by protonation. Whenpositively charged, the surface can retain anions such as Cl~, SO4

2~, andPO4

3~. In the case of multivalent anions such as SO42~ and PO4

3~, a bridgecan form between the anion and the surface, and there is a potential forcovalent bonds to form between the anion and the surface by loss of waters(Sposito and Goldberg 1984). Anions that bridge in this way are essentiallypart of the new surface, are likely to be held tenaciously, and would po-tentially change the original surface from an anion to a cation exchanger.

Effect of Organic Matter on SO42~ Adsorption

Organic anions appear to compete directly with SO42~ for retention sites

(Johnson and Todd 1983), although in comparisons of subsoils of highly


(Parfitt and Smart 1978;

'ion

ig soil pH (Kamprath eti Coleman 1974; Coutoil. 1987). Thus, if soilscesses, they may retain;ntration in solution forere artificially acidified

adsorption begins de-5O4

:~ adsorption begane aluminum, indicating.nd Al hydroxy-sulfatesmart 1978; Singh 1984),he primary mechanismDn was associated withat lower pH and release

ts of pH on SO42~ ad-

m responsible for SO42~

t retention mechanismsin adsorption with de-

chanism (simple anionDrption of SO4

2~ wouldisearchers found SO4

2~:e positive charge (Van;e observations of these.sidered to be separatedlange capacity gave an

eral surface gains pos-pporting a net negativei sites is changed fromby protonation. When:h as Cl~, SO4

2~, and!~ and PO4

3~, a bridgehere is a potential for[face by loss of watershis way are essentially.ously, and would po-) a cation exchanger.

lorption

*~~ for retention sitesof subsoils of highly

contrasting physical and chemical properties it is can be observed that SO4:

adsorption is directly proportional to carbon content (Singh 1984). Observedpositive correlations between SO4

2~ adsorption and organic matter contentsappear to result from the relative stability of organic matter-Al surface com-plexes in many soil systems. Thus organic matter content may be directlycorrelated with extractable Al and Fe in subsoils (Harrison et al. 1989).

The mechanism for organic matter competing for SO42~ adsorption sites

can be either that of an anion adsorbing onto and blocking SO42~ adsorption

sites or of solubilizing and removing surface Al and Fe. Solubility of Feand Al from soil surfaces is typically greatly increased by the addition oforganic acids such as citrate and oxalate (Schnitzer 1969; Bartlett and Riego1972; Hue et al. 1988). Thus, the solubility and removal of SO4

2~ adsorptionsites may be important in explaining reductions in SO4

2~ adsorption withincreasing levels of organic matter in soils.

As mentioned previously, the role of organic matter in SO42~ adsorption

may be easily confused with other soil factors, such as surface area, or thepresence of Fe and Al oxides, which may increase organic matter retentionas well as SO4

2~ retention. Studies that have looked at the role of organicmatter content in similar soils have typically found a negative relationshipbetween the two. Surface soils, for instance, typically have the highest levelsof organic matter and exhibit little SO4

2~ adsorption.

Effects of Competing Anions on Sulfate Adsorption

The primary anion competing with SO42~ for inorganic adsorption in soils

is phosphate. A detailed discussion of the interactions of sulfate and phos-phate is included in Chapter 7.

Observations from the IPS Study

Several soil properties appeared to be correlated with observed SO42~ ad-

sorption. The concentration of oxalate extractable Al was most closely cor-related with SO4

2~ adsorption for Spodosols and Inceptisols. This obser-vation has been seen in several previous studies (Barrow 1969; Haque andWalmsley 1973; Johnson and Todd 1983; Nommik et al. 1984; Singh 1984).For example, Figure 5.30 shows the significant relationship (0.01 level) be-tween SO4

2~ adsorption and oxalate-extractable Al for Spodosols and In-ceptisols. Very noticeable in this figure is the lack of high levels of oxalateAl in the weathered Ultisols despite a wide range of SO4

2~ adsorption. Theregression coefficient for Inceptisols and Spodosols of SO4

2~ adsorption ver-sus the Al was .63 (r2).

None of the Al or Fe extractions measured correlated well with observedSO4

2~ adsorption for soil horizons collected from Ultisols, although it hasbeen shown in other studies that extractable Fe may be important (Chao etal. 1962; Aylmore et al. 1967; Parfitt and Smart 1978; Johnson et al. 1980;Singh 1984).


"3= 5

Spodosols and InceptisolsUltisols

Sulfate adsorbed =- 0.26 -i- 0.0033 oxalate AJ!- = 0.63

200 400 600 800oxalate AI (mmol/kg)

1000

Figure 5.30. Sulfate adsorbed from 0.25 mAf CaSOj solution versus oxalate-ex-tractable Al for subsoils from IPS sites; regression line for Spodosols and Inceptisolscombined.

It was impossible to assess the effect of organic matter on SO42 adsorp-

tion in this study. Organic matter levels were directly correlated with ex-tractable Al, so the apparent relationship between SO4

2~ adsorption and or-ganic matter was positive and very similar in appearance to Figure 5.30.

A significant positive relationship (0.01 level) between SO42~ adsorption

and pH was seen when all soil samples were plotted simultaneously (Figure5.31). This direct relationship is in contrast to what has been seen in severalother studies of SO4

2~ adsorption where soil samples have been acidified byaddition of acid solutions (Chao et al. 1964; Harward and Reisenauer 1966;Couto et al. 1979; Huete and McColl 1984). In other studies, sulfate isdisplaced by the addition of base.

Below about pH 3.5, additional drops in pH have been seen to lowerSO4

2~ adsorption because of dissolution of Al and possible destruction ofadsorption sites (Nodvin et al. 1986; Dongsen and Harrison 1990). Becausea range in pH was achieved by natural soil development processes and notby adding acid or base to the same samples of the soil material over a rel-atively short time period, it is difficult to directly compare the observationsof this study with those of the other studies.

Organic Sulfur Dynamics Including Mineralization andImmobilization of Various Organic Fractions

J.W. Fitzgerald and A.R. Autry

Central to assessing the deleterious effects of acidic precipitation on forestedecosystems is a thorough understanding of sulfur accumulation and retentionmechanisms in these systems. Sulfate is extremely mobile in the soluble


soils using 33S in laboratory incubations have shown that all IPS soils havethe capacity to retain additional sulfur inputs via microbial immobilization(pp. 119-129) the importance of which varies among soils.

The classification of sites based on retention or loss of sulfur as well asthe more detailed analyses of sites subjected to different levels of sulfur inputshows that the IPS sites exhibit differences in the relative importance ofbiotic versus abiotic processes in affecting sulfur dynamics. For those siteswith elevated loadings of sulfur associated with atmospheric pollutants, thesulfur dynamics during the period of the EPS project can generally be ex-plained by analyses of abiotic sulfate adsorption-desorption processes as hasbeen concluded by previous studies (Johnson et al. 1980; Johnson 1984;Rochelle et al. 1987). However, the role of irreversible sulfate adsorptionneeds further attention, because our results suggested imbalances of sulfurinputs and outputs for some sites, including net losses of sulfur (CategoryIV). In addition, for those sites with lower levels of SO4

2~ inputs, biotictransformations by uptake, litter production, and storage by forest vegetationas well as microbial transformations (mineralization-immobilization) (Cat-egory II) can influence sulfur dynamics because a significant fraction ofsulfur flux is associated with passage through biotic constituents (Mitchellet al. 1991). Furthermore, for most IFS sites, the organic sulfur pool in themineral soil is the dominant sulfur constituent, but the long-term dynamicsof this pool including formation rates and net losses are difficult to quantify.Changes in this pool may have important consequences relating to SO4

2~loss and retention in forested ecosystems, and, in conjunction with processesaffecting the reversibility of sulfate adsorption, will influence how changesin sulfur loadings from the atmosphere will affect the concentrations andfluxes of SO4

2~ in soil and associated surface waters.

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