Decomposit...rom Scots pine black cherry

ORIGINAL PAPER
Klaus Lorenz Æ Caroline M. Preston Æ Susan Krumrei
Karl-Heinz Feger
Decomposition of needle/leaf litter from Scots pine, black cherry,
common oak and European beech at a conurbation forest site
Received: 5 February 2004 / Accepted: 7 May 2004 / Published online: 3 July 2004
Ó Springer-Verlag 2004
Abstract Litter decomposition was studied for 2 years in
a mixed forest serving as a water protection area (Rhine-
Neckar conurbation, SW Germany). Two experiments
differing in initial dry weight equivalent in litterbags
were set up: one to compare decomposition of European
beech leaves (Fagus sylvatica) with common oak leaves
(Quercus robur), and the other comparing decomposi-
tion of Scots pine needles (Pinus sylvestris) with black
cherry leaves (Prunus serotina Ehrh.), respectively. Mass
losses were greater for oak litter than for beech (75.0
versus 34.6%), and for cherry litter than for pine (94.6
versus 68.3%). In both experiments, a strong initial loss
of soluble compounds occurred. The changes in litter N
and P concentrations and the decrease in C-to-N ratio
coincided with changes in residual mass. However, nei-
ther tannin and phenolic concentrations nor NMR
could explain the pronounced variation in mass loss
after 2 years. Differences in litter palatability and
toughness, nutrient contents and other organic com-
pounds may be responsible for the considerable differ-
ences in residual mass between litter types. The fast
decay of black cherry leaves appears to play a major role
in the present humus dynamics at the studied site. Since
black cherry has a high N demand, which is mainly met
by root uptake from the forest floor, this species is
crucial for internal N cycling at this conurbation forest
site. These effects together may significantly contribute
to prevent nitrate leaching from the forest ecosystem
which is subject to a continuous N deposition on an
elevated level.
Keywords Litter quality Æ 13
C CPMAS NMR
spectroscopy Æ Nutrient cycling Æ Nitrogen retention
Introduction
Forest soils in conurbation regions can accumulate
nutrients (notably nitrogen, phosphorus) but also heavy
metals and other pollutants originating from atmo-
spheric deposition (Lovett et al. 2000; Pouyat et al. 2002;
Pouyat and McDonell 1991). Few studies have reported
effects of the urban environment on soils in adjacent
forests. Significantly affected were the soil fungal com-
munity, decomposers, litter decomposition rates, net
nitrification rates and soil carbon pools and fluxes
(Baxter et al. 2002; Goldman et al. 1995; Markkola et al.
2002; Pouyat and McDonell 1991; Pouyat et al. 1997;
Zhu and Carreiro 1999). Forests in conurbation areas
often play a prominent role for public drinking water
supply from groundwater (Dudley and Stolton 2003).
Hence, such forests require special attention in order to
protect related functions of the soil as filter, buffer, and
transformer. Notably the transformations of organic
matter in the top soil are crucial in preventing a release
of nitrate and other potential pollutants into the
hydrosphere.
Exotic plants are often introduced into conurbation
forests for ornamental reasons (Pouyat et al. 2002). For
example, during the first half of the 20th century, the
North American species black cherry (Prunus serotina
Ehrh.) was introduced to forests in Germany. There
were multiple motivations, but fire protection, insect
control and site amelioration were the most prominent
(cf. Starfinger 1990; Haag und Wilhelm 1998). However,
black cherry grows fast and shrubby, increasingly
K. Lorenz (&) Æ S. Krumrei
Institut fu¨ r Bodenkunde und Standortslehre,
Universita¨ t Hohenheim, Emil-Wolff-Str. 27,
70599 Stuttgart, Germany
E-mail: [email protected]
Tel.: +49-711-4593669
Fax: +49-711-4593117
C. M. Preston
Pacific Forestry Centre, Natural Resources Canada,
506 West Burnside Rd., Victoria,
BC, V8Z 1M5, Canada
S. Krumrei Æ K.-H. Feger
Institut fu¨ r Bodenkunde und Standortslehre,
Technische Universita¨ t Dresden, Pienner Str. 19,
01735 Tharandt, Germany
Eur J Forest Res (2004) 123: 177–188
DOI 10.1007/s10342-004-0025-7
hampering silvicultural operations, notably the
re-introduction of European beech and common oak
beneath the pine canopy.
On the positive side, cherry litter decomposes quickly,
thereby improving soil quality by promoting the decay
of mor humus layers and an increase in soil pH (von
Wendorff 1952). Furthermore, black cherry accumulates
nutrients, in particular N, K, Ca and Mg (Feger et al.
2003). Due its superiority in competition on poor sites
with sandy soils, Prunus serotina is now widely spread in
Germany with the main focus in the lowlands of N
Germany and the upper Rhine valley (SW Germany)
(Meersschaut et al. 1999; Starfinger 1990). Forestry
practice, however, regards black cherry as a weed that
should be removed to ensure growth and performance of
the dominant trees and to allow the re-introduction of
natural deciduous species (notably beech and oak) under
the canopy of conifers (cf. Haag and Wilhelm 1998). So
far there is no evidence demonstrating negative effects of
such removal measures.
The biodegradation of plant litter is greatly influ-
enced by its chemical characteristics (Almendros et al.
2000). Initial litter N and P contents are often positively
correlated with early decay rates (Berg 2000; Vesterdal
1999). Negative correlations between initial polyphenol
contents and rates of decomposition have been reported
(Loranger et al. 2002; Palm and Sanchez 1991). The rate
of nutrient cycling can also be reduced by tannins, which
amount up to 20% of the plant dry weight
(Ha¨ ttenschwiler and Vitousek 2000; Kraus et al. 2003b;
Northup et al. 1998; Preston 1999; Yu and Dahlgren
2000). Determination of C fractions in litter by
solid-state 13
C nuclear magnetic resonance (NMR)
spectroscopy has proven useful in characterizing litters
with respect to their potential to decompose and release
nutrients. In particular, the content of alkyl C (waxes
and cutin), as determined by NMR, increases during
decomposition. Cutin is highly resistant to decomposi-
tion and is related to physical toughness (Gallardo and
Merino 1993; Preston et al. 1997). Therefore the content
of alkyl C may be a useful indicator of litter decom-
posability (Baldock and Preston 1995). Identification of
litter characteristics that are consistently closely related
to decomposability has proven surprisingly difficult.
Across a broad range of litter types, the C-to-N ratio
appears to be the best predictor of decay rate (Enriquez
et al. 1993; Pe´ rez-Harguindeguy et al. 2000).
The litterbag method has been used frequently in
many decomposition studies (Swift et al. 1979). In
comparison with mini-container, cotton-strip, 15
N, 14
C,
13
C isotopes and bait-lamina methods, all relevant bio-
logical, microbiological, chemical and physical mea-
surement endpoints for the determination of organic
matter (OM) breakdown could be applied for the
litterbag method (Knacker et al. 2003). However, the
litterbag method has also several disadvantages; e.g. its
robustness has not been studied systematically, but the
method has been taken for granted by scientists.
Furthermore, a systematic approach to studying its
reproducibility has not yet been undertaken, and no
interlaboratory comparison or ring test has been carried
out. Other OM (e.g. roots, fungal hyphae) might enter
the litterbags during exposure in the field and hamper
the interpretation of changes in C, N, P and K con-
centrations. However, despite the disadvantages, the
litterbag method at present is the most appropriate
technique available to study OM breakdown in decom-
position studies under field conditions (Knacker et al.
2003).
We followed decomposition over 2 years in two lit-
terbag studies in a conurbation forest in Germany. One
experiment was set up with leaf litter from two decidu-
ous species, European beech (Fagus sylvatica) and
common oak (Quercus robur), and the second with
needle and leaf litter from Scots pine (Pinus sylvestris)
and black cherry (Prunus serotina Ehrh.), respectively. In
addition to changes in litter mass and concentrations of
C, N and P, we characterized litter samples using specific
analyses for condensed tannins (CT) and polyphenols,
and solid-state 13
C nuclear magnetic resonance spec-
troscopy with cross-polarization and magic angle spin-
ning.
Materials and methods
Study site
Litter decomposition was studied in the Mannheim-
Ka¨ fertal forest, located in the Rhine-Neckar conurba-
tion very close to the city of Mannheim (northern upper
Rhine valley, SW Germany, Table 1). It is a vast forest
area which is of major importance for the public
Table 1 Site properties of the study area (Ries et al. 2000; Feger
et al. 2001, 2003)
Easting 3464988
Northing 5490800
Elevation 88 m a.s.l.
Mean annual temperaturea
10.6 °C
Annual precipitationa
659 mm
Parent material Aeolian sand covering Rhine terrace
Soil profile (FAO) Dystric Cambisol
Humus type (Mull)-moder
pH (H2O)
Of 4.4
Oh 4.0
0–5 cm 3.9
5–10 cm 4.0
Present atmospheric deposition
Nitrogen $25 kg haÀ1
yearÀ1
Sulphur $10 kg haÀ1
yearÀ1
Tree speciesb
Overstorey: 100% Pinus sylvestris
Understorey: 60% Prunus serotina
20%Fagus sylvatica
10% Quercus robur
10% Tilia cordata
Stand age 70–100 years
a
Averages (30-year), meteorological station Mannheim-Mu¨ hl-
schleuse (Schirmer und Vent-Schmidt 1979)
b
Haag und Wilhelm (1998)
178
drinking water supply for 0.5 million people. The cli-
mate is warm and relatively dry. Soil pH is very low even
in the subsoil. However, due to an elevated rate of
atmospheric N deposition, humus forms are relatively
favorable (mull-moder types) and appear to have
improved during recent decades (Krumrei et al. 2003).
The stand consists of ca. 90-year-old Scots pine (Pinus
sylvestris), whereas black cherry (Prunus serotina) forms
a second layer which is very dense. Therefore, other
deciduous species (notably beech and oak), despite being
promoted by forest management during recent decades,
form only a minor portion of the understorey. Nutrition
of pine shows suboptimal supply with N and slight
tendencies for disharmonies with respect to P and K
(Feger et al. 2003). The deposition history in the con-
urbation is clearly reflected in elevated pools of N, S and
heavy metals (Ries et al. 2000).
Litter preparation and sampling
In November 1999, freshly fallen senescent leaves from
beech (Fagus sylvatica), oak (Quercus robur), black
cherry (Prunus serotina Ehrh.) and brown needles from
pine (Pinus sylvestris) were collected by hand and nets
beneath mature trees at the study site. Two litterbag
experiments were set up, one to compare the decompo-
sition of beech versus oak leaves, and the other to
compare pine needles with black cherry leaves. Beech
and oak litter (5 g dry weight equivalent) and 2.5 g of
cherry and pine litter were put into polyester-net litter-
bags (12·12 cm; 1 mm mesh), respectively. In Novem-
ber 1999, 35 bags of each litter were pinned to the forest
floor surface in 7 blocks (0.5 m2
). From each of these
blocks, 7 litterbags were collected in May 2000 (after
0.5 years) and November 2001 (after 2 years), respec-
tively. Litterbags were handsorted to remove mesofauna
and external debris. Litter aliquots from each bag were
dried at 70 °C to constant mass and weighed for calcu-
lation of mass loss. For the proanthocyanidin and Folin
Ciocalteu assays and for chemical analysis, dried com-
posite samples for each species and date of collection
were ground in a Wiley mill.
Analysis of C, N, and P
Total C and N concentrations of ground litters were
analysed in duplicate by oxidative flash combustion
using a Vario EL Foss Heraeus. Total phosphorus
concentrations were determined in duplicate in extracts
after wet digestion with HNO3 using a AES-ICP Spectro
Ciros CCD.
Proanthocyanidin (PA) assay
The procedure was modified slightly from those reported
previously (Lorenz et al. 2000). A standard solution
(0.5 mg mlÀ1
in methanol) was prepared using purified
condensed tannin from the needles of black spruce
(Picea mariana (Mill.) B.S.P.) (Lorenz and Preston
2002). Litter samples were analysed in two stages to
determine extractable and residual tannins. Extraction
was performed twice by adding acetone/water (70:30) (v/
v), and the extracts were combined. The insoluble resi-
due was air dried for analysis of residual tannins. Sample
concentrations were determined from the calibration
curve established for black spruce tannin. Total tannin
was expressed as the sum of extractable and residual
tannin. The response in the assay depends on the pro-
anthocyanidin:prodelphinidin ratio of the tannin, hence
tannin concentrations should be regarded relative to the
tannin standard (Kraus et al. 2003a).
Folin Ciocalteu assay
The procedure was modified slightly from those reported
previously (Preston et al. 1997; Preston 1999). The
reagent, prepared each day, was 20% (w/v) sodium
carbonate solution. A stock solution (0.1 mg mlÀ1
in
water) was prepared using a pure catechol (k 1480, Sig-
ma). Sample concentrations in the combined acetone/
water extracts of the proanthocyanidin assay were
determined from the absorbances of prepared catechol
standards at 750 nm. Concentrations of phenolics
should be regarded relative to the standard catechol.
However, purifying standards from the same litter
material would be helpful for the interpretation of sam-
ple absorbances (Appel et al. 2001). The Folin Ciocalteu
method provides a broad measure of all readily oxidized
compounds including tannins, non-tannin phenolics and
non-phenolic compounds (Schofield et al. 1998).
13
C CPMAS NMR spectroscopy
Solid-state 13
C NMR spectra of litter with cross-polar-
ization and magic-angle spinning (CPMAS NMR) were
obtained using a Bruker MSL 300 spectrometer (Bruker
Instruments Inc., Karlsruhe, Germany) operating at
75.47 MHz. Dry, powdered samples were spun at
4.7 kHz in a 7-mm OD rotor. Spectra were acquired
with 1 ms contact time, 2 s recycle time, and 6,000 scans,
and were processed using 30–40 Hz line-broadening and
baseline correction. Chemical shifts are reported relative
to tetramethylsilane (TMS) at 0 ppm, with the reference
frequency set using adamantane. Dipolar dephased
(DD) spectra were generated by inserting a delay period
of 40–50 ls without 1
H decoupling between the cross-
polarization and acquisition portions of the CPMAS
pulse sequence. All DD spectra were obtained using the
TOSS sequence for Total Suppression of Spinning
Sidebands.
The NMR spectra of litters were divided into
chemical shift regions as follows: 0–50 ppm, alkyl C;
50–60 ppm, methoxyl C; 60–93 ppm, O-alkyl C;
93–112 ppm, di-O-alkyl C and some aromatics; 112–
179
140 ppm, aromatic C; 140–165 ppm, phenolic C; and
165–190 ppm, carboxyl C. Boundaries were adjusted
slightly to correspond with spectral minima. Areas of the
chemical shift regions were determined after integration
and expressed as percentages of total area (‘‘relative
intensity’’). There are limitations in the quantitative
reliability of CPMAS spectra, but it is appropriate to use
them to compare intensity distributions among similar
samples (Preston et al. 1994, 1997). The DD spectra
were used only for qualitative discussion.
Difference spectra (2-year minus initial spectra) were
used to get an impression of the nature of the more
resistant C structures accumulating during decomposi-
tion. These were obtained by subtracting the initial from
the 2-year spectra, using the MSL software. In this
qualitative procedure, the heights of the largest peaks
were matched (mainly the O-alkyl signal), and then the
2-year spectrum subtracted. Small adjustments were
made in the height of the 2-year spectrum to generate a
difference spectrum that best maintained overall phasing
and minimized distortion, in the eyes of the operator.
Statistical analysis
Values for remaining mass are based on arithmetic
means (±standard deviation) of the 7 samples collected
after 0.5 and 2 years of exposition. For statistical
validity of the results, the nonparametric Mann-Whitney
U-test (Sachs 1978) was applied to compare pairwise
two means (beech/oak, pine/cherry). The significance
level of 95% was used. Comparison of mean values was
performed with the statistical program package SPSS.
The tannin, total phenolics and elemental analyses were
carried out on composite samples and are interpreted
qualitatively.
Results
Mass, chemical and biochemical changes
Litter mass remaining after 0.5 and 2 years of decom-
position are given in Table 2. Remaining mass after
0.5 years ranged from 31.7–85.5%, and was significantly
lower for oak litter than for beech, and for cherry litter
than for pine. These differences persisted for the second
sampling, with only 5.4% of initial mass remaining for
cherry, while 31.7% of the initial mass was still
remaining for pine. After 2 years, the remaining mass
for oak litter was also significantly lower compared with
beech.
Total C concentrations in fresh litter were similar for
all deciduous species and showed similar decreases for
beech and oak (Table 3). For cherry, C concentrations
initially increased and then also decreased. Pine had the
highest initial C concentration of all litter (539 mg gÀ1
)
and remained fairly unchanged after 0.5 years and de-
creased for the second sampling but was still higher than
for cherry. Initially, oak was the highest in N and P
followed by beech, cherry and pine, while C-to-N ratios
increased in the same order. In contrast to carbon, N
concentrations generally increased during decomposi-
tion and concomitantly C-to-N ratios decreased. The
biggest increase in N concentration was found for
cherry, while smaller changes were observed for pine,
oak and beech. The C-to-N ratio for each sampling re-
mained higher for beech litter than for oak, and for pine
litter compared with cherry. Phosphorus concentrations
distinctly increased for pine and cherry after 0.5 and
2 years, while they only slightly increased for oak but
decreased for beech.
Table 4 summarizes data for total and percentage
extractable condensed tannins and total phenolics in
plant litter before and after two periods of decomposi-
tion. Owing to the lack of specific standards for each
species, the absolute values are not reliable, although the
analyses reflect the pattern of changes in concentrations
of tannins and phenolics with time (Kraus et al. 2003a).
Fresh beech litter was distinctively higher in condensed
tannins (CT) compared with oak, while percentage
extractable tannins and total phenolics were fairly sim-
ilar. This is probably due to oak having a higher pro-
portion of hydrolysable tannins (HT) that are detected
by the phenolic assay, but not by the proanthocyanidin
assay specific for condensed tannins. Cherry litter was
distinctively higher in total tannins and phenolics
compared with pine, while differences in the extractable
tannin portion were small.
Tannin concentrations and particularly the extract-
able fraction decreased very fast for all litters during
Table 2 Remaining mass during litter decomposition, n=7a
Species 0.5-year decomposition 2-year decomposition
Remaining mass (%) Remaining mass (%)
Beech 85.5 (4.0) a 65.4 (10.5) a
Oak 60.5 (5.1) b 25.0 (9.0) b
Pine 49.6 (4.2) a 31.7 (12.1) a
Cherry 31.7 (5.6) b 5.4 (3.5) b
a
Standard deviations in parentheses. Means followed by different
letters are significantly different (P