production of dissolved organic carbon in forested catchments

Production of dissolved organic carbon in forested catchments

D. Hongve*

National Institute of Public Health, PO Box 4404 Torshov, N-0403 Oslo, Norway

Received 30 November 1998; accepted 19 August 1999

Abstract

Leaf litter is an important source of natural dissolved organic carbon (DOC) in forested catchments. Rainwater percolatingthrough fresh litter obtains higher concentrations of DOC and colour than from older forest floor material and organic soils.Chemical characterisation using DOC fractionation and tests of biodegradability show that natural litter percolates containsignificant fractions of coloured and highly refractory hydrophobic acids (humic substances) and variable fractions of biode-gradable compounds. Deciduous leaf litter imparts high DOC concentrations in the autumn, while coniferous litter and organicsoils release DOC more evenly. Leaching from fresh deciduous litter may explain the seasonality in the concentration of DOCin discharge from forested catchments. Countrywide Norwegian data show very poor relationships between occurrences ofmires in the catchment of lakes and water colour. High concentrations of DOC may occur in lakes where the catchment has ahigh proportion of surface runoff due to thin or impermeable soils or swamp areas.q 1999 Elsevier Science B.V. All rightsreserved.

Keywords:Organic materials; Surface water; Dissolved organic carbon (DOC); Forest soils; Litter; Leaching

1. Introduction

The concentrations of coloured dissolved organiccarbon (DOC) in streams and lakes draining forestedcatchments vary within wide limits. Within the Nordicregion (Fenno-Scandia) the concentrations are overalllow in western Norway where high precipitation givesrapid flushing of catchments and dilution of DOC, andhigh in the eastern boreal areas of Sweden andFinland. High DOC values also occur in the oceaniccoastal areas of Norway (Henriksen et al., 1998). HighDOC concentrations are usually associated withcatchments with a high percentage of mires. However,many coloured lakes are also found in catchmentswithout notable wet areas. In southeast Norway such

locations are often characterised by shallow mineralsoil or just a blanket of mosses and lichens underlainby Pre-Cambrian gneisses and granites. To my knowl-edge, no explanation of the high DOC concentrationsin these localities has been given.

A marked seasonality in the concentration andcharacter of DOC in discharge from forested catch-ments has been reported (Meili, 1992; Ivarsson andJansson, 1994). Autumnal runoff events give oftensignificant DOC concentration peaks (McDowelland Likens, 1988; David et al., 1992; Easthouse etal., 1992).

Since waters of high colour and DOC concentra-tions are typical of regions rich in swamps, it is gener-ally believed that coloured DOC in surface water isderived from peat and organic soils. However, it is notpossible to infer the DOC production rate in a swampfrom in situ concentrations unless water retentiontimes are known. Mitchell and McDonald (1992)

Journal of Hydrology 224 (1999) 91–99

0022-1694/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-1694(99)00132-8

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* Tel.: 147-2204-2692; fax:147-2204-2686.E-mail address:[email protected] (D. Hongve)

and Vogt and Muniz (1997) have shown that leachingfrom moorland peat after prolonged drought leads toincreased DOC concentrations. In forested catch-ments the main production of DOC takes place inthe upper forest floor (McDowell and Likens, 1988;Qualls et al., 1991) and deciduous litter is an impor-tant source for DOC and colour in runoff and soilwater (Thurman, 1985; Stevenson, 1994; Zsolnay,1996). High concentrations of DOC in unpollutedsurface waters originate almost exclusively from thecatchment (Thurman, 1985; Mitchell, 1990). Aroundone half of the DOC concentration will usually equalan operational definition of aquatic humic substancesbased on hydrophobic absorption on Amberlite XAD-8 resin (Thurman and Malcolm, 1981). Other majorDOC fractions are hydrophilic acids and neutralcompounds (mainly carbohydrates) (Leenheer, 1981;Aiken, 1985; Thurman, 1985).

The aim of the present study was to identify themajor sources, other than mires, for DOC in forestedcatchments. DOC production in organic soils variesconsiderably with experimental conditions, such aswater extraction pattern (Christ and David, 1996a),temperature and moisture (Christ and David,1996b). Therefore, it was decided to expose testsamples to leaching by percolating precipitationunder conditions as close to natural field conditions

as possible. The qualities of the percolates arediscussed in relation to hydrological pathways andprocesses that control DOC concentrations within acatchment. It has not been possible, within the limitsof this project, to make estimates of the relative abun-dance of the various sources to DOC in the discharge.This will usually show major differences betweencatchments. The discussion highlights processes andproperties of organic substrates that have been givenlittle attention in studies of the relations betweencatchment hydrology and water quality. Othersubstrates than those treated here, for instance leach-ing by throughfall (Qualls et al., 1991) and litter ofgrass, herbs, lichens, etc. may also be of importance tothe runoff quality from forest ecosystems.

2. Materials and methods

Organic soils were collected as intact pieces of theOh-horizon (2–3 cm thick) from a ferric podzol in anold natural stand of Norway spruce (Picea abies(L.)Karst.). Fibric histosols (HSf) from twoSphagnummires were collected at the surface, 5–10 and 20–25 cm depth, which correspond to H1, H2 and H3,respectively, on the von Post decomposition scale.Leaf litter of birch (Betula pubescensEhrh.), willow(Salix auritaL.), alder (Alnus incana(L.) Moench.),Scots pine (Pinus silvestrisL.) and Norway sprucewere sampled from a natural mixed forest in Enebakk,southeast of Oslo, Norway (October 1992). The leaveswere taken directly from many different trees afterseveral cold nights when they had been exposed tofreezing.

Litter samples and undisturbed slices of the soilcores were installed in lysimeters (Fig. 1) where therelease of DOC to percolating precipitation could bemonitored. The lysimeter experiment was performedclose to the sampling location. The design and posi-tion of the lysimeters, placed level with the soilsurface in the shadow of trees, but with no hindrancefor falling precipitation, ensured that temperature andmoisture conditions in the litter samples were as closeas possible to natural field conditions. TheSphagnumand the soil samples were, however, probably exposedto more desiccation than under natural conditions. Nopre-treatment (drying, grinding, etc.) was employed.Aliquots were used for determination of dry weight

D. Hongve / Journal of Hydrology 224 (1999) 91–9992

Fig. 1. Lysimeter used for collection of soil and litter percolates.The underground collector protects the water sample from sunlightand temperature fluctuations.

(1058C). Initially each lysimeter contained about thesame substrate volume, corresponding to 5–20 g dryweight organic matter. All percolation occurringduring the next 12 months was sampled and analysed.

The weather during the first autumn shifted betweenrain and snow episodes and the substrate temperatureswere in the range 0–58C. Three sets of water samples,each representing multiple precipitation events, were

D. Hongve / Journal of Hydrology 224 (1999) 91–99 93

Table 1Accumulated production of DOC and water colour (as mg Pt) per 10 g dry weight during an annual sampling sequence starting with fresh litter

Sample specimen DOC (mg) Colour (mg Pt/l) Colour:DOC ratio

Deciduous litter 665 1600 2.4Coniferous litter 107 442 4.1Peat 43 202 4.7O-horizon 92 359 3.9Sphagnuma 35 83 2.4

a Only autumn and winter samples because the specimens died of desiccation in spring.

Colour

1

10

100

1000

10000

1 Oct 1 Jan 1 Apr 1 Jul 1 Oct

mg

Pt/l

012345

precipitation mm

/day

Deciduous litter Coniferous litterPeat, H3 Oh-horizonSphagnum, H1 Precipitation

DOC

1

10

100

1000

10000

1 Oct 1 Jan 1 Apr 1 Jul 1 Oct

mg/

l

Fig. 2. Average precipitation intensities for each collection period and variation in colour and DOC concentrations in percolates from litters(average values for different forest tree species) and organic soils. The aqueous concentrations pertain to 10 g dry weight of each substrate.

collected before 6 December when the substratesfroze and were buried under a snow cover that lasteduntil April. Water samples were collected every timepercolation occurred during the following spring andsummer, but many minor rain events did not result inpercolation. Mean day temperatures during thesummer months were mostly in the range 12–188C.Earthworms became numerous in some leaf littersamples late in the second autumn.

Pore water samples from 5 cm sections of a mirecore were squeezed out by hand using light pressure.After that, each sample was washed with excess ofpurified water until the filtered (0.45mm) pore waterwas colourless and showed no absorption of ultravio-let light �l � 254 nm�. The samples were saturatedwith purified water and incubated in glass beakers indarkness at 208C for 60 days before, again, the porewater was squeezed out.

Dissolved components were analysed on filteredsamples (0.45mm). Colour is given as mg Pt/l(Hazen units). DOC was analysed using a Techni-con AutoAnalyzerII, Industrial method 415-76W,employing ultraviolet irradiation and persulfate fordigestion of organic compounds. Fractionation ofDOC into hydrophobic and hydrophilic acids, basesand neutrals was done after the procedure of Leenheer

(1981) and slightly modified by Easthouse et al.(1992). Tests of aerobic biodegradability were madeusing a modification of ISO 10707 (InternationalOrganisation for Standardisation, 1994). Unfilteredpercolate samples were diluted to 10–20 mg/l DOCto avoid overgrowth of micro-organisms and incu-bated with continuous stirring for 2–3 months atroom temperature (23̂ 28C). Inorganic nutrientswere added according to ISO 10707 but no inoculumwas used. Subsamples were withdrawn regularly forDOC measurements.

Lake water and catchment characteristics werecollected from the national register of waterworks atthe National Institute of Public Health, Oslo. Lakesserving as raw water sources for waterworks are, arule, pristine with a minimum of human activities inthe catchment except some forestry.

3. Results

3.1. DOC leaching

Total annual productions of colour and DOC fromvarious types of solid organic matter are shown inTable 1. Fig. 2 shows the temporal variation in colourand DOC in the percolates. The various tree speciesyielded different concentrations, birch on average 5%less and willow 35% less than alder. The differencebetween parallel samples was on an average 22% forlitter and 45% for soil samples. Fresh litter fromdeciduous trees produced high DOC and colour valuesduring the first autumn, but the concentrationsdeclined rapidly. The first rain event after snowmeltproduced a minor increase, but after that thesesamples showed values similar to those for the othermaterials. In total, 13% of the initial content oforganic carbon in deciduous litter was recovered asDOC during an annual leaching sequence. Pine littergave twice as high concentrations as spruce during theautumn and similar concentrations during the follow-ing summer. In total, coniferous litter gave only 0.1times as high DOC concentrations as deciduous litterduring the first autumn, and a total of 2% of the carbonwas recovered in the percolates. The colour and DOCvalues increased during the summer.

In comparison with litter, the organic soil samplesimparted less DOC and colour per mass solid organic


0

50

100

DOC

Colour

0

50

100

Deciduouslitter

Coniferouslitter

Peat O-horizon

autumn - snowmelt spring - second autumn

% o

f tot

al

Fig. 3. Percentages of DOC and water colour leached from variousorganic substrates during the autumn–winter period and the follow-ing snow-free season.

matter. The concentrations in percolates from the soilswere highest during the summer. The O-layerconsisted mainly of identifiable remains of spruceneedles and gave almost as high yield as fresh sprucelitter, around 2% of the total solid carbon during thestudy period, while the yield from peat was less than1%. During the autumn months, live moss (Sphag-num) gave percolates with around the same colourintensities and higher DOC concentrations than peatfrom 10 to15 cm depth from the same mire. Fig. 3shows the accumulated seasonal production of DOC

and colour production in the various types of litter andorganic soils.

Results of the incubation experiment with moss andpeat are shown in Fig. 4. The original pore watercontained moderate DOC concentrations and thecolour increased with the depth. After the water hadbeen changed and the samples incubated for2 months, the peat from 20–25 cm depth producedthe same DOC concentration as before while thevalues for DOC and colour in the surface layer weresix and eight times higher than found originally.

3.2. DOC fractions

Selected litter percolates were analysed for DOCfractions. Fig. 5 shows percentages of hydrophobicand hydrophilic acids and hydrophilic neutrals.Other fractions did not exceed the limits of quantifica-tion. Hydrophobic and hydrophilic acids dominated inthe first percolates from deciduous litter and, afterthat, hydrophilic neutrals became the dominant frac-tion. Hydrophobic compounds were most abundant inconiferous litter percolates, and the composition didnot change much with time.

3.3. Biodegradability

Biodegradability of DOC from litter percolates wasdetermined from October and May (Fig. 6). Thedegradation rates were fastest for percolates of decid-uous and pine litter in the autumn. Around 45% of theDOC concentrations were mineralised during the firstweek, and around 30% remained in solution after7 weeks. In May, the degradation rates were slowerand around 50% was mineralised during 7 weeks. TheDOC concentrations in spruce percolates were notsignificantly reduced (#5%) in these tests. Thereduction rates for water colour were much lowerthan for DOC and more than 90% of the colourremained after 7 weeks incubation for all types oflitter percolates.

3.4. Water colour vs. catchment characteristics

The national waterworks register gives informationon water colour and catchment coverage of mires andforest areas for 86 lakes in counties with predomi-nantly inland climate (southeast Norway) and 414lakes from the counties along the western Atlantic


0 cm

5 cm

10 cm

15 cm

20 cm

25 cm

moss, H1

peat, H2

peat, H3

Original pore New pore

DOC colour DOC colour

25 69 150 570

10 25 54 245

14 179 15 290

Depthwater water

Fig. 4. A peat core from aSphagnummire. The figures show naturalDOC (as mg/l) concentrations and colour values (as mg Pt/l) of thepore water and concentrations 60 days after the pore water had beenexchanged with purified water.

0

50

100

28 O

ct

18 N

ov

5 M

ay

28 O

ct

18 N

ov

5 M

ay

% o

f tot

al D

OC

Hydrophobic acids Hydrophilic acids Hydrophilic neutrals

Deciduous litter Coniferous litter

Fig. 5. Percentages of DOC fractions in litter percolates. Averagevalues for litter from various species in each category.

coast (Rogaland, Finmark) which have a more oceanicclimate. The mean coverage of mires is 8% for theinland lakes and 16% for the coastal lakes. For inlandlakes there was no significant statistical relationshipbetween water colour and percentage of mires in thecatchment (r2 � 0:008,p . 0:1). For the coastal lakesthere was a weak, but statistically significant, increasein water colour with increasing percentage of mires(r2 � 0:099,p , 0:001) (Fig. 7).

4. Discussion

The experimental conditions in this study, regard-ing sample humidity, temperature and leaching rates,largely depended on the weather conditions of theactual year. Christ and David (1996a,b) showed thatsuch conditions determined the yields in experimentalextraction of DOC from soils. The yields under fieldconditions may, therefore, be highly variable. Accord-ingly, only major seasonal variations and differencesbetween substrate types will be dealt with here. Theamounts of litter used in the leaching experimentscorrespond to 1–2 kg/m2, which is higher than usualin forested catchments after leaf fall. In Fig. 2 thepercolate concentrations are scaled to 2 kg/m2 for allsubstrates and are, accordingly, higher than undernatural conditions with regard to litter. However, themost important point is to show the relative differ-ences between the substrate types. The influence ofsubstrate to water ratio for the result of leachingexperiments was not studied but is assumed to beminor compared with the differences between thesubstrate types. The results show that fresh deciduouslitter has a very high potential for production of DOCin the short term compared to coniferous litter andorganic soils. The leaching rate was highest forfresh litter from leaf fall to early spring while biode-gradation is retarded by low temperature. Undernatural humid conditions the material is processedby organisms in the forest floor during the followingwarm season and the lysimeter results with deciduouslitter from this period may, therefore, not be compar-able with natural field processes. Leaching from coni-ferous litter varied less than for deciduous litter duringthe first year and, as for organic soils, may be


0

20

40

60

80

100

0 20 40 60Days

DO

C, %

May

Oct.

Fig. 6. Biodegradability test results for percolates from spruce, birch, willow and alder litter collected on 28 October and 5 May. The symbolsindicate mean values, total ranges and geometric regression lines.

Southeast inland

mire %

Col

our,

mg

Pt/l

0 10 20 30 400

30

60

90

120

150

West coast

0 20 40 60 80 1000

30

60

90

120

150

Fig. 7. Lake water colour vs. per cent cover of mires in the catch-ments for 86 lakes in the inland district of southeast Norway and 414lakes in the counties along the Atlantic coast.

governed by the temperature. A difference betweenleachates from spruce and pine needles during thefirst autumn is probably due to differences in theneedle shedding pattern. Old pine needles die andfall off in the autumn while dead spruce needlesmay sit on the trees for an extended time. DOC leach-ing from peat was highest during the summer monthsand both soil types produced more coloured runoff athigher temperatures (Fig. 3).

Litter production is abundant in all forested catch-ments, whatever the amount of soil and soil types, andlitter leaching can explain why highly coloured lakesare also found in catchments without swamps. Waterpercolating through litter is considerably enriched inDOC, but then other factors control whether thesehigh DOC concentrations actually reach the streamor lake. The water pathways in the catchment areprobably most important for aquatic DOC concentra-tions. When the soil solution percolates through adeep soil DOC is selectively reduced (Qualls andHaines, 1981; Cronan and Aiken, 1985; Thurman,1985; Qualls et al., 1991; Easthouse et al., 1992).The B-horizon effectively controls DOC concentra-tions (McDowell and Wood, 1984). Sorption ontoinorganic stream bed substrates may also accountfor rapid removal of DOC from the water (Dahm,1981). Lakes in forested catchments with permeableinorganic soils may, as a result, receive water withlow DOC concentrations (Cole et al., 1984) whilecatchments with scarce soil cover can have lakeswith high DOC concentrations.

In contrast to what may be a common view ofcoloured waters since they are often called “bogwater” or “swamp water” (Mitchell, 1990), the influ-ence of mires on the water’s content of colouredorganic substances was found to be insignificant. Itshould be evident from Fig. 7 that the occurrence ofmires is hardly the primary reason for the watercolour. The effects that waterlogged mires sometimeshave on DOC concentrations in runoff may dependmore on their influence on water pathways than onleaching of DOC from the peat. The deep layers ofmires can be quite impermeable and the dominantflow will accordingly pass laterally through the lessdecomposed superficial layer (Haldorsen et al., 1992).Since mire runoff has little, if any contact with inor-ganic soils, the concentration is still high when thewater is discharged into a stream or lake. Both the

lysimeter and saturation experiments indicate thatthe potential for DOC production in peat deposits islow compared with freshly produced organic matteron the surface. The high DOC concentrations usuallyfound in deep peat layers is, therefore, probably notdue to high DOC production in these layers, but is aresult of long water retention times.

DOC from fresh litter contains hydrophobic acidsthat resemble humic substances in several respects(Qualls and Haines, 1991; Qualls et al., 1991). Thisstudy shows that litter percolates of common foresttrees are more or less refractory, and consist of DOCfractions in proportions not unlike soil extracts (Christand David, 1996a) or coloured lake water (e.g. Matt-son et al., 1998). DOC in coloured lakes is recalci-trant, representing the less labile fraction from naturalprocessing of leachates through natural decay, selec-tive absorption, and precipitation processes in soils,streambeds and in the water column. Radiocarbonstudies have shown that most of the DOC in streamsand lakes is derived from parent material of muchyounger age than the bulk organic soil material(Malcolm, 1985; Stevenson, 1985; Schiff et al.,1990). This too supports a hypothesis that DOC insurface water is to a great extent derived from recentterrestrial primary production, and disagrees with theconclusions of McDowell and Likens (1988) whosuggest that most of the DOC in streams is derivedfrom the older forest floor material.

Integrated hydrological and hydrochemical modelshave been used with success to model chemical waterquality in catchments with variable hydrological path-ways (e.g. Grieve, 1991; Taugbøl et al., 1994). Duringwet conditions most water follows shallow flow paths,while during dry conditions, water passes throughdeeper soil layers. Autumn rain and spring snowmelttherefore coincide with runoff low in ionic strengthand high DOC values.

5. Conclusions

Leaching from freshly produced organic substrates,in particular deciduous litter, may explain much of themarked seasonality in the concentration and characterof DOC in discharge from forested catchments andwhy autumnal runoff events have especially signifi-cant DOC peaks. Although deciduous litter is seldom


the most abundant organic material in forested catch-ments, it is most easily degraded. Because of theseasonal occurrence of fresh leaf litter, wet episodesduring the summer leach higher proportions of DOCfrom organic soils and coniferous litter. High concen-trations of DOC occur in lakes where the catchmenthas a high proportion of surface runoff due to shallowor impermeable soils and/or swamp areas.

Acknowledgements

I wish to thank Prof. Stephen Norton, University ofMaine, USA, Dr Ian C. Grieve, University of Stirling,UK, Prof. John Brittain, University of Oslo, Norway,and one anonymous reviewer for valuable commentsto the manuscript. Thanks also to Prof. Tore Krogstad,Agricultural University of Norway, for help withdetermination of soil samples.

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production of dissolved organic carbon in forested catchments

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