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