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ARTICLE IN PRESS
Available at www.sciencedirect.com
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5
0961-9534/$ - see frodoi:10.1016/j.biomb
�Corresponding aSE-750 07 Uppsala,
E-mail address: A
http://www.elsevier.com/locate/biombioe
Wood fuel quality of two Salix viminalis stands fertilisedwith sludge, ash and sludge–ash mixtures
Anneli Adlera,b,�, Ioannis Dimitrioua, Par Aronssona, Theo Verwijsta, Martin Weiha
aDepartment of Crop Production Ecology, Swedish University of Agricultural Sciences (SLU), P.O. Box 7043, SE-750 07 Uppsala, SwedenbInstitute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Riia 181, 51014 Tartu, Estonia
a r t i c l e i n f o
Article history:
Received 6 September 2006
Received in revised form
15 December 2007
Accepted 8 January 2008
Available online 7 March 2008
Keywords:
Age of shoot population
Bark
Biofuel
Heavy metals
Planting density
Nutrients
Structure of shoot population
Willow
nt matter & 2008 Elsevieioe.2008.01.013
uthor at: Department ofSweden. Tel.: +46 18 6723
a b s t r a c t
This study assessed the effects of stand structure and fertilisation with wood ash and/or
sludge on wood fuel quality of Salix viminalis. The relative proportions of bark and wood in
1-, 2- and 3-year-old shoot populations were determined. The concentrations of essential
elements (N, P, K) and heavy metals (Cu, Zn, Cd, Ni) in bark and wood were used to assess
the wood fuel quality in harvestable shoot biomass. Controlled field experiments were
conducted on two newly harvested commercial short-rotation willow coppice fields. Five
treatments were applied: sewage sludge at the maximum legally permitted amount; ash;
two sludge–ash mixtures supplying the maximum and twice the maximum permitted
sludge–ash amount; and a control receiving mineral nutrients only.
The proportion of bark in the willow stands was decreasing with the age of the shoot
population. The shoot population with few large stems, compared to that with many small
stems, had a lower proportion of element-rich bark in the harvestable shoot biomass,
meaning better quality of the wood fuel. Overall, wood fuel quality in terms of mineral
concentrations was influenced by the age of the shoot population at harvest, stand
structure, management practices (e.g. planting density, fertilisation) and site conditions
(soil type, element availability). Our results imply that harvestable shoot biomass of
willows grown as few large stems have better wood fuel quality, compared to harvestable
shoot biomass of many small stems. Increased length of cutting cycle improves the wood
fuel quality.
& 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Fast-growing plantations of Salix and Populus are today a
viable alternative source of bio-energy, and the profitability of
Salix culture can be significantly improved by the use of by-
products such as wood ash and sludge [1]. For example, wood
ash mixed with sewage sludge can be applied to the energy
plantation to meet the nutrient requirements of the Salix
stand. The production cost of Salix biomass fuel decreases
when the nutrient requirements of the crop can be supplied
without artificial fertilisers [2].
r Ltd. All rights reserved.
Crop Production Ecology,06; fax: +46 18 672890.A. Adler).
During combustion of woody biomass, a large proportion of
sulphur (S) and nitrogen (N) are gasified, while the majority of
nutrients, trace elements and the acid-neutralising capacity
remain in the ash [3,4]. Consequently, the ash can be recycled
and used as a fertiliser. However, high concentrations of zinc
(Zn) and cadmium (Cd) in biomass fuels can cause problems
in ash recycling, since these metals accumulate in the ash
during combustion or are emitted to the atmosphere as
particulates [5]. Sewage sludge is the solid by-product of
wastewater treatment plants. It contains large amounts of
phosphorus (P), some N (mainly organically bound), very little
Swedish University of Agricultural Sciences (SLU), P.O. Box 7043,
ARTICLE IN PRESS
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 915
potassium (K) and heavy metals. For sustainable biomass
utilisation, it is essential to circulate the material flows and to
integrate the biomass ash and sewage sludge within the
natural cycles. Therefore, the minerals in the nutrient cycle
from soil/ash and sludge—to willow roots/stems—to com-
bustion—to ash—to soil should be utilised as completely as
possible. However, since sewage sludge and wood ash may
also contain rather high concentrations of heavy metals [5,6],
the natural cycle of minerals within the process of energy
production from biomass may be disturbed by deposition of
heavy metals in soils under energy forests.
The bark of small-diameter stems can form a large
proportion of the harvestable shoot biomass produced in
short-rotation crops. Bark and wood differ in the relative
amounts of cellulose, hemicellulose, lignin, inorganic materi-
als and extractives, and behave therefore differently during
thermal decomposition [7]. One of the main differences in
chemical composition of wood and bark is the concentration
of inorganic ions; the concentrations of macronutrients and
some heavy metals are significantly higher in bark than in
wood [8–17]. Consequently, the variation in proportion of bark
in a short-rotation willow stand may have pronounced effects
on the characteristics of the wood fuel. High concentrations
of elements decrease the quality of the wood fuel. The
inorganic ions are known to exert a great influence on the
thermal degradation of polysaccharides and lignin [18,19]. It
has been found that a number of ions act as catalysts,
resulting in lowering of decomposition temperature and an
increase in char yield [20,21]. High concentrations of K in
wood fuel decrease the ash melting point [22].
On the other hand, a high proportion of bark may play an
important role in removal of elements from willow vegetation
filters. In vegetation filters, we wish to remove as many
elements as possible through harvesting shoots, but the
biomass fuel should still be of high quality, i.e. the concen-
trations of elements in harvestable shoot biomass should be
low. Hence, proportion of bark plays an important role when
the aim is to remove elements from a vegetation filter and
burn the harvested wood fuel for energy. Taking into account
the above-mentioned differences between bark and wood, a
comparative analysis of different biomass fractions is essen-
tial for evaluation of wood fuel quality.
Earlier studies on short-rotation willow coppice have
assessed the effects of different site conditions and planting
densities on stand structure and harvestable shoot biomass
yields [23–28], while the dynamics of bark proportions in
willow stands and its effect on wood fuel quality has not been
focused yet.
The present study focuses on the particular aspects of wood
fuel quality that can be influenced by management practices
such as the frequency of harvests and the fertilisation with
wastes. Wood fuel quality of willows in terms of proportion of
bark, concentrations of elements in bark and wood (N, P, K
and Zn, Cd, Cu, Ni) and the biomass yield was investigated in
two commercial willow plantations. These plantations had
been established with different planting densities and were
fertilised with sewage sludge, wood fuel ashes and various
mixtures of these. The relationship between the proportion of
bark in the destructively measured shoots and the shoot
diameter at 55 cm height was used to calculate the dry weight
(DW) of bark in non-destructively measured shoots [16]. The
shoots were sampled in wintertime according to commercial
harvesting practices. Our hypothesis was that wood fuel with
a higher proportion of bark would contain more essential
elements and heavy metals compared to wood fuel with
lower proportion of bark, and would thus be of poorer quality.
2. Materials and methods
2.1. Sites
Two newly harvested commercial willow plantations (Salix
viminalis L., clone 78-021), located at Linnes Hammarby
(Hammarby) and Lundby Gard (Lundby) near Uppsala in
central Sweden, were studied. The plantations were estab-
lished in 1997 in a twin row formation with densities of 17,700
and 25,000 cuttings ha�1, at Hammarby and Lundby, respec-
tively. At Hammarby, the distance between the rows alter-
nated at 150 and 75 cm and the distance between the plants in
the rows was approximately 50 cm. At Lundby, the distance
between the rows alternated at 125 and 75 cm and the
distance between the plants in the rows was approximately
40 cm. Both plantations were in the beginning of their second
cutting cycle when we started the experiment.
The soil at Hammarby contained 43% clay (o0.002 mm),
47% silt (0.002–0.06 mm) and 10% sand (0.06–2 mm). The total
N content was 0.19%, the organic C content was 2.08%, and
the pH around 6.6. The soil at Lundby contained 57% clay, 34%
silt and 9% sand. The total N content was 0.27%, the organic C
content was 2.73%, and the pH around 6.2. The mean length
of the growing season in the area is 180–200 days, the mean
rainfall 500–600 mm year�1; the mean air temperature during
the growing season (April–September) is 13 1C (data for
1961–1990) [29].
2.2. Treatments
A randomised block design with five fertilisation treatments
and four replicates was laid out in each of the two fields
(Hammarby and Lundby) in spring 2001. Each plot was
9�9 m2 in size. Fertilisation treatments comprised different
combinations of stabilised dewatered sludge from a nearby
municipal wastewater treatment plant, and wood fuel
ash from the district heating plant in Enkoping in central
Sweden (Table 1). Control plots received 14.5 kg P and
48 kg K ha�1 year�1 as mineral fertiliser during the 3 years of
the experiment. All plots, including control plots, were
additionally fertilised with 100 kg N ha�1 year�1 according to
recommendations [30]. The plant-available fractions of Zn, Ni
and Cd in the soil were less available in the treatments
containing ash compared to the sludge and control treat-
ments (Table 2). In contrast, the plant-available fraction of Cu
in the soil was less available in the sludge and control
treatments compared to the treatments containing ash.
2.3. Proportion of bark in sample pieces and shoots
Shoots were sampled each winter after the first, second and
third growing season. At each site, 15 shoots were sampled to
ARTICLE IN PRESS
Ta
ble
1–
Am
ou
nts
of
dew
ate
red
sta
bil
ised
slu
dg
ea
nd
wo
od
fuel
ash
ap
pli
ed
ind
iffe
ren
ttr
eatm
en
tsa
tH
am
ma
rby
an
dLu
nd
by
inC
en
tra
lS
wed
en
,a
nd
sup
ply
of
ma
cro
nu
trie
nts
(kg
ha�
1)
an
dh
eav
ym
eta
ls(g
ha�
1)
toth
eso
ilby
slu
dg
ea
nd
ash
ap
pli
cati
on
Tre
atm
ent
Typ
eo
ffe
rtil
iser
Ap
pli
ed
as
fresh
weig
ht
(th
a�
1)
Ap
pli
ed
as
dry
weig
ht
(th
a�
1)
NP
KC
dC
uN
iZ
nS
oil
pH
aft
er
ap
pli
cati
on
[17]
Lu
nd
by
Ha
mm
arb
y
Slu
dge
Slu
dge
8.6
2.6
72
.89
12
.94
.72
00
25
52
27
56
.30
6.5
3
Ash
Ash
8.6
5.5
0.6
99
18
7.0
5.2
42
49
91
32
06
.49
6.8
8
Slu
dge+
ash
Slu
dge
4.3
1.3
36.4
46
1.4
2.3
1001
27
1138
Ash
4.3
2.8
0.3
50
95.2
2.6
216
50
672
To
tal
8.6
4.1
36
.79
69
6.6
5.0
12
17
78
18
10
6.4
96
.78
(Slu
dge+
ash
)�2
Slu
dge
8.6
2.6
72.6
91
2.9
4.7
1996
54
2268
Ash
8.6
5.5
0.6
100
188
5.2
426
100
1327
To
tal
17
.28
.17
3.1
19
01
90
.99
.92
42
21
54
35
95
6.5
56
.87
Co
ntr
ol
Min
era
l
fert
ilis
er
––
–4
4144.0
––
––
6.1
86
.47
No
teth
at
all
plo
tsre
ceiv
ed
ina
dd
itio
n100
kg
Nh
a�
1y
ea
r�1.
So
ilp
Hin
the
0–1
5cm
lay
er
3y
ea
rsa
fter
slu
dge
an
d/o
ra
sha
pp
lica
tio
nis
pre
sen
ted
as
lea
stsq
ua
rem
ea
ns,
SE¼
0.0
6.
Bo
ldn
um
bers
den
ote
the
tota
la
mo
un
tsfo
rea
chtr
ea
tmen
t.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5916
describe the relationship between the shoots’ diameter at
55 cm height (D55) and the proportion of bark (BPshoot). The
shoots were selected in such a way that the whole range of
D55 found in each stand was represented. The height of the
shoots was measured. The shoots were divided into age
fractions, and from each age fraction three 5-cm long sample
pieces were taken for more detailed analyses. Bark and wood
were separated in all sample pieces. The fresh weight of the
sample pieces was determined. After drying at 85 1C for
4 days, the corresponding oven DW of bark and wood was
determined. The DW of the rest of the age fractions was also
determined. Bark DW of each age fraction was calculated
using the average proportion of bark in three sample pieces of
the respective age. Thereafter, bark DW and wood DW in each
age fraction were added to give the total shoot DW (TDWshoot).
Proportion of bark in the whole shoot (BPshoot) was calculated
as the ratio of bark DW of the shoot (BDWshoot) to TDWshoot.
Non-linear regression between D55 and BPshoot of the destruc-
tively measured shoots was established to estimate the bark
proportion of non-destructively measured shoots:
BPshoot ¼ aþ b�Dc55, (1)
where a, b and c were the estimated parameters (Fig. 1).
2.4. Proportion of bark in willow stands
Another 30 shoots were selected from the stand for establish-
ing the allometric relationship between D55 and TDWshoot.
The selected shoots represented the whole range of D55 found
in the plantation. Non-linear regression of TDWshoot on D55
was used to estimate the DW of non-destructively measured
shoots [31]:
TDWshoot ¼ d� Dg55, (2)
where d and g were the estimated parameters.
To estimate the total stand biomass and shoot size
frequency distribution at Lundby and Hammarby, 12 stools
(i.e. plants that had re-sprouted after harvest) were labelled
within each treatment plot (i.e. n ¼ 12 stools �5 treatments
�4 replications ¼ 240 stools per site). D55 of all living shoots
of the selected stools were measured during the winter after
each of the three growing seasons. Stool mortality was
estimated by counting the number of living stools of a subplot
(4.5�4.5 m2 in the middle of each 9�9 m2 treatment plot) at
the beginning and end of the experiment.
The mean proportion of bark in the whole stand (BPstand)
was estimated using Eq. (1) to calculate BPshoot for non-
destructively measured shoots in each plot and using Eq. (2)
to calculate the TDWshoot of these shoots. Thereafter,
BDWshoot was calculated according to
BDWshoot ¼ BPshoot � TDWshoot (3)
and BPstand was obtained using the following equation:
BPstand ¼
PnBDWshoot � 100P
nTDWshoot, (4)
where n is the number of non-destructively measured shoots.
Thus, the proportion of bark in non-destructively measured
shoots was assumed to represent the proportion of bark in
the whole shoot population.
ARTICLE IN PRESS
Table 2 – Average plant-available fractions of heavy metals in the 0–15 cm soil layer at Lundby and Hammarby threegrowing seasons after the applications (n ¼ 12) [17]
Site Treatment Zn (mg kg�1 DM) Ni (mg kg�1 DM) Cd (mg kg�1 DM) Cu (mg kg�1 DM)
Lundby Sludge 0.39 0.23 0.009 0.09
Ash 0.22 0.15 0.006 0.10
Sludge+ash 0.22 (0.29) 0.15 (0.18) 0.007 (0.004) 0.13 (0.07)
(Sludge+ash)� 2 0.22 0.15 0.005 0.15
Control 0.36 0.22 0.009 0.09
Hammarby Sludge 0.30 0.10 0.005 0.13
Ash 0.19 0.07 0.003 0.15
Sludge+ash 0.31 (0.22) 0.11 (0.09) 0.004 (0.007) 0.16 (0.11)
(Sludge+ash)� 2 0.21 0.06 0.003 0.17
Control 0.29 0.15 0.006 0.13
Plant-available fractions of heavy metals in the 0–15 cm soil layer at both sites before the experiment are shown in brackets (n ¼ 2, while an
analysed sample consisted of 5 sub-samples).
40
Lundby Hammarby
BP s
hoot
20
30
D55 D55
10 20 3010
00 10 20 30
Fig. 1 – Relationship between D55 (stem diameter at 55 cm
height) and proportion of bark (BPshoot) for 1-year-old
(crosses), 2-year-old (open circles, solid line) and 3-year-old
shoots (filled circles, dashed line) in the second cutting cycle
of Salix viminalis grown at two sites in central Sweden: (a)
2-year-old shoots: BPshoot ¼ �2.0+73.0�D55(�0.35), R2
¼ 0.54;
3-year-old shoots: BPshoot ¼ �2.0+66.60�D55(�0.32), R2
¼ 0.60;
(b) 2-year-old shoots: BPshoot ¼ �2.0+70.14�D55(�0.35), R2
¼ 0.86;
3-year-old shoots: BPshoot ¼ �2.0+52.48�D55(�0.25), R2
¼ 0.81.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 917
2.5. Chemical analyses
The concentrations of N, P and K were analysed for composite
samples of bark or wood for 15 entire shoots per site. Three
shoots from each treatment (one per plot) were collected,
i.e. 15 shoots were sampled per site and age class of the
shoots. Analysis of stem fractions was designed to take into
account concentration differences in different age fractions of
Salix stems. The bark or wood fractions of the three sample
pieces per age fraction were pooled together. The composite
samples of bark for entire shoots were calculated according to
the weight proportion of each age fraction of the shoot. The
composite samples of wood for entire shoots were obtained in
a similar way according to the weight proportion of each age
fraction. Total N, P and K concentrations in bark/wood
samples of the destructively measured 1- and 3-year-old
shoots were analysed by gas chromatography in a Carlo Erba
Elemental Analyser NA 1500 [32]. In total, composite samples
of bark/wood in 60 shoots were analysed.
2.6. Calculations and statistical analyses
The concentration of heavy metals in bark and wood in
different treatments of this experiment, published by Dimi-
triou et al. [16], and the concentrations of N, P and K in bark
and wood, determined in this study, were used to calculate
the wood fuel quality of harvestable shoot biomass. Wood
fuel quality (i.e. element concentration per tonne harvestable
shoot biomass) was calculated as
Wood fuel quality ¼ ½element�bark �BPstand
100
þ ½element�wood � 1�BPstand
100
� �, (5)
where [element]bark and [element]wood are the concentrations
of an element in the bark and wood, respectively, in a shoot
population, BPstand/100 is the mean proportion of bark in the
whole stand and 1�(BPstand/100) is the mean proportion of
wood in the whole stand.
The assumption that BPshoot is independent of treatment
was verified by means of analysis of covariance (ANCOVA),
i.e. by testing the effects of the factors site, age (i.e. age of the
shoot) and treatment on BPshoot, where D55 was used as a
covariate. The response variable BPshoot was power trans-
formed (BPshoot�2 ) prior to the analysis for normality of data.
The effect of site on the height of the destructively measured
shoots was tested by means of ANCOVA using age of the
shoot as a covariate. The effects of site and treatment on the
number of shoots per stool and the number of stools per plot
were tested by means of ANCOVA using age of the shoot as a
covariate.
The effects of site, tissue, treatment and age of the
shoots on the concentrations of N, P and K in the composite
bark/wood samples (see Section 2.5) of the destructively
measured shoots were tested by general linear models
(GLMs). The concentrations of K were log-transformed prior
to the analyses for normality of the data.
ARTICLE IN PRESS
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5918
GLM was used to test the effects of age of the shoot
population, site and treatment on the BPstand and quality of
the wood fuel. The concentrations of P and Zn were square
root-transformed; the concentration of N was inverse-trans-
formed and the concentrations of K, Cd, Cu and Ni were log-
transformed prior to the analyses for normality of the data.
All statistical analyses were performed with the statistical
packages SA System v. 8.1 [33].
3. Results
3.1. Proportion of bark in shoots
The BPshoot was significantly affected by D55 (Table 3a),
while D55 in turn was determined by the age of the shoot
(po0.001, R2¼ 0.74, n ¼ 100). The variation of BPshoot in
relation to D55 decreased with increasing shoot age (Fig. 1).
The BPshoot of 1-year-old shoots varied greatly and the non-
linear regression described only 32% and 37% of the variation
in BPshoot at Lundby and Hammarby, respectively. The
corresponding non-linear regression model (Fig. 1) after the
second growing season explained 54% of the measured
variation in BPshoot at Lundby and 86% of the variation in
BPshoot at Hammarby. The BPshoot after the third growing
season showed relatively high variability at Lundby, where
BPshoot varied between 21% and 30% (R2¼ 0.60; Fig. 1a). Willow
shoots of the same clone and age from the Hammarby site
had proportions of bark values in the range 19–28% (R2¼ 0.81;
Fig. 1b).
Table 3 – Summary of (a) ANCOVA results for effects ofage of the shoot, site, treatment and stem diameter at55 cm height (D55, covariate) on shoot proportion of bark(n ¼ 3 or 4 per treatment) in the 1-, 2- and 3-year-oldshoots from plantations at Lundby and Hammarby(the model was significant at po0.001, R2
¼ 0.67.The interactions were not significant and are notpresented.) (b) GLM results for the effects of age of theshoot population, site and treatment on proportion ofbark in these willow stands (the model was significant atpo0.001, R2
¼ 0.95)
Factor df F-Value p-Value
(a) Shoot proportion of bark (BPshoot)
D55 (covariate) 1 90.34 0.00001
Age 1 0.30 Ns. (0.58)
Site 1 2.19 Ns. (0.14)
Treatment 4 1.29 Ns. (0.28)
(b) Proportion of bark in the stand (BPstand)
Age 1 1268 0.00001
Site 1 22.99 0.00001
Age�Site 1 4.49 o0.05
Site�Treatment 4 1.11 Ns. (0.36)
Treatment 4 0.36 Ns. (0.84)
Age�Treatment 4 0.22 Ns. (0.93)
Ns. ¼ non-significant.
3.2. Element concentrations in bark and wood
The concentrations of K in the bark and wood were
significantly higher at Lundby compared to Hammarby
(the effect of site: po0.001; the model was significant at
po0.001, R2¼ 0.93, df ¼ 108). The concentrations of K were
significantly higher in the 1-year-old shoots compared to the
3-year-old shoots (the effect of age: po0.001). The 1-year-old
shoots contained 7.7370.26 mg K g�1 DM (dry matter) in the
bark and 1.7870.26 mg K g�1 DM in the wood at Lundby, and
7.2970.25 mg K g�1 DM in the bark and 1.6870.25 mg K g�1 DM
in the wood at Hammarby. The 3-year-old shoots contained
6.1070.26 mg K g�1 DM in the bark and 1.3670.26 mg K g�1 DM
in the wood at Lundby, and 5.5470.25 mg K g�1 DM in the bark
and 0.9770.25 mg K g�1 DM in the wood at Hammarby. The
concentrations of K in the shoots were significantly affected
also by the treatments (the effect of treatment: po0.001). The
mean K concentrations in the bark and wood of 3-year-old
shoots were 6.8170.90 and 1.2770.12 mg g�1 DM, respectively,
in the (sludge+ash)�2 treatment at Hammarby. These values
were significantly higher compared to the mean K concentra-
tions in the bark and wood of the rest of the treatments at the
same site (5.2270.20 and 0.9070.03 mg g�1 DM, respectively).
At Lundby, the mean K concentration was 6.1070.31 mg g�1
DM in the bark and 1.3670.06 mg g�1 DM in the wood in all
treatments.
The concentrations of P in the bark and wood were
significantly higher at Lundby compared to Hammarby and
were affected by age of the shoots (the effect of site: po0.001;
the effect of age: po0.001; the model was significant at
po0.001, R2¼ 0.83, df ¼ 108). The effect of treatment on P
concentrations in the bark and wood was not significant
(p ¼ 0.11). The mean P concentrations in the 1-year-old shoots
were 2.1270.07 mg g�1 DM in the bark and 0.9570.07 mg g�1
DM in the wood at Lundby; and 1.8070.07 mg g�1 DM in the
bark and 0.7870.07 mg g�1 DM in the wood at Hammarby. The
mean P concentrations in the 3-year-old shoots were
1.4870.07 mg g�1 DM in the bark and 0.6070.07 mg g�1 DM
in the wood at Lundby, and 1.1670.07 mg g�1 DM in the bark
and 0.3870.07 mg g�1 DM in the wood at Hammarby.
The concentrations of N were significantly higher in the
bark compared to wood and were significantly higher in
the 1-year-old shoots compared to the 3-year-old shoots
(the effect of tissue: po0.001; the effect of age: po0.001; the
model was significant at po0.001, R2¼ 0.90, df ¼ 108). The
effects of site and treatment on the N concentrations in
the bark and wood were not significant (p ¼ 0.96 and p ¼ 0.90,
respectively). The mean N concentrations of the 1-year-old
shoots were 15.4170.27 mg g�1 DM in the bark and
3.9670.27 mg g�1 DM in the wood. The mean N concentra-
tions of the 3-year-old shoots were 10.1370.27 mg g�1 DM in
the bark and 2.2370.27 mg g�1 DM in the wood.
3.3. Proportion of bark and biomass dynamics atstand level
A higher proportion of shoots with small diameter were
observed at Lundby compared to Hammarby in each year
during the 3 years of the experiment (Fig. 2). After the third
growing season, 41% and 37% of the living shoots recorded in
ARTICLE IN PRESS
900
600
300
00 10 20 30 40
0 10 20 30 40
900
600
300
0
0.1
0
0
0.2
0.1
0.2
0.3
Prop
ortio
nPr
opor
tion
Cou
ntC
ount
D55
3rd1st 2nd
Fig. 2 – Shoot size distribution of non-destructively
measured shoots in two willow plantations in Central
Sweden. Diameters of 55 cm height (D55) of all shoots in 240
stools after the 1st, 2nd and 3rd year of the second cutting
cycle at (a) Lundby and (b) Hammarby.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 919
the beginning of the experiment were present in the
plantations at Lundby and Hammarby, respectively.
The number of shoots per stool decreased with the
increasing age of the shoot population (the effect of age as
covariate: po0.001), and was significantly higher at Lundby
compared to Hammarby (the effect of site: po0.001; the
model was significant at po0.001, R2¼ 0.90, df ¼ 119). The
shoots were significantly taller at Hammarby compared to
Lundby after each of the three growing seasons (the model
was significant at po0.001, R2¼ 0.81, n ¼ 69). The height of
the 3-year-old shoot population was 458.1721 cm at Ham-
marby and 377.1722 cm at Lundby. The number of stools per
plot was significantly higher at Lundby compared to Ham-
marby (the effect of site: po0.001; the model was significant
at po0.001, R2¼ 0.37, df ¼ 79; Table 4). After the third growing
season 91% and 93% of the stools recorded in the beginning of
the experiment were present in the plantations at Lundby and
Hammarby, respectively.
BPstand decreased with the increasing stand age and was
significantly higher at Lundby compared to Hammarby after
each of the three growing seasons (ANCOVA, the effect of site
po0.0001, R2¼ 0.94, df ¼ 39; Tables 3b and 4; Fig. 3). The
treatments did not have any significant effect on the BPstand.
The harvestable shoot biomass (yield) increased with the
age of shoot population and was significantly higher at
Hammarby than at Lundby (the effect of site: po0.05; the
model was significant at po0.001, R2¼ 0.87, df ¼ 69; Table 4).
The BPstand decreased with increasing yield (Fig. 4).
3.4. Wood fuel quality
Changes in the proportion of bark in the two willow stands
during the experimental period had a significant effect on the
quality of the wood fuel. BPstand significantly affected the
wood fuel quality in terms of the concentrations of P, K and
Cd in the harvestable shoot biomass (Tables 5 and 6). The
concentrations of all the studied elements per tonne harvest-
able shoot biomass decreased with the increasing age of the
shoot population (Table 5, the effect of age; Table 6). The wood
fuel quality in terms of the concentrations of P, Zn and Cd per
tonne harvestable shoot biomass was significantly higher at
Lundby compared to Hammarby (Table 5, the effect of site;
Table 6). The harvestable shoot biomass yield and the total
energy content of the plantations were higher at Hammarby
compared to Lundby (Table 4).
4. Discussion
The data support the hypothesis that wood fuel with a higher
proportion of bark compared to wood fuel with the lower
proportion of bark contains more essential elements and
heavy metals and is thus of poorer quality. The stand
structure with the higher proportion of small-sized shoots
resulted in the higher proportion of bark in the plantation and
consequently a poorer quality of wood fuel in terms of high
element concentrations.
4.1. Proportion of bark and stand structure
The denser spacing of willow plants at Lundby most likely
promoted the development of more shoots per coppiced plant
(i.e. stool) in this plantation compared to the plantation at
Hammarby (Fig. 2). The proportion of the shoots from the
smaller size classes that had died by the end of the third
growing season (Fig. 2) was higher at Hammarby compared to
Lundby. As a result, the amount of large stems with low
proportion of bark was high at Hammarby. The topography of
the willow field could also have influenced the stand
structure at Lundby. This plantation had a higher proportion
of relatively short and small-sized (low D55) shoots compared
to Hammarby. The plantation at Lundby was covered with
water during early spring each year, indicating that the
experimental site was lying on a waterlogged soil. In an
earlier study, multiple-stem formation (i.e. polycormism) in
mountain birch (Betula pubescens ssp. tortusa (Lebed.) Nyman)
ARTICLE IN PRESS
Table 4 – Characteristic parameters of the studied shoot populations in two willow plantations at Lundby and Hammarby,central Sweden (means7SE): shoot diameter at 55 cm height (D55) and the number of non-destructively measured shoots(n), the mean proportion of bark in the whole stand (BPstand), bark dry weight (BDW), wood dry weight (WDW), number ofliving shoots per stool, number of stools per hectare, the yield, net energy content of harvestable shoots on fresh weightbasis (EFW)
Site/age ofshootpopulation
Treatment D55 (n) BPstand
(%)BDW
(t ha�1)WDW(t ha�1)
Livingshoots
(stool�1)
Stools(plot�1)
Yield(t ha�1)
EFW
(GJ ha�1)
Lundby, 1 year Sludge 6.070.1
(752)
27.870.2 4.470.3 11.470.8 15.771.0 2971 15.871.1 3272
Ash 5.870.1
(741)
25.370.5 3.970.2 11.670.7 15.471.2 3071 15.570.9 3173
Sludge+ash 5.970.1
(726)
28.170.2 4.070.4 10.371.1 15.171.0 2672 14.371.5 2773
(Sludge+ash)�2 6.270.1
(797)
28.270.2 4.870.5 12.371.6 16.671.1 2771 17.172.1 3474
Control 5.570.1
(725)
27.870.2 3.570.2 9.271.6 15.171.0 3171 12.772.2 2872
Hammarby, 1
year
Sludge 6.670.1
(472)
23.670.2 4.270.5 13.871.7 9.870.6 2873 18.072.2 2372
Ash 6.970.1
(465)
23.770.2 4.570.5 14.771.8 9.770.5 2572 19.272.3 2474
Sludge+ash 6.770.1
(510)
24.070.2 4.770.4 15.071.6 10.670.7 2373 19.772.3 2475
(Sludge+ash)�2 7.070.1
(511)
23.770.2 4.970.2 15.670.5 10.670.7 2573 20.570.7 2571
Control 8.870.2
(465)
23.770.1 4.270.3 13.371.1 9.770.7 2671 17.571.5 2978
Lundby, 3
years
Sludge 13.870.3
(319)
24.770.2 3.970.3 11.970.8 6.670.5 2771 15.871.1 162711
Ash 13.070.3
(356)
25.370.5 3.970.2 11.670.7 7.470.5 2671 15.570.9 1597 9
Sludge+ash 13.270.3
(335)
25.070.6 3.670.3 10.771.2 7.070.6 2572 14.371.5 146715
(Sludge+ash)�2 13.570.3
(349)
24.670.4 4.270.5 12.971.7 7.370.5 2671 17.172.1 175722
Control 13.770.4
(253)
24.770.6 3.170.5 9.671.7 5.370.4 2772 12.772.2 130722
Hammarby, 3
years
Sludge 19.270.5
(170)
22.070.3 3.970.4 14.171.8 3.570.3 2572 18.072.2 184723
Ash 19.170.5
(191)
22.070.1 4.270.5 15.071. 4.070.3 2371 19.272.3 197724
Sludge+ash 18.170.4
(213)
22.270.2 4.470.4 15.371.6 4.570.3 2471 19.772.0 201721
(Sludge+ash)�2 18.770.4
(214)
22.170.2 4.570.2 15.970.5 4.570.3 2371 20.570.7 2107 7
Control 17.571.5
(183)
22.070.2 3.870.3 13.771.2 3.870.3 2371 17.571.5 179715
EFW was calculated by using a formula of net heating value for fresh material [5] and the yield of the experimental plots.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5920
at relatively high soil pH values was proportional to the
severity of nutrient and/or oxygen deficiency in the soil
and was associated with waterlogging of the soil [34]. In
addition to the effect on stand structure, waterlogged soils
may also affect the yield of plantations. Labreque et al. [25]
found that the dry biomass yield at poorly drained willow
plantations was 66% of the yield produced at well-drained
willow plantations in Montreal, Canada. Similar results were
found in this study. The aboveground shoot biomass at
Lundby was 80% of the yield produced in a sloping field at
Hammarby.
The applied treatments did not have any significant
effect either on the number of shoots per stool or on the
number of stools per plot. Consequently, the applied
treatments did not have any effect on the number of shoots
in each plot. This is in accordance with a study conducted
on an abandoned farmland in Southern Quebec, Canada,
where wastewater sludge application in a willow plantation
ARTICLE IN PRESS
30
35
20
0
25
HammarbyLundbySite
1 2 3Age (years)
BP sh
oot
Fig. 3 – Proportion of bark in the willow stands (BPstand) after
the 1st, 2nd and 3rd year of the 2nd cutting cycle at Lundby
and Hammarby. Means (7SE) for 5 treatments are shown.
35
00 10 20 30
20
25
30
BP s
tand
(%)
Yield (t ha-1)
Fig. 4 – Relationship between BPstand and harvestable shoot
biomass (yield) after the 1st, 2nd and 3rd year of the second
cutting cycle in two commercial willow plantations at
Lundby and Hammarby (R2¼ 0.44, po0.0001).
BPstand ¼ 35.6+yield(�103). Data are the plot means; n ¼ 3
years�2 sites�20 plots.
Ta
ble
5–
Eff
ect
so
fth
ep
rop
ort
ion
of
ba
rkin
the
sta
nd
(BP
sta
nd),
the
ag
eo
fth
esh
oo
tp
op
ula
tio
n,s
ite
an
dtr
eatm
en
to
nth
eq
ua
lity
of
the
wo
od
fuel,
kg
t�1
(N,P
,K)a
nd
gt�
1
(Zn
,N
i,C
d,
Cu
),g
row
na
tLu
nd
by
an
dH
am
ma
rby
inC
en
tra
lS
wed
en
(n¼
2si
tes�
2a
ges�
5tr
eatm
en
ts�
4re
pli
cati
on
s)
Eff
ect
df
NP
KZ
nN
iC
dC
u
Fp
Fp
FP
Fp
Fp
Fp
Fp
BP
sta
nd
11.2
5N
s.5.2
6o
0.0
56.1
6o
0.0
50.0
2N
s.1.8
7N
s.8.8
5o
0.0
11.9
4N
s.
Age
150.1
4o
0.0
01
17.2
2o
0.0
01
5.0
5o
0.0
517.1
3o
0.0
01
43.2
4o
0.0
01
16.9
0o
0.0
01
166.8
0.0
01
Sit
e1
0.4
8N
s.26.4
7o
0.0
01
2.8
7N
s.379.3
o0.0
01
0.7
3N
s.561.3
o0.0
01
0.5
7N
s.
Tre
atm
en
t4
2.3
8N
s.8.3
7o
0.0
01
12.8
1o
0.0
01
19.8
6o
0.0
01
7.9
5o
0.0
01
3.5
0o
0.0
514.3
8o
0.0
01
Ns.¼
no
tsi
gn
ifica
nt.
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 921
did not change the number of stems per plot compared to
control plots [35]. In contrast, wood ash application signifi-
cantly changed the structure of a willow bio-energy planta-
tion in New York State, USA, where wood ash increased
the size of stems and decreased the number of stems,
while the plot biomass production was not changed [36].
In the plantations at Lundby and Hammarby, wood ash
and/or sludge application did not change the structure of
the plantations, while the DW of the shoots in the
(sludge+ash)�2 treatment was increased, resulting in the
increased aboveground shoot biomass in this treatment
(Table 4).
ARTIC
LEIN
PRES
S
Table 6 – Wood fuel quality of harvestable shoot biomass from two willow plantations in Central Sweden, kg t�1 (N, P, K) and g t�1 (Zn, Ni, Cd, Cu), for the various treatments,if a potential harvest occurred annually or every 3 years
Site, treatment/harvest
interval
N P K Zn Ni Cd Cu
1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years 1 year 3 years
Lundby
Sludge 6.7370.04 4.0770.03 1.2670.01 0.7670.01 3.6470.03 2.4970.02 73.170.5 56.870.6 2.9470.02 0.9870.01 1.8970.01 1.1470.01 7.9870.01 2.8470.01
Ash 6.8170.04 3.8370.07 1.1470.01 0.7770.01 3.2670.02 2.3370.05 63.170.3 53.571.3 2.7170.01 0.9670.02 1.7070.01 1.3270.02 7.0370.01 2.7270.02
Sludge+ash 9.0170.03 3.9370.10 1.5070.01 0.8770.01 3.4670.01 2.4970.05 68.270.2 54.371.6 3.3170.01 1.5870.03 1.5870.01 1.1470.02 7.8370.01 3.0270.03
(Sludge+ash)�2 7.7670.03 3.7370.05 1.3770.01 0.8570.01 4.2870.02 2.5570.04 77.870.3 57.071.0 3.2270.01 1.0970.01 1.9470.01 1.1970.01 10.4570.01 3.1970.01
Control 7.8270.04 4.6970.11 1.1370.01 0.8470.01 3.2370.02 2.9970.08 63.270.3 48.871.4 2.5970.01 1.3670.03 1.7870.01 1.2070.02 8.0170.01 2.8670.03
Hammarby
Sludge 7.9070.09 4.0670.04 1.2770.01 0.5870.01 3.5870.04 1.9870.03 39.270.4 33.170.5 2.7970.03 1.0370.01 0.9070.01 0.4870.01 8.4370.02 2.9070.02
Ash 6.6770.02 4.2770.01 0.9070.01 0.5570.01 3.1170.01 1.8870.01 35.970.1 33.170.1 3.0470.01 0.9170.01 0.8370.01 0.4370.01 8.5370.01 2.8770.01
Sludge+ash 7.0070.02 4.0470.03 1.2270.01 0.5370.01 3.5770.01 1.7870.01 47.370.2 32.670.3 3.8470.02 0.8970.01 0.9470.01 0.4870.01 9.3170.01 2.4770.01
(Sludge+ash)�2 6.8370.07 4.3070.03 1.0270.01 0.5470.01 3.4670.04 2.5070.02 40.770.5 32.070.3 2.5570.02 0.7670.01 0.8270.01 0.5570.01 10.0770.01 2.7270.01
Control 6.6670.21 4.0670.03 0.9570.02 0.5570.01 2.6370.10 1.7870.01 39.171.5 25.170.2 2.4470.07 1.0770.01 0.7270.02 0.4970.01 7.5170.06 2.3470.01
The means (7SD) were calculated on the basis of the treatment plots (n ¼ 4).
BIO
MA
SS
AN
DB
IO
EN
ER
GY
32
(20
08
)9
14
–9
25
92
2
ARTICLE IN PRESS
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5 923
4.2. Wood fuel quality
Age of the shoot population had a major effect on the quality
of the wood fuel (Table 5). As the proportion of element-rich
bark decreased with the increasing age of the shoot popula-
tion, the concentrations of elements in the harvestable shoot
biomass decreased during the 3 years, improving the quality
of the wood fuel (Table 6). The lower biomass yield at Lundby
resulted in higher proportion of bark in the willow stand
(Fig. 4). The higher BPstand at Lundby (Fig. 3; Table 4) resulted
in higher concentrations of the elements in the harvestable
shoot biomass and, consequently, poorer quality of wood fuel
at this site compared to the wood fuel at Hammarby (Table 6).
The higher proportion of bark means also that the wood fuel
has a higher moisture content compared to the wood fuel
with lower proportion of bark. Moisture content usually
strongly influences the efficiency of the combustion system,
i.e. efficiency decreases with increasing moisture content.
Energy spent on vaporising water can only be recovered if
the combustion system is equipped with an appropriate
heat recovery device, for example a flue gas condensation
unit [5,37].
Fertilisation, soil type and soil pH are important factors
affecting biomass production in willows [24]. Fertilisation
with sludge and/or ash supported the harvestable biomass
production similarly as in the control treatment, where
mineral fertilisers were used (Tables 1 and 4), but reduced
the quality of the wood fuel by increasing concentrations of
the studied elements (except N) in the harvestable shoot
biomass (Table 5). The wood fuel quality was particularly
lower after the first growing season in terms of P and K
concentrations in the treatments containing sludge (Table 6).
However, the ability of fast-growing trees to accumulate
elements that reduce the quality of wood fuel seems to be
limited. The concentrations of Zn in the composite stem
samples of I-214 and Eridano poplar clones (50–80 g t�1)
exposed to industrial waste [38] were in a similar range as
the Zn concentrations in the harvestable shoot biomass of
willows at Lundby (Table 6). Fertilisation of these poplars with
industrial waste increased the total Zn concentration in the
soil by 20%, while the uptake of Zn by the trees was similar to
the uptake of Zn by willows at Lundby. At Lundby, fertilisation
with sludge in legally allowed doses increased the plant-
available fractions of Zn in the soil only by 8% compared to
the control treatments (Table 2).
Due to different absorption capacity for nutrients and trace
elements among species and clones, there are essential
differences in the chemical composition of biofuels. Watson,
Pulford and Riddell-Black [39] found that the Salix clones
S. burjatica ‘Germany’, S. x dasyclados, S. candida and S. spaethii
were more resistant to elevated concentrations of heavy
metals (i.e. produced more biomass on contaminated sites)
and had higher heavy metal concentrations in the wood
compared to the less resistant clones S. viminalis and
S. triandra. These less resistant clones had greater concentra-
tions of Cu and Ni in the bark compared to the wood. The old
Salix clone used in our study (78-021) had also significantly
higher concentrations of all studied elements in the bark
compared to wood. This indicates that the clone used in our
experiments was not an effective accumulator of heavy
metals. An interesting subject to investigate in the future
would be whether clones that accumulate heavy metals in
higher concentrations in wood compared to bark produce
more biomass on contaminated sites. However, higher
concentrations of heavy metals in wood would lower the
wood fuel quality. In this context, plant breeding and genetic
improvements of energy crops offer one possibility to
optimise yield and fuel quality in the future, e.g. to grow
Salix species with high yield and low concentrations of alkali
and heavy metals and other elements that have negative
effect on wood fuel quality (e.g. S and Cl) [5].
The concentrations of N, K (this study) and Cd were ca
4 times higher in the bark compared to wood both in 1- and
3-year-old shoots, and the concentrations of Zn were ca. 8
times higher in the bark compared to wood in the shoots of
both ages [17]. This means that, to ensure good quality of the
wood fuel, willows should be harvested when the proportion
of bark in the stand is relatively low. High N concentrations in
biomass fuels contribute to increased emissions of N oxides
during combustion of N-rich wood fuel, especially bark [37],
but the magnitude is affected by the technology of the
combustion process [5]. Problems with emission of N oxides
can be avoided in larger heat and power plants by the use of
flue gas filters, with which modern district heating plants in
Sweden are usually equipped. In the current study, the
differences in element concentrations between the bark and
wood were relatively larger than those between the treat-
ments. Consequently, the proportion of bark in the stand
should have a greater effect on the wood fuel quality
compared to the fertilisation with sludge and/or wood ash.
Still, the results concerning treatment effects on element
concentrations in this study should be viewed with caution,
since the number of shoots analysed per treatment was
relatively small (n ¼ 3–4). Site significantly affected the
concentrations of the heavy metals in willow bark and wood
in this experiment. The higher concentrations of Zn, Cd and
Ni in the clayey soil at Lundby compared to Hammarby
possibly resulted in higher concentrations of these metals in
the willow bark and wood at this site [17].
High ash content of bark lowers the heat transfer during
combustion [5,22]. The quantity of ash is thus strongly
influenced by the proportion of bark in wood fuels [40,41].
We did not investigate whether the fertilisation with sludge
and/or ash causes the enrichment with mineral elements in
the ashes. These questions should be addressed in the
following studies.
In conclusion, the proportion of bark in the studied willow
stands determined the quality of the wood fuel, since the bark
had significantly higher concentration of majority of the
elements. The proportion of bark in the wood fuel was
dependent on the age of the shoot population and shoot size
frequency distribution of the stand. The application of sludge
and/or ash decreased the wood fuel quality in terms of
increased concentrations of P, K and heavy metals in the bark
and wood. The proportion of bark in a willow stand can be
influenced by the choice of clone and site, planting density
and the frequency of harvests. For example, establishment of
willow fields with relatively low plant densities and extension
of the length of cutting cycles should decrease both the
concentrations of heavy metals in the composite wood fuel
ARTICLE IN PRESS
B I O M A S S A N D B I O E N E R G Y 3 2 ( 2 0 0 8 ) 9 1 4 – 9 2 5924
(kg t�1) and the emissions of nitrous oxides during combus-
tion due to relatively low proportion of bark. Accordingly,
shortening of rotation length should result in an increased
concentration of the above-mentioned elements in the
wood fuel.
Acknowledgements
The authors wish to thank R. Childs, E.-M. Fryk and
C. Segerqvist for technical assistance. Two anonymous
referees and Dr. Almir Karacic commented the manuscript.
Funding for this research was provided by the Swedish
National Energy Administration (project no. P12302-1) and
by the Royal Swedish Academy for Agriculture and Forestry,
which is gratefully acknowledged. Ultuna Egendom and Lars
Helgstrand are gratefully acknowledged for allowing us to use
their willow plantations at Linnes Hammarby and Lundby
Gard, respectively, for our experiments. Johannes Forkman
advised in statistics. Mary McAfee revised the language of an
earlier version of the manuscript.
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