transport of dissolved organic matter through a sandy forest soil

DIVISION S-7—FOREST & RANGE SOILS

Transport of Dissolved Organic Matter through a Sandy Forest SoilM. G. Dosskey* and P. M. Bertsch

ABSTRACTWe assessed the transport of dissolved organic matter (DOM)

through a sandy, Ultisol forest soil on the southeastern U.S. CoastalPlain, and contrasted the results with similar studies from other forestregions, to test the hypothesis that DOM transport is greater throughsandy Ultisols than finer textured Ultisols and Spodosols. Within asmall headwater catchment, concentrations of dissolved organic car-bon (DOC), a measure of DOM, were measured in soil solution atthree depths (10, 30, and 90 cm) in sand A and E horizons of soilprofiles, and in sand along the valley bottom at 198- to 264-cm depthin shallow groundwater. Water samples were collected after everyrainfall event for 21 mo using zero-tension lysimeters, suction sam-plers, and piezometers. Mean concentration of DOC in soil waterdecreased from 25.5 mg C L ' at 10-cm depth to 13.7 mg C L ' at30 cm, and to 1.8 mg C L ' at 76 to 99 cm, before contacting clay-enriched horizons. All valley bottom stations consistently averagedbetween 0.3 and 2.1 mg C I '. We did not find significant seasonalpatterns, nor a correlation between DOC concentration and magni-tude of rainfall events. We estimate that the flux of DOC decreasedmore sharply with soil depth than concentration due to attenuationof water transport through the soil profile. Compared with literaturedata from other forest regions, our results do not support the hypothe-sis that there is greater DOM transport through sandy upland soilson the Coastal Plain. Our results suggest that this is due to strongDOM retention within deep sand E horizons of these soils. Thus,strong DOM retention in forest soils appears to occur across a broaderrange of soil types than those exhibiting podzolization or having highclay content.

r I \IE PRODUCTION AND MOVEMENT OF DOM through-L forest soils is linked to energy and nutrient budgets

of aquatic systems (Fisher and Likens, 1973; Meyer,1990) and to facilitated transport of various soluble andinsoluble constituents of soils (e.g., Cronan et al., 1978;Schnitzer, 1978; Quails et al., 1991; Herbert andBertsch, 1995).

Numerous studies have quantified DOM concentra-tions in forest soil solutions, but under a narrow rangeof soil conditions. Most previous studies have concernedcool, moist forests on Spodosol soils or Inceptisols ex-hibiting podzolization (e.g., Ugolini et al., 1977, 1988;Dawson et al., 1978, 1981; McDowell and Wood, 1984;Cronan and Aiken, 1985; Driscoll et al., 1985; Moore,1989; Dalva and Moore, 1991). These studies indicatethat, in general, large quantities of DOM leach intomineral soil from the forest floor, producing solutionconcentrations up to 122 mg C L"1 (Dawson et al.,1978). Concentrations typically decrease sharply with

M.G. Dosskey, Dep. of Forestry, Fisheries, and Wildlife, Univ. of Ne-braska, IOI Plant Industry, Lincoln, NE 68583-0814; and P.M. Bertsch,Division of Biogeochemistry, Savannah River Ecology Lab., Univ. ofGeorgia, Drawer E, Aiken, SC 29802. Received 19 Mar. 1996. "'Corre-sponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 61:920-927 (1997).

increasing soil depth, with B horizons solutions havingconcentrations, on average, about one-fifth of valuesobserved in forest floor solutions (Cronan, 1990; Dalvaand Moore, 1991). Podzolization processes, involvinginteraction of DOM with soil minerals and its depositionin B horizons by sorption and precipitation, are theprimary mechanisms of DOM retention in these soils(e.g., Dawson et al., 1978; McDowell and Wood, 1984;Cronan and Aiken, 1985; Moore et al., 1992; David etal., 1995). Strong DOM sorption within B horizons isconsidered to be the primary limitation to DOM trans-port from upland forest to streams and lakes in spodo-solic soil regions (McDowell and Likens, 1988; Davidand Vance, 1991). Podzolization also explains DOMretention in Spodosols and Inceptisols in boreal forest,tundra, and alpine regions (Litaor, 1987; Ugolini etal, 1987).

A few studies of medium-textured Ultisols, and re-lated Inceptisols, have shown a similar pattern of highforest floor production and sharp attenuation of DOMconcentration with soil depth (Cronan et al., 1990;Quails and Haines, 1991; Richter et al., 1995). Retentionof DOM in Ultisols has been attributed primarily tosorption to soil clays (Jardine et al., 1989).

Sandy Ultisols in warm, moist forests on the CoastalPlain in the southeastern USA may be more conduciveto DOM transport than Spodosols and finer texturedUltisols. Relatively high DOM concentration in the re-gion's streams (commonly referred to as blackwaterstreams) are thought to result, in part, from low abilityof sand soils to immobilize DOM from percolating soilwater (Meyer, 1986). In this and other warm temperateand tropical regions, blackwater streams tend to corre-late with watersheds dominated by sandy soils, whileclear water streams, with lower DOM concentrations,drain watersheds dominated by clayey soils (Leenheer,1980; St. John and Anderson, 1982; Meyer, 1986; Nelsonet al., 1993).

Other evidence, however, indicates DOM transportthrough Coastal Plain soils may be low despite the sandycondition. Groundwater DOM concentrations on theCoastal Plain are generally lower than in other regionsof the USA (Leenheer et al., 1974), and much lower thanthe blackwater streams to which groundwater flows. Arecent analysis estimated that an upland forest areacontributed only 7% of the organic matter in a blackwa-ter stream, with streamside wetlands accounting for therest (Dosskey and Bertsch, 1994). These results implythat upland forest floors do not release much DOM tosoil, or that DOM is effectively immobilized in the soilbefore reaching groundwater.

Abbreviations: DOM, dissolved organic matter; DOC, dissolved organiccarbon; PVC, polyvinyl chloride.

920

DOSSKEY & BERTSCH: TRANSPORT OF DISSOLVED ORGANIC MATTER 921

Our objectives were to quantify DOM transportthrough a sandy forest soil on the southeastern U.S.Coastal Plain and to compare the results with similardata from other forest regions, so we could test thehypothesis that DOM transport is greater through theseCoastal Plain landscapes. Specifically, we (i) quantifiedDOM in the soil water pathway from uplands toward ablackwater stream, (ii) determined the pattern of DOMattenuation through the soil, (iii) evaluated seasonalpatterns in DOM concentration and its relationship tosize of rainfall events, and (iv) compared these charac-teristics with published reports from other forestregions.

METHODSSite Description

This study was conducted at the Savannah River Site, an802-km2 U.S. Department of Energy reservation, located inthe Sandhill region of the upper Atlantic Coastal Plain inSouth Carolina. The topography is gently rolling with broadflat ridges separating moderately sloping valleys with low-gradient blackwater streams. Upland soils generally havestrongly acid, sand surface horizons (up to 2-m depth) withlow organic matter content (<20 g kg"1) over clay-enriched(sandy loam to sandy clay loam) subsoils (Rodgers, 1990).The broad ridges are dominated by pine plantations (longleafpine [Pinus palustris Miller], loblolly pine [P. taeda L.], slashpine [P. elliottii Engelm.]) and natural mixed pine-oak forest.Southern mixed deciduous forest (oak [Quercus spp.], hickory[Carya spp.], sweetgum [Liquidambar styraciflua L.], amongothers) dominates in the valleys near stream courses (Work-man and McLeod, 1990). Mean annual rainfall is 1210 mmand is evenly distributed throughout the year (National Oce-anic and Atmospheric Administration, 1982). Rainfall infiltra-tion is rapid, and overland flow events are uncommon evenon steeper slopes (Williams and Finder, 1990).

Sampling was conducted in a 4-ha, moderately sloping head-water catchment covered with =40-yr-old mixed pine-oak for-est (Fig. 1). Soils within the catchment are primarily Blanton(loamy, siliceous, thermic Grossarenic Paleudult) and Fuquayseries (loamy, siliceous, thermic Arenic Plinthic Paleudult;Rodgers, 1990). Soil variability is mainly in the depth of sandto the Bt horizon. Soil profiles in the upper elevations (StationsA-E in Fig. 1) consist of 5 to 8 cm of litter (O) over 8 to 15cm of grayish-brown sand (A), 42 to 85 cm of yellowish-brownsand (El and E2), 10 to 20 cm of yellowish-brown sandy loam(Btl), and yellowish-red sandy clay loam (Bt2) of undeter-mined thickness. Along the valley bottom (Stations 1-6 in Fig.1), sand E horizons are much thicker (>150 cm), with grayishmottling appearing in the upper Bt2 midway down the valley(at Station 4 in Fig. 1) becoming closer to the surface untilthe subsoil becomes entirely light grayish sand at Station 6.At Station 6, the water table fluctuated between 2 and 150cm below the soil surface during our 21-mo study period. Anintermittent seep, below Station 6, marks the lower end ofour study catchment and the beginning of a riparian wetlandalong Fourmile Branch, the second-order blackwater streamto which it drains.

Groundwater flow path analysis of the general area of thestudy site indicates that there is a downward hydraulic gradientfrom the water table aquifer toward underlying poorly con-fined aquifers (below 61-m elevation), but that horizontal flowtoward and discharge into Fourmile Branch is dominant overdownward migration (Cummins et al., 1990).

Fig. 1. Topography and locations of sampling stations in the studycatchment on the upper Atlantic Coastal Plain in South Carolina.The catchment is located 200 m northwest of the intersection ofRoad C and Road 4 on the U.S. Department of Energy's SavannahRiver Site. Topography was derived from an engineering surveymap of scale 1 in = 100 ft and 2-ft contour interval.

Sampling and AnalysesDissolved organic C was used throughout this study as a

measure of DOM because the various forms of plant, soil,and aquatic organic matter have generally similar C contents(Schnitzer, 1978; Thurman, 1985). We measured DOC concen-trations in soil water at several locations along its presumedpathway toward the riparian wetland: (i) vertically through soilprofiles, and (ii) laterally down the valley bottom in shallowgroundwater above the Bt horizon (Fig. 1). In general, watersamples were collected after every rainfall event for 21 mo,between March 1993 and December 1994.

Vertical profile sampling was replicated at six stations in thecatchment (Stations A-F in Fig. 1). At each station, throughfallamount was measured by rain gauge (0.2-mm divisions; All-Weather Gauge, Productive Alternatives, Fergus Falls, MN)in order to quantify water volume entering the soil profile.Samples of throughfall solution were collected during selectedevents by glass funnel into 2-L amber glass bottles. Soil waterwas sampled near the bottom of the A horizon at 10 cm, andin the El horizon at 30 cm, using zero-tension lysimeterssimilar to those described by Jordan (1968). Each lysimeterwas 45.6 by 20.3 cm constructed entirely of stainless steel.The lysimeters were installed parallel to the soil surface intoexcavated tunnels, which were backfilled. Each lysimeter

922 SOIL SCI. SOC. AM. J., VOL. 61, MAY-JUNE 1997

drained downslope through 2 m of buried PVC pipe into a20-L glass bottle located in a 100-cm-deep covered access hole.Water volumes collected by the zero-tension lysimeters wererecorded, but considered unreliable for quantifying waterfluxes through soil horizons due to inherent under- and over-collection problems (Haines et al., 1982; Jemison and Fox,1992).

At Stations A through E (Fig. 1), soil water was also sam-pled just above the Bt horizon, at 76- to 99-cm depth (average90 cm), using porous cup suction samplers (Model 1900 fittedwith 0.5 bar high-flow ceramic, Soil Moisture Equipment,Santa Barbara, CA). These samplers were installed byaugering a 7.6-cm-diam. hole to the appropriate depth, andsetting the sampler cup into the hole with a slurry of lowerE2 horizon material before backfilling. Each sampler was fit-ted with a 15-cm-diam. collar, made of sheet aluminum, at5-cm depth into the A horizon to prevent vertical flow downthe sides of the sampler tubes. Soil water was drawn into thesamplers 18 to 36 h after rain events by applying 30 kPa suctionfor 1 h. Consequently, these samplers drew water only aftera few larger rainfall events, which temporarily perched wateratop the Bt horizon.

Along the valley bottom, soil water was sampled at sevenstations (Stations la-lb, 2-6 in Fig. 1) using porous cup suctionsamplers (Model 1900 fitted with 0.5 bar high-flow ceramic)and piezometers (1.9-cm-diam. PVC with a 23-cm-long slottedscreen zone), installed in the same manner as for suctionsamplers at Stations A through E (described above). At Sta-tions la and Ib, one suction sampler was located just abovethe Bt horizon at 198-cm depth. At Stations 2 through 4, asuction sampler was paired with a piezometer just above theBt horizon at 215- to 264-cm depth. These suction samplerswere useful for obtaining water samples (using 30 kPa for1 h) during periods when the water table was too deep forcollecting sufficient volume from the piezometers, but the soilaround the samplers was still saturated or nearly so. Pairedsuction samplers and piezometers also provided a side-by-sidecomparison of DOC sampling of water table groundwater forevaluating potential artifacts associated with suction samplers

(e.g., Haines et al., 1982). At Stations 5 and 6, piezometersonly were located in sand at two depths, 112 to 122 and 221to 244 cm. Water table depths in all piezometers were recordedprior to water sampling. Additional grab samples were col-lected from the seep, whenever it was flowing.

All piezometers and suction samplers were installed 7 to8 mo prior to sampling for this study. All lysimeters wereinstalled 4 wk prior to sampling for this study.

Water samples were collected within 18 to 36 h after allrainfall events, which yielded measureable quantities in theA horizon lysimeters. At those times, water samples weretaken from all samplers from which we could draw water. Inthe field, water samples were siphoned directly from samplersthrough precombusted glass-fiber filters (Whatman GF/C, 1.2-|jLm retention) into 120-mL amber glass bottles with Teflon-lined caps. The samples were refrigerated immediately. Dis-solved organic C concentrations were measured within 1 wk ofcollection by combustion-nondispersive, infrared gas analysisusing a Shimadzu TOC-500 Organic Carbon Analyzer (Shi-madzu Scientific Instruments, Columbia, MD). The resultantvalues for DOC concentration probably include colloid-associ-ated organic C in the 0.1- to 1.2-|jim size range, which hasbeen found to be mobile in these soils (Kaplan et al., 1993;Seaman et al., 1995).

RESULTSDissolved Organic Carbon Concentrations

in the Soil Water PathwayThere were 56 rainfall events during the 21-mo study

period that produced measurable percolation throughthe A horizon. Rainfall events ranged from 20-min-longthunderstorms to 3-d-long frontal storms, producingfrom 0.4 to 88.8 mm of throughfall. Table 1 reportsmean DOC concentrations in soil water collected ateach soil profile and valley bottom sampling stationafter these rainfall events. For comparison, we have also

Table 1. Concentration of dissolved organic carbon (DOC) in soil solution collected in soil profiles and in shallow groundwater in asmall upland catchment on the upper Atlantic Coastal Plain in South Carolina. Samples were collected during a 21-mo period afterevery rainfall event that yielded solution in the A horizon lysimeters. Values for soil profile locations are based on one average value(from 5-6 replicate samplers) from each event. Valley bottom locations were not replicated.

Location

Stations A-FThroughfallAElE2

Stations la.lbStation 2Station 3Station 4Station 5

Station 6

SeepGroundwater#Streamtt

Pathway

soil profilesoil profilesoil profilesoil profilevalley bottomvalley bottomvalley bottomvalley bottomvalley bottom

valley bottom

valley bottomlocallocal

Samplertypet

BLLSS

S,PS,PS,PPPPPGMG

Depthcm

surface1030

76-99198215239264112221122244

surface300 to 3000

surface

Mean

35.025.513.71.82.11.3H0.6H0.6H1.70.31.20.41.1

<15.3

DOC

Std. dev.i- mg C L-' —————

21.87.16.10.31.21.40.81.01.40.71.00.60.6

—-

«§

8563856

211823215542569

—-

T B — bottle with funnel; L = zero-tension lysimeter; S = suction sampler; P = piezometer; G = grab; M = U.S. Department of Energy groundwatermonitoring wells.

I Sample standard deviation.§ Number of rainfall events from which samples were obtained.fl DOC values computed from suction sampler data only. Piezometer results were similar, but from fewer samples.n Cummins et al., 1990; Dosskey and Bertsch, 1994.ft Fourmile Branch; Newman, 1986; Dosskey and Bertsch, 1994.


included published values for local groundwater andstream DOC.

Results in Table 1 show the highest DOC concentra-tions were measured near the soil surface (mean 25.5mg C L-1), followed by a sharp decline with depth tothe bottom of the E2 horizon (76-99-cm depth; mean1.8 mg C L"1). Variation in DOC concentration amongevents decreased similarly with depth in the soil profile.Concentrations of DOC in the E2 horizon along thevalley bottom (range of means 0.3-2.1 mg C L"1) weresimilar to the mean value for the E2 horizon at profilesampling stations. Concentration of DOC in the E2 hori-zon did not change materially along the valley bottomto the seep, nor did deeper samples at Stations 5 and 6differ materially from shallower counterparts. Concen-trations measured along the valley bottom appear simi-lar to values reported for deeper water-table ground-water, but lower than values reported for FourmileBranch, the nearby stream to which our catchmentdrains.

Sample sizes differed among sampling locations (Ta-ble 1). Lysimeters in the El horizon did not collect soilwater from some smaller events that yielded measurablequantities in the A horizon. We attribute this to soilstorage of water in the intervening 20-cm-thick soillayer. Suction samplers did not withdraw sufficient vol-ume unless the soil surrounding the ceramic cup wassaturated or nearly so. In particular, we obtained fewsamples from suction samplers at 76- to 99-cm depth inprofile plots and at Stations la and Ib, even after somelarger rainfall events. The Bt horizon at these stationsdid not sustain perched water tables for long periods.At valley bottom stations, lower sample sizes reflectperiods when the groundwater table had receded belowa level where sufficient volume could be obtained usingour samplers. The seep flowed only after nine eventsduring the study period.

Figure 2 shows water table elevations measured atvalley bottom Stations 2 through 6 during the studyperiod. Gaps in the record for each station reflect peri-ods when the water table had receded below the bottom

82

80

78

C 76O

ra 74>0)

Q] 72

70

68

\ Sta 2

Sta 3

Sta 4

Mar Jul Jan Jul Dec

1993 1994Fig. 2. Water table elevations measured at valley bottom stations 2

to 6 during the study period March 1993 to December 1994. Gapsin the record for Stations 2, 3, and 4 represent periods when thewater table was below the bottom of the piezometers at thesestations.

of the piezometer. These data show that for all samplingdates when measurements were made, there was a con-sistent hydraulic gradient down the valley toward theseep, consistent with our initial assumption that shallowgroundwater flows laterally along the valley bottom to-ward Fourmile Branch.

Results reported for Stations 2 through 4 in Table 1are from suction samplers only, because the sample sizewas somewhat larger than for the corresponding pi-ezometers at these stations. We found no material differ-ence in DOC concentrations between water samplescollected by piezometers and suction samplers whensamples were obtained from both (Table 2). Severalhypotheses have been forwarded to explain reports ofceramic-sampler-related effects on water chemistry(e.g., Haines et al., 1982; Litaor, 1988). We attributelack of effects in this study to: (i) the use of large-pore-size ceramic, (ii) installation of samplers several monthsprior to use, and (iii) sampling sandy soil horizons thatwere saturated or nearly so.

Since sample sizes differed among samplers (Table1), a more accurate illustration of the decrease in DOCconcentration as water percolates vertically through thesoil profile is made by comparing mean DOC concentra-tions for only those rainfall events yielding samples fromboth A and E horizon lysimeters. For the 38 events thatyielded samples from the El horizon (mean 13.7 mg CL"1), the corresponding mean DOC concentration fromA horizon samples was 24.8 mg CL"1. This value isvery similar to the overall A horizon mean reported inTable 1 (25.5 mg C L"1).

Dissolved Organic Carbon Concentrationvs. Magnitude of Rainfall Event

In general, the magnitude of a rainfall event had noeffect on the concentration of DOC in percolating soilwater in this study (Fig. 3). A linear regression of Ahorizon DOC concentrations for events ranging from6.6 to 88 mm of throughfall shows no significant slope(P = 0.37), indicating that soil water DOC concentra-tions were, on average, remarkably unaffected by thequantity of water entering the soil profile.

Samples collected from four very small events (0.4-1.5mm of throughfall), however, consistently had lowerDOC concentrations than samples from larger events(Fig. 3). All four very small events occurred during early

Table 2. Concentration of dissolved organic carbon (DOC) insoil solution samples collected by both suction sampler andpiezometer at the same location.

Location

Station 2

Station 3

Station 4

«t

18

17

23

Sampler type

suction samplerpiezometersuction samplerpiezometersuction samplerpiezometer

Mean

1.01.20.70.80.60.7

DOC

Std. dev4mg C L-' ———

1.01.60.80.81.00.7

t Number of times samples were obtained from both piezometer andsuction sampler at a location.

t Sample standard deviation.


O•f(O

50

40

30

CS 20OOO 10 *OQ

20 40 60 80

Throughfall Amount (mm)Fig. 3. Concentration of dissolved organic carbon (DOC) in soil solu-

tion collected from the A horizon (10-cin depth) plotted againstthroughfall amount, for 56 events during a 21-mo period. Eachsymbol represents the mean of six replicate lysimeters and eightthroughfall gauges. The regression line includes 52 events wherethroughfall was a 6.6 mm.

summer (May-June), after long dry spells (no rain inprevious 6-36 d, <10 mm in previous 13-51 d). Volumescollected from A horizon lysimeters after these eventswere a much greater fraction of throughfall than allother events. This phenomenon was also observed inEl horizon samples, but with a much less dramatic effecton DOC concentration. Whether these measurementsreflect real processes, or are an artifact of our collectionsystem, is uncertain. Some plausible hypotheses include:(i) dilution of small samples by condensation in fieldcollection bottles, and (ii) facilitated water percolationattributable to hydrophobic coatings on soil particles.The explanation for these events, however, is peripheralto our goal, since these events are too small to contributesignificantly to an overall landscape C budget.

Seasonally of Dissolved OrganicCarbon Concentration

There was no clear seasonally to soil water DOCconcentrations in our study (Table 3). Although theretended to be lower DOC concentrations in winter events(January and February), there were no statistically sig-nificant differences in mean DOC concentration in A

Table 3. Concentration of dissolved organic carbon (DOC) insoil solution samples collected after rainfall events at 10-cindepth during different periods of the year. Comparisons wereperformed using one average value (from up to six replicatesamplers) to represent each event. There were no significantdifferences among means (P = 0.27).

DOCMonth nt Mean Std. dev4

Jan.-Feb.Mar.-Apr.May-JuneJuly-Aug.Sept.-Oct.Nov.-Dec.Annual

878129852

24.227.430.726.027.425.826.8

- mg C IT1 ——————2.17.15.35.97.03.45.6

t Number of rainfall events sampled.$ Sample standard deviation.

horizon lysimeters among 2-mo increments of the calen-dar year (P = 0.27). The four very small events discussedabove were not included in the seasonal analysis, hence,the higher overall mean and lower standard deviationfor the A horizon than reported in Table 1.

DISCUSSIONDissolved Organic Matter Concentrations

in Soil WaterBased on measurements of DOC, concentrations of

DOM in our sandy, Coastal Plain soil profile appearsimilar to those in other forest regions (Table 4). In thisstudy, leachate under the A horizon (10-cm depth) hadan average concentration of 25.5 mg C L"1, which iswell within the normal range observed in other regions(Table 4; see also reviews by Cronan, 1990; Dalva andMoore, 1991; Herbert and Bertsch, 1995). The sharpdecline of DOM concentration with soil depth at ourstudy site, to 13.7 mg C LT1 at 30 cm and 1.8 mg C L"1

at 79 to 99 cm, is also similar to reports from otherregions. A major difference between this study and pre-vious reports is that the decline in DOM concentrationwith soil depth occurred entirely within sandy E hori-zons, rather than in spodic or argillic B horizons. Thus,there appears to be substantial DOM retention withinmineral horizons of our Coastal Plain soil despite lowclay content and lack of spodic development.

Although we were able to obtain samples from onlya few, large, rainfall events at the 76- to 99-cm depthat profile sampling stations, we believe that the resultantDOC concentrations are representative of more generalconditions in soil water percolating to this depth fromthe overlying soil. Our results indicate that (i) there wasno dilution of DOC concentration by larger rainfallevents, and (ii) ceramic cups of suction samplers hadno material effect on sample DOC concentrations. Fur-thermore, we had intentionally located our profile sam-pling stations at higher elevations in the catchment inorder to minimize the risk that perched soil water wouldbe diluted by return flow from groundwater. If somereturn flow occurred, the larger rainfall events fromwhich we obtained samples would be least affected.

A combination of chemical and physical processesmay explain the strong retention of DOM in our sandyUltisol soil. Sorption to soil clays, particularly underlow organic matter content and acidic conditions, isthought to play a dominant role in DOM retention bysoils (Jardine et al., 1989; Moore et al., 1992; Vance andDavid, 1992; Herbert and Bertsch, 1995). In Ultisols,DOM partitions to a greater extent to Fe oxides andkaolinite clays than to 2:1 phyllosilicates (Jardine et al.,1989). At our study site, the A and E horizons haverelatively low clay content. However, analyses of similarsoil profiles nearby suggest that the clay fraction is domi-nated by kaolinite, with substantial quantities of Fe andAl oxides situated as coatings on sand grains (Kaplan,1993; Kaplan et al., 1993; Seaman et al., 1995). Loworganic matter content, presumably maintained by rapidmicrobial decomposition under a thermic temperatureregime, combined with strongly acidic pH, may max-imize sorptive capacity of limited clay surfaces. Further-


Table 4. Estimated average annual dissolved organic carbon (DOC) concentration, water flux, and DOC flux through different forestsoils. Parenthetical values are our estimates based on the following water flux assumptions: throughfall is 75% of rainfall (for coarseUltisol only); logarithmic decrease through a root zone of 100 cm, with no subsequent change to groundwater.

Soil/Forest type/Locationf

Coarse UltisolPine-oak forestSC, Coastal Plain

Fine UltisolPine forestSC, Piedmont

SpodosolHardwood-conifer forestNH, White Mountains

Mean annualrainfall

mm1210

1170

1280

Horizon

under Ain Elunder E2under Oin Ein Btunder BtinCin Ein Bsunder B

Depthcm1030900

1560

1756009-151830

Texture

sandsandsandorganicsandy loamclay loamclaysaproliteloamy sand to sandy loamloamy sand to sandy loamloamy sand to sandy loam

PH

4.5-6.04.5-5.54.5-5.5

4.34.13.94.23.93.84.6

DOC Cone.mg C L '

25.513.71.8

34.023.81.51.00.6

28.55.93.0

Water flux$L m~2 yr~'

(501)(403)(306)737

(649)(624)(616)616923897781

DOC flux

kg C ha ' yr-'(128)

(55)(6)

251(154)

(9)(6)4

2635323

t Coarse Ultisol (this study); fine Ultisol (Richter et al., 1994,1995; Richter and Markewitz, 1996); Spodosol (McDowell and Likens, 1988).J Coarse Ultisol water flux = 704 - 88.4 In (depth) from 3 cm above mineral soil in the O horizon. Fine Ultisol water flux = 790 - 37.9 In (depth) from

4 cm above mineral soil in the O horizon.

more, preferential water and solute flow, which canoccur through macropores in highly structured soils(Jardine et al., 1990), may be lacking in sand horizons.Thus, interaction between limited clay surfaces andDOM in percolating soil solution may be maximized insand horizons at our study site. Abundant clays in Bthorizons of the subsoils may contribute additional DOMretention, and become important where surface hori-zons are thinner than at our site.

While DOM concentration near the soil surface (10cm) was highly variable among events, we could notaccount for the variation based on seasonal or rainfallevent-size relationships that have been reported in otherregions. Higher concentrations have been observed dur-ing the growing season in cooler forest regions (Meyerand Tate, 1983; McDowell and Wood, 1984; Cronan andAiken, 1985). Lack of seasonality at our study site maybe due to relatively short, mild winters without concen-trated growth and litterfall periods. We were surprised,however, at the lack of an event size effect on DOCconcentration. Such a relationship has been suggested,though not directly observed, in a Spodosol forest soil(McDowell and Wood, 1984). Complex production, re-lease, and retention mechanisms may be at work tobuffer DOM concentrations in soil solutions (Cronan,1990; Dalva and Moore, 1991).

Along the valley bottom, soil water above the Bthorizon had consistently low DOC concentration. Wedo not know the degree to which shallow groundwateralong the valley bottom represents a mixture of: (i) soilwater percolating vertically from the soil surface, (ii)lateral flow from valley sides of perched water atop theBt horizon, and (iii) return flow of deeper groundwaterinto sandy surface horizons. Regardless of the specificcontributing pathways, consistently low DOC concen-trations along this flow path supports the contentionthat only relatively low DOC water drains from ourupland catchment to the local waterway.

Dissolved Organic Matter Flux through SoilInsofar as DOC concentration results suggest that the

sandy soils at our study site retain DOM similarly toforest soils in other regions, they do not directly indicatehow similar the quantities of transported DOM might

be. A better comparison for this purpose would be ofDOC flux, which accounts for amount of water flowthrough soil.

Although we did not measure water flow through oursoil profile, we can estimate it well enough to approxi-mate DOC fluxes at our study site (Table 4). On anannual basis, the amount of water flow decreases withsoil depth due to water uptake by plant roots (McDowelland Likens, 1988). To obtain water flow, we estimatedannual average throughfall to the soil surface to be 75%of average rainfall (Lull, 1964). Water percolation belowthe root zone was estimated to be that required to sus-tain annual average groundwater base flow to a nearbystream (297 L m~2 yr"1). Groundwater base flow wascomputed as 92% of annual average discharge (4.07GL yr~') from the 12.6-km2 Fourmile Branch watershed(Williams and Finder, 1990; Dosskey and Bertsch, 1994).Between these two depths, water flux was assumed todecrease in proportion to fine root biomass (McDowelland Likens, 1988). Studies of mixed pine-hardwood for-ests on sandy soils show that fine root biomass decreaseslogarithmically with soil depth, and that at least 95%of fine root biomass occurs in the upper 100 cm of soil(Farrish, 1991; Ehrenfeld et al., 1992). Based on thesereports, we assumed the effective bottom of the rootzone to be at 100 cm. Finally, we estimated the averageannual DOC flux through each soil depth by multiplyingthe mean DOC concentration (from Table 1) by ourestimate of average annual water flow through eachdepth.

Based on our analysis, we estimate that the DOC fluxthrough our Coastal Plain soil decreases from about 128kg C ha'1 yr^ at 10-cm depth to 6 kg C ha"1 yr'1 at 90-cm depth, approximately the bottom of the E2 horizon(Table 4). The results illustrate that the flux of DOMdecreases with soil depth even more sharply than con-centration due to attenuation of water flow. At ourstudy site, this decrease occurs entirely within the sandportion of the soil profile. Potential for error in ourestimates of DOC flux is expected to be greatest nearthe soil surface, where throughfall has been estimated,DOC concentration exhibits the greatest variability, andchange in water flux with depth is greatest. Potentialerror decreases with soil depth due to the logarithmic


function for water flux through the root zone, such thatour estimates are relatively insensitive to assumptionsregarding throughfall and rooting depth. On this basis,our estimate for DOC flux through the A horizon shouldbe viewed with particular caution. However, we aremore confident that our estimate for DOC flux belowthe E2 horizon is a reasonable representation of ourstudy site.

For comparison, we have found only two other pub-lished studies containing DOM fluxes along with ade-quate site characterization (Table 4). One study is of amixed hardwood-conifer forest on a Spodosol soil inthe White Mountains of New Hampshire (McDowelland Likens, 1988). The other is of a loblolly pine foreston a finer textured Ultisol on the Piedmont in SouthCarolina (Richter et al., 1994, 1995; Richter and Mar-kewitz, 1996). For the Piedmont site, we added someestimates for water and DOC fluxes through the uppermineral horizons of the soil profile in order to presenta more thorough comparison. To do so, we assumedthe same logarithmic decrease in water flow through aroot zone of 100 cm as we did for our study, in orderto scale existing water flux data between the O and theC horizons. We think this approximation is reasonablefor the purposes of our general comparison, since thesetwo South Carolina sites have similar forest type andclimate.

To the extent that results from the three studiesin Table 4 are representative of their respective forestregions (mixed forest on a Spodosol, pine forest on afine-textured Ultisol, mixed forest on a coarse-texturedUltisol), we find no clear evidence for major differencesamong them in the quantities of DOM being transportedthrough soil profiles.

Our comparison of these three sites indicates thatthere are similarly large quantities of DOM leachinginto surface soils, as well as similarly strong attenuationof DOM in mineral soil horizons. In each case, com-pared with DOM inputs to soil, the quantity of DOMleaching to groundwater is very small. This comparisonsuggests that, in general, DOM transport through sandyUltisols is similar to that in finer textured Ultisols andSpodosols in other forest regions.

CONCLUSIONThe results of this study do not support the hypothesis

that there is greater DOM transport through sandy,upland forest soils on the southeastern U.S. CoastalPlain than in other forest regions.

Previous studies have indicated that strong DOMretention in forest soils is related to podzolization inSpodosols and high clay content in Ultisols. Our resultssuggest that similarly strong DOM retention may alsooccur within deep sand E horizons of Ultisols. Thus,strong DOM retention by forest soils appears to occuracross a broader range of soil types than those exhibitingpodzolization or having high clay content.

ACKNOWLEDGMENTS

This research was supported by Financial Assistance Awardno. DE-FC09-96SR18546 from the U.S. Department of En-

ergy to the University of Georgia Research Foundation. Addi-tional support for manuscript preparation was provided bythe USDA Forest Service, National Agroforestry Center, Lin-coln, NE.

transport of dissolved organic matter through a sandy forest soil

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