9
Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages Axel Don * , Karsten Kalbitz Department of Soil Ecology, Bayreuth Institute for Terrestrial Ecosystem Research (BITO ¨ K), University of Bayreuth, D-95440 Bayreuth, Germany Received 18 October 2004; received in revised form 18 February 2005; accepted 29 March 2005 Abstract Litter is one of the main sources of dissolved organic carbon (DOC) in forest soils and litter decomposition is an important control of carbon storage and DOC dynamics. The aim of our study was to evaluate (i) effects of tree species on DOC production and (ii) relationships between litter decomposition and the amount and quality of DOC. Five different types of leaves and needles were exposed in litterbags at two neighboring forest sites. Within 12 months we sampled the litterbags five times and leached aliquots of field moist litter in the laboratory. In the collected litter percolates we measured DOC concentrations and recorded UV and fluorescence spectra in order to estimate the aromaticity and complexity of the organic molecules. Furthermore, we investigated the biodegradability of DOC from fresh and decomposed litter during 6 weeks incubations. Fresh sycamore maple litter released the largest amounts of DOC reaching about 6.2% of litter C after applying precipitation of 94 mm. We leached 3.9, 1.6, 1.0 and 3.3% carbon from fresh mountain ash, beech, spruce and pine litter, respectively. In the initial phase of litter decomposition significantly decreasing DOC amounts were released with increasing litter mass loss. However, after mass loss exceeds 20% DOC production from needle litter tended to increase. UV and fluorescence spectra of percolates from pine and spruce litter indicated an increasing degree of aromaticity and complexity with increasing mass loss as often described for decomposing litter. However, for deciduous litter the relationship was less obvious. We assume that during litter decomposition the source of produced DOC in coniferous litter tended toward a larger contribution from lignin-derived compounds. Biodegradability of DOC from fresh litter was very high, ranging from 30 to 95% mineralized C. DOC from degraded litter was on average 34% less mineralizable than DOC from fresh litter. Taking into account the large DOC production from decomposed needles we can assume there is an important role for DOC in the accumulation of organic matter in soils during litter decomposition particularly in coniferous forests. q 2005 Elsevier Ltd. All rights reserved. Keywords: Dissolved organic carbon; Litter decomposition; Biodegradability; Fluorescence spectroscopy 1. Introduction Foliar litter is the main input of organic carbon into forest soils and litter decomposition is one of the most important processes determining the amount of organic C remaining in the forest floor (Berg et al., 2001). During litter decompo- sition parts are leached from the litter layer and percolate as dissolved organic carbon (DOC) into the mineral soil (Qualls and Haines, 1991; Kalbitz et al., 2000). In some studies, the litter horizon is considered to be the most important source of DOC in soils (Qualls and Haines, 1991; Michalzik and Matzner, 1999). During litter decomposition the specific surface area and the permeability of the litter will increases, leading to increased DOC leaching. DOC plays an important role in transport, mineralisation and stabilization of C in soils (Kalbitz et al., 2000). There is a lack of knowledge concerning the effect of different tree species on the amount and quality of leached DOC from litter. In the laboratory, Kuiters and Mulder (1993) found the highest amounts of DOC leached from deciduous litter, whereas Strobel et al. (2001) extracted highest DOC amounts from spruce forest floor compared to beech, oak and grand fir. Michalzik et al. (2001) did not find a general trend comparing DOC concentrations and fluxes of coniferous and deciduous temperate forests. Besides tree species, the decomposition degree of litter should affect the amount and quality of DOC. Fresh litter releases highly Soil Biology & Biochemistry 37 (2005) 2171–2179 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.03.019 * Corresponding author. Address: Max-Planck-Institute for Biogeochem- istry, P.O. Box 10 01 64, D-07701 Jena, Germany. Tel.: C49 3641 576184; fax: C49 3641 577863. E-mail address: [email protected] (A. Don).

Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

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Page 1: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Amounts and degradability of dissolved organic carbon

from foliar litter at different decomposition stages

Axel Don*, Karsten Kalbitz

Department of Soil Ecology, Bayreuth Institute for Terrestrial Ecosystem Research (BITOK), University of Bayreuth, D-95440 Bayreuth, Germany

Received 18 October 2004; received in revised form 18 February 2005; accepted 29 March 2005

Abstract

Litter is one of the main sources of dissolved organic carbon (DOC) in forest soils and litter decomposition is an important control of

carbon storage and DOC dynamics. The aim of our study was to evaluate (i) effects of tree species on DOC production and (ii) relationships

between litter decomposition and the amount and quality of DOC. Five different types of leaves and needles were exposed in litterbags at two

neighboring forest sites. Within 12 months we sampled the litterbags five times and leached aliquots of field moist litter in the laboratory. In

the collected litter percolates we measured DOC concentrations and recorded UV and fluorescence spectra in order to estimate the

aromaticity and complexity of the organic molecules. Furthermore, we investigated the biodegradability of DOC from fresh and decomposed

litter during 6 weeks incubations. Fresh sycamore maple litter released the largest amounts of DOC reaching about 6.2% of litter C after

applying precipitation of 94 mm. We leached 3.9, 1.6, 1.0 and 3.3% carbon from fresh mountain ash, beech, spruce and pine litter,

respectively. In the initial phase of litter decomposition significantly decreasing DOC amounts were released with increasing litter mass loss.

However, after mass loss exceeds 20% DOC production from needle litter tended to increase. UV and fluorescence spectra of percolates from

pine and spruce litter indicated an increasing degree of aromaticity and complexity with increasing mass loss as often described for

decomposing litter. However, for deciduous litter the relationship was less obvious. We assume that during litter decomposition the source of

produced DOC in coniferous litter tended toward a larger contribution from lignin-derived compounds. Biodegradability of DOC from fresh

litter was very high, ranging from 30 to 95% mineralized C. DOC from degraded litter was on average 34% less mineralizable than DOC

from fresh litter. Taking into account the large DOC production from decomposed needles we can assume there is an important role for DOC

in the accumulation of organic matter in soils during litter decomposition particularly in coniferous forests.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Dissolved organic carbon; Litter decomposition; Biodegradability; Fluorescence spectroscopy

1. Introduction

Foliar litter is the main input of organic carbon into forest

soils and litter decomposition is one of the most important

processes determining the amount of organic C remaining in

the forest floor (Berg et al., 2001). During litter decompo-

sition parts are leached from the litter layer and percolate as

dissolved organic carbon (DOC) into the mineral soil

(Qualls and Haines, 1991; Kalbitz et al., 2000). In some

studies, the litter horizon is considered to be the most

0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.soilbio.2005.03.019

* Corresponding author. Address: Max-Planck-Institute for Biogeochem-

istry, P.O. Box 10 01 64, D-07701 Jena, Germany. Tel.: C49 3641 576184;

fax: C49 3641 577863.

E-mail address: [email protected] (A. Don).

important source of DOC in soils (Qualls and Haines, 1991;

Michalzik and Matzner, 1999). During litter decomposition

the specific surface area and the permeability of the litter

will increases, leading to increased DOC leaching. DOC

plays an important role in transport, mineralisation and

stabilization of C in soils (Kalbitz et al., 2000).

There is a lack of knowledge concerning the effect of

different tree species on the amount and quality of leached

DOC from litter. In the laboratory, Kuiters and Mulder

(1993) found the highest amounts of DOC leached from

deciduous litter, whereas Strobel et al. (2001) extracted

highest DOC amounts from spruce forest floor compared to

beech, oak and grand fir. Michalzik et al. (2001) did not find

a general trend comparing DOC concentrations and fluxes

of coniferous and deciduous temperate forests. Besides tree

species, the decomposition degree of litter should affect the

amount and quality of DOC. Fresh litter releases highly

Soil Biology & Biochemistry 37 (2005) 2171–2179

www.elsevier.com/locate/soilbio

Page 2: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–21792172

soluble and readily degradable organic compounds (Norden

and Berg, 1990). In turn, the relative enrichment of lignin-

derived compounds during litter decomposition (Norden

and Berg, 1990; Baldock et al., 1992) should result in DOC

of a more aromatic character. These expected changes in

DOC composition should affect its microbial stability

because lignin-derived compounds are the stable ones

(Kalbitz et al., 2003a). However, effects of litter decompo-

sition on DOC composition have not yet been studied

because soil water sampled in situ contains a mixture of

DOC derived from organic matter with different decompo-

sition stages. Therefore, the effect of the litter decompo-

sition on DOC could not be distinguished.

In our study we combined field and laboratory-experi-

ments. Litter was decomposed in-situ in litterbags, and was

leached in the laboratory to release DOC. The aim of our

study was to examine the effect of decomposition of five

different types of litter on the amount and spectroscopic

properties of DOC and its microbial degradability.

2. Material and methods

2.1. Field-exposure of litter

Air-dried leaves and needles from sycamore maple (Acer

pseudoplatanus L.), European beech (Fagus sylvatica L.),

mountain ash (Sorbus aucuparia L.), Norway spruce (Picea

abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) were

incubated in litterbags at two neighboring forest sites for up

to 12 months. The sites were located at Coulissenhieb in the

Fichtelgebirge mountains in North-East Bavaria (775 m

a.s.l., 50808 035 00N, 11852 010 00E). One site was a 160 y old

spruce stand, the other site a neighboring 3 y old clear-

cutting (Kalbitz et al., 2004). The climate in this area is

characterized by high precipitation (1100 mm yrK1), an

annual mean temperature of around 5 8C and a persistent

snow cover during winter season (site descriptions are

available by Gerstberger et al., 2004). Senescent litter

samples collected by meshes were derived from trees

adjacent to the study site except for the pine litter. Pine litter

originated from the nutrient poor Jardaas-site in central

Sweden (site descriptions by Axelsson and Brakenhielm,

1980). In previous studies on litter decomposition the same

pine litter was used as model litter (Couteaux et al., 1998;

Berg, 2000). We did not cut or grind the litter samples

except for maple leaves, which were cut into small pieces

(2–4!2–4 cm2) to fit into the litter bags.

Each litterbag (mesh size 0.5!1 mm2) contained two

separate parts. The first part was filled with 2 g of litter from

one tree species to determine the mass loss and the second

part, with 10 g for DOC-extraction. The bias of the

decomposition process due to the exclusion of soil fauna

in litterbags can be expected to be relatively small because

the decomposing community of acidic coniferous ecosys-

tems, like the Coulissenhieb site consists mainly of

microorganisms, generally lacking a meso- and macrofauna

(e.g. Wallwork, 1976).

In total 24 spatial replications (12 at each site) were

installed to cope with the high variability of the decompo-

sition process. The field-exposure started in June 2001. After

1, 3, 5, 9 and 12 months 120 litterbags were always harvested

(five litter species in 12 replications at both sites) and the

litter was cleaned manually to remove fungi hyphae, roots,

sprouts and small animals. The mass loss of the litter was

determined as the difference between initial and actual dry

weight (48 h drying at 80 8C). The water content of the litter

was determined as difference between field moist and dried

litter weight. Four samples of the 12 replicates with similar

mass loss were mixed each time to three composite samples

for each litter species. Thus, the variability of the

mineralisation process is maintained for further investigation

and not evened out as it is done with randomized pooling.

Part of the litter samples were ground and the C- and

N-content was determined (CHN-O Rapid, Heraeus Ele-

mentar). Additionally, the content of heavy metals and

macro-nutrients was measured in the fresh undecomposed

litter (hot HNO3-extraction, ICP-AES, GCP Electronics).

2.2. DOC extraction and characterization

A sub sample of 5.5 g field moist litter (dry matter base)

was exposed to an extraction scheme with ultrapure water at

5 8C, the mean annual temperature of the study site. Content

and properties of DOC are affected by season with mostly

larger concentrations in the growing season (Kalbitz et al.,

2000, 2004). Furthermore, after dry periods DOC concen-

trations are increasing with increasing portions of microbial

products (Zsolnay et al., 1999; Kaiser et al., 2001). We

wanted to reveal effects of litter decomposition and tree

species but we were not interested in seasonal effects on

DOC concentrations and composition. Therefore, we

considered our first extraction step as a pre-extraction in

order to equalize seasonal differences at least in the water

content of the samples. For this pre-extraction the litter

samples were soaked in 250 ml ultra pure water in beakers

for 24 h. Thereafter, the litter samples were spread out

evenly in Buchner funnels (12.8 cm dia.). The solution

phase of the pre-extraction was obtained and analyzed as

described below. After allowing the litter 7 d to acclimatize

at 5 8C, it was extracted 3 d via sprinkler irrigation. During

each of these 3 d, 320 ml of ultra pure water was sprayed

over the sample using a time of 20 h. The solutions were

filtered through a pre-washed 0.45 mm cellulose acetate

membrane filter and the DOC concentration (high TOC

Elementar) was measured. The remainders of samples from

the first irrigation day were immediately frozen for further

analyses of spectroscopic properties and biodegradability.

We considered that concentrations and composition of DOC

in the first percolates after the pre-extraction and the 1-week

conditioning period represented best the effect of different

litter species and stages of litter decomposition with minor

Page 3: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Table 1

Total extractable amounts of DOC (mg g CK1) cumulative from four extraction steps (94 mm) after 0 to 12 months of field incubation of the litter in a forest

stand (s) and a clear-cut site (c)

Fresh 1 month 3 months 5 months 9 months 12 months

Maple (s) 62.4 (4.34) 15.3 (0.40) 11.5 (1.69) 10.7 (1.35) 6.5 (0.22) 6.8 (1.42)

Maple (c) 28.3 (2.46) 17.9 (2.43) 11.6 (0.82) 7.1 (1.25) 10.1 (1.11)

Mountain ash (s) 39.3 (2.45) 13.5 (1.46) 6.2 (0.97) 5.0 (0.56) 4.8 (0.71) 5.2 (0.85)

Mountain ash (c) 19.0 (5.34) 9.5 (1.00) 4.5 (0.33) 4.4 (0.12) 4.6 (0.30)

Europ. beech (s) 16.0 (0.07) 4.4 (0.21) 3.1 (0.49) 2.7 (0.57) 4.2 (0.56) 4.1 (0.46)

Europ. beech (c) 9.0 (0.94) 6.2 (1.14) 4.3 (0.32) 3.5 (0.76) 5.4 (0.82)

Norway spruce (s) 9.5 (0.21) 6.2 (0.75) 3.6 (0.18) 5.5 (1.53) 5.9 (0.63) 11.2 (5.17)

Norway spruce (c) 4.8 (0.45) 5.0 (0.55) 4.7 (0.75) 2.6 (0.42) 5.9 (3.21)

Scots pine (s) 33.3 (0.45) 8.0 (0.64) 3.7 (0.41) 4.4 (1.40) 5.5 (0.47) 9.4 (4.07)

Scots pine (c) 10.1 (0.63) 4.2 (0.26) 4.3 (1.11) 3.8 (0.89) 3.3 (0.51)

Standard deviation in brackets; nZ3.

A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–2179 2173

reflection of seasonal effects. However, we also calculated

the cumulative amounts of released DOC including pre-

extraction and the three irrigation steps (Table 1). The

cumulative amount of water (1210 ml) was equivalent to

94 mm precipitation.

Spectroscopic properties of DOC as UV radiation

absorbance and fluorescence spectra are powerful tools to

characterize high numbers of samples (Kalbitz et al., 1999).

UV absorbance is a successful predictor for the aromaticity

of DOC (Chin et al., 1994; Kalbitz et al., 2003b) whereas

fluorescence spectra give some additional information about

the complexity and condensation of the organic molecules

(Zsolnay et al., 1999; McKnight et al., 2001). For each of

these samples UV absorbance at lZ280 nm (UVIKON 930,

BIO-TEK Instrument) and fluorescence spectra using

emission mode were recorded (BIO-TEK Instrument;

Kalbitz et al., 2003a,b). Humification indices were deduced

from the fluorescence spectra as an indicator of the

complexity of the molecules. The humification index

deduced from emission spectra (HIX) is defined as the

area in the upper quarter (S 435–480 nm) of the usable

emission spectra divided by the area in the lower usable

quarter (S 300–345 nm) (Zsolnay et al., 1999). Measure-

ments and calculations followed the methods described by

Zsolnay et al. (1999), Kalbitz and Geyer (2001).

2.3. Incubation of DOC to measure biodegradability

To quantify the potential biodegradability of DOC,

samples extracted from all five species of fresh and

decomposed litter from the forest stand site (5 months of

field exposure, samples from the first irrigation) were

incubated in 120 ml sealed flasks at 20 8C for 6 weeks with

six to nine replications. The samples with higher concen-

trations than 10 mg C lK1 were diluted to 10 mg C lK1 to

avoid excessive microbial growth (Hongve et al., 2000) and

effects of different concentrations on DOC mineralization

(Zsolnay, 2003). The C/N/P ratio was adjusted to 100/10/1

by adding NH4NO3 and K2HPO4, which was close to the

nutrient concentrations measured in soil solution at the

given site (Kalbitz, data unpublished).

To facilitate equivalent start conditions for the microbial

community within the litter percolates, a mixed inoculum

was added to every flask. We used 0.5 g of litter from each

litter species, added 6–7 ml of ultrapure water and

aerobically incubated at 20 8C for 12 d to reactivate the

microorganisms. Thereafter the litter was extracted twice

with a 4 mM CaCl2 solution (litter: solution 1:5) and passed

through a 5 mm membrane filter to remove grazing

microfauna (Jandl and Sollins, 1997). The inoculum was

added to the sample in a sample/inoculum volume ratio of

100:1. The chosen method aimed to quantify the potential

biodegradable DOC. The mineralization dynamic was

quantified by daily CO2 measurements in the head space

during the first week, and afterwards at increasing intervals

(up to once per week). The CO2 concentration was

measured with a gas chromatography system (HP 6890,

Hewlett Packard, thermal conductivity detector). The CO2

evolution by mineralization of the inoculum C was

determined in a control sample (ultra pure water, inoculum

and nutrients) and subtracted from the other samples.

A double exponential model was fitted to the data sets

estimating an easily degradable (labile) and a more stable

pool (Kalbitz et al., 2003b; Wang et al., 2004):

mineralized Cð% of total CÞ

Z a � ð1 KeKk1�tÞC ð100 KaÞ � ð1 KeKk2�tÞ

aZsize of the labile pool [% of total C], k1, k2Zmineralization rate of the labile and stable pool.

Half life (t1/2) and the size of the labile and stable pool

were calculated. Biodegradability of the samples was

defined as C-mineralization after 6 weeks of incubation.

2.4. Data treatment and statistics

Effects of litter species on litter decomposition were

tested to be significant for each sampling date with ANOVA

procedure and the Fisher-LSD test (P!0.05). For these tests

all of the 12 replicates per site, litter species and sampling

day were used. Differences between the two sites were

tested to be significant using t-test (P!0.05). Properties of

Page 4: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Table 2

Mass loss (%) of five litter species at five sampling dates incubated in a forest stand (s) and an adjacent clear-cut site (c)

Time of harvesting Summer 2001 Autumn 2001 Autumn 2001 Spring 2002 Summer 2002

Months after start of

the experiment

1 month 3 months 5 months 9 months 12 months

Maple (s) 5.6 (1.5) 16.4 (2.9) 23.5 (2.8) 25.2 (3.9) 28.9 (6.1)

Maple (c) 2.7 (1.4) 11.5 (2.4) 19.9 (2.5) 22.7 (3.4) 29.9 (7.4)

Mountain ash (s) 12.2 (4.3) 29.3 (4.4) 37.1 (3.8) 36.2 (4.4) 40.3 (2.5)

Mountain ash (c) 9.0 (4.5) 26.1 (3.9) 35.0 (4.3) 36.4 (3.6) 41.2 (5.2)

Europ. beech (s) K0.1 (1.4) 2.5 (1.0) 8.6 (1.1) 8.6 (2.5) 5.1 (2.0)

Europ. beech (c) K0.1 (0.9) 5.2 (2.6) 10.1 (1.7) 10.6 (1.5) 11.5 (3.6)

Norway spruce (s) 5.8 (1.1) 15.0 (1.6) 22.3 (2.0) 23.2 (1.4) 29.3 (2.9)

Norway spruce (c) 3.7 (0.6) 9.9 (1.1) 18.0 (1.5) 22.5 (6.2) 23.0 (2.0)

Scots pine (s) 4.8 (1.0) 13.6 (1.7) 22.7 (6.1) 25.0 (3.9) 28.9 (3.6)

Scots pine (c) 2.5 (0.7) 8.5 (0.9) 19.5 22.5 25.1 (6.7)

Standard deviation in brackets; nZ12.

A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–21792174

dissolved organic matter (UV, humification indices, biode-

gradability) were tested to be statistically different as

described above using the six replicates (both sites) for each

litter species and sampling day (P!0.05). Furthermore, we

calculated linear regressions between different DOC proper-

ties and the mass loss of the litter.

Fig. 1. Extractable DOC amounts of maple and pine litter after 3 months of

field incubation. Four extraction steps-one pre-extraction with 250 ml

(batch, 19 mm) and three irrigation with 25 mm deionised water; nZ6

(litter from both sites).

3. Results

3.1. Litter decomposition

After 12 months of litter decomposition mountain ash

had significantly the largest mass loss of all litter types

(40.8G4.0%, mean and standard deviation of all replicates

from both sites; Table 2). At the other end of the scale beech

litter lost only 8.3%G4.3% of its mass. Coniferous litter

from pine, spruce and maple leaves had a mass loss of

between 26.2 and 29.4% with no statistically significant

differences between these three species. There was a slightly

decreased decomposition rate during the winter season

compared to the summer season for all litter species.

The forest site where the litter was exposed (forest stand

vs. clear-cut site) partly affected the rate of litter decompo-

sition. Spruce litter in the forest stand showed significant

higher mass loss at four out of five sampling dates. Pine and

maple litter decomposition was also faster (P!0.05) in the

forest stand during the first 3 and first 5 months, respectively.

The decomposition of these litter species seemed to decline at

the clear-cut site especially during summer. In contrast beech

litter degraded generally faster at the clear-cut site (three out

of five sampling dates were significant) but with an overall

very low mass loss (Table 2). There was no significant

difference between the two field sites in the mass loss of

mountain ash leaves at any time of sampling.

3.2. Leaching of DOC from litter

DOC concentrations were largest in solutions obtained by

the pre-extraction and declined during the three irrigation steps

(Fig. 1). However, DOC amounts of the pre-extraction and the

three irrigation steps were obtained in similar proportions to

each other for different sampling dates. Therefore, we have to

state that the applied pre-extraction was not necessary or not

fully successful to equalize for seasonal effects on DOC

release. That also means, it does not matter whether we are

using DOC concentrations from one of the extractions or the

cumulative amounts of released DOC to discuss effects of litter

decomposition on DOC dynamics. For future studies we

recommend to skip the complicated extraction and irrigation

scheme and use just the single batch extraction.

Fresh leaves released the largest amounts of DOC with

the highest values for maple and mountain ash litter

reaching 62.4 and 39.3 mg g CK1, respectively (Table 1).

DOC release decreased during the first months of litter

decomposition for all five species. However, DOC

especially from needles tended to increase reaching up to

11.2 mg g CK1 for spruce after exceeding a mass loss of

20% which was even larger than DOC release from fresh

spruce litter. Extractable DOC from spruce and pine almost

doubled from the fourth to the fifth sampling dates. In

contrast, DOC release barely altered with increasing

decomposition of maple and mountain ash litter.

After 5 months of field exposure, DOC release from needles

Page 5: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Fig. 2. Humification indices deduced from fluorescence emission spectra of coniferous (A) and deciduous (B) litter percolates from the first irrigation step.

DOC extracts from fresh litter and five sampling dates. No significant trend for mountain ash and beech.

A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–2179 2175

in the forest site was more than from the clear-cut site.

Concurrently, needle litter in the forest stand had a larger

mass loss than in the clear-cut site (Table 2). In contrast to

needle litter, the generally larger extracted amounts of DOC

from leaves incubated at the clear-cut site compared to the

forest stand were not accompanied by larger mass losses.

3.3. Spectroscopic properties of DOC

Our experiments showed that specific UV absorbance at

lZ280 nm and humification indices (HIX) deduced from

fluorescence emission spectra of litter percolates were

distinctive for tree species and the decomposition stage of

the litter. UV absorbance and HIX showed on average the

same tendencies; therefore, only HIX data are presented.

They indicated the largest aromaticity and complexity of

DOC from maple litter. At the other end of the scale very

small values were measured for DOC from pine litter. This

was especially the case for fresh needles (Fig. 2).

Fig. 3. Mineralisation dynamic of DOC from fresh litter measured as CO2-product

species were incubated in three replicates.

For percolates of all litter species we measured

increasing values of the spectroscopic properties with

increasing decomposition time. Percolates from the two

coniferous species showed especially steadily increasing

UV absorbance and HIX with increasing mass loss of litter

or decomposition time (Fig. 2). For mountain ash percolates

there was no significant trend toward higher HIX with

increasing mass loss of the litter. Due to low mass loss of

beech litter the correlation between the HIX and the mass

loss was also not significant.

3.4. DOC mineralisation

As observed for spectroscopic properties, the biodegrad-

ability of the litter percolates depended strongly on litter

species. DOC from the fresh coniferous samples showed the

highest biodegradability. About 95% DOC of pine litter and

77% of spruce litter was mineralized within 42 d. In contrast

only 33% of maple DOC was converted to CO2 (Fig. 3).

ion fitted with a double exponential model. Three percolates from each litter

Page 6: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Table 3

Quantitative measures for the mineralization of DOC leached from fresh and decomposed litter (5 months litter decomposition) after 42 days incubation

Biodegrad abilitya (%) Labile fractionb (%) Labile pool t1/2c (h) Stable pool t1/2

d (h) r2 e (%)

DOC extracted from fresh litter

Maple 33 10 75 2288 98.5

Mountain ash 73 48 83 1068 98.7

Beech 54 25 128 1392 99.6

Norway spruce 77 31 97 634 99.3

Scots pine 95 33 36 260 95.5

Mean values 67 29 84 1128 98.3

DOC extracted from decomposed litter

Maple 24 5 30 3180 99.2

Mountain ash 65 52 14 2159 99.6

Beech 43 28 43 3122 98.9

Norway spruce 31 3 21 2063 98.8

Scots pine 50 3 18 1060 98.6

Mean values 43 18 22 1979 99.0

Three percolates from each litter species were each incubated in three replicates.a Mineralized portion (biodegradability).b Size of the labile DOC pool.c Half-live of the labile DOC pool.d Half-live of the stable DOC pool.e Coefficient of determination of the double exponential model.

A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–21792176

The dynamic of DOC mineralization in most samples was

well fitted by a double exponential model (Table 3). The

calculated stable pool varied between 52 and 90% with the

highest value for DOC from maple litter. Half-lives of the

stable pool ranged between 11 and 95 d whereas half-lives

of the labile pool were very short, between 1.5 and 5.3 d.

Five months of litter decomposition significantly affected

the mineralization of the extracted DOC. In general, DOC

from decomposed litter was 34% less degradable than DOC

extracted from fresh litter (see Table 3). The biodegradability

of DOC from coniferous litter especially decreased above-

average from 95 to 50% (pine) and from 77 to 31% (spruce).

Both showed an almost linear degradation dynamic with a

small labile fraction of only 3% of total DOC (calculated

from the double exponential model). The strongest decrease

in biodegradability of DOC from coniferous litter coincided

with largely increasing values of spectroscopic properties

(see above). Furthermore, DOC mineralization correlated

highly to the measured spectroscopic properties (Fig. 4).

Thus, high values of the spectroscopic properties correspond

to low DOC degradability.

Fig. 4. Correlation between humification index deduced from fluorescence

emission spectra with degradability of litter DOC. DOC from fresh litter

(RZ0.96) and DOC from litter after five months of decomposition (RZ0.83).

4. Discussion

4.1. Litter decomposition

To understand the dynamics of DOC extracted from litter

it is important to consider the decomposition status of the

litter species. However, it is beyond the scope of this study

to reveal and quantify controlling factors of litter decompo-

sition. We related the C/N ratios and contents of nutrients

and heavy metals of the fresh litter to the observed mass

loss. The C/N value is expected to have great explanatory

power for litter decomposition with larger mass losses at

low C/N values, at least in the initial decomposition phase

(Ellenberg, 1986; Berg and Matzner, 1997). However, in

our study we could not confirm this. None of the measured

chemical variables such as C/N ratio, nutrient and heavy

metal content explained the differences in mass loss among

the litter species. In turn, physical conditions such as the

water content of the litter seem to determine how fast litter

decomposes (Chadwick et al., 1998). The variability of

mass loss among the five litter species correlated well with

the water content of the samples (Fig. 5). Especially during

the summer season, when water becomes a limiting factor

for microbial activity, the water content of the litter

samples showed a significant correlation with their mass

loss (RZ0.54 in Autumn 2001 after 3 months of field

exposure; Fig. 5). This confirms the importance of moisture

for decomposition and for making organic compound

available to microorganisms through dissolution.

Page 7: Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages

Fig. 5. Correlation between water content of the litter and litter mass loss

after 3 months of field incubation (Autumn 2001) for all five litter species;

nZ120.

A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–2179 2177

There was a tendency of lower mass loss of the litter at

the clear-cut site compared to the forest stand for all litter

species, except beech litter. The absence of a forest canopy

allows the litter to dry more frequently, which slows down

decomposition (Prescott et al., 2000).

4.2. Effect of litter decomposition on DOC leaching

Litter is considered to be an important source of soil

DOC (Kalbitz et al., 2000). Furthermore, litter decompo-

sition and DOC release are mainly biotic processes

controlled by microorganisms (e.g. Salamanca et al.,

2003). Therefore, it can be assumed that both processes

are closely related.

After large DOC release from fresh litter, DOC

production decreased significantly during the initial phase

of litter decomposition. As in our study, Magill and Aber

(2000) found that maple litter released a high pulse of DOC

from fresh litter. Hongve et al. (2000) measured rapidly

declining leaching rates from the litter layer of deciduous

forests after high fluxes in autumn. Obviously, the pool of

highly soluble organic compounds in fresh litter, respon-

sible for its large DOC release, was exhausted after a few

months. The size of this pool can reach 39% of litter

C (Qualls and Haines, 1991; Magill and Aber, 2000). We

extracted with three irrigation steps and one pre-extraction

6.2% litter C at maximum (Table 1). Higher extractable

C-amounts reported by other studies may be due to the

cutting of the leaves into small pieces prior to extraction

and high extraction temperatures. We used the mean

annual temperature of our field site (5 8C) for DOC

extraction in the laboratory in order to mimic field

conditions with respect to amount and composition of

leached DOC.

After exceeding a certain decomposition stage DOC

production tended to increase especially from coniferous

litter. This may be explained by the structure of the tissue

and the chemical composition of needles as compared to

leaves. While deciduous litter has only a thin epidermic

layer needles have a thick epidermic and hypodermic layer

which may protect parts of the tissue from being broken

and leached (Esau, 1965). This could be the reason why

leaves often release more DOC than needles (Kuiters and

Mulder, 1993). However, already after 12 months of litter

decomposition needles released more DOC than leaves.

This partly explains higher DOC fluxes in coniferous forests

compared to deciduous ones (Currie and Aber, 1997). Our

results emphasize that decomposed litter is an important

source of DOC. Thus, different decomposition stages should

be taken into account while interpreting litter as a source of

DOC.

The comparison of the two different sites further

emphasizes the observed difference between leaves and

needles. The larger mass loss of needles at the forest

stand in comparison to the clear-cutting resulted in

increasing extractable amounts of DOC with increasing

litter mass loss. Generally however, larger extracted

amounts of DOC from leaves incubated at the clear-cut

site compared to the forest stand were not accompanied

by larger mass losses. Thus, decomposition of needles

seems to promote DOC release whereas decomposition of

leaves does not. The only explanation we have might be

the different morphology of leaves and needles as

discussed above.

Additionally, there seems to be a seasonal pattern

superimposed which is connected with accelerated litter

decomposition pattern and in some cases increased DOC

release. It cannot be separated from the decomposition

dynamic itself. These seasonal effects comprise dry–wet and

freeze–thaw cycles both are known to accelerate DOC

release from soils (Kalbitz et al., 2000). Precipitation

patterns before harvesting the litterbags did not exert a

significant effect on the amount of extractable DOC.

4.3. Litter decomposition and properties and degradability

of DOC

Amounts and properties of DOC derived from litter are

affected by microbial litter decomposition in different ways.

High activity of the decomposing community may decrease

DOC due to respiratory losses of DOC as CO2 (Moore and

Dalva, 2001). This should be particularly pronounced

during the initial phase of litter decomposition. Carbo-

hydrates as a main component of soluble litter material

(Norden and Berg, 1990) are readily available for

mineralization and often have only short half-lives. They

contribute mainly to CO2 generations and less to DOC

production, particularly in decomposing litter (Park et al.,

2002). On the other hand, decomposition of litter may lead

to additional extractable amounts of DOC due to disruption

of cell walls and other membranes preventing DOC

leaching. Furthermore, other components of litter as

cellulose or lignin are oxidized by microorganisms and

contribute to DOC release. Therefore, increased contri-

bution of lignin-derived compounds to DOC at later stages

of litter decomposition can be assumed (Kalbitz et al.,

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A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–21792178

2004). However, dead microbial cells and soluble microbial

products may additionally enlarge the DOC pool which

differ very much in their properties compared to the lignin-

derived ones (Zsolnay, 2003).

For DOC from spruce and pine needles we measured

increasing humification indices and UV absorbance as the

needles decomposed. Our findings indicate a significant

change of litter percolate quality toward larger complexity

and aromaticity of the organic compounds with increased

decomposition of the litter. After 12 months of field

exposure their mean UV absorbance were two and three

times as high as at the beginning of the decomposition

process. This cannot be explained by a relative accumu-

lation of recalcitrant organic matter with a high aromaticity

but with changed source of DOC within the litter itself. It

seems that at later stages of litter decomposition lignin-

derived compounds (larger UV absorbance and humifica-

tion indices) contribute more to DOC release than at the

initial phase. This is confirmed by decreased degradability

of DOC after litter decomposition because lignin-derived

DOC components are the more stable ones (Kalbitz et al.,

2003a). This increase was especially pronounced in

coniferous litter where UV absorbance and humification

indices increased concurrently, the amount of leached DOC

also re-increased after exceeding a mass loss of 20% (Fig. 2,

Table 1).

In contrast to DOC from coniferous litter, we did not

always find a continuous increase in aromatic and

complex compounds for DOC from deciduous litter.

The thin epidermis of leaves might not only be

responsible for the absence of an increase in DOC

production but also for unchanged DOC properties. The

measured spectroscopic properties indicated that the pool

of organic matter involved in DOC production did not

change during decomposition of leaf litter as observed

for needles.

The potential biodegradability of DOC in the different

litter species could be adequately estimated using spectro-

scopic properties like the humification index deduced from

fluorescence emission spectra (Fig. 4). DOC from fresh and

decomposed maple litter was by far the least biodegradable

and concurrently had the highest aromaticity and complex-

ity of the molecules. The strong correlation between the

humification index derived from fluorescence emission

spectra and DOC mineralisation supports findings from

Kalbitz et al. (2003b) and illustrates the suitability of these

spectroscopic methods (UV, fluorescence) to estimate DOC

biodegradability.

Beside physical protection, intrinsic chemical charac-

teristics of the organic matter determine its stability in

soils. A high production of DOC with high stability may

lead to a relevant C input into the mineral soil. We found

with coniferous litter in particular that increasing amounts

of DOC were released as decomposition progressed.

Additionally, spectroscopic properties and the incubation

experiments showed an increasing stability of the DOC

from decomposed needles (Fig. 2, Table 3). The turnover

rates of the stable pool decreased by 75% from fresh to

decomposed litter percolates and the pool size increased.

However, we found higher turnover rates of the labile

pool of percolates from decomposed litter compared to

fresh litter. Turnover rates calculated from the double

exponential model should be elucidated together with the

pool size. The pool sizes and their turnover rates are

reciprocally related (Wang et al., 2004) leading to per se

higher turnover rates of the labile pool by small pool

sizes. In general, the smaller size of the labile DOC pool

from decomposed litter indicated a larger stability of

the whole DOC despite larger turnover rates of the

labile pool.

Litter percolates with high rates of DOC mineraliz-

ation did not generally derive from litter with high

decomposition rates (mass loss). DOC from maple leaves

mineralized relatively slowly but the leaves themselves

decompose in intermediate rates compared to other litter

types. In turn, DOC from pine litter was with high rates

of DOC mineralisation but the litter itself showed only

intermediate mass loss. Although litter decomposition

and DOC dynamic are related processes, these differ-

ences emphasize that the stability of leached DOC only

partly explains the complex decomposition process of

litter.

4.4. Conclusions

Leaching of DOC from litter is affected by litter

decomposition. Large amounts of DOC can be released

from both fresh and highly decomposed litter. The quantity

and especially the quality of released DOC depend on the

litter species. DOC from degraded litter was on average

34% less mineralizable than DOC from fresh litter.

Therefore, the importance of DOC for the C accumulation

in the mineral soil should increase during litter decompo-

sition. This is particularly true for species with an

increasing release of DOC at later stages of litter

decomposition as observed for coniferous litter. Thus,

while studying C dynamics of forests it seems to be

important to take into account the influence of the tree

species and the degradation stages of litter on the DOC

dynamic.

Acknowledgements

We would like to thank Bjorn Berg for his helpful

advice and the members of the Central Analytical

Department of BITOK for analytical support. This study

was funded by German Ministry of Education and

Research (BMBF).

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A. Don, K. Kalbitz / Soil Biology & Biochemistry 37 (2005) 2171–2179 2179

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