chemistry response of two forested watersheds to acid atmospheric deposition

WATER RESOURCES BULLETINVOL. 26, NO.5 AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 1990

CHEMISTRY RESPONSE OF TWO FORESTED WATERSHEDSTO ACID ATMOSPHERIC DEPOSITION'

M. W. Kress, R. Baker, and S. J. Ursic2

ABSTRACT The deposition and chemistry of precipitation wereestimated for one year in two forest ecosystems in the South-Central United States. Precipitation, throughfall, litter leachate,and soil leachate were analyzed for a small catchment of pine-hardwoods in southeastern Oklahoma and for a catchment of loblol-ly pines (Pinus taeda L.) in northern Mississippi. In thepine-hardwood forest, 98 percent of the acid deposition was neutral-ized, 50 percent in the forest canopy, and 48 percent in the forestfloor. In the pine forest, 75 percent of the acid deposition was neu-tralized, all in the forest floor. The pine-hardwood ecosystem accu-mulated sulfate, nitrate, and ammonia ions, and lost base cations.During seasons of deficient precipitation, dry deposition appearedto enrich the concentrations of hydrogen, nitrate, sulfate, andammonia ions in throughfall samples at both locations.(KEY TERMS: acid deposition; nutrient flux; South-Central UnitedStates; forest ecosystem; cation exchange; forest canopy.)

INTRODUCTION

Considerable research has been conducted on theeffects of acid deposition in the forest environment.The impetus for much of this work came from fearsthat acid deposition is causing productivity declinesin forests (Engstrom et al., 1971) and acidification ofsurface waters (Galloway et al., 1978). However,investigations have largely neglected the south-cen-tral region of the United States.

The South-Central United States includes impor-tant forests of the Upper Coastal Plain and theOuachita Uplands. Here, soils are largely Ultisolshaving low base saturation and high susceptibility toacidification (Binkley et al., 1989). Although aciddeposition is moderate with annual pH's ranging from4.6 to 5.0 (NADP, 1987; Kress et al., 1988), this regionis potentially susceptible to the effects of acid deposi-tion. Therefore, a study was undertaken to determinethe disposition of acid deposition in two forest

ecosystems in the South-Central United States, andto estimate base levels for deposition chemistry inthose ecosystems.

Site Descriptions

METHODS

Two forested watersheds were selected as studysites, one in Oklahoma and the other in Mississippi(Figure 1). The Oklahoma watershed covers 7.7hectares near Clayton Lake in the KiamichiMountains of southeastern Oklahoma. It has an over-story of shortleaf pine (Pinus echinata Mill.) andmixed hardwoods, principally oak (Quercus sp.) andhickory (Carya sp.). The hardwoods constitute 54 per-cent of the total tree basal area; the pines constitute46 percent. This watershed has a southwest aspect.The soil on the upper slope is Pirum stony, fine sandyloam (Typic Hapluduits, fine-loamy, siliceous, ther-mic) with slopes of 10 percent to 25 percent. ThePrium soil grades into Octavia stony, fine sandy loam(Typic Paleudults, fine-loamy, siliceous, thermic) withslopes of 3 percent to 5 percent. Precipitation falls pri-marily as rain with spring and fall wet-seasons and awinter dry-season (Figure 2). The 1613 mm of precipi-tation that fell during the sampling period, July 30,1985, to July 29, 1986, was 32 percent above the 1221mm annual mean (NOAA, 1987). Thirty-six percent ofthis precipitation volume resulted in streamfiow(Figure 2). The stream flowed intermittently fromNovember through June with little or no flow fromJuly through October (Figure 2).

'Paper No. 90038 of the Water Resources Bulletin. Discussions are open until June 1, 1991.2Respectively, Senior Research Specialist, Oklahoma State University, Department of Forestry, 008 Agriculture Hall, Stiliwater, Oklahoma

74078; Ecologist, U.S. Forest Service, Wildlife Habitat Laboratory, P.O. Box 7600, Naoogdoches, Texas 75962; and Research Forester, U.S.Forest Service, Southern Forest Experiment Station, Forest Hydrology Laboratory, P.O. Box 947, Oxford, Mississippi 38655.

747 WATER RESOURCES BULLETIN

No 50 100

Kress, Baker, and Ursic

The Mississippi watershed, located near Oxford,covers 1.45 ha. This plantation, predominantly loblol-ly (Pinus taeda L.), was established in 1939 on severe-ly eroded soils. The soil on the upper slope isLexington silty clay loam (Typic Paleudalf, fine-silty,mixed, thermic) with slopes of 5 percent to 12 percent.The Lexington soil grades into Orangeburg-Rustonsandy loam (Typic Paleudaif, fine-loamy, siliceous,thermic) with slopes of 12 percent to 17 percent. Asmall area near the outlet is covered by recent alluvi-um. The watershed has a southern aspect.Precipitation, essentially all rain, is well distributedthroughout the year (Figure 3). The 1137 mm of pre-cipitation that fell during the sampling period, July30, 1985, to July 29, 1986, was 18 percent below the1386 mm annual mean (NOAA, 1987). Precipitationwas below average from November through April(Figure 3). Streamflows are infrequent, since the soilsare deep and well drained. Streamfiow accounted forless than 2 percent of the disposition of total precipi-tation, and this streamfiow was not analyzed.

Sample Collection

At both watersheds, precipitation was sampledwith wetfall collectors and bulk precipitation collec-tors (Galloway and Likens, 1976). A throughfall col-lector, a litter lysimeter, and a soil lysimeter wereinstalled at each of ten, randomly-selected, sites oneach watershed. The throughfall collectors were 102cm above ground. Tension-free lysimeters (Jordan,1968) were placed at two soil depths: 1) between the

forest floor and the mineral soil, and 2) 15 cm belowthe surface of the mineral soil. A weighing-bucketrain gage recorded the precipitation at the Oklahomawatershed; three standard gages were used at theMississippi watershed. Streamflow was measured ina 1.2-meter H-flume at Oklahoma and a 0.91-meterH-flume at Mississippi, both equipped with a Belfortwater-level-recorder.

At the Oklahoma site, an ISCO sampler tookstream samples at 3 cm intervals of stage. These dis-crete-stage samples were collected after each stormand frozen. All other water samples were collectedweekly and composited. Field personnel determinethe pH, conductivity, and volume of each compositesample, froze 500 ml of each sample, and transportedall samples to the laboratory at Oklahoma StateUniversity for further analysis.

It was necessary to preserve the Oklahoma sam-pies due to the large number of samples and the dis-tance between the field site and the laboratory. Thesesamples were preserved by freezing at —17 Celsius.some researchers using a variety of sample matrixesfound slow freezing to be a satisfactory method of pre-serving samples (Proctor, 1962; Morgan and Clarke,1964; Marvin and Proctor, 1965; Jenkins, 1968;Harms et al., 1974; Klingaman and Nelson, 1976;Muller et al., 1981; Ross and Bartlett, 1990).

The field procedures were similar in Mississippi.However, samples were collected on a storm basisrather than weekly. Samples were refrigerated at 4°Crather than frozen. Chemical analyses wereconducted by the U.S. Forest Service Hydrologylaboratory at Oxford. Samples at both locations werecollected from July 30, 1985 to July 29, 1986.

WATER RESOURCES BULLETIN 748

IFigure 1. Location of the Two Research Sites for Acid Deposition in the South-Central United States.

300

250

i50100

50

0

1985—86 Runoff

1980—87 Means

,R l,R 11R , I,R .R I,R I,R I,Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul

Chemistry Response of Two Forested Watersheds to Acid Atmospheric Deposition

Oklahoma Precipitallon

1 985—86 Precip1943—87 Means

Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun JulMonth

Oklahoma Streamfiow300

250

200

150

c 100

50

0

Month

Figure2. Monthly Precipitation and Runoff at the Oldahoma Watershed.


C00c0a)

0

Laboratory personnel conducted the following anal-yses according to methods described in StandardMethods (1985):

MethodTest

pHCond.

N03SO4—2

NH4Ca2Mg2KNa

Oklahoma

MeterMeterCadmium Reduction (manual)ThrbidimetricElectrodeAtm. Abs. Spec.

Atm. Abs. Spec.

Atm. Abs. Spec.

Atm. Abs. Spec.

MeterMeter

Ion ChromatographyIon ChromatographyMethylthymol Blue (auto.)Atm. Abs. Spec.

Atm. Abs. Spec.

Atm. Abs. Spec.

Atm. Abs. Spec.


Mississippi Precipitation

1985—86 Precipitation1894—1987 Means

300

250

200

1 50

100

50

0Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul

Month

Figure 3. Monthly Precipitation at the Mississippi Watershed.

Laboratory Procedures

Mississippi



The Oklahoma laboratory ran quality control stan-dards from the Environment Protection Agency alongwith each set of samples. The pH electrode was cali-brated daily with two buffers and checked with a low-ionic solution.: This solution was formulated by theNational Atmospheric Deposition Program.Measurements in this solution were consideredacceptable if they had a pH range of 4.3 ± 0.1.

Calculations

All chemical concentrations were volume weighted.For flux calculations, we estimated missing data withflow-weighted seasonal means. Since this study wasnot designed to accurately estimate the volumes of lit-ter leachate or soil leachate, flux calculations werelimited to wetfall, bulk precipitation, throughfall, andstreamflow. Hydrogen-ion concentrations were calcu-lated from pH. Since the ionic strength of the sampleswas less than 0.001 M, we assumed that hydrogen-ionactivity equaled concentration.

We defined the seasons as follows:

Summer —

Fall

July 30, 1985, to September 17, 1985,and June 24, 1986 to July 29, 1986.

— September 17, 1985 to December 24,1985.

Winter — December 24, 1985 to March 25, 1986.

Spring — March 25, 1986 to June 24, 1986.

RESULTS AND DISCUSSION

Chemical Concentrations

The forests at both locations effectively reduced theacidity of deposition before it reached the soil. In thepine-hardwood forest of Oklahoma, the hydrogen-ionconcentration of bulk precipitation was reduced 50percent in the forest canopy and an additional 48 per-cent in the forest floor (Table 1). In the pine forest ofMississippi, the hydrogen-ion concentration of bulkprecipitation was increased 15 percent in the forestcanopy and decreased 91 percent in the forest floor(Table 2). We suspect that a difference in forest typescaused the difference in trends at the two sites. TheMississippi forest is predominantly loblolly pine,while hardwoods make up half of the basal area oftrees on the Oklahoma watershed. Other workersfound that conifers increase the acidity of throughfallwhile hardwoods decrease the acidity of throughfall(Cronan and Reiners, 1983; Mahendrappa, 1983;Johannes et al., 1985). The increased acidity inconifer throughfall is thought to be caused by leaching

TABLE 1. Ion Concentrations for the Oklahoma Watershed, July 1985 through July 1986.

Sample TypeWater SO4-2 NO3- NH3 Ca'2 Mg"2 K"

(nun) (p.eqfL) (j.teqfL) (j.ieq/L) (teqfL) (peq/L) (ieq/L)

Na'(ieq/L)

H(teq/L)

Wetfall 1611 27 13 11 8 2 1 7 16

Bulk 1559 32 16 12 21 2 3 6 20

Throughfall 1206 36 27 14 50 10 16 11 10

Litter Leachate 28 23 6 109 43 32 13 0.5Soil Leachate 104 15 2 135 50 29 15 0.4Runoff 577 78 1 0 36 46 15 13 1.1

TABLE 2. Ion Concentrations for the Mississippi Watershed, July 1985 through July 1986.

Sample TypeWater SO4-2 NO3- NH3 Ca'2 Mg'2 K'(mm) (jteqL) (teqIL) (jteq/L) (jteqfL) (teq/L) (jteqlL)

Na'(jteqlL)

H'(teqIL)

Wetfall 1137 28 14 15 10 5 6 10 24Bulk 1137 29 15 16 14 6 9 13 26

Throughfall 924 52 29 24 43 27 45 20 30Litter Leachate 65 23 40 131 91 71 23 6.4Soil Leachate 77 10 36 40 101 52 27 4.5


0a)3£0

-4-0L-4-Ca-)000

0a)

£0

-4-0L-I-Ca)0C00


Figure 4. Seasonal Chemical Concentrations in Throughfall for the Oklahoma and Mississippi Watersheds.


Oklahoma Throughfall

S urn me rFallWinierSpring

60

50

40

30

20

10

0

1 00

80

60

40

20

0

HYDROGEN SULFATE NITRATE AMMONIA

Mississippi Throughfall

LIN\1

SummerFallWinfer

Spring

HYDROGEN SULFATE NITRATE AMMONIA


of organic acids and washoff of dry deposition (Reussand Johnson, 1986). Dry deposition can account formore than one-half of the total deposition of hydrogenions (Lindberg et al., 1986), and organic acids cancomprise 20 percent of the strong acids in surfacewaters (Eshleman and Hemond, 1985).

At the Mississippi watershed most of the increasedacidity in throughfall occurred during the winter(Figure 4), a period of low precipitation (Figure 3). Awinter increase also occurred in Oklahoma, but it wasless pronounced (Figure 4). Mississippi had lowerwinter precipitation than Oklahoma, 165 mm com-pared to 205 mm. Lower precipitation during the win-ter resulted from extended periods betweenprecipitation events and apparently produced accu-mulations of acidic, dry deposition. Sulfate, nitrate,and ammonia concentrations in throughfall alsoincreased during the winter months (Figure 4). Longperiods between precipitation allow dry deposition toaccumulate on the foliage (Moore, 1983), thus thesamples may have been greatly enriched by subse-quent wash off. In addition, biological activitydeclines during the winter (Donahue et al., 1983) socanopy uptake of nitrogen is reduced (Grennfelt andHultberg, 1986).

Cation exchange is in part responsible for loweringacidity in throughfall and litter leachate(Mecklenburg et al., 1966; Lovett et al., 1985). InOklahoma, base cations increased from 32 p.eq/L to87 jteq/L to 197 peqfL in bulk precipitation, through-fall, and litter leachate, while the hydrogen-ion con-centration decreased from 20 j.teqfL to 10 i.eq/L to 0.5ieq/L (Table 1). In Mississippi, base cations increasedfrom 42 p.eq/L to 135jieq/L to 316 p.eqfL in bulk precip-itation, throughfall, and litter leachate, while thehydrogen-ion concentration changed from 26 peqfL to30 p.eq/L to 6.4 .teq/L (Table 2). Apparently, basecations replaced hydrogen ions in the Oklahomathroughfall and litter leachate and in the Mississippilitter leachate. However, these solutions gained from5 to 11 base cations for each hydrogen ion lost. Thisionic imbalance requires either a proportionalincrease in anions or decrease in other cations. Therequired ionic balance is absent in our analysis ofthroughfall and litter-leachate chemistry; therefore,we must conclude that one or more important ions aremissing from this analysis, probably bicarbonate,chloride, and organic anions. Cole and Johnson (1977)found that bicarbonate was the dominant anion at allcollection levels except for forest floor. We suspectthat organic acids were also important anions in ourthroughfall and litter leachate samples, since weobserved a yellow-brown color in these samples, andthis coloration is associated with organic acids inwater (Gjessing, 1976).

At the Oklahoma watershed, calcium concentra-tions consistently exceeded the combined total concen-trations of magnesium, potassium, and sodium inprecipitation, throughfall, litter leachate, and soilleachate (Table 1 and Figure 5). Therefore, we wouldexpect calcium concentrations to dominate in thestreamfiow also. In streamfiow, magnesium dominat-ed, 46 xeq/L Mg2 to 36 p.eq/L Ca2 (Table 1 andFigure 5). However, the lower soil horizons remainedunmonitored, since the soil lysimeters collected wateronly from the A horizon. Therefore, we suspect thelower soil horizons control the shift to magnesium,probably with large quantities of exchangeable mag-nesium ions and root uptake of calcium ions. If thelower soil horizons control this cation shift, then theflow paths through the lower horizons probably domi-nate the movement of water to the stream.

Chemical Fluxes

No analyses of chemical fluxes were calculated forthe Mississippi data, because we lacked watershedoutput data. Less than 2 percent of precipitation vol-ume resulted in stream flow.

The Oklahoma watershed exported base cations inexcess of inputs from bulk precipitation. Bulk precipi-tation imported 49.5 meq/m base cations whilestreamfiow exported 64.3 rneq/m2 base cations (Table3). This cation imbalance comes primarily from thenet loss of magnesium, 24.4 meg/rn2, while calciumgained 12.0 meq/m2 (Table 3). This loss of magnesiumis probably indicative of significant magnesiumreserves in the lower soil horizons. The loss of magne-sium rather than calcium may be significant to thelong-term health of the forest vegetation since calci-um is the primary cation lost in throughfall.Apparently, calcium remains available for plantuptake. our analysis of cation export fails to includeinputs from soil weathering and inputs from dry depo-sition. Bulk precipitation collectors accumulate only aportion of dry deposition, and dryfall may account formore than half of the total inputs of Ca2 and K(Lindberg et al., 1986). Therefore, we probably overes-timated net cation loss.

The Oklahoma watershed accumulated nitrate andammonia ions. Bulk precipitation accounted for 25.3meg/rn2 NO3— and 18 meg/rn2 NH3, while streamfiowaccounted for 0.8 meg/rn2 N03and 0.0 meg/m2 NH3(Table 3). Nitrogen retention probably results frombiological uptake (Johnson et al., 1986).

Reuss and Johnson (1986) compared 16 headwaterwatersheds and found that the Ultisols of the south-eastern United States generally accumulate sulfateions. This trend appears to hold for the Ultisols at theOklahoma watershed. This watershed gained 50


0a)

C0

-4--aL.

-4-Ca)0C0

C)

Sample TypeWater(mm)

SO42(meqlm2)

NO3-(meqlm2)

NH3(meq/m2)

Ca'2(meq/m2)

M?2(meq/m2)

K1

(meq/m2)

Na(meq/m2)

H'(meq/m2)

Wetfall 1611 43 20.2 17 12.6 3.9 1.1 10.5 25.4

Bulk 1559 50 25.3 18 33.0 2.4 4.4 9.7 31.4

Throughfall 1206 43 32.9 17 60.0 12.3 19.4 12.8 12.4

Runoff 577 46 0.8 0 21.0 26.8 8.9 7.6 0.65

med/rn2 sulfate ions in bulk precipitation and lost 46meq/m2 sulfate ions in streamfiow (Table 3). However,actual sulfate accumulation may be greater, becausebulk precipitation may underestimate total depositionof sulfate (Lindberg et al., 1986; Lindberg and Garten,1988). On the Oklahoma watershed, sulfate apparent-ly passes through the A horizon and adsorbs to theclay in the heavier-textured lower horizons.

CONCLUSIONS

Both forests effectively reduce the acidity of precip-itation before it reaches the mineral soil. TheOklahoma watershed appears to accumulate acidanions and lose base cations. However, this rate ofloss may be insignificant compared to the relativeuncertainty of the data. The net loss of magnesium



Oklahoma Cations

CalciumI ] MagnesiumN\ Potassiumt<x?l Sodium

1 40

1 20

1 00

80

60

40

20

0PRECIP THRUFALL LITTER SOIL STREAM

Figure 5. Concentrations of Base Cations for the Oklahoma Watershed.

TABLE 3. Ion Fluxes for the Oklahoma Watershed, July 1985 through July 1986.


may be indicative of dominant flow paths throughlower soil horizons. The accumulation of depositionduring dry periods strongly influences throughfallchemistry

The ability of the soil to accumulate sulfate and itsapparent reserve of magnesium leads us to concludethat present levels of acid deposition may take manydecades to produce any significant effect on the soilchemistry, the stream-water chemistry or the vegeta-tion. Two major questions arise from this researchand need to be addressed: 1) How are soil formationprocesses affecting the balance between acid inputsand cation outputs? and 2) Will long-term changes inthe soil chemistry result from acid deposition?

ACKNOWLEDGMENTS

The research described in this paper was supported by the U.S.Forest Service Southern Forest Experiment Station; OklahomaState University, Forestry Department; and by the United StatesDepartment of Interior as authorized by the Water ResourcesDevelopment Act of 1979 (PL 95-467). Journal Article J-5780 of theOklahoma Agricultural Experiment Station, Oklahoma StateUniversity, Stillwater, Oklahoma.

We wish to thank Dr. P. J. Wigington for initially organizing thisstudy.

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chemistry response of two forested watersheds to acid atmospheric deposition

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