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Effects of planting orientation and density on the soil solution chemistry and growth of willow cuttings
b i o m a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5 e1 7 3
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http://www.elsevier.com/locate/biombioe
Effects of planting orientation and density on the soil solution chemistry 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, China b Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources, Yangling 712100, China c School of Forest Sciences, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland d The Finnish Forest Research Institute, Joensuu Unit, P.O. Box 68, FI-80101 Joensuu, Finland e Department of Biology, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
article info
abstract
Article history:
Short rotation coppice (SRC) willow are established conventionally by inserting cuttings
Received 7 June 2011
vertically into the soil, but their ability to reproduce vegetative has also been demonstrated
Received in revised form
by planting cuttings horizontally. There is a lack of knowledge about the biomass
5 July 2012
production, root characteristic, and nutrient leaching of plantations established through
Accepted 4 September 2012
horizontally planted cuttings. A plot experiment was conducted to compare the soil
Available online 24 September 2012
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
Keywords:
7500 and 22,500 cuttings ha1). The horizontally planted cuttings achieved the same stem
Willow
yields (4.08 t ha1) as the vertically planted cuttings (4.86 t ha1). The stem biomass was
Fine roots
doubled to a planting density of 22,500 cuttings ha1 (6.34 t ha1) compared to at
Production
7500 cuttings ha1 (3.36 t ha1). The effect of planting orientation or density had no effect
Leaching
on the root biomass or production. Willows decreased the conductivity, (NO2þNO3)eN and
Horizontal orientation
the dissolved total N in the soil solution compared with unplanted plots, but the influence
Lysimeter
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
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
heat and power and hydro thermal upgrading, into a variety of 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
* Corresponding author. 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, China. Tel.: þ86 (0) 15389245368. E-mail address: [email protected] (Y. Cao). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.09.006
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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 application 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 solution were lower in the pots that contained willow [9]. The large-scale movement 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 characteristics make willow suitable for this kind of expansion in the range of its applications in biomass production and the environmental programme [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 the willow 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 stabilization and restoration of the substrate [12]. Recently, a lay-flat planting machine was designed in order to increase planting speed and to reduce the establishment cost of SRC willow. In contrast to the conventional SRC
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 propagation materials 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 length of willow, no differences between the two planting orientations were found after 16 weeks in the stem yield or in the leaf and fine 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 the horizontally 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 biomass was 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 orientation 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。350 N, 29。460 E). The
Table 1 e Texture of the soil at different soil depths. The mean values of samples from the four blocks. Fraction size and content in percentage (%) Depth (cm) 0e10 10e20 20e30 30e40
Clay (<2 mm)
Silt (2e20 mm)
Coarse silt (20e63 mm)
Sand (63e200 mm)
Coarse sand (200e2000 mm)
2.0 2.5 3.4 3.4
14.9 16.6 17.3 18.5
8.1 7.8 5.3 6.1
11.0 12.5 10.2 12.3
50.9 58.4 60.8 55.3
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experiment area was established on a former lawn. The major 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 randomized block design. In June 2008, the experiment area was ploughed and harrowed, and divided into four blocks. The plot size was 2 2 m, and each plot was separated by plastic (0.2 m depth below the soil surface) to avoid any major 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 ha1). 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, which would 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 occasionally 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.
2.2.
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Measurements of stem production
The height and the number of living shoots were measured each month 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 C until attaining constant weight.
2.3. Measurements of the biomass and production of fine 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 ingrowth 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 individual 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 the mesh 2 cm from each side of the nets were 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 was measured 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
Fig. 1 e Daily precipitation (mm) and mean daily temperature ( C) during the growing period in 2009 and 2010.
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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 laboratory, 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 l1) was measured within 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 standard methods of the Finnish Forest Research Institute. The dissolved total nitrogen (DTN, mg l1), ammonium (NH4eN, mg l1) and the sum of the nitrite and nitrate (NO2þNO3, mg l1) were mesasured within 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].
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 Bonferroni method was used in the model 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 ha1 than at 7500 cuttings ha1 (P < 0.001, Table 2). However, there was no difference in the stem biomass for either planting orientation, 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 individual shoots was significantly higher at the density of 7500 cuttings ha1 than at 22,500 cuttings ha1 (P < 0.01 for both, Table 2).
3.2. 2.5.
Fine root biomass production
Statistics 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
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 height of the tallest shoot, the number of living stems, and the mean diameter and dry weight of individual shoots for the different treatments in the two growing years (Standard error of the mean in parentheses, n [ 3). 2009
1
Stem dry biomass production (t ha ) Mean height of the tallest shoots (cm) Number of shoots per stool Mean diameter of individual shoots (mm) Mean dry weight of individual shoots (g)
2010
H þ LD
V þ LD
H þ HD
V þ HD
H þ LD
V þ LD
H þ HD
V þ HD
1.4 (0.07) 316(9) 3.2(0.3) 11.2(0.6) 65.0 (7)
1.4(0.2) 303(7) 4.5(0.3) 10.4(0.6) 57.0(6)
2.5(0.1) 331(14) 4.2(0.7) 10.3(0.4) 52.3(4)
3.3(0.3) 309(7) 5.9(0.4) 9.6(0.4) 45.7(3)
5.5(0.5) 390(9) 13.5(1.2) 12.1(0.4) 102.0(8)
5.1(0.9) 357(12) 11.6(1) 12.0(0.5) 102.4(9)
8.2(0.1) 354(4) 11.3(0.9) 11.2(0.3) 78.2(5)
10.4(0.9) 372(8) 14.3(0.9) 11.2(0.3) 88.5(3)
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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).
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 of water collected in 2010 with the zero tension lysimeters was 332 mm, which included 212 mm 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 cm was higher for the CT treatments (398.7 134.4 mS cm1) than for the planted plots (65.1 9.9 mS cm1) (P ¼ 0.04), but no differences were detected between planted treatments. In contrast, the annual mean DOC concentration was lower for the CT treatments
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(21.6 10.2 mg l1) than for the planted plots (45.6 5.0 mg l1) (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 l1) was significantly higher than in the irrigated months (0.6 and 0.4 mg l1 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 l1) than in the planted treatments (0.7 mg l1) (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 l1) was significantly higher than in July (0.5 mg l1) and August (0.4 mg l1) 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 l1 for July and August, respectively) than in April (24.3 mg l1), 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 l1, respectively) than in autumn months (7.4 and 6.9 mg l1 for September and October, respectively). In 2010, in all of the plots, the mean monthly concentrations of (NO2þNO3)eN were significantly decreased
Fig. 3 e Annual fine root production at different soil depths estimated using the root net method in the single growing season of 2009 (a) And in two consecutive growing seasons of 2009 and 2010 (b). Error bars indicated standard error of the mean (n [ 4).
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Fig. 4 e Weekly concentrations (mg lL1) of NH4eN (a), NO2DNO3eN (b) And DOC (c) In soil leachates collected from zero tension lysimeters (at a depth of 25 cm) in different treatment plots in 2010. Error bars indicated standard error of the mean (n [ 3).
from April, May and June (3.1, 3.0 and 2.8 mg l1, respectively) to July (1.8 mg l1), and they decreased significantly to their lowest concentration in August, September and October (1.4, 0.6 and 0.5 mg l1, respectively).
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 concentrations of DOC did not differ between 2009 and 2010, and no
Fig. 5 e Mean weekly concentrations (mg lL1) of NH4eN (a), (NO2DNO3)-N (b) And DOC (c) In soil water collected by using tension lysimeters (at a depth of 60 cm) in the different treatment plots. Error bars indicated standard error of the mean (n [ 3).
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differences between treatments were observed on an annual level. In 2009, in all of the plots, the mean monthly concentrations had decreased significantly from June (76.7 mg l1) to July and August (39.4 and 36.4 mg l1, respectively), and also to September and October (28.4, 24.0 mg l1, respectively). During the irrigation period of 2010, the mean monthly concentrations of DOC in July (27.2 mg l1) was significantly higher than in the other months (19.9, 22.0, 26.9, 23.3, 21.0, and 19.7 mg l1 for April, May, June, August, September and October, respectively) (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 cm1, 53.7 11.2 mg l1 for conductivity and DTN, respectively) than for the planted plots (92.4 13.5 mS cm1, 2.9 0.8 mg l1 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 vertically and 0.9 m long cuttings horizontally with the same planting density (1000 cuttings ha1) 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 ha1 produced significantly less stem biomass than did 2 m long willow rods planted at a density of 5000 cuttings ha1. 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 horizontally 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 ha1 as high as at 7500 cuttings ha1 in the present field experiment. The explanation suggested in earlier experiments was that the standing stem biomass production finally becomes independent of planting density up a certain range of planting densities, e.g. 20,000 cuttings ha1 [23,24]. Hence, the current practice planting density is a planting density of 15,000 cuttings ha1 in the UK, and 12,000 cuttings ha1 in Sweden [23,25]. In contrast to the situation with stands
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production, the mean diameter and weight of individual plants was significantly larger at the density of 7500 cuttings ha1 than at 22,500 cuttings ha1. 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, variability 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 aboveground 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 plantations [27]. Willow roots were located primarily within the top 30 cm of the soil, whereas their depth in soil may extend to 1.3 m, and occasionally to a depth of 3 m [28]. The root characteristics 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 core method [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 availability 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 elements were still observed in water samples following the installation of lysimeters for one month in 2009. Subsequently, the concentrations of the elements measured decreased with time, and the harvest action in winter 2009 did not affect the concentrations 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.
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However, the concentrations were much smaller than those figures (NO3eN <11 mg l1, NO2eN <0.15 mg l1, NH4eN <0.4 mg l1) 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 We would 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 the manuscript 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-Sa¨a¨tio¨ (Finland), the Koneen-Sa¨a¨tio¨ (Finland), and the Academy of Finland (project 214545).
references
[1] Keoleian GA, Volk TA. Renewable energy from willow biomass crops: life cycle energy, environmental and economic performance. Crit Rev Plant Sci 2005;24:385e406. [2] Kuzovkina YA, Volk TA. The characterization of willow (Salix L.) varieties for use in ecological engineering applications: co-ordination of structure, function and autecology. Ecol Eng 2009;35:1178e89. [3] Dimitriou I, Aronsson P. Nitrogen leaching from shortrotation willow coppice after intensive irrigation with wastewater. Biomass Bioenergy 2004;26:433e41. [4] Goodlass G, Green M, Hilton B, McDonough S. Nitrate leaching from short-rotation coppice. Soil Use Manag 2007;23:178e84. [5] Adegbidi HG, Volk TA, White EH, Abrahamson LP, Briggs RD, Bickelhaupt DH. Biomass and nutrient removal by willow clones in experimental bioenergy plantations in New York state. Biomass Bioenergy 2001;20:399e411. [6] Bo¨rjesson P, Berndes G. The prospects for willow plantations for wastewater treatment in Sweden. Biomass Bioenergy 2006;30:428e38. [7] Elowson S. Willow as a vegetation filter for cleaning of polluted drainage water from agricultural land. Biomass Bioenergy 1999;16:281e90.
[8] Volk TA, Abrahamson LP, Nowak CA, Smart LB, Tharakan PJ, White EH. The development of short-rotation willow in the northeastern United States for bioenergy and bioproducts, agroforestry and phytoremediation. Biomass Bioenergy 2006; 30:715e27. [9] Cao Y, Lehto T, Repo T, Silvennoinen R, Pelkonen P. Effects of planting orientation and density of willows on nutrient leaching in a greenhouse experiment. New Forests 2011;41:361e77. [10] Kuusemets V, Mander U, Lohmus K, Ivask M. Nitrogen and phosphorus variation in shallow groundwater and assimilation in plants in complex riparian buffer zones. Water Sci Technol 2001;44:615e22. [11] Perttu KL, Kowalik PJ. Salix vegetation filters for purification of waters and soils. Biomass Bioenergy 1997;12:9e19. [12] Kuzovkina YA, Quigley MF. Willows beyond wetlands: uses of Salix L. species for environments projects. Water Air Soil Pollut 2005;162:183e204. [13] Gray DH, Sotir RB. Biotechnical and bioengineering slope stabilization. New York: Wiley; 1996. [14] Lovett S, Price P. Principles for riparian lands management. Canberra: Land Water Australia; 2007. [15] Vervaeke P, Luyssaert S, Mertens J, De Vos, Speleers L, Lust N. Dredged sediment as a substrate for biomass production of willow trees established using the SALIMAT technique. Biomass Bioenergy 2001;21:81e90. [16] Lowthe-Thomas SC, Slater FM, Randerson PF. Reducing the establishment costs of short rotation willow coppice (SRC)a trial of a novel layflat planting system at an upland site in mid-Wales. Biomass Bioenergy 2010;34:677e86. [17] McCracken AR, Moore JP, Walsh LRE, Lynch M. Effect of planting vertical/horizontal willow (Salix spp.) cuttings on establishment and yield. Biomass Bioenergy 2010;34:1764e9. [18] Lukac M, Godbold DL. Fine root biomass and turnover in southern taiga estimated by root inclusion net. Plant Soil 2010;331:505e13. [19] Piirainen S, Finer L, Mannerkoski H, Starr M. Leaching of cations and sulphate after mechanical site preparation at a boreal forest clear-cut area. Geoderma 2009;149:386e92. [20] Chan YH. Biostatistics 301A: repeated measurement analysis (mixed models). Singapore Medical J 2004;45:456e60. [21] Dieterich B, Martin P. Influence of planting depth and orientation on sprouting of willow cuttings. Aspects Appl Biol 2008;90:233e8. [22] Bullard MJ, Mustill SJ, Carver P, Nixon PMI. Yield improvements through modification of planting density and harvest frequency in short rotation coppice Salix spp. Resource captures and use in two morphologically diverse varieties. Biomass Bioenergy 2002;22:27e39. [23] Wilkinson JM, Evans EJ, Bilsborrow PE, Wright C, Hewison WO, Pilbeam DJ. Yield of willow cutivars at different planting densities in a commercial rotation coppice in the north of England. Biomass Bioenergy 2007;31:469e74. [24] Bergkvist P, Ledin S. Stem biomass yields at different planting designs and spacings in willow coppice systems. Biomass Bioenergy 1998;14:149e56. [25] Mola-Yudego B, Aronsson P. Yield models for commercial willow biomass plantation in Sweden. Biomass Bioenergy 2008;32:829e37. [26] Bullard MJ, Mustill SJ, McMillan SD, Nixon PMI, Carver P, Britt CP. Yield improvements through modification of planting density and harvest frequency in short rotation coppice Salix spp. Yield response in two morphologically diverse varieties. Biomass Bioenergy 2002;22:15e25. [27] Volk TA, Abrahamson LP, White EH. Root dynamics in willow biomass crops. Syracuse, New York: SUNY College of Environmental Science and Forestry; 2001. [28] Crow P, Houston TJ. The influence of soil and coppice cycle on the rooting habit of short rotation poplar and willow coppice. Biomass Bioenergy 2004;26:497e505.
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[29] Heinsoo K, Merilo E, Petrovits M, Koppel A. Fine root biomass and production in a Salix viminalis and Salix dasyclados plantation. Estonian J Ecol 2009;59:27e37. [30] Amato M, Bitella G, Rossi R, Gomez JA, Lovelli S, Gomes JF. Multi-electrode 3D resistivity imaging of alfalfa root zone. Eur J Agron 2009;31:213e22. [31] Cao Y, Repo T, Silvennoinen R, Lehto T, Pelkonen P. An appraisal of the electrical resistance method for assessing root surface area. J Exp Bot 2010;61:2491e7.
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[32] Cao Y, Repo T, Silvennoinen R, Lehto T, Pelkonen P. Analysis of the willow root system by electrical impedance spectroscopy. J Exp Bot 2011;62:351e8. [33] Lehmann J, Schroth G. Nutrient leaching. In: Schroth G, Sinclair FL, editors. Trees, crops and soil fertility. CAB International; 2003. [34] Zotarelli L, Scholberg JM, Dukes MD, Carpena RM. Monitoring of nitrate leaching in sandy soil: comparison of three methods. J Environ Qual 2007;36:953e62.
Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
Effects of planting orientation and density on the soil solution chemistry 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, China b Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources, Yangling 712100, China c School of Forest Sciences, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland d The Finnish Forest Research Institute, Joensuu Unit, P.O. Box 68, FI-80101 Joensuu, Finland e Department of Biology, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
article info
abstract
Article history:
Short rotation coppice (SRC) willow are established conventionally by inserting cuttings
Received 7 June 2011
vertically into the soil, but their ability to reproduce vegetative has also been demonstrated
Received in revised form
by planting cuttings horizontally. There is a lack of knowledge about the biomass
5 July 2012
production, root characteristic, and nutrient leaching of plantations established through
Accepted 4 September 2012
horizontally planted cuttings. A plot experiment was conducted to compare the soil
Available online 24 September 2012
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
Keywords:
7500 and 22,500 cuttings ha1). The horizontally planted cuttings achieved the same stem
Willow
yields (4.08 t ha1) as the vertically planted cuttings (4.86 t ha1). The stem biomass was
Fine roots
doubled to a planting density of 22,500 cuttings ha1 (6.34 t ha1) compared to at
Production
7500 cuttings ha1 (3.36 t ha1). The effect of planting orientation or density had no effect
Leaching
on the root biomass or production. Willows decreased the conductivity, (NO2þNO3)eN and
Horizontal orientation
the dissolved total N in the soil solution compared with unplanted plots, but the influence
Lysimeter
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
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
heat and power and hydro thermal upgrading, into a variety of 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
* Corresponding author. 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, China. Tel.: þ86 (0) 15389245368. E-mail address: [email protected] (Y. Cao). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.09.006
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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 application 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 solution were lower in the pots that contained willow [9]. The large-scale movement 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 characteristics make willow suitable for this kind of expansion in the range of its applications in biomass production and the environmental programme [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 the willow 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 stabilization and restoration of the substrate [12]. Recently, a lay-flat planting machine was designed in order to increase planting speed and to reduce the establishment cost of SRC willow. In contrast to the conventional SRC
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 propagation materials 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 length of willow, no differences between the two planting orientations were found after 16 weeks in the stem yield or in the leaf and fine 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 the horizontally 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 biomass was 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 orientation 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。350 N, 29。460 E). The
Table 1 e Texture of the soil at different soil depths. The mean values of samples from the four blocks. Fraction size and content in percentage (%) Depth (cm) 0e10 10e20 20e30 30e40
Clay (<2 mm)
Silt (2e20 mm)
Coarse silt (20e63 mm)
Sand (63e200 mm)
Coarse sand (200e2000 mm)
2.0 2.5 3.4 3.4
14.9 16.6 17.3 18.5
8.1 7.8 5.3 6.1
11.0 12.5 10.2 12.3
50.9 58.4 60.8 55.3
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experiment area was established on a former lawn. The major 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 randomized block design. In June 2008, the experiment area was ploughed and harrowed, and divided into four blocks. The plot size was 2 2 m, and each plot was separated by plastic (0.2 m depth below the soil surface) to avoid any major 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 ha1). 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, which would 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 occasionally 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.
2.2.
167
Measurements of stem production
The height and the number of living shoots were measured each month 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 C until attaining constant weight.
2.3. Measurements of the biomass and production of fine 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 ingrowth 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 individual 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 the mesh 2 cm from each side of the nets were 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 was measured 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
Fig. 1 e Daily precipitation (mm) and mean daily temperature ( C) during the growing period in 2009 and 2010.
168
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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 laboratory, 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 l1) was measured within 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 standard methods of the Finnish Forest Research Institute. The dissolved total nitrogen (DTN, mg l1), ammonium (NH4eN, mg l1) and the sum of the nitrite and nitrate (NO2þNO3, mg l1) were mesasured within 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].
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 Bonferroni method was used in the model 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 ha1 than at 7500 cuttings ha1 (P < 0.001, Table 2). However, there was no difference in the stem biomass for either planting orientation, 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 individual shoots was significantly higher at the density of 7500 cuttings ha1 than at 22,500 cuttings ha1 (P < 0.01 for both, Table 2).
3.2. 2.5.
Fine root biomass production
Statistics 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
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 height of the tallest shoot, the number of living stems, and the mean diameter and dry weight of individual shoots for the different treatments in the two growing years (Standard error of the mean in parentheses, n [ 3). 2009
1
Stem dry biomass production (t ha ) Mean height of the tallest shoots (cm) Number of shoots per stool Mean diameter of individual shoots (mm) Mean dry weight of individual shoots (g)
2010
H þ LD
V þ LD
H þ HD
V þ HD
H þ LD
V þ LD
H þ HD
V þ HD
1.4 (0.07) 316(9) 3.2(0.3) 11.2(0.6) 65.0 (7)
1.4(0.2) 303(7) 4.5(0.3) 10.4(0.6) 57.0(6)
2.5(0.1) 331(14) 4.2(0.7) 10.3(0.4) 52.3(4)
3.3(0.3) 309(7) 5.9(0.4) 9.6(0.4) 45.7(3)
5.5(0.5) 390(9) 13.5(1.2) 12.1(0.4) 102.0(8)
5.1(0.9) 357(12) 11.6(1) 12.0(0.5) 102.4(9)
8.2(0.1) 354(4) 11.3(0.9) 11.2(0.3) 78.2(5)
10.4(0.9) 372(8) 14.3(0.9) 11.2(0.3) 88.5(3)
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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).
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 of water collected in 2010 with the zero tension lysimeters was 332 mm, which included 212 mm 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 cm was higher for the CT treatments (398.7 134.4 mS cm1) than for the planted plots (65.1 9.9 mS cm1) (P ¼ 0.04), but no differences were detected between planted treatments. In contrast, the annual mean DOC concentration was lower for the CT treatments
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(21.6 10.2 mg l1) than for the planted plots (45.6 5.0 mg l1) (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 l1) was significantly higher than in the irrigated months (0.6 and 0.4 mg l1 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 l1) than in the planted treatments (0.7 mg l1) (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 l1) was significantly higher than in July (0.5 mg l1) and August (0.4 mg l1) 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 l1 for July and August, respectively) than in April (24.3 mg l1), 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 l1, respectively) than in autumn months (7.4 and 6.9 mg l1 for September and October, respectively). In 2010, in all of the plots, the mean monthly concentrations of (NO2þNO3)eN were significantly decreased
Fig. 3 e Annual fine root production at different soil depths estimated using the root net method in the single growing season of 2009 (a) And in two consecutive growing seasons of 2009 and 2010 (b). Error bars indicated standard error of the mean (n [ 4).
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Fig. 4 e Weekly concentrations (mg lL1) of NH4eN (a), NO2DNO3eN (b) And DOC (c) In soil leachates collected from zero tension lysimeters (at a depth of 25 cm) in different treatment plots in 2010. Error bars indicated standard error of the mean (n [ 3).
from April, May and June (3.1, 3.0 and 2.8 mg l1, respectively) to July (1.8 mg l1), and they decreased significantly to their lowest concentration in August, September and October (1.4, 0.6 and 0.5 mg l1, respectively).
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 concentrations of DOC did not differ between 2009 and 2010, and no
Fig. 5 e Mean weekly concentrations (mg lL1) of NH4eN (a), (NO2DNO3)-N (b) And DOC (c) In soil water collected by using tension lysimeters (at a depth of 60 cm) in the different treatment plots. Error bars indicated standard error of the mean (n [ 3).
b i o m a s s a n d b i o e n e r g y 4 6 ( 2 0 1 2 ) 1 6 5 e1 7 3
differences between treatments were observed on an annual level. In 2009, in all of the plots, the mean monthly concentrations had decreased significantly from June (76.7 mg l1) to July and August (39.4 and 36.4 mg l1, respectively), and also to September and October (28.4, 24.0 mg l1, respectively). During the irrigation period of 2010, the mean monthly concentrations of DOC in July (27.2 mg l1) was significantly higher than in the other months (19.9, 22.0, 26.9, 23.3, 21.0, and 19.7 mg l1 for April, May, June, August, September and October, respectively) (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 cm1, 53.7 11.2 mg l1 for conductivity and DTN, respectively) than for the planted plots (92.4 13.5 mS cm1, 2.9 0.8 mg l1 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 vertically and 0.9 m long cuttings horizontally with the same planting density (1000 cuttings ha1) 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 ha1 produced significantly less stem biomass than did 2 m long willow rods planted at a density of 5000 cuttings ha1. 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 horizontally 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 ha1 as high as at 7500 cuttings ha1 in the present field experiment. The explanation suggested in earlier experiments was that the standing stem biomass production finally becomes independent of planting density up a certain range of planting densities, e.g. 20,000 cuttings ha1 [23,24]. Hence, the current practice planting density is a planting density of 15,000 cuttings ha1 in the UK, and 12,000 cuttings ha1 in Sweden [23,25]. In contrast to the situation with stands
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production, the mean diameter and weight of individual plants was significantly larger at the density of 7500 cuttings ha1 than at 22,500 cuttings ha1. 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, variability 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 aboveground 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 plantations [27]. Willow roots were located primarily within the top 30 cm of the soil, whereas their depth in soil may extend to 1.3 m, and occasionally to a depth of 3 m [28]. The root characteristics 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 core method [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 availability 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 elements were still observed in water samples following the installation of lysimeters for one month in 2009. Subsequently, the concentrations of the elements measured decreased with time, and the harvest action in winter 2009 did not affect the concentrations 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.
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However, the concentrations were much smaller than those figures (NO3eN <11 mg l1, NO2eN <0.15 mg l1, NH4eN <0.4 mg l1) 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 We would 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 the manuscript 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-Sa¨a¨tio¨ (Finland), the Koneen-Sa¨a¨tio¨ (Finland), and the Academy of Finland (project 214545).
references
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