– © WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0323–4320/00/0203–0077 $ 17.50+.50/077 82 77Acta hydrochim. hydrobiol. 28 (2000) 2

Dissolved Organic Carbon in Seepage Water –Production and Transformation during SoilPassage

Gelöster organisch gebundener Kohlenstoff im Sickerwasser – Produktionund Transformation während der Bodenpassage

B. Ludwig, B. Heil, H. Flessa,and F. Beese 1

1 PD Dr. Bernard Ludwig, Bálint Heil, Dr. Heiner Flessa, Prof.Dr. Friedrich Beese, University of Göttingen, Institute of SoilScience and Forest Nutrition, Büsgenweg 2, D-37077 Göttin-gen, Germany

Correspondence to B. LudwigE-mail: bludwig@gwdg.de

Keywords: Calamagrostis epigeios, Decomposition, Dis-solved Organic Carbon, Epilobium angustifolium, SeepageWater, Soil Column, Sorption, Stable Isotope

Summary: Dissolved organic carbon (DOC) in seepage watercan combine with organic pollutants, with Al and heavy metalions and transport them through the soil profile with a poten-tial to contaminate groundwater. We studied the production ofDOC in aerobic decomposition experiments at 8 °C and mois-ture close to field capacity in soils from two sites with differ-ent microbial activities (spodic dystric Cambisols with moder(SLB) and mor-moder (SLS) layers) using 13C-depleted plantsof differing decomposability (Epilobium angustifolium andCalamagrostis epigeios). Additionally, we investigated theDOC transformation during soil passage in decomposition ex-periments and in the field for the sites SLB and SLS. For SLS,decomposition of Epilobium resulted in a cumulative CO2 pro-duction of 14% of the added C within 128 days. Priming ef-fects were negligible. CO2 production for the experiments us-ing Calamagrostis was less with 11% for SLB and 10% forSLS. Cumulative DOC production was markedly high in theEpilobium decomposition experiment, being 25 g m–2, out ofwhich 11 g m–2 were Epilobium-derived (2% of the added C).For the Calamagrostis experiments, cumulative productions ofDOC and Calamagrostis-derived DOC (0.1% of the added Cfor SLS and SLB) were much less. During the soil passage,much of the DOC was removed by sorption or decompositionprocesses. Field studies at SLS and SLB using 13C naturalabundance showed that 13C distribution of soil organic matterincreased with depth, probably mainly due to a discriminationof C isotopes by decomposing microorganisms. DOC, how-ever, showed a depletion of 13C from –28‰ PDB to –29‰(SLB at 40 cm) or –28 to –30‰ (SLS at 20 cm) with depth,owing to preferential decomposition of 13C-enriched substanc-es or preferential adsorption. This study indicates that DOCproduction is strongly affected by litter composition and thatsignificant changes in DOC composition may occur during itspassage through a soil depth of 40 cm.

Schlagwörter: Abbau, Bodensäule, Calamagrostis epigeios,Epilobium angustifolium, gelöster organisch gebundener Koh-lenstoff, Sickerwasser, Sorption, stabiles Isotop

Zusammenfassung: Gelöster organisch gebundener Kohlen-stoff (DOC) im Sickerwasser kann mit organischen Schadstof-fen, Al- und Schwermetallionen Bindungen eingehen unddurch das Bodenprofil transportiert werden mit dem Potential,Grundwasser zu kontaminieren. Wir untersuchten die DOC-Produktion in aeroben Abbauexperimenten bei 8 °C undFeuchtegehalten nahe der Feldkapazität in Böden zweierStandorte mit unterschiedlichen mikrobiellen Aktivitäten(spodic dystric Cambisols mit Moder- (SLB) und rohhumus-artiger Moder-Auflage (SLS)) bei Verwendung 13C-abgerei-cherter Pflanzen unterschiedlicher Abbaubarkeit (Epilobiumangustifolium und Calamagrostis epigeios). Zusätzlich unter-suchten wir die DOC-Transformation während der Bodenpas-sage in Abbauexperimenten und im Freiland für die StandorteSLB und SLS. Für SLS wurde beim Abbau von Epilobium ei-ne kumulative CO2-Produktion von 14% des zugegebenenKohlenstoffs innerhalb von 128 Tagen gefunden. Priming-Effekte waren vernachlässigbar. Die CO2-Produktion beimAbbau von Calamagrostis war geringer: 11% für SLB und10% für SLS. Die kumulative DOC-Produktion war beträcht-lich in dem Abbauexperiment mit Epilobium: 25 g m–2, wobei11 g m–2 Epilobium-bürtig waren (2% des zugegebenen C).Für die Abbauexperimente mit Calamagrostis waren die ku-mulativen Produktionen an DOC und an Calamagrostis-bürti-gem DOC (0.1% des zugegebenen C bei SLS und SLB) erheb-lich geringer. Während der Bodenpassage wurde ein Großteildes DOC durch Sorption oder Abbauprozesse aus dem Sicker-wasser entfernt. Freilandstudien bei SLS und SLB unter Ver-wendung der natürlichen 13C-Häufigkeit zeigten, dass die 13C-Verteilung der organischen Bodensubstanz mit zunehmenderTiefe anstieg, vermutlich hauptsächlich aufgrund einer Diskri-minierung der C-Isotope durch zersetzende Mikroorganis-men. DOC wies jedoch mit zunehmender Tiefe eine 13C-Ab-reicherung von –28‰ PDB bis zu –29‰ (SLB bei 40 cm)oder von –28 bis zu –30‰ (SLS bei 20 cm) auf, aufgrund ei-nes bevorzugten Abbaus 13C-angereicherter Substanzen oderbevorzugter Adsorption. Diese Studie zeigt, dass die DOC-Produktion erheblich von der Qualität der Streu abhängt unddass signifikante Änderungen in der DOC-Zusammensetzungwährend der Bodenpassage bis zu einer Tiefe von 40 cm auf-treten können.

Acta hydrochim. hydrobiol. 28 (2000) 2 –77 8278

1 IntroductionDissolved organic carbon (DOC) in the seepage water is

an important component of terrestrial ecosystems, because itcan combine with Al and heavy metal ions and organic pollu-tants and transport them through the soil profile with a poten-tial to contaminate groundwater.

DOC production seems to be a function of the microbialactivity in the forest floor and soil. Therefore, its productionmay be mainly governed by factors which also control biologi-cal activity [1], such as temperature (DOC maxima were ob-served in summer [2]); moisture (DOC production increasedwith increasing moisture contents [1]); and other conditionsfor the decomposer such as pH and concentration of nutrients[3], anaerobic or aerobic conditions [4] and concentration oftoxic substances. Additionally, litter biodegradability may in-fluence DOC production [5]. In field studies, enhanced DOCproduction was observed with natural drying and rewettingcycles of soils [6] and has been partly attributed to lysis of mi-crobial cells [1].

During soil passage DOC is mainly sorbed in mineral hori-zons of soils. Jardine et al. [7] reported an increased DOC ad-sorption with increasing soil profile depth whereby hydropho-bic acids were preferentially sorbed on clay-sized particles.Retention of dissolved organic matter in mineral soils has beenreported for a variety of soil types and textures, ranging fromSpodosols and Inceptisols exhibiting podzolization, medium-textured Ultisols to sandy Ultisols [8].

Decomposition of DOC has not been studied as much asits sorption [9–10]. However, the few studies available indicatethat decomposition of DOC plays only a minor role in themarked decrease of DOC concentrations with profile depth.Only up to 30% of the DOC from seepage water of differentmineral horizons were decomposed after an incubation periodof 134 days at 295 to 297 K [9]. Qualls and Haines [9] suggest-ed that after the adsorption of DOC, bacteria, and fungi occur-ring close to the adsorbed surfaces exude exoenzymes that canhydrolyze it. This decomposition is a slow process whichclears the adsorption sites and the adsorption capacity is re-newed. The concept of a slow decomposition was supported byfindings that ferrihydrite and aluminum hydroxide reducedconsiderably the decomposition of carbohydrates [11]. How-ever, the concept of slow decomposition was challenged bySchiff et al. [12] and Trumbore et al. [13] who studied the fateof DO14C and DO13C in seepage water during soil passage.They concluded that, although some of the DOC is transportedto and sorbed in the upper B horizon, the ultimate fate of DOCis decomposition to CO2 and loss to the atmosphere.

We studied the CO2 production and DOC production andtransformation during soil passage in decomposition experi-ments with Epilobium angustifolium and Calamagrostis epi-geios using soils from two sites of different litter qualities. Weused 13C-depleted plant material in order to determine theamounts of plant-derived DOC and CO2 during the experi-ment. Epilobium and Calamagrostis were chosen, becausethey are common species in forests and they differ consider-ably in their decomposability [14–15].

For the decomposition experiments we used an automatedmicrocosm system [16] which restricts the investigation ofDOC transformation during soil passage to a depth of approxi-mately 20 cm. In order to study DOC transformation at greaterdepths down to 80 cm, an additional field experiment was car-ried out which included the determination of the 13C distribu-tion of DOC using ceramic cups.

Objectives were to study (i) the DOC production as a func-tion of litter quality and site conditions and (ii) the transforma-tion of DOC in seepage during soil passage under laboratoryconditions (using 13C-depleted plants) and in the field (usingnatural 13C abundance).

2 Materials and MethodsSites

Soils were taken from two sites in the Solling, Germany, oneunder beech (SLB) and the other under spruce (SLS). The soil typefor both sites is a spodic dystric Cambisol. Organic layers aremoder (SLB) and mor-moder (SLS). For SLB, pHH2O values of or-ganic layers are 3.3 (OH layer) and 4.0 (OL layer) and for SLS,pHH2O values are 3.8 (OH) and 4.1 (OL). C/N ratios for SLB rangefrom 21 to 33 and from 23 to 28 for SLS. SLS has less amounts offine roots, micorrhizial fungi, and animals when compared to SLB[17]. Microbial biomass ranges from 20.7 g kg–1 (OL) to 3.5 g kg–1

(OH) for SLB [18] and from 6.6 g kg–1 (OL) to 1.3 g kg–1 (OH) forSLS.

Plant material

Calamagrostis epigeios (from rhizomes) and Epilobium an-gustifolium (from seeds) were grown in a chamber using 13C-de-pleted CO2 (–48‰ PDB). The temperature was 15 °C and dailylight radiation was 12 h. Plants were harvested after three months.13C distributions were –70.5‰ PDB (shoot) and –70.1 (roots) forCalamagrostis and –76.7 (shoot) and –73.3 (roots) for Epilobium.Carbon contents were 43.8% for Calamagrostis and 43.7% forEpilobium. C/N ratios were 44 for Epilobium and 55 for Calama-grostis.

Laboratory decomposition experiments

Twenty one samples were taken from the sites SLS and SLB inplastic columns of 14.4 cm diameter and placed in an automaticmicrocosm system [16]. Prior to the decomposition experiments,all columns were irrigated with 4 mm d–1 (water contents wereclose to field capacity) at 8 °C for two months. Compositions of theirrigation solutions were adjusted to the throughfall compositionsfrom the respective sites. Continuous suction was applied at thebottom of each microcosm to maintain unsaturated conditions.Leachate was collected in glass bottles connected to the suctionsystem. The following variants were considered:

A) columns containing undisturbed soil organic layers fromthe sites SLS and SLB. The height of the litter layers were 7 cm(SLS) or 4 cm (SLB).

B) columns containing undisturbed organic layers and 17 cmdeep-mineral soils, resulting in total heights of 24 cm (SLS) or21 cm (SLB).

C) same as A), except that 30 g of C. epigeios was added on thecolumn surface. For SLS, an additional experiment included addi-tion of 25 g of E. angustifolium.

D) same as B), except that 30 g of C. epigeios was added on thecolumn surface.

After plant addition, decomposition was studied for 128 days(irrigation: 4 mm d–1, 8 °C). CO2 fluxes were measured daily andDOC production below the column was determined fortnightly.Additionally, DO13C and 13CO2 contents were measured usingmass spectrometry. The fraction f, of C coming from the 13C-de-pleted plants and released into CO2 and DOC was calculated byconsidering the measured δ values of CO2 and DOC, the measuredδ values of the respective control columns [δ(CO2) and δ(DOC atthe respective depths)], the measured δ values of the 13C-depletedplants and the measured δ values of the organic layers of SLS andSLB, respectively.

Acta hydrochim. hydrobiol. 28 (2000) 2 –77 82 79

Analytical procedures

DOC and soil organic matter (SOM) samples were acidified toremove inorganic C, dried at 40 °C (DOC: using a vacuum rotatingevaporator) and measured for their 13C contents using an ElementalAnalyzer Delta C Finnigan mass spectrometer. 13CO2 contentswere measured using a GC Delta plus Finnigan mass spectrometer.DOC contents of seepage water were measured using a C/N analyz-er (Heraeus). CO2 fluxes were determined by gas chromatography(Shimadzu) [16].

Field study

Samples for the determination of 13C contents of SOM wereobtained every centimeter down to 11 cm (SLB) or down to 30 cm(SLS) and additionally from the depths (in cm) 15, 25, 40, 60 and85 (SLB) or 35, 45, 55, 65, 75 and 85 (SLS) (n = 5). The 13C distri-bution of DOC was studied using ceramic cups installed in 0 cm,10 cm, 20 cm, 40 cm and 80 cm (SLB) or in 0 cm, 10 cm, 20 cm and80 cm (SLS). Samples (n = 3) were obtained in October 1997 andbulked.

3 Results and Discussion3.1 CO2 and DOC Production

Figure 1 shows the cumulative CO2 production curves(corrected for CO2 production in control plots) in the decom-position experiments using Epilobium and Calamagrostis inpercent of added C. For SLS, the highest CO2 production wasfound for the easily decomposable material Epilobium where14% of the added C was respired after 128 days incubation. AtSLS, the site with reduced microbial activity, only 10% of theadded Calamagrostis-C was respired after 128 days, whereasat SLB, C mineralization was slightly higher (11% of the add-ed Calamagrostis-C respired). The C mineralization ratesfound in this study are similar to the ones reported for beechlitter decomposition at a moder site [19]. Pöhhacker and Zech[19] found that at 5 °C the pool of organic C decreased by ap-proximately 15% within 90 days. The highest decompositionrates during the two-year study were investigated in the first 30to 90 days [19].

The CO2 productions reported here are small compared tothe values reported for agricultural ecosystems [20], probablymainly owing to reduced microbial activities when comparedto agricultural soils. Another reason for the small CO2 produc-tion is that plant residues were added superficially and not in-corporated.

We studied the 13C contents of the produced CO2 to deter-mine whether the increase in CO2 production was completelydue to the added plant materials or whether an increased de-composition of the organic layer (priming effect) occurred.The fractions of Epilobium-derived C obtained by 13C massspectrometry were in good agreement with the fractions ofEpilobium-derived C estimated using the CO2-fluxes (CO2

fluxes of the Epilobium experiment minus the CO2 fluxes fromthe control plot divided by the CO2 fluxes of the Epilobium ex-periment) (Fig. 1, below). This agreement shows that a prim-ing effect was negligible in the experimental conditions (siteconditions, C/N ratios of plant materials and organic layers).

The DOC production was much less than the CO2 produc-tion at the given aerobic conditions (Fig. 2, Table 1). The gooddecomposability of Epilobium resulted not only in a largerCO2 production but also in a markedly increased DOC produc-tion when compared to Calamagrostis. After 128 days, DOC

0

5

10

15

0 50 100 150

CO

2 pr

oduc

tion

in %

of a

dded

C

Days

Epilobium (SLS)

Calama-grostis (SLB)

Calamagrostis (SLS)

56 d

40 d

28 d

24 d

14 d

7 d

0.4

0.5

0.6

0.7

0.8

0.4 0.5 0.6 0.7 0.8Fraction of Epilobium-derived CO 2 (estimated by CO 2 fluxes)

Fra

ctio

n of

Epi

lobi

um-d

eriv

ed C

O2

(det

erm

ined

by

13C

-MS

)

r2 = 0.92

Fig. 1: Cumulative CO2 production (in % of added C by consider-ing the CO2 production of the organic layer) for different decompo-sition experiments (above). Fraction of Epilobium-derived CO2

(determined by MS) versus the one estimated by CO2 fluxes for dif-ferent days (below).

Kumulative CO2-Produktion (in % des zugegegebenen C beiBerücksichtigung der CO2-Produktion der organischen Auflage)für verschiedene Abbauexperimente (oben). Anteil des Epilobium-bürtigen CO2 (bestimmt mittels MS) gegen den Anteil des Epilo-bium-bürtigen CO2, der über CO2-Flüsse abgeschätzt wurde(unten). Daten werden für verschiedene Termine gezeigt.

production was 24.7 g m–2 C for Epilobium, 14.0 g m–2 C forCalamagrostis and 10.1 g m–2 C for the control (organic layeronly) (Fig. 2). These differences in DOC production for Epilo-bium and Calamagrostis were accompanied by similar largedifferences in the plant-derived DOC contents. For Epilobium,11.4 g m–2 C stemmed from the added plant (2% of the addedC) after 128 days, whereas only 0.9 g m–2 C were Calamagros-tis-derived (0.1% of the added C) (Table 1). Gale and Gilmour[4] also reported only small amounts of DOC production(2.4 mg C) after 30 days (corrected considering the DOC pro-duction of control plot without plant additions) at an incuba-tion experiment with 0.5 g alfalfa in 100 g of an Ap horizon ofan Ultisol under aerobic conditions (at 25 °C and water con-tents being 50% of the water holding capacity).

For Epilobium and Calamagrostis, the increases in DOCproduction from 65 days onwards were higher than theamounts of plant-derived C (cf. Fig. 2 and Table 1). DOC pro-duction at SLB accounted for 8.7 g m–2 C (organic layer andCalamagrostis) and 6.9 g m–2 C (organic layer only) after

Acta hydrochim. hydrobiol. 28 (2000) 2 –77 8280

0

4

8

0 65 130

SLS

0

4

8

0 65 130

SLB

0

5

10

15

20

25

0 65 130

Epilobium

Calamagrostis

Control

SLS

0

5

10

15

20

25

0 65 130

SLB

DaysDays

Cum

ulat

ive

DO

C r

elea

se fr

om

the

min

eral

soi

l in

g m

-2C

umul

ativ

e D

OC

rel

ease

from

th

e or

gani

c la

yer

in g

m-2

Fig. 2: Cumulative DOC releases from the organic layers (above)and mineral soils (below) for different treatments (addition ofCalamagrostis, Epilobium or no addition). Error bars refer tostandard errors.

Kumulative DOC-Freisetzung aus den organischen Auflagen(oben) und Mineralböden (unten) für verschiedene Behandlungen(Zugabe von Calamagrostis, Epilobium oder keine Zugabe). DieFehlerbalken stellen Standardfehler dar.

Table 1: Cumulative Calamagrostis- and Epilobium-derived amounts of C in CO2 production and DOC output from the organic layer for thesites SLS and SLB. Added amounts of Calamagrostis-C were 801.7 g m–2 and added amount of Epilobium-C was 666.3 g m–2.

Kumulative Calamagrostis- und Epilobium-bürtige Mengen an C an der CO2-Produktion und DOC-Freisetzung aus derAuflage für die StandorteSLS und SLB. Zugegebene Mengen an Calamagrostis-C waren 801.7 g m–2 und die zugegebene Menge an Epilobium-C war 666.3 g m–2.

Day afterplant addition

Calamagrostis-derived C (SLB)

g m–2

Calamagrostis-derived C (SLS)

g m–2

Epilobium-derived C (SLS)

g m–2

DOC-C CO2-C DOC-C CO2-C DOC-C CO2-C8 0.2 3.9 0.1 2.6 0.1 2.5

22 0.3 16.8 0.1 10.7 0.9 12.635 0.3 27.7 0.2 18.3 1.8 24.265 0.5 52.2 0.2 38.2 4.5 46.692 0.7 71.8 0.5 57.5 7.6 67.4

106 0.8 79.8 0.7 65.6 9.3 77.4128 1.0 90.9 0.9 76.2 11.4 91.3

128 days of incubation which was slightly less than at SLS(Fig. 2). However, the amount of Calamagrostis-derived DOCwas similar for both sites (Table 1).

The ratios of plant-derived CO2 to plant-derived DOC af-ter 128 days were around 90 for Calamagrostis at SLS andSLB, but only 8 for Epilobium (Table 1), indicating that thespecific rates of DOC production differ considerably depend-ing on the plant from which the DOC is derived.

3.2 Changes of DOC Composition duringSoil Passage

The cumulative values of DOC released after the passagethrough the mineral soil are considerably lower than thosefrom the organic layers, owing to sorption and decompositionprocesses occurring in the mineral soil. In experiments withCalamagrostis decomposition the cumulative DOC values forSLS and SLB were not significantly higher (p < 0.05) than inthe control plots (Fig. 2). A comparison between the DOC re-leased from the organic layers and the mineral soils shows thatduring the passage of seepage water through the upper mineralhorizon (at 17 cm depth), the DOC concentration decreased onan average by 54% for SLS or by 34% for SLB. The Calama-grostis-derived carbon outputs are only very small (notshown).

Brandt et al. [21] also reported only very small plant-de-rived DOC outputs from the mineral soil (40 cm depth) with14C-labelled oat straw which was incorporated in the upper5 cm of a Luvisol and irrigated with increased amounts ofrainfall (1700 mm a–1). After 100 days, the oat-derived DOCoutputs accounted for only 0.4% of the added amounts.

3.3 Changes of DOC and SOM Compositionin the Field

13C mass spectrometry investigations of soil organic mat-ter showed a δ 13C enrichment with increasing depth from–28‰ PDB (O layer) to –26‰ (Bv/IIBv horizon) for SLB andfrom –28‰ (O layer) to –25‰ (I–II Btv) for SLS (Fig. 3). Themain process responsible for this enrichment may be the dis-crimination of C isotopes by decomposing microorganisms[22]. However, other processes may include a decrease in at-mospheric δ 13C abundance of CO2 or translocation of fulvicacids enriched with 13C [23].

For both sites, DOC concentrations in the seepage waterdecreased markedly from around 20 mg L–1 (10 cm) to6 mg L–1 (100 cm) (not shown). DOC showed a depletion of13C from –28 to –29‰ PDB (SLB) or –28 to –30‰ PDB

Acta hydrochim. hydrobiol. 28 (2000) 2 –77 82 81

SLS

-90

-70

-50

-30

-10

10

-30 -29 -28 -27 -26 -25 -24

-90

-70

-50

-30

-10

10

-30 -29 -28 -27 -26 -25 -24

SOMDOC

SLB

Dep

th in

cm

δ 13C in O/OO PDB

Fig. 3: Changes in the 13C distribution of DOC and SOM (n = 5)with depth for the sites SLB and SLS. Error bars refer to standarderrors.

Tiefenweise Änderungen der 13C-Verteilung des DOC und SOM(n = 5) für die Standorte SLB und SLS. Die Fehlerbalken stellenStandardfehler dar.

(SLS) (Fig. 3). A depletion of 13C in DOC with depth was alsoreported for other sites and was assigned to preferential de-composition of 13C-enriched substances or preferential ad-sorption [24, 25]. However, preferential adsorption of hydro-phobic acids during soil passage [7] and the observation thatDOC sorption is quantitatively more important than DOC de-composition [9] may not explain the 13C depletion of DOCwith depth, because hydrophobic acids may be 13C-depletedcompared to the bulk-SOM ([24]; B. Ludwig, unpublished).Thus, it is possible that some of the easily decomposable com-pounds of the DOC (e.g., hemicellulose-derived compoundswhich are enriched in 13C) are removed by sorption and fol-lowed by decomposition, whereas the more stable DOC com-pounds (cellulose- and lignin-derived) which are depleted in13C remain in seepage water. The possibility of decompositionprocesses in the subsoil is also considered by Guggenbergerand Kaiser [26] who stated that in the subsoil, organic aciditymay partly be neutralized by microbial decomposition of theorganic acids.

Similar to our findings, a change in DOC composition inseepage water has also been reported by Guggenberger andZech [27] in the transitional zone between Oh and A horizonsunder spruce. In this depth, dissolved organic carbon was

found to represent a continuum ranging from slightly modi-fied plant-derived carbohydrates to strongly modified lignin-derived compounds and to products of microbial resynthesis.Our results, however, suggest that significant changes in DOCcomposition can occur in deeper depths of 40 cm in SLB soil(Fig. 3).

3.4 Implications of the Study for DOCin Seepage Water

Application of 13C mass spectrometry was useful to quan-tify the sources of DOC and its transformation during soil pas-sage. Production of DOC was related to CO2 production, butthe plant-derived CO2 to DOC production rates differed con-siderably between Calamagrostis and Epilobium, indicatingthat the specific rates of DOC production depend on the litterquality. However, further studies, especially those relatingplant organic compounds (soluble carbohydrates, starch, cel-lulose, hemicellulose, tannins and lignin) to DOC production[28], are necessary to gain improved insights in the mecha-nisms of DOC production.

DOC transformation during soil passage was significantin the decomposition experiments and field studies. Complex-ation and transport of Al and heavy metals will affect DOCtransformation and is being currently studied.

AcknowledgementsThis study was funded by the Deutsche Forschungsge-

meinschaft (DFG) as part of the superfund network ROSIG(Refraktäre organische Säuren in Gewässern). B. Heil re-ceived a German Academic Exchange Scholarship (DAAD)during the study period. We wish to thank Dr. P. K. Khanna forcritical comments on the manuscript.

References[1] Christ, M. J., David, M. B.: Temperature and moisture effects

on the production of dissolved organic carbon in a Spodosol.Soil Biol. Biochem. 28, 1191–1199 (1996).

[2] Grieve, I. C.: Variations in chemical composition of the soilsolution over a four-year period at an upland site in southwestScotland. Geoderma 46, 351–362 (1990).

[3] Guggenberger, G.: Acidification effects on dissolved organicmatter mobility in spruce forest ecosystems. Environ. Inter-national 20, 31–41 (1994).

[4] Gale, P. M., Gilmour, J. T.: Net mineralization of carbon andnitrogen under aerobic and anaerobic conditions. Soil Sci.Soc. Am. J. 52, 1006–1010 (1988).

[5] Olsen, M. W., Frye, R. J., Glenn, E. P.: Effect of salinity andplant species on CO2 flux and leaching of dissolved organiccarbon during decomposition of plant residue. Plant Soil 179,183–188 (1996).

[6] Matzner, E., Thoma, E.: Auswirkungen eines saisonalen Ver-sauerungsschubes im Sommer/Herbst 1982 auf den che-mischen Bodenzustand verschiedener Waldökosysteme. Allg.Forstz. 39, 677–683 (1983).

[7] Jardine, P. M., Weber, N. L., McCarthy, J. F.: Mechanisms ofdissolved organic carbon adsorption on soil. Soil Sci. Soc.Am. J. 53, 1378–1385 (1989).

[8] Dosskey, M. G., Bertsch, P. M.: Transport of dissolved organicmatter through a sandy forest soil. Soil Sci. Soc. Am. J. 61,920–927 (1997).

[9] Qualls, R. G., Haines, B. L.: Biodegradability of dissolvedorganic matter in forest throughfall, soil-solution, andstreamwater. Soil Sci. Soc. Am. J. 56, 578–586 (1992).

Acta hydrochim. hydrobiol. 28 (2000) 2 –77 8282

[10] Boissier, J. M., Fontvielle, D.: Biodegradable dissolved or-ganic carbon in seepage waters from two forest soils. SoilBiol. Biochem. 25, 1257–1261 (1993).

[11] Miltner, A., Zech, W.: Carbohydrate decomposition in beechlitter as influenced by aluminium, iron and manganeseoxides. Soil Biol. Biochem. 30, 1–7 (1998).

[12] Schiff, S. L., Aravena, R., Trumbore, S. E., Hinton, M. J.,Elgood, R., Dillon, P. J.: Export of DOC from forested catch-ments on the Precambrian Shield of Central Ontario: Cluesfrom 13C and 14C. Biogeochemistry 36, 43–65 (1997).

[13] Trumbore, S. E., Schiff, S. L., Aravena, R., Elgood, R.: Sourc-es and transformation of dissolved organic carbon in the HarpLake forested catchment: The role of soils. Radiocarbon 34,626–635 (1992).

[14] Landhäusser, S. M., Lieffers, V. J.: Competition betweenCalamagrostis canadensis and Epilobium angustifolium un-der different soil temperature and nutrient regimes. Can. J.For. Res. 24, 2244–2250 (1994).

[15] Rozema, J., Tosserams, M., Nelissen, H. J. M, van Heer-waarden, K., Broekman, R. A., Flierman, N.: Stratosphericozone reduction and ecosystem processes: Enhanced UV-Bradiation affects chemical quality and decomposition ofleaves of the dune grassland species Calamagrostis epigeios.Plant. Ecol. 128, 284–294 (1997).

[16] Hantschel, R. E., Flessa, H., Beese, F.: An automated micro-cosm system for studying soil ecological processes. Soil Sci.Soc. Am. J. 58, 401–404 (1994).

[17] Ellenberg, H., Mayer, R., Schauermann, J.: Ökosystemfor-schung – Ergebnisse des Sollingprojektes. Ulmer, Stuttgart,1986.

[18] Wang, C. P: Soil and temperature influence on microbiologi-cal nitrogen transformations. Rep. Forest EcosystemResearch Center at the Göttingen Univ. B 156 (1993).

[19] Pöhhacker, R., Zech, W.: Mikrokosmenversuche zum Einflußvon Temperatur und Sauerstoffverfügbarkeit auf den mikro-biellen Streuabbau. Mittlg. Dtsch. Bodenkundl. Ges. 71,217–220 (1993).

[20] Recous, S., Robin, D., Darwis, D., Mary, B.: Soil inorganic Navailability: effect on maize residue decomposition. SoilBiol. Biochem. 27, 1529–1538 (1995).

[21] Brandt, S., Pütz, T., Führ, F.: Formation and translocation ofrefractory organic substances (ROS) after straw amendmentto soils. In: Frimmel, F. H., Abbt-Braun, G. (Eds.): Symposi-um on Refractory Organic Substances in the Environment –ROSE, October 6–8, 1997, University of Karlsruhe, Abstractsof Oral and Poster Papers. Veröffentlichungen des Lehrstuhlsfür Wasserchemie und der DVGW-Forschungsstelle amEngler-Bunte-Institut der Universität Karlsruhe 35. Karls-ruhe, 1997, pp. 40–43.

[22] Becker-Heidmann, P.: Die Tiefenfunktionen der natürlichenKohlenstoff-Isotopengehalte von vollständig dünnschicht-weise beprobten Parabraunerden und ihre Relation zurDynamik der organischen Substanz in diesen Böden. Ham-burger Bodenkdl. Arbeiten 13 (1989), pp. 228.

[23] Huggins, D. R., Clapp, C. E., Allmaras, R. R., Lamb, J. A.,Layese, M. F.: Carbon dynamics in corn-soybean sequencesas estimated from carbon-13 abundance. Soil Sci. Soc. Am. J.62, 195–203 (1998).

[24] Schiff, S. L., Aravena, R., Trumbore, S. E., Dillon, P. J.: Dis-solved organic carbon cycling in forested watersheds: a car-bon isotope approach. Water Resour. Res. 26, 2949–2957(1990).

[25] Dörr, H.: Isotopendatierung (14C, 13C) von DOC in der un-gesättigten Bodenzone. In: Frimmel, F. H., Abbt-Braun, G.(Eds.): Refraktäre organische Säuren in Gewässern, VCH,Weinheim, 1993, pp. 253–265.

[26] Guggenberger, G., Kaiser, K.: Significance of DOM in thetranslocation of cations and acidity in acid forest soils. Z.Pflanzenernähr. Bodenk. 161, 95–99 (1998).

[27] Guggenberger, G., Zech, W.: Dissolved organic carbon in for-est floor leachates: simple degradation products or humicsubstances? Sci. Total Environ. 152, 37–47 (1994).

[28] Wershaw, R. L., Leenher, J. A., Kennedy, K. R., Noyes, T. I.:Use of 13C NMR and FTIR for elucidation of degradationpathways during natural litter decomposition and compost-ing. I. Early stage leaf degradation. Soil Sci. 161, 667–679(1996).

received 23 June 1999accepted 14 January 2000