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Effects of planting orientation and density on the soil solutionchemistry and growth of willow cuttings

Yang Cao a,b,c,*, Tarja Lehto c, Sirpa Piirainen d, Jussi V.K. Kukkonen e, Paavo Pelkonen c

a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F

University, Yangling 712100, Chinab Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources, Yangling 712100, ChinacSchool of Forest Sciences, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, FinlanddThe Finnish Forest Research Institute, Joensuu Unit, P.O. Box 68, FI-80101 Joensuu, FinlandeDepartment of Biology, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland

a r t i c l e i n f o

Article history:

Received 7 June 2011

Received in revised form

5 July 2012

Accepted 4 September 2012

Available online 24 September 2012

Keywords:

Willow

Fine roots

Production

Leaching

Horizontal orientation

Lysimeter

* Corresponding author. State Key LaboratorConservation, Northwest A&F University, Ya

E-mail address: yang.cao@nwsuaf.edu.cn0961-9534/$ e see front matter ª 2012 Elsevhttp://dx.doi.org/10.1016/j.biombioe.2012.09.

a b s t r a c t

Short rotation coppice (SRC) willow are established conventionally by inserting cuttings

vertically into the soil, but their ability to reproduce vegetative has also been demonstrated

by planting cuttings horizontally. There is a lack of knowledge about the biomass

production, root characteristic, and nutrient leaching of plantations established through

horizontally planted cuttings. A plot experiment was conducted to compare the soil

solution chemistry and the growth of stem and roots of willow cuttings (Salix schwerinii)

with vertical or horizontal planting orientation at two planting densities (corresponding to

7500 and 22,500 cuttings ha�1). The horizontally planted cuttings achieved the same stem

yields (4.08 t ha�1) as the vertically planted cuttings (4.86 t ha�1). The stem biomass was

doubled to a planting density of 22,500 cuttings ha�1 (6.34 t ha�1) compared to at

7500 cuttings ha�1 (3.36 t ha�1). The effect of planting orientation or density had no effect

on the root biomass or production. Willows decreased the conductivity, (NO2þNO3)eN and

the dissolved total N in the soil solution compared with unplanted plots, but the influence

was not detected systematically at each sampling depth or in each year. The differences in

soil water concentrations between planting treatments remained small. In conclusion, we

have shown that both planting orientation methods, horizontal and vertical, can be used

for preventing nutrient leaching and maximizing biomass production. It will also be

interesting to expand the application of horizontally planted willow materials in order to

stabilize slops, control erosion and reclaim contaminated sites.

ª 2012 Elsevier Ltd. All rights reserved.

1. Introduction heat and power and hydro thermal upgrading, into a variety of

Short rotation coppice (SRC) willow has been widely accepted

as a renewable energy source [1,2]. Willow biomass can be

converted by a wide range of technologies, such as combined

y of Soil Erosion and Drngling 712100, China. Te(Y. Cao).ier Ltd. All rights reserved006

energy forms and carriers [1]. To achieve the target set under

the Kyoto Protocol for energy production from renewable

sources, large areas of former agricultural land have been

proposed for use as SRC willow plantations [3]. About

yland Farming on the Loess Plateau, Institute of Soil and Waterl.: þ86 (0) 15389245368.

.

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3166

15,000 ha of SRC willow have been established in Sweden [1].

In the UK, in 2002 there were some 1500 ha SRC willow, which

had increased to 23,000 ha by 2006 [4]. Intensive research into

fast-growing SRC willow has led to a rapid expansion appli-

cation of SRC willow in terms of environmental performance.

The large-scale use of SRC willow as vegetation filters for

wastewater irrigation has been tested in southern Sweden

since 1993 [5,6]. The irrigation of willow with nutrient-rich

wastewater can lead to a substantial increase in yield and to

a reduction in the costs of fertilization and sewage treatment

[3,7]. However, the risks of element leaching to groundwater

and water-courses and the capability of willow to prevent

leaching should be known.

The ability of growth vigorously after coppice and the

extensive fine root system are important attributes of willow,

making it ideal for reducing nutrients entering streams [8]. In

our previous greenhouse experiment, we noticed that the

total N and nitrate concentrations of a soil solutionwere lower

in the pots that contained willow [9]. The large-scale move-

ment of non-point source pollution from agricultural land to

watercourses through the soil and in surface run-off is

complex and difficult to control. The natural defense system,

vegetated buffer zones, is a practical strategy for the control of

non-point pollution resulting from agriculture. Compared

with grass and tree buffer strips, willow crop is an ideal

vegetation type for the construction of riparian buffers [10].

Many willow buffers have been established along the banks of

streams, e.g. in the USA and in Sweden [8,11].

The essential ecophysiological characteristicsmakewillow

suitable for this kind of expansion in the range of its appli-

cations in biomass production and the environmental pro-

gramme [2]. This includes the ease of vegetative propagation

through the use of cuttings, its rapid growth and high yield

obtained on short rotations, and also its diffuse fibrous root

systems and its high tolerance of water-saturated soils.

Vegetative propagation can be achieved by thewillow cuttings

being placed vertically or horizontally in the soil [12]. The

method of placing cuttings vertically in the soil is commonly

used for planting SRC willow. However, the horizontal

planting of willow materials has been used only in the slope

stabilization and site restoration of stream banks, and in

relation to contaminated sediments [13e15]. On particular

sites where traditional vertical planting method is impossible

because of waterlogging, only bunched willow materials

planted horizontally into the sediments can provide stabili-

zation and restoration of the substrate [12].

Recently, a lay-flat plantingmachinewas designed in order

to increase planting speed and to reduce the establishment

cost of SRC willow. In contrast to the conventional SRC

Table 1 e Texture of the soil at different soil depths. The mean

Fraction size and content in percentage (%)

Depth (cm) Clay (<2 mm) Silt (2e20 mm) Coarse silt (20e

0e10 2.0 14.9 8.1

10e20 2.5 16.6 7.8

20e30 3.4 17.3 5.3

30e40 3.4 18.5 6.1

planting method, the lay-flat planter places willow rods

(1e2 m) horizontally in the ground at a depth of 8e10 cm [16].

LowtheeThomas et al. have shown [16] that the lay-flat

planting (horizontal) not only achieves the equivalent yield

as the traditionally planted SRC (vertical), but it also reduce

planting costs by 48%. Although no different biomass

production has been detected in the plot experiment between

planting willow cuttings vertically and horizontally,

McCracken et al. [17] still argue that planting 20 cm cuttings

vertically is the best practice for the establishment of SRC

willow. The reason for their decision is that approximately

330%more propagationmaterials were required in the case of

the horizontally planted willow rods (2 m) than the vertically

planted cuttings (20 cm) in plots of the same size.

In a previous greenhouse experiment to investigate the

effects of horizontal or vertical planting orientation using the

same lengthofwillow,nodifferencesbetweenthe twoplanting

orientations were found after 16 weeks in the stem yield or in

the leaf andfine root biomass [9]. An interesting point observed

in this greenhouse experiment was that there was a delay in

the first two growing weeks in the appearance of the shoots of

thehorizontally planted cuttings compared to the shoots of the

vertically planted cuttings. More coarse roots were also

observed in the pots containing the horizontally planted

cuttings. The stem yield also increased with planting density.

However, in this greenhouse experiment no effect of planting

density on root biomasswas observed. This is explained by the

relatively small pots used,which restricted the fast growth and

extension of the willow roots. The effects of planting orienta-

tion and density on the root system under field conditions

involving a long observation period remained unknown.

In this present study, the previous greenhouse experiment

was repeated under field conditions. The objective of the

study was to discover the effects of planting orientation and

density on the growth of the stem and root system of willow

cuttings using a two-year observation period under field

conditions, and also the influence of different treatments on

nutrient leaching in the rooting zone. The hypothesis of this

study was that a horizontal planting orientation and/or high

planting density would have a positive effect on the root

system of willow, thereby reducing nutrient leaching.

2. Materials and methods

2.1. Study area, experimental design and management

This experiment was conducted at the Botanic Gardens of the

University of Eastern Finland (62。350N, 29。460E). The

values of samples from the four blocks.

63 mm) Sand (63e200 mm) Coarse sand (200e2000 mm)

11.0 50.9

12.5 58.4

10.2 60.8

12.3 55.3

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3 167

experiment area was established on a former lawn. Themajor

part of the soil was coarse sand (Table 1). The experimental

design was a full-factorial design with planting orientation

and density two factors. The planting orientation included

both vertical (V) and horizontal (H) levels. The planting density

consisted of two levels: low density (LD) and high density (HD).

In addition, there was one unplanted plot (CT) in each block.

All of the treatments were replicated four times in a random-

ized block design.

In June 2008, the experiment area was ploughed and har-

rowed, and divided into four blocks. The plot size was 2 � 2m,

and each plot was separated by plastic (0.2 m depth below the

soil surface) to avoid anymajor interaction between plots. The

distance between each plot was 0.5 m to provide a pathway.

Cuttings of Salix schwerinii (0.25 m in length) were planted by

hand with either a vertical (V) or a horizontal (H) planting

orientation, with either 3 or 9 cuttings per plot (corresponding

to 7500 (LD) and 22,500 (HD) cuttings ha�1). The tops of the

vertical cuttings were 5 cm above the soil surface, while the

horizontal cuttings were placed 5 cm below the soil surface.

The cuttings were planted 50 cm from each other in the plot.

After planting, the experiment area was fenced to prevent

the access of hare intent on browsing on the plants. During

the winter 2008e2009, however, the shoots were completely

grazed by vole. The effect was equal to cutting back the

shoots, whichwould in any case have been performed in order

to promote more shoots per stool in 2009. In May 2009, each

plot was covered with black polythene mulch, which allowed

rainwater infiltration while also controlling the growth of

weeds. Manual weeding of the paths were carried out occa-

sionally at the beginning of the growth period in each year.

The plantations were not fertilized, but they were irrigated

throughout the summer of 2008 using sprinkler equipment. In

summer 2009, no irrigation was used which caused a limited

number and volume of soil leachate samples. In the summer

2010, the plots were irrigated each Sunday with 40 mm of

water from 4 July to 29 August mainly for getting adequate

water samples. The groundwater level remained 1.6 m below

the soil surface from 19 April to 9 May 2010, but thereafter it

could not be detected from the groundwater well down to

a depth of 2 m. The meteorological conditions were recorded

from the Linnunlahti station of the Finnish Meteorological

Institute Network within 2 km of the Botanic Gardens (Fig.1).

The effective temperature sum (daily mean temperature

above þ5 �C) during the growing period was 1276 �C d in 2009

and 1513 �C d in 2010, while the precipitation from April to

October was 343 mm in 2009 and 324 mm in 2010.

Fig. 1 e Daily precipitation (mm) and mean daily tempera

2.2. Measurements of stem production

The height and the number of living shoots were measured

eachmonth throughout the growing seasons of 2009 and 2010

in each plot. The annual stem production was measured by

harvesting the shoots in both autumns when no leaves were

present. The diameters of the living willow shoots were

measured at shoots 30 cm above the shoot base and then cut

back to 5 cm high stumps. The samples were dried at 105 �Cuntil attaining constant weight.

2.3. Measurements of the biomass and production offine roots

Fine root growth and turnover play a crucial role in carbon,

nutrient and water cycles. Therefore, it is of importance to

accurately estimate the standing biomass and production of

fine roots. To limit soil disturbance, a modification of the in-

growth core method, the root mesh net method where only

a two-dimensional net is inserted into the soil, was used to

estimate the fine root production [18]. In June 2009, 4 indi-

vidual nylon mesh nets (10 cm width and 30 cm length, 2 mm

mesh size) were inserted vertically into the soil, with the aid of

a steel plate and a hammer, in a single row about 25 cm from

the cuttings in each plot. Two root mesh nets were extracted

in the October of both 2009 and 2010. To extract the nets,

blocks of soil that contained the mesh nets were lifted using

a narrow garden spade. The fine roots that had grown through

themesh 2 cm from each side of the netswere defined in order

to estimate the root production. Two samples of the same soil

profile depth were pooled into a single sample. The fine roots

were washed out of the soil manually and dried at 105 �C until

attaining constant weight.

The standing root biomass wasmeasured in the autumn of

2010 using the auger method with a 3.5 cm core diameter at

three depths: 0e10, 10e20 and 20e30 cm. The coring locations

were situated around the central willow plant in each plot.

Four soil cores were sampled from four directions at 25 cm

distance from the central willow plant, and samples from the

same soil profile depth were pooled into a single sample. The

fine roots (�2 mm in diameter) were washed out of the soil

manually, and dried at 105 �C until attaining constant weight.

2.4. Soil solution sampling and laboratory analyses

In the middle of May 2009, one zero tension and one tension

lysimeter were installed in each plot of three blocks at

ture (�C) during the growing period in 2009 and 2010.

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3168

different depths so that the soil solution could be sampled.

Any potential disturbance of the willow plants was avoided.

The zero tension lysimeter installed at a depth of 25 cm below

the soil surface had been constructed of a polythene plastic

funnel filled with quartz sand. It had a collecting area of

299 cm2 and it was fitted to a 2 L sample collection bottle. The

tension lysimeter consisted of a P80 ceramic cup (67 mm in

length and 12 mm in diameter, Hoechst CeramTec AG,

Germany), a plastic pipe (connected by a nylon connector

Swagelok� PFA) and a glass bottle. Tension lysimeters were

installed at a depth of 60 cm, and a tension of 60 kPa was

maintained on a fixed regular basis with an electrical pump

that was used at intervals of 6 h (6 h on, 6 h off). Soil solutions

were sampled weekly from 15 June to 12 October 2009 and

from 19 April to 11 October 2010. The samples were stored

overnight in a cold room (4 �C) at the Botanic Gardens. The

following morning they were then transferred to the labora-

tory, where their pH (PHM 92 Radiometer) and conductivity

(CDM 92 Conductivitymeter) were measured from unfiltered

samples. In subsequent analyses, the samples were filtered

(Schleicher & Schuell GF 52 glass wool filter) and stored in

a freezer (�18 �C). The dissolved organic carbon (DOC mg l�1)

wasmeasuredwithin 2 or 3 days from samples stored in a cold

room (4 �C) using a TOC-5000A (Total organic Carbon

Analyzer, Shimadzu) in 2009 and a Multi N/C 2100 (Analytik

Jena, Germany) in 2010 according the standardmethods of the

Finnish Forest Research Institute. The dissolved total nitrogen

(DTN,mg l�1), ammonium (NH4eN,mg l�1) and the sum of the

nitrite and nitrate (NO2þNO3,mg l�1) weremesasuredwithin 6

months using a FIA-star 5000 analyzer (FOSS TECATOR) from

frozen samples. If the concentrations were smaller than the

detection limit, a value half of the detection limits was used as

a substitute [19].

2.5. Statistics

Repeated measures ANOVA was used to compare stand stem

production, the mean height of the tallest shoot, the number

of living stems, and root production between 2009 and 2010.

Root biomass and production were compared using two-way

ANOVA according to the respective planting orientation,

planting density including taking into consideration the

different soil depths as a repeated measure. The significant

differences in the soil water chemical concentrations between

treatments were tested using a mixed linear model [19,20]. In

the model, the treatment was set as a fixed factor, while the

Table 2 e Annual stem dry biomass production, the mean heigmean diameter and dryweight of individual shoots for the diffethe mean in parentheses, n [ 3).

2009

H þ LD V þ LD H

Stem dry biomass production (t ha�1) 1.4 (0.07) 1.4(0.2) 2

Mean height of the tallest shoots (cm) 316(9) 303(7) 3

Number of shoots per stool 3.2(0.3) 4.5(0.3) 4

Mean diameter of individual shoots (mm) 11.2(0.6) 10.4(0.6) 10

Mean dry weight of individual shoots (g) 65.0 (7) 57.0(6) 52

plot was a covariate, block and interaction between block and

treatment were random factors. In addition, the sampling

week was a repeated factor. The emmeans subcommand

using the Bonferronimethodwas used in themodel for testing

the differences between treatments in each year or month or

changes in time. Concentrations undergoing logarithmic

transformation were used in the model. The statistical

significance was assessed at a level of 0.05, and the statistical

analyses were performed using PASW software (PASW,

ver.18.0, USA).

3. Results

3.1. Stem production

The annual stem production, the mean height of the tallest

shoot, the number of living stems and the diameter of the

stems had obviously increased in 2010 in comparison with

2009 (P < 0.0001, Table 2). Twice as much stem biomass was

produced at the planting density of 22,500 cuttings ha�1 than

at 7500 cuttings ha�1 (P < 0.001, Table 2). However, there was

no difference in the stem biomass for either planting orien-

tation, or in the interaction between it with planting density.

There was also no difference in the effect of the respective

treatments on the mean height of the tallest shoots. The

vertical planting orientation produced a higher number of

living shoots than the horizontal planting orientation

(P ¼ 0.001, Table 2). The mean diameter and weight of indi-

vidual shoots was significantly higher at the density of

7500 cuttings ha�1 than at 22,500 cuttings ha�1 (P < 0.01 for

both, Table 2).

3.2. Fine root biomass production

The fine root biomass as determined from the core samples

(Fig. 2) and the fine root production as determined from the

root nets (Fig. 3) declined with increasing soil depth. The

surface soil layer (0e10 cm) contained a higher amount of fine

root biomass than the other two soil layers (P ¼ 0.003 for both,

Fig. 2). The root production with a soil layer of 20e30 cm was

significantly lower than with the other upper soil layers

(P < 0.004). The fine root production at 0e10 cm soil layer was

obviously higher during the two consecutive growing seasons

of 2009 and 2010 than in the single growing season of 2009

(P ¼ 0.035, Fig. 3). Neither planting orientation nor planting

ht of the tallest shoot, the number of living stems, and therent treatments in the two growing years (Standard error of

2010

þ HD V þ HD H þ LD V þ LD H þ HD V þ HD

.5(0.1) 3.3(0.3) 5.5(0.5) 5.1(0.9) 8.2(0.1) 10.4(0.9)

31(14) 309(7) 390(9) 357(12) 354(4) 372(8)

.2(0.7) 5.9(0.4) 13.5(1.2) 11.6(1) 11.3(0.9) 14.3(0.9)

.3(0.4) 9.6(0.4) 12.1(0.4) 12.0(0.5) 11.2(0.3) 11.2(0.3)

.3(4) 45.7(3) 102.0(8) 102.4(9) 78.2(5) 88.5(3)

Fig. 2 e Fine root biomass in different soil depths in the

different treatments estimated using soil coring in October

2010. Error bars indicated the standard error of the mean

(n [ 4).

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3 169

density affected the fine root biomass, but the largest fine root

biomass was observed in the horizontally planted cuttings in

the high density treatment (Fig. 2).

3.3. Soil solution chemistry

During the sampling period, the total of precipitation in 2009

between 15 June and 12 October was 303 mm, while in 2010 it

was 324 mm between 19 April and 11 October. The number of

soil leachate samples collected with zero tension lysimeters

from below the depth of 25 cm was small in 2009 (n ¼ 42),

whereas in 2010, as a result of irrigation, the number of soil

leachate samples increased (n ¼ 120). The total of irrigation in

the period of July and August was 440 mm. The mean volume

ofwater collected in 2010with the zero tension lysimeterswas

332mm,which included 212mm in July and August. However,

several differences between the treatments were detected in

2009, e.g. the mean annual conductivity of the soil leachates

collected from below the depth of 25 cmwas higher for the CT

treatments (398.7 � 134.4 mS cm�1) than for the planted plots

(65.1� 9.9 mS cm�1) (P¼ 0.04), but no differences were detected

between planted treatments. In contrast, the annual mean

DOC concentration was lower for the CT treatments

Fig. 3 e Annual fine root production at different soil depths est

season of 2009 (a) And in two consecutive growing seasons of 2

mean (n [ 4).

(21.6� 10.2mg l�1) than for the planted plots (45.6� 5.0mg l�1)

(P ¼ 0.02), but no differences were detected between the

planted treatments. No significant differences were observed

for pH, DTN, NH4eN, (NO2þNO3)eN concentrations in 2009.

In 2010, there were no statistically significant differences

between the planted and CT treatments in terms of their

annual pH, conductivity, DTN, NH4eN or DOC concentrations

of soil leachates collected with zero tension lysimeters at

a depth of 25 cm. The monthly mean concentration of NH4eN

in the soil leachates from all of the plots in April (1.1 mg l�1)

was significantly higher than in the irrigated months (0.6 and

0.4 mg l�1 for July and August, respectively) (P ¼ 0.001). The

annual mean (NO2þNO3)eN concentration of soil leachates

collected from below the 25 cm was higher in the control

treatment (9.2 mg l�1) than in the planted treatments

(0.7 mg l�1) (P ¼ 0.04, Fig. 4), but no differences were found

between the planted plots. The mean monthly concentration

of (NO2þNO3)eN in April (1.3 mg l�1) was significantly higher

than in July (0.5 mg l�1) and August (0.4 mg l�1) in the planted

treatments (P ¼ 0.001). The mean monthly concentration of

DOC from the zero lysimeters was slightly higher during the

irrigating period (28.6 and 30.0 mg l�1 for July and August,

respectively) than in April (24.3 mg l�1), but no statistical

differences were detected between the different months.

In the soil leachate samples collected by tension lysimeters

from a depth of 60 cm, the NH4eN concentration was high on

the first sampling occasions in 2009 (Fig. 5). The mean annual

concentrations of NH4eN did not differ between treatments.

The monthly mean concentrations of NH4eN were gradually

decreased in 2009 and remained low in 2010, but on an annual

level the difference was not significant (Fig. 5).

The concentrations of (NO2þNO3)eN in soil water collected

by means of tension lysimeters from a depth of 60 cm were

high in the first samples collected after installation in 2009

(Fig. 5). The mean annual concentrations of (NO2þNO3)eN did

not differ between 2009 and 2010, and no differences between

the treatments were observed on an annual level. In 2009, in

all of the plots, the mean monthly concentrations during the

summer months (June, July, and August) were significantly

higher (8.4, 8.7 and 8.2 mg l�1, respectively) than in autumn

months (7.4 and 6.9 mg l�1 for September and October,

respectively). In 2010, in all of the plots, the mean monthly

concentrations of (NO2þNO3)eN were significantly decreased

imated using the root net method in the single growing

009 and 2010 (b). Error bars indicated standard error of the

Fig. 4 eWeekly concentrations (mg lL1) of NH4eN (a), NO2DNO3eN (b) AndDOC (c) In soil leachates collected fromzero tension

lysimeters (at a depth of 25 cm) in different treatment plots in 2010. Error bars indicated standard error of the mean (n [ 3).

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3170

from April, May and June (3.1, 3.0 and 2.8 mg l�1, respectively)

to July (1.8 mg l�1), and they decreased significantly to their

lowest concentration in August, September and October (1.4,

0.6 and 0.5 mg l�1, respectively).

Fig. 5 e Mean weekly concentrations (mg lL1) of NH4eN (a), (NO

tension lysimeters (at a depth of 60 cm) in the different treatme

(n [ 3).

The concentration of DOC in soil water collected by means

of tension lysimeters from a depth of 60 cm was high in the

first sampling conducted in 2009. The mean annual concen-

trations of DOC did not differ between 2009 and 2010, and no

2DNO3)-N (b) And DOC (c) In soil water collected by using

nt plots. Error bars indicated standard error of the mean

b i om a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3 171

differences between treatments were observed on an annual

level. In 2009, in all of the plots, the mean monthly concen-

trations had decreased significantly from June (76.7 mg l�1) to

July and August (39.4 and 36.4mg l�1, respectively), and also to

September and October (28.4, 24.0mg l�1, respectively). During

the irrigation period of 2010, the mean monthly concentra-

tions of DOC in July (27.2 mg l�1) was significantly higher than

in the other months (19.9, 22.0, 26.9, 23.3, 21.0, and 19.7 mg l�1

for April, May, June, August, September and October, respec-

tively) (P ¼ 0.001). Only the mean annual conductivity and

concentration of DTN in the soil water collected using tension

lysimeters were higher in 2010 for the CT plots

(755.4 � 187.4 mS cm�1, 53.7 � 11.2 mg l�1 for conductivity and

DTN, respectively) than for the planted plots

(92.4� 13.5 mS cm�1, 2.9� 0.8mg l�1 for conductivity and DTN,

respectively) (P ¼ 0.01 for both). However, there were no

differences detected between the planted plots.

4. Discussion

The possibility of establishing a willow plantation using the

horizontally planted cuttings was proved again in the present

study. While we used the same length of cuttings throughout

this experiment, the horizontally planted cuttings produced

a stem yield that was similar to that of the vertically planted

cuttings, as was also found in our earlier pot experiment [9].

Earlier studies have compared horizontal and vertical planting

methods using cuttings of different length. However, our

results are also consistent with those previous experiments

that involved planting different length cuttings. Similar stem

yields were produced by planting 0.25 m long cuttings verti-

cally and 0.9 m long cuttings horizontally with the same

planting density (1000 cuttings ha�1) and by planting 0.2 m

long cuttings vertically and 2 m long cuttings horizontally

with different planting densities [16,17]. The plot experiment

conducted by McCracken et al. [17] proved, however, that

horizontally planted 0.1 m long willow cuttings at a density of

25 000 cuttings ha�1 produced significantly less stem biomass

than did 2 m long willow rods planted at a density of

5000 cuttings ha�1. It was also found that, in addition to the

length of the cuttings, their planting depth in the soil also had

an influence on the growth of willow cuttings. A UK study has

shown that cuttings of equal lengths (20 cm) planted hori-

zontally at a depth of 15 cm produced a greater shoot length

than did those planted at a depth of 5 cm [21].

In the present experiment, a significant difference in the

stem biomass yields for the two planting densities was

observed. This result is consistent with previous research into

the effect of planting density on stem biomass yields [22,23].

However, only twice as much stem biomass was produced

with the planting density of 22,500 cuttings ha�1 as high as at

7500 cuttings ha�1 in the present field experiment. The

explanation suggested in earlier experiments was that the

standing stem biomass production finally becomes indepen-

dent of planting density up a certain range of planting

densities, e.g. 20,000 cuttings ha�1 [23,24]. Hence, the current

practice planting density is a planting density of

15,000 cuttings ha�1 in the UK, and 12,000 cuttings ha�1 in

Sweden [23,25]. In contrast to the situation with stands

production, the mean diameter and weight of individual

plants was significantly larger at the density of

7500 cuttings ha�1 than at 22,500 cuttings ha�1. The results

produced by Bullard et al. [22,26] have shown that there is

a negative non linear relationship between the weight of

individual plants and their planting density.

In this present study, roots, which were less than 2 mm in

diameter, were measured only down to a depth of 30 cm. In

the present field, experiment and our earlier pot experiment,

there was also no significant influence of planting orientation

and density on the fine root biomass [9]. Nevertheless, vari-

ability was very high in this study, and the fine root biomass

was found to be highest in horizontally planted cuttings with

high planting density. Such results contrast with the previous

assumption that planting density should change the growth

and development of a root system just as it would above-

ground growth and development [27]. However, no other

specific research has been done on the effect of planting

density on root distribution and the growth of willow plan-

tations [27]. Willow roots were located primarily within the

top 30 cmof the soil, whereas their depth in soilmay extend to

1.3 m, and occasionally to a depth of 3 m [28]. The root char-

acteristics of willow coppice are influenced by numerous

factors, including soil conditions, management, coppice cycle

and species [29]. Fertilization, for example, significantly

reduces the biomass and annual production of fine roots [29].

Quantifying the fine root biomass and production is, however,

labor intensive, costly, and destructive. The root mesh net

method has been used to limit soil distribution in estimating

fine root production. This method has been found to produce

the same results as the in-growth coremethod [18]. Some new

non-destructive techniques, such as electrical resistance

tomography (ERT) and electrical impedance with a single

frequency or multi-frequencies, are under investigation for

their ability in observing root systems and their response

under different growing conditions [30e32]

Zero lysimeters are open at the top and rely on gravity to

collect water [33]. Hence, in contexts of low soil water avail-

ability and dry conditions, such as the summer months of

2009 and 2010 in the present study, it is often impossible to

obtain a sufficient volume of samples to make measurements

using zero tension lysimeters. In contrast to zero tension

lysimeters, however, tension lysimeters can be supplied with

a vacuum. In consequence, tension lysimeters can easily be

used to collect water percolation under large-area field

conditions and from below a specific soil depth, e.g. from

beneath a root zone [33]. The installation of lysimeters may,

however, result in appreciable soil disturbance [34]. In the

present study, high concentrations of chemical elementswere

still observed in water samples following the installation of

lysimeters for one month in 2009. Subsequently, the concen-

trations of the elements measured decreased with time, and

the harvest action in winter 2009 did not affect the concen-

trations of elements in soil water in 2010. Previous studies

have shown that relatively high leaching occurs only during

the establishment period and at the final removal plants

period [4]. Results from the present study and the previous

greenhouse experiment [9] indicate that willows are plants

that may potentially decrease the N concentrations in the soil

leachate, especially at 25 cm soil layer in present study.

b i om a s s an d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5e1 7 3172

However, the concentrations were much smaller than those

figures (NO3eN <11 mg l�1, NO2eN <0.15 mg l�1, NH4eN

<0.4 mg l�1) set as limits for drinking water in Finland.

In conclusion, this three-year field experiment has

demonstrated that horizontally planted willow cuttings can

achieve the same performances in terms of stem yield and

root biomass, and they can have a similar effect on nutrient

leaching as conventional vertically planted willow cuttings.

Hence, the horizontal planting orientation may well serve as

an alternative planting method aimed at achieving biomass

production. It will be interesting in further experiments to

explore the influence of willow clones, the length or diameter

size of cuttings, and the planting depth of horizontally planted

willow materials in the stabilization of slops, the control of

erosion, the reclamation of contaminated sites, and the

mitigation of leaching in riparian zones.

Acknowledgment

Wewould like to thank Dr Aki Villa and the laboratory staff at

the School of Forest Sciences and Department of Biology,

University of Eastern Finland, for their contribution to this

study, and also the laboratory of the Finnish Forest Research

Institute, Joensuu Research Unit, for conducting the water

nutrient analysis. We also greatly appreciate the contribution

made by the staff of the Botanic Gardens of the University of

Eastern Finland. In addition, we should like to thank Docent

Tapani Repo (Finnish Forest Research Institute, Joensuu unit)

for his comments on themanuscript and Dr John A Stotesbury

(University of Eastern Finland) for the English revision of the

manuscript. Financial support for this study was provided by

the China Scholarship Council (CSC, China), the Niemi-Saatio

(Finland), the Koneen-Saatio (Finland), and the Academy of

Finland (project 214545).

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