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Willow biomass obtained from different soils as a feedstock for energy
Industrial Crops and Products 75 (2015) 114–121
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Willow biomass obtained from different soils as a feedstock for energy a,∗ ˙ Michał Krzyzaniak , Mariusz J. Stolarski a , Stefan Szczukowski a , Józef Tworkowski a , b Arkadiusz Bieniek , Mirosław Mleczek c a
Department of Plant Breeding and Seed Production, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-724 Olsztyn, Poland Department of Soil Science and Soil Protection, University of Warmia and Mazury in Olsztyn, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-724 Olsztyn, Poland c Department of Chemistry, University of Life Sciences in Poznan, Wojska Polskiego 75, 60-625 Pozna´ n, Poland b
a r t i c l e
i n f o
Article history: Received 14 January 2015 Received in revised form 2 June 2015 Accepted 8 June 2015 Available online 22 June 2015 Keywords: Salix viminalis Cultivars Thermo-physical and chemical properties Energy value Biomass
a b s t r a c t Perennial energy crops, including willow, should be highly productive and the biomass should possess a high energy value, allowing farmers to obtain large amounts of energy per ha each year. The aim of this study was to determine the morphological traits, biomass productivity and quality of willow coppice grown on two different soil sites and harvested in a three-year cycle. The experiment was performed on a commercial willow plantation owned by the University of Warmia and Mazury in Olsztyn (the UWM). The plantation, located in north-eastern Poland, covered 4.7 ha. The study demonstrated that the UWM 043 cultivar grown in a three-year rotation system on Haplic Cambisols (Eutric) soil can produce plants over 8 m tall with shoots over 4 cm in diameter. On the other hand, plants of Salix spp. grown on much poorer soil are much shorter and have thinner shoots. Under optimal production conditions, it is possible to harvest in a three-year rotation system as much as 16.12 Mg ha−1 year−1 of dry biomass per year with moisture equal to 47.56%. The yield of the UWM 043 cultivar was significantly (40%) higher than that of the cultivar Turbo. The biomass of the UWM 043 clone proved to be better fuel owing to a higher bulk density, lower moisture, higher lower heating value and a lower content of nitrogen, sulphur and chlorine. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The agricultural land area in Poland is estimated to be about 18.7 million hectares (GUS, 2014). Four plant species: willow (Salix spp.), poplar (Populus), Virginia mallow (Sida hermaphrodita R.) and giant miscanthus (Miscanthus x giganteus), seem most likely to ˙ enter the biomass production market in Poland (Jezowski, 2008; ˙ Krasuska and Rosenqvist, 2012; Jezowski, 2008; Stolarski et al., 2014). Among all the perennials cultivated in Poland, naturally ˙ growing willow is the most promising crop (Krzyzaniak et al., 2014; Stolarski et al., 2014a). The area of its cultivation totaled about 6.2 thousand hectares, whereas all energy crops, including willow, were cultivated on an area of about 10 thousand hectares (Gajewski, 2010). It is predicted that in 2020 the land under energy crops would cover 2 million hectares, including about 500 thousand ha under perennial energy crops. Such a volume is required to meet the objectives outlined in the Directive 2009/28/EC on the
∗ Corresponding author. ˙ E-mail address: [email protected] (M. Krzyzaniak). http://dx.doi.org/10.1016/j.indcrop.2015.06.030 0926-6690/© 2015 Elsevier B.V. All rights reserved.
promotion of the use of energy from renewable sources (European Commission, 2009; Ku´s and Faber, 2009). In Poland, Salix is represented by 28 species and numerous hybrids (Tomanek, 1994). There have been several studies conducted into the properties of new willow varieties intended for energy production. Research has been conducted in Sweden, the UK, the USA and in many other countries (Larsson, 1998; Lindegaard and Barker, 1996; Smart et al., 2005). In Poland, there is no single, central programme for research into the cultivation of SRWC willow, whereas the number of varieties registered with the Polish Research Centre for Cultivar Testing (15 Polish proprietary varieties) is a sign of interest in the subject (COBORU, 2014). Of the varieties mentioned, ten were registered by the research team of the Department of Plant Breeding and Seed Production of the University of Warmia and Mazury in Olsztyn. The dominant system of Short Rotation Woody Crops (SRWC) cultivation on farmland is a short, three-year cycle (Kopp et al., 1997; Labrecque and Teodorescu, 2003; Kopp et al., 1997; Stolarski et al., 2014b). Soil preparation before the planting of willow cuttings comprises typical soil tillage treatments: deep ploughing, harrowing, mechanical and
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chemical weed control, mineral fertilization, etc. Another willow biomass production method is known as the Eko-Salix system. It consists of simplified, ploughless soil tillage and the planting of unrooted pole cuttings (Stolarski et al., 2011a). Perennial energy crops, including willow, should be highly productive and the biomass should possess high energy value, allowing farmers to obtain large amounts of energy per ha each year. The productivity potential of perennial crops is highly varied and can range from a few to several tonnes of dry matter per ha annually. The yield depends on the soil quality, plant species and cultivar, crop management, planting density and rotation cycle (Labrecque and Teodorescu, 2005; Quaye and Volk, 2013; Stolarski et al., 2014a). Apart from the biomass productivity obtained in a given year under given agro-climatic conditions, another important factor is the plantation life, on the basis of which we can estimate the accumulated biomass production of the plantation of studied perennial crop. Knowledge of the yields that can be expected on a commercial scale is crucial for clone selection and management optimization processes. Studies of this aspect of Short Rotation Woody Crops (SRWC) production have been done in Poland in recent years ˙ et al., 2014; Stolarski et al., 2014a, 2013). The inter(Krzyzaniak action of all of the above factors has a significant effect on both biomass productivity as well as the quality of energy feedstock, which is of paramount importance for the development of lignocellulosic crop production in the future. Short rotation woody crops should be established on poor, marginal or contaminated soils, and shall not compete with the production of edible crops. Nevertheless, farmers producing biomass will seek the highest yield from a unit area. It was hypothesized that biomass productivity for two selected cultivars would differ between cultivars and the cultivars productivity would be different on two soil types. In view of the above, the purpose of the present research has been to determine the morphological traits, biomass production and quality from two willow cultivars grown on a commercial scale at two different stands on loam and loamy-sandy soil.
2. Material and methods 2.1. Characteristics of the experimental area The years 2009–2011, when the research was carried out, were similar in the mean air temperatures (from 7.1 to 8.4 ◦ C) (Fig. 1). The total rainfall in each year was higher during the growing season than the analogous values calculated for the multi-year period (Fig. 1). In turn, the average annual precipitation in 2009 and 2010 was higher, and in 2011 it was lower than the multi-year average. The rainfall distribution was variable. In each year, April was drier than the multi-year average, which could inhibit the early growth and development of willow plants. The study area is characterized by gently undulating land, where differences in relative height were less than 15 m. The experiments were located on an eastward slope with a 10% gradient. The tests were carried out at two sites located on different soils. The first SRWC willow stand is located on loam soil: Haplic Cambisols (Eutric) (IUSS Working Group WRB, 2007) developed from loam with underlying clay loam at the depth of 88 cm. The second site was on Brunic Arenosols (Dystric) (WRB 2007) formed from loamy sand deposited on sand (Table 1). The actual moisture (% of the volume) at both sites was strongly differentiated. In the loam soil site it was within 20.9–30.7% vol. In the loamy sand soil site there was less water (7.2–10.1% vol.). At A and Bw levels of the loam soil site, water was easily available to plants (pF 2.0–3.0), whereas at the bedrock level it was poorly
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Table. 1 Characteristics of soil texture. Horizon
Depth (cm)
Percent of fraction of diameter (mm) >2.0
2.0–0.05
0.05–0.002
Loam soil 0–30 A 30–88 Bw C 88–150
0 0 0
42 45 30
47 45 51
13 10 19
Loamy sand soil A 0–28 28–65 Bv 65–150 C
0 0 0
85 74 88
10 22 11
5 4 1
Textural group
<0.002 Loam Loam Clay loam Loamy sand Loamy sand Sand
A – humic horizon; Bw – horizon enriched with non-alluvial iron deposits; Bv – in situ iron enrichment; C – bedrock.
accessible (pF 3.0–4.2). In the loamy sand site, water was poorly accessible to plants (pF 3.0–4.2) at all levels. The soils at the experimental sites were characterized by extremely different air and water conditions. There were few macropores in the loam soil site, whereas the loamy sand soil site was poor in micropores. Consequently, the loamy sand soil site suffered from periodic water deficits, which had an adverse effect on the growth and development of plants, an event unobserved at the other site. 2.2. Establishment and management of the plantation In the third decade of April 2008, a commercial willow plantation was established on 4.7 hectares of land at the Research Station ˙ owned by the University of Warmia and Mazury in Olszin Łe˛ zany, tyn (the UWM) (N: 53◦ 58 E: 21◦ 8 ). The preceding crop was winter triticale grown in a crop rotation system. After the triticale was harvested, in late autumn 2007, deep winter ploughing to the depth of 35 cm was done with a subsoiler. In spring 2008, soil smoothing was carried out and willow cuttings of the UWM 043 and Turbo cultivars bred at the UWM in Olsztyn were planted. Each cultivar was planted on two types of soil and on separate plots, which in total covered 2.35 ha of land. Cuttings, 25 cm in length and 0.9–1.1 cm in diameter, were planted manually from 20 to 22 April 2008 at a density of 18,000 cuttings per ha. A strip cropping system was used. The distance between double rows was 1.5 m and the distance between single rows in a double row was 0.75 m. The distance between cuttings in a row was 0.5 m. During first week after planting, weeds were controlled chemically (Guardian Complete MIX 664 SE: 3.5 l ha−1 with acetochlor and terbuthylazine: 300 g l−1 ). The one-year-old shoots were harvested after the 2008 growing season to increase the number of shoots from each stump in the following year. Mineral fertilizer was applied in the first decade of April 2009 in doses corresponding to N 100, P2 O5 30 and K2 O 60 kg ha−1 . The results of the study since 2009 are presented, in which the first factor consists of cultivars of willow species Salix viminalis: UWM 043 and Turbo. The second factor is the type of soil site: loam and loamy sand. For each stand and cultivar, experimental blocks measuring 60 m × 150 m were designed, on which plots measuring 3.0 × 10.0 m (30 m3 each) were randomly chosen in four replications (Fig. 2). After three years, in the first decade of January 2012, the plant density of three-year-old willow stands per 1 ha and the number of living shoots with height >1.5 m were counted on the designated plots. Next, the density of plants per hectare was calculated from plots. The following biometric measurements were performed on 10 plants within the plot: the stem height and diameter of stems at 0.5 m height of the stem. In order to obtain more detailed yield determination, in the first decade of January 2012, three-year-old
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Fig. 1. Weather conditions during the experiment period 2009–2011 and multi-year period 1998–2007; bars represent precipitation; curves represent air temperatures.
shoots were manually cut down 5–10 cm above the ground, using a DCS520 (Makita) chainsaw. The shoots harvested within each small plot were weighed with a precision of ±1 kg, respectively, thus determining fresh matter biomass. They were then chopped into chips using a Junkkari HJ 10 G wood chipper (Junkkari), coupled to a 96 kW powered tractor (New Holland). The willow dry biomass was calculated based on the fresh mass of shoots harvested from small plots and their moisture content. The dry biomass of the shoots was estimated using the fresh weight of the shoots. Additionally, the energy value of the biomass was estimated by multiplying the fresh biomass by its lower heating value (a detailed description in Section 2.3). 2.3. Harvested biomass quality evaluation
Fig. 2. Schematic overview of the experimental design.
Representative samples of willow chips were taken from each combination clone × site. The samples were placed in plastic bags and transported to the laboratory of the Department of Plant Breeding and Seed Production, the University of Warmia and Mazury in Olsztyn. The following characteristics were determined in the samples: moisture, higher heating value (HHV), lower heating value (LHV), ash content, volatile matter and fixed carbon, bulk density and concentrations of C, H, S, N, Cl. Each analysis was performed in three replications. Bulk density was determined by weighing chips in a 0.8 m3 container with a known weight. Next, the moisture was determined with the oven-dry method. For this aim, a sample of biomass was dried at 105 ◦ C to a constant weight (PN (Polish Standard) 80/G-04511). Next, the biomass was ground and the HHV was determined with the dynamic method using an IKA C 2000 calorimeter (IKA Werke Gmbh&Co. KG) in accordance with the PN-81/G-04513 standard. The LHV of the biomass was calculated according to Kopetz et al. (2007). Volatile matter, fixed carbon and ash content were determined at 550 ◦ C with an ELTRA TGA-THERMOSTEP automatic thermogravimetric analyzer (PN-ISO 562). The content of nitrogen was analyzed with the Kjeldahl method using a K-435 unit and B-324 BÜCHI distiller (BÜCHI Labortechnik AG), while the content of chlorine was determined using an Eschka mixture.
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0.0000 0.0000 0.0000
16.12 ± 0.28a 3.28 ± 0.41c 11.29 ± 0.61b 2.53 ± 0.28d 9.70 ± 6.87a 6.91 ± 4.70b 13.70 ± 2.62a 2.91 ± 0.51b 8.31 ± 5.87
0.0000 0.0000 0.0000
0.84a 1.22c 1.83b 0.85d 20.62a 14.10b 7.86a 1.54b 17.60 ± ± ± ± ± ± ± ± ± 48.36 9.84 33.86 7.60 29.10 20.73 41.11 8.72 24.92
0.0000 0.0000 0.0000
1.73a 2.33c 3.64b 1.73c 39.16a 27.80b 13.60a 2.68b 33.62 ± ± ± ± ± ± ± ± ± 92.23 19.05 67.27 15.50 55.64 41.39 79.75 17.27 48.51
0.0008 0.3027 0.9902
0.55 0.36 0.13 0.19 0.44b 0.18a 0.44 0.18 0.52 ± ± ± ± ± ± ± ± ± 1.64 1.83 2.42 2.60 1.73 2.51 2.03 2.22 2.12
0.0009 0.0000 0.9570
5.11 2.01 4.03 0.13 11.18a 10.82b 5.87a 416b 11.31 ± ± ± ± ± ± ± ± ± 40.38 20.56 32.81 13.19 30.47 23.00 36.59 16.88 26.73
0.0000 0.0000 0.9593
0.18 0.25 0.34 0.14 1.89a 1.89b 0.84a 0.82b 1.98 ± ± ± ± ± ± ± ± ± 8.07 4.55 6.56 3.06 6.31 4.81 7.32 3.80 5.56
P-value Variety (A) Soil (B) AB
Mean for experiment
Mean for soils
Mean for varieties
Turbo
±Standard deviation; a, b, c ... homogeneous groups; Significant at P < 0.05.
0.0152 0.0011 1.0000
± ± ± ± ± ± ± ± ±
1.78 2.52 1.78 2.52 3.19b 3.19a 2.33b 2.86a 3.47 0.0152 0.0011 1.0000
Number of plants (plants ha−1 )
Loam soil Loamy sand soil Loam soil Loamy sand soil UWM 043 Turbo Loam soil Loamy sand soil
The bulk density and moisture of biomass differed significantly between the cultivar (P = 0.0000) and soil type (P = 0.0007; and P = 0.0002 respectively). No significant effect was observed between cultivar and soil interactions (P = 0.7708; and P = 0.1691 respectively) (Table 2). The bulk density of fresh chips of UWM 043 cultivar was significantly higher (321.28 kg m−3 ) than from the Turbo cultivar (Table 4). The average moisture of the biomass of three-year-old willow plants was 49.14%, ranging from 47.96% for the UWM 043 cultivar to 50.31% for the Turbo cultivar. The higher heating value of willow biomass was significantly higher in the Turbo cultivar, where it accounted for an average of 19.61 MJ kg−1 of dry matter (P = 0.0014). The biomass obtained from the light soil was characterized by significantly higher HHV compared to the biomass obtained from the loam soil site (P = 0.0000).
Parameter
3.3. Evaluation of the biomass quality
Table 2 Plant density, biometric traits and yield of three-year-old willow stands.
The fresh biomass productivity and dry wood yield differed significantly between the variety (P = 0.0000), soil type (P = 0.0000) and between their interactions (P = 0.0000) (Table 2). The willow dry biomass obtained from the UWM 043 grown in three-year rotation in the field experiment on the commercial plantation was high (9.70 Mg ha−1 year−1 on average), ranging from 3.28 Mg ha−1 year−1 on loamy sand soil to 16.12 Mg ha−1 year−1 on loam soil. In turn, the cultivar Turbo yielded on average 40.4% less, although on loam soil it produced an average dry biomass of over 11 Mg ha−1 year−1 d.m. Our analyses demonstrated that the dry biomass was positively correlated with the number of plants (0.77), their height (0.96) and diameter of shoots (0.94) (Table 3).
Mortality (%)
3.2. Biomass productivity
2.78 7.41 5.86 10.49 5.09 8.18 4.32 8.95 6.64
Plant height (m)
Shoot diameter (mm)
Number of shoots
Fresh biomass production (Mg ha−1 )
The number of three-year-old willow plants was on average 16 806 plants ha−1 , thus being 6.64% lower than the initial number (Table 2). Significantly higher plant mortality was recorded for the Turbo cultivar than for the UWM 043 (P = 0.0152). A similar dependence for this trait was found on both sandy loam soil as well as loam soil (P = 0.0011). The average height of three-year-old plants in the experiment was 5.56 m. The plants of the UWM 043 cultivar were significantly higher (6.31 m) than those of Turbo cultivar (4.81 m) (P = 0.0000). The plants growing on loam soil were on average nearly twice as tall as the plants on loamy sand soil (P = 0.0000). The UWM 043 cultivar grown on loam soil yielded plants which – at the height of 8.07 m – were 5 m taller than Turbo plants grown on loamy sand soil. The cultivar Turbo produced significantly more shoots than the UWM 043 cultivar (P = 0.0008), whereas the average diameter of shoots from the UWM 043 was 30.47 mm, being 7.5 mm larger than in the case of Turbo. The plants grown on loam soil produced significantly thicker shoots than the plants grown on loamy sand soil (P = 0.0000). In conclusion, the UWM 043 cultivar produced fewer shoots, but the shoots of this cultivar had a larger diameter and larger height, which had a positive effect on the greater yield of this cultivar compared to the Turbo cultivar.
321 454 321 454 575a 575b 420a 514b 625
3.1. Plant mortality and biometric traits
± ± ± ± ± ± ± ± ±
Dry matter yield (Mg ha−1 )
3. Results
UWM 043
All statistical analysis were performed using STATISTICA 9.0 software package (StatSoft Inc.). A multiple SNK (Student NewmanKeuls) test at a significance level alpha = 0.05 was applied in order to analyse homogenous groups for the studied features of the biomass.
17500 16667 16944 16111 17083 16528 17222 16389 16806
Dry matter yield (Mg ha−1 year−1 )
2.4. Statistical analysis
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Table 3 The correlation coefficients for the analysed traits. Number of plants
Number of shoots
Plant height
Shoot diameter
Fresh biomass
Dry biomass
Biomass energy value
Number of plants Number of shoots Plant height Shoot diameter Fresh biomass Dry biomass Biomass energy value
1.00 −0.40 0.81* 0.75* 0.76* 0.77* 0.77*
-0.40 1.00 −0.48 −0.36 −0.35 −0.37 −0.37
0.81* −0.48 1.00 0.95* 0.96* 0.96* 0.96*
0.75* −0.36 0.95* 1.00 0.94* 0.94* 0.94*
0.76* −0.35 0.96* 0.94* 1.00 1.00* 1.00*
0.77* −0.37 0.96* 0.94* 1.00* 1.00 1.00*
0.77* −0.37 0.96* 0.94* 1.00* 1.00* 1.00
*
Correlation coefficients significant at the level of ≤0.05.
Table 4 Thermophysical properties and chemical composition of the biomass obtained from three-year-old willow stems. Bulk density (kg m−3 )
Moisture content (%)
HHV (MJ kg−1 d.m.)
LHV (MJ kg−1 )
Ash (% d.m.)
Fixed carbon (% d.m.)
Volatile matter (% d.m.)
C (% d.m.)
H (% d.m.)
N (% d.m.)
S (% d.m.)
Cl (% d.m.)
Loam soil Loamy sand soil Loam soil Loamy sand soil UWM 043
310.00 ± 2.00 332.55 ± 9.45
47.56 ± 0.34 48.36 ± 0.42
19.38 ± 0.02c 19.66 ± 0.04b
9.00 ± 0.07 8.97 ± 0.12
1.23 ± 0.01d 1.56 ± 0.00b
19.19 ± 0.07c 20.06 ± 0.02b
78.88 ± 0.06a 78.14 ± 0.01c
49.98 ± 0.13 50.11 ± 0.02
5.98 ± 0.01 5.89 ± 0.02
0.50 ± 0.00b 0.40 ± 0.01d
0.024 ± 0.001 0.023 ± 0.001
0.006 ± 0.000 0.008 ± 0.001
262.50 ± 4.50 287.78 ± 11.48
49.66 ± 0.17 50.96 ± 0.02
19.40 ± 0.03c 19.83 ± 0.04a
8.55 ± 0.05 8.48 ± 0.02
1.41 ± 0.01c 1.68 ± 0.02a
20.01 ± 0.25b 21.42 ± 0.05a
78.71 ± 0.03b 76.62 ± 0.06d
49.77 ± 0.12 50.14 ± 0.26
5.92 ± 0.00 5.96 ± 0.09
0.62 ± 0.02a 0.47 ± 0.00c
0.029 ± 0.001 0.025 ± 0.001
0.008 ± 0.001 0.010 ± 0.000
321.28 ± 13.78a 47.96 ± 0.56b
19.52 ± 0.16b
8.98 ± 0.09a
1.39 ± 0.18b
19.62 ± 0.48b
78.51 ± 0.41a
50.04 ± 0.11
5.93 ± 0.05
0.45 ± 0.06b
0.023 ± 0.001b
0.007 ± 0.001b
Turbo Loam soil
275.14 ± 15.89b 50.31 ± 0.072a 286.25 ± 26.20b 48.61 ± 0.18b
19.61 ± 0.24a 19.39 ± 0.02b
8.52 ± 0.05b 8.78 ± 0.25
1.55 ± 0.15a 1.32 ± 0.10b
20.71 ± 0.79a 19.60 ± 0.48b
77.67 ± 1.14b 78.80 ± 0.10a
49.96 ± 0.27 49.88 ± 0.16b
5.94 ± 0.06 5.95 ± 0.03
0.54 ± 0.08a 0.56 ± 0.06a
0.027 ± 0.002a 0.026 ± 0.003a
0.009 ± 0.001a 0.007 ± 0.001b
310.16 ± 26.26a 49.66 ± 1.45a
19.74 ± 0.10a
8.73 ± 0.28
1.62 ± 0.07a
20.74 ± 0.75a
77.38 ± 0.83b
50.12 ± 0.17a
5.92 ± 0.07
0.44 ± 0.04b
0.024 ± 0.002b
0.009 ± 0.001a
298.21 ± 27.96 0.0000 0.0007 0.7708
19.56 ± 0.20 0.0014 0.0000 0.0053
8.75 ± 0.25 0.0000 0.2598 0.6509
1.47 ± 0.18 0.0000 0.0000 0.0035
20.17 ± 0.84 0.0000 0.0000 0.0084
78.09 ± 0.93 0.0000 0.0000 0.0000
50.00 ± 0.20 0.3768 0.0267 0.2193
5.93 ± 0.05 0.8313 0.3792 0.0517
0.50 ± 0.08 0.0000 0.0000 0.0025
0.025 ± 0.003 0.0002 0.0019 0.0693
0.008 ± 0.002 0.0012 0.0012 1.0000
Parameter UWM 043
Turbo
Mean for varieties Mean for soils
Loamy sand soil Mean for experiment Variety (A) Soil (B) AB
49.14 ± 1.37 0.0000 0.0002 0.1691
±Standard deviation; a, b, c... homogeneous groups; Significant at P < 0.05.
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Parameter
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However, when the moisture was taken into account, it was shown that the LHV of biomass from both sites was at the same level (P = 0.2598). In turn, the UWM 043 cultivar had a significantly higher LHV (8.98 MJ kg−1 ) than the Turbo cultivar (P = 0.0000). The ash content, fixed carbon and volatile matter in biomass differed significantly between the cultivar (P = 0.0000), soil type (P = 0.0000) and their interactions (Table 4). The biomass of the UWM 043 contained significantly less ash (1.39% d.m.) compared to the Turbo. The content of fixed carbon ranged from 19.19 to 21.42% d.m., whereas the content of volatile matter was between 76.62 and 78.88% d.m. The content of carbon determined in threeyear-old willow wood was on average 50.04% d.m. in the UWM 043 cultivar and 49.96% d.m. in Turbo (P = 0.3768). Among the analyzed soil sites, significantly more carbon was found in the biomass obtained from the plants grown on loamy sand (P = 0.0267). The content of hydrogen in the willow biomass was not differentiated by the experimental factors (5.93% d.m., on average). In contrast, the content of nitrogen, sulphur and chlorine was significantly higher in the biomass of cultivar Turbo. The concentrations of sulphur and chlorine in the willow biomass determined in this experiment were low, i.e. 0.025% d.m. and 0.008% d.m., respectively. 3.4. Biomass energy value The energy value of the biomass was significantly differentiated by the willow variety (P = 0.0000), soil type (P = 0.0000) and between their interactions (P = 0.0000) (Fig. 3). The highest biomass energy value was found for UWM 043 cultivar grown on loam soil (276.7 GJ ha−1 year−1 ). The amount of energy in the cultivar Turbo grown on the same site was 85 GJ ha−1 year−1 lower. The significantly lowest value of this feature was found in the Turbo cultivar cultivated in loamy sand soil (only 43.8 GJ ha−1 year−1 ). With an average energy value of coal equal to 25 GJ Mg−1 , it can be concluded that the energy contained in the biomass of cultivars UWM 043 and Turbo plants grown on loam soil correspond to 11.1 and 7.7 Mg of coal, respectively. 4. Discussion Dry biomass productivity of SRWC willow cultivated in a three-year rotation cycle varied significantly, depending on the cultivar and soil conditions. Out of the two cultivars bred at the UWM, the UWM 043 was more productive. The studies carried out in Poland showed that the willow yield from short rotation plantations tested in field trials under optimal conditions reached up to 30 Mg ha−1 year−1 d.m. (Stolarski et al., 2008; Stolarski et al., 2011b). The average experimental yields, in turn, were between 10 and 12 Mg ha−1 year−1 d.m. (Tworkowski et al., 2006). However, yields from large-scale plantations were, in general, significantly lower than those obtained from large (70–300 ha) plantations tested by the UWM researchers, and ranged from 4 to 10 Mg ha−1 year−1 d.m. (Tworkowski et al., 2010). The reasons why yields from large-scale plantations were low were the difficulties in selecting and preparing the land area, mistakes made while setting up plantations, planting randomly chosen willow clones, ineffective weed control and inadequate fertilization. Searle and Malins (2014) also state that yields from field trials are much higher than from large scale plantations, due to the edge effect, easier and more frequent crop management, etc. On large-scale plantations, yields are also lower due to biomass losses, difficulties in crop management and harvesting inefficiency. For example, in Sweden, experimental plantations gave high willow yields, which were not successfully repeated on largescale commercial plantations (Melin and Larsson, 2005). The highest average yields (12 Mg ha−1 year−1 d.m.) were achieved
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on non-irrigated commercial plantations of willow harvested in three-year rotation (Melin and Larsson, 2005). This relationship has been verified by the data obtained from 1512 plantations kept from 1986 to 2000, where the average willow yields were just 2.67 Mg ha−1 year−1 d.m. and the highest ones reached 20.54 Mg ha−1 year−1 d.m. (Mola-Yudego, 2011). Finally, typical yields produced by Salix viminalis plants on commercial plantations in the United Kingdom are from 6 to 11 Mg ha−1 year−1 d.m. (Bullard et al., 2002; Powlson et al., 2005). In a study conducted in Denmark with eight willow cultivars grown on five different soils with different methods of crop management and levels of fertilization in a three-year harvest rotation, the willow yield levels were highly differentiated and ranged from 2.9 to 10.2 Mg ha−1 year−1 d.m. (Larsen et al., 2014). In another study, conducted in the USA, 18 cultivars of willow grown on two plots (three-year harvest cycle) were shown to have significantly different yields, ranging from 3.54 to 13.58 Mg ha−1 year−1 d.m. (Serapiglia et al., 2013). A high yield of 4 willow clones (average 14.1 Mg ha−1 year−1 d.m.) grown on very good soil and fertilized with mineral fertilizers was obtained in a four-year harvest cycle in another study by Stolarski et al., 2013. In Poland, the most productive is the cultivation of willow on soils abundant in nutrients, where the groundwater table is high (down to 2 m). On a large-scale plantation of willow set up on alluvial soil in the Vistula river valley, where different willow cultivars were grown on a well-managed plantation and harvested in short (2- or 3-year) cycles, the average yield was 20 Mg ha−1 year−1 d.m. (Tworkowski et al., 2010). In the authors’ own study, the growth and yield of willow plants varied significantly according to cultivar and the soil conditions. Particularly large differences were observed during long periods without precipitation. A large water deficit occurred at a loamy sand soil site due to the absence of groundwater capillarity between the lower layer of sand and the upper layer of loamy sand in the soil profile. Roots of willow plants had therefore much worse conditions for the uptake of water than those of plants growing on loam soil site, where the permeation of water through soil capillaries to the rhizosphere was constant. It has been demonstrated that the cultivar, type of soil and water access are significant yield-forming factors in willow growth, which was confirmed by other authors (Bungart and Hüttl, 2001; Labrecque and Teodorescu, 2003; Fischer et al., 2005; Tworkowski et al., 2006). Sevel et al. (2012) also found that willow yield was significantly differentiated by clones and soil types, indicating that different clones may be better suited for different soil types. They found that that dry matter production ranged from 5.2 Mg ha−1 year−1 d.m. for Inger cultivated on the sandy soil to 8.8 Mg ha−1 year−1 d.m. for Tora cultivated on the organic soil. Cultivars Inger, Sven, Tora and Tordis cultivated on sandy soil yielded at the same level, whereas Inger and Tora cultivated on organic soil produced significantly higher yields than Sven and Tordis. The literature generally recommends harvesting SRWC willow every three years, justifying this recommendation not only by higher productivity versus shorter rotation systems but also by the good quality of the harvested biomass as fuel (Kopp et al., 1997; Labrecque et al., 1997; Adler et al., 2005; Labrecque and Teodorescu, 2003; Smaliukas et al., 2007). Biomass to be used as solid fuel should be characterized by a low content of moisture, ash and alkaline elements, but have a high energy value. For processes of biochemical conversion of biomass to liquid fuels, the ratio of cellulose to lignin and hemicelluloses content is also important (McKendry, 2002). In the authors’ own experiment, the UWM 043 cultivar was characterized by a higher LHV of biomass and a higher specific density compared to Turbo. This variety also had significantly less ash, nitrogen, sulphur and chlorine than the Turbo cultivar, which makes it a prospective choice as an energy crop. One reason for a better quality of the UWM 043 biomass was the fact that it produced
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Fig. 3. The energy value of SRWC biomass. Significant at P < 0.05; P-value: Variety (A) – 0.0000; Soil (B) – 0.0000; AB – 0.0000
fewer shoots, which were larger in diameter than shoots produced by Turbo. Adler et al. (2005) found that thinner shoots contain more bark than thicker shoots and the bark has much more N, P, K, Mg, Ca, Cd, Pb, Co and Zn than wood. This feature affects the combustion process and can accelerate corrosion of furnaces. The content of ash in willow biomass is directly dependent on the biomass content of alkaline elements. The lower their content in fuel, the less ash is generated during its combustion (Tharakan et al., 2003). In the authors’ previous studies, a significant effect of the cultivar and willow clone on the content of ash and elements was found (Stolarski, ˙ 2009; Krzyzaniak et al., 2015).
5. Conclusions The biometric traits, yield and quality of biomass produced by Salix spp. varied significantly according to both the cultivar and the soil site. This experiment showed that willow Salix viminalis grown in three-year cycle on good soils (in this case, loam) under the conditions present in north-eastern Poland can produce plants over 8 m tall with shoots over 4 cm in diameter. In turn, production of Salix spp. on much poorer soils (in this study, loamy sand) resulted in lower average plant height and shoot diameter. These factors directly influence willow yield. Under optimal conditions, on a commercial plantation of willow in north-eastern Poland, established on heavy soil in a three-year rotation system, it is possible to obtain up to 16.12 Mg ha−1 year−1 of dry biomass. The UWM 043 cultivar yielded significantly higher than the Turbo cultivar. The biomass produced by the UWM 043 proved to be more valuable fuel owing to a higher bulk density, lower moisture content, higher LHV and lower content of nitrogen, sulphur and chlorine. The present research has indicated that studies on the selection of willow cultivars or clones with respect to soil conditions should be continued, because this factor has a significant influence on yield volumes, energy value and biomass quality.
Acknowledgments The authors wish to acknowledge the financial support for the studies provided by project grant N R12 0065 10, from the National Centre for Research and Development.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Willow biomass obtained from different soils as a feedstock for energy a,∗ ˙ Michał Krzyzaniak , Mariusz J. Stolarski a , Stefan Szczukowski a , Józef Tworkowski a , b Arkadiusz Bieniek , Mirosław Mleczek c a
Department of Plant Breeding and Seed Production, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-724 Olsztyn, Poland Department of Soil Science and Soil Protection, University of Warmia and Mazury in Olsztyn, University of Warmia and Mazury in Olsztyn, Plac Łódzki 3, 10-724 Olsztyn, Poland c Department of Chemistry, University of Life Sciences in Poznan, Wojska Polskiego 75, 60-625 Pozna´ n, Poland b
a r t i c l e
i n f o
Article history: Received 14 January 2015 Received in revised form 2 June 2015 Accepted 8 June 2015 Available online 22 June 2015 Keywords: Salix viminalis Cultivars Thermo-physical and chemical properties Energy value Biomass
a b s t r a c t Perennial energy crops, including willow, should be highly productive and the biomass should possess a high energy value, allowing farmers to obtain large amounts of energy per ha each year. The aim of this study was to determine the morphological traits, biomass productivity and quality of willow coppice grown on two different soil sites and harvested in a three-year cycle. The experiment was performed on a commercial willow plantation owned by the University of Warmia and Mazury in Olsztyn (the UWM). The plantation, located in north-eastern Poland, covered 4.7 ha. The study demonstrated that the UWM 043 cultivar grown in a three-year rotation system on Haplic Cambisols (Eutric) soil can produce plants over 8 m tall with shoots over 4 cm in diameter. On the other hand, plants of Salix spp. grown on much poorer soil are much shorter and have thinner shoots. Under optimal production conditions, it is possible to harvest in a three-year rotation system as much as 16.12 Mg ha−1 year−1 of dry biomass per year with moisture equal to 47.56%. The yield of the UWM 043 cultivar was significantly (40%) higher than that of the cultivar Turbo. The biomass of the UWM 043 clone proved to be better fuel owing to a higher bulk density, lower moisture, higher lower heating value and a lower content of nitrogen, sulphur and chlorine. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The agricultural land area in Poland is estimated to be about 18.7 million hectares (GUS, 2014). Four plant species: willow (Salix spp.), poplar (Populus), Virginia mallow (Sida hermaphrodita R.) and giant miscanthus (Miscanthus x giganteus), seem most likely to ˙ enter the biomass production market in Poland (Jezowski, 2008; ˙ Krasuska and Rosenqvist, 2012; Jezowski, 2008; Stolarski et al., 2014). Among all the perennials cultivated in Poland, naturally ˙ growing willow is the most promising crop (Krzyzaniak et al., 2014; Stolarski et al., 2014a). The area of its cultivation totaled about 6.2 thousand hectares, whereas all energy crops, including willow, were cultivated on an area of about 10 thousand hectares (Gajewski, 2010). It is predicted that in 2020 the land under energy crops would cover 2 million hectares, including about 500 thousand ha under perennial energy crops. Such a volume is required to meet the objectives outlined in the Directive 2009/28/EC on the
∗ Corresponding author. ˙ E-mail address: [email protected] (M. Krzyzaniak). http://dx.doi.org/10.1016/j.indcrop.2015.06.030 0926-6690/© 2015 Elsevier B.V. All rights reserved.
promotion of the use of energy from renewable sources (European Commission, 2009; Ku´s and Faber, 2009). In Poland, Salix is represented by 28 species and numerous hybrids (Tomanek, 1994). There have been several studies conducted into the properties of new willow varieties intended for energy production. Research has been conducted in Sweden, the UK, the USA and in many other countries (Larsson, 1998; Lindegaard and Barker, 1996; Smart et al., 2005). In Poland, there is no single, central programme for research into the cultivation of SRWC willow, whereas the number of varieties registered with the Polish Research Centre for Cultivar Testing (15 Polish proprietary varieties) is a sign of interest in the subject (COBORU, 2014). Of the varieties mentioned, ten were registered by the research team of the Department of Plant Breeding and Seed Production of the University of Warmia and Mazury in Olsztyn. The dominant system of Short Rotation Woody Crops (SRWC) cultivation on farmland is a short, three-year cycle (Kopp et al., 1997; Labrecque and Teodorescu, 2003; Kopp et al., 1997; Stolarski et al., 2014b). Soil preparation before the planting of willow cuttings comprises typical soil tillage treatments: deep ploughing, harrowing, mechanical and
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chemical weed control, mineral fertilization, etc. Another willow biomass production method is known as the Eko-Salix system. It consists of simplified, ploughless soil tillage and the planting of unrooted pole cuttings (Stolarski et al., 2011a). Perennial energy crops, including willow, should be highly productive and the biomass should possess high energy value, allowing farmers to obtain large amounts of energy per ha each year. The productivity potential of perennial crops is highly varied and can range from a few to several tonnes of dry matter per ha annually. The yield depends on the soil quality, plant species and cultivar, crop management, planting density and rotation cycle (Labrecque and Teodorescu, 2005; Quaye and Volk, 2013; Stolarski et al., 2014a). Apart from the biomass productivity obtained in a given year under given agro-climatic conditions, another important factor is the plantation life, on the basis of which we can estimate the accumulated biomass production of the plantation of studied perennial crop. Knowledge of the yields that can be expected on a commercial scale is crucial for clone selection and management optimization processes. Studies of this aspect of Short Rotation Woody Crops (SRWC) production have been done in Poland in recent years ˙ et al., 2014; Stolarski et al., 2014a, 2013). The inter(Krzyzaniak action of all of the above factors has a significant effect on both biomass productivity as well as the quality of energy feedstock, which is of paramount importance for the development of lignocellulosic crop production in the future. Short rotation woody crops should be established on poor, marginal or contaminated soils, and shall not compete with the production of edible crops. Nevertheless, farmers producing biomass will seek the highest yield from a unit area. It was hypothesized that biomass productivity for two selected cultivars would differ between cultivars and the cultivars productivity would be different on two soil types. In view of the above, the purpose of the present research has been to determine the morphological traits, biomass production and quality from two willow cultivars grown on a commercial scale at two different stands on loam and loamy-sandy soil.
2. Material and methods 2.1. Characteristics of the experimental area The years 2009–2011, when the research was carried out, were similar in the mean air temperatures (from 7.1 to 8.4 ◦ C) (Fig. 1). The total rainfall in each year was higher during the growing season than the analogous values calculated for the multi-year period (Fig. 1). In turn, the average annual precipitation in 2009 and 2010 was higher, and in 2011 it was lower than the multi-year average. The rainfall distribution was variable. In each year, April was drier than the multi-year average, which could inhibit the early growth and development of willow plants. The study area is characterized by gently undulating land, where differences in relative height were less than 15 m. The experiments were located on an eastward slope with a 10% gradient. The tests were carried out at two sites located on different soils. The first SRWC willow stand is located on loam soil: Haplic Cambisols (Eutric) (IUSS Working Group WRB, 2007) developed from loam with underlying clay loam at the depth of 88 cm. The second site was on Brunic Arenosols (Dystric) (WRB 2007) formed from loamy sand deposited on sand (Table 1). The actual moisture (% of the volume) at both sites was strongly differentiated. In the loam soil site it was within 20.9–30.7% vol. In the loamy sand soil site there was less water (7.2–10.1% vol.). At A and Bw levels of the loam soil site, water was easily available to plants (pF 2.0–3.0), whereas at the bedrock level it was poorly
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Table. 1 Characteristics of soil texture. Horizon
Depth (cm)
Percent of fraction of diameter (mm) >2.0
2.0–0.05
0.05–0.002
Loam soil 0–30 A 30–88 Bw C 88–150
0 0 0
42 45 30
47 45 51
13 10 19
Loamy sand soil A 0–28 28–65 Bv 65–150 C
0 0 0
85 74 88
10 22 11
5 4 1
Textural group
<0.002 Loam Loam Clay loam Loamy sand Loamy sand Sand
A – humic horizon; Bw – horizon enriched with non-alluvial iron deposits; Bv – in situ iron enrichment; C – bedrock.
accessible (pF 3.0–4.2). In the loamy sand site, water was poorly accessible to plants (pF 3.0–4.2) at all levels. The soils at the experimental sites were characterized by extremely different air and water conditions. There were few macropores in the loam soil site, whereas the loamy sand soil site was poor in micropores. Consequently, the loamy sand soil site suffered from periodic water deficits, which had an adverse effect on the growth and development of plants, an event unobserved at the other site. 2.2. Establishment and management of the plantation In the third decade of April 2008, a commercial willow plantation was established on 4.7 hectares of land at the Research Station ˙ owned by the University of Warmia and Mazury in Olszin Łe˛ zany, tyn (the UWM) (N: 53◦ 58 E: 21◦ 8 ). The preceding crop was winter triticale grown in a crop rotation system. After the triticale was harvested, in late autumn 2007, deep winter ploughing to the depth of 35 cm was done with a subsoiler. In spring 2008, soil smoothing was carried out and willow cuttings of the UWM 043 and Turbo cultivars bred at the UWM in Olsztyn were planted. Each cultivar was planted on two types of soil and on separate plots, which in total covered 2.35 ha of land. Cuttings, 25 cm in length and 0.9–1.1 cm in diameter, were planted manually from 20 to 22 April 2008 at a density of 18,000 cuttings per ha. A strip cropping system was used. The distance between double rows was 1.5 m and the distance between single rows in a double row was 0.75 m. The distance between cuttings in a row was 0.5 m. During first week after planting, weeds were controlled chemically (Guardian Complete MIX 664 SE: 3.5 l ha−1 with acetochlor and terbuthylazine: 300 g l−1 ). The one-year-old shoots were harvested after the 2008 growing season to increase the number of shoots from each stump in the following year. Mineral fertilizer was applied in the first decade of April 2009 in doses corresponding to N 100, P2 O5 30 and K2 O 60 kg ha−1 . The results of the study since 2009 are presented, in which the first factor consists of cultivars of willow species Salix viminalis: UWM 043 and Turbo. The second factor is the type of soil site: loam and loamy sand. For each stand and cultivar, experimental blocks measuring 60 m × 150 m were designed, on which plots measuring 3.0 × 10.0 m (30 m3 each) were randomly chosen in four replications (Fig. 2). After three years, in the first decade of January 2012, the plant density of three-year-old willow stands per 1 ha and the number of living shoots with height >1.5 m were counted on the designated plots. Next, the density of plants per hectare was calculated from plots. The following biometric measurements were performed on 10 plants within the plot: the stem height and diameter of stems at 0.5 m height of the stem. In order to obtain more detailed yield determination, in the first decade of January 2012, three-year-old
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Fig. 1. Weather conditions during the experiment period 2009–2011 and multi-year period 1998–2007; bars represent precipitation; curves represent air temperatures.
shoots were manually cut down 5–10 cm above the ground, using a DCS520 (Makita) chainsaw. The shoots harvested within each small plot were weighed with a precision of ±1 kg, respectively, thus determining fresh matter biomass. They were then chopped into chips using a Junkkari HJ 10 G wood chipper (Junkkari), coupled to a 96 kW powered tractor (New Holland). The willow dry biomass was calculated based on the fresh mass of shoots harvested from small plots and their moisture content. The dry biomass of the shoots was estimated using the fresh weight of the shoots. Additionally, the energy value of the biomass was estimated by multiplying the fresh biomass by its lower heating value (a detailed description in Section 2.3). 2.3. Harvested biomass quality evaluation
Fig. 2. Schematic overview of the experimental design.
Representative samples of willow chips were taken from each combination clone × site. The samples were placed in plastic bags and transported to the laboratory of the Department of Plant Breeding and Seed Production, the University of Warmia and Mazury in Olsztyn. The following characteristics were determined in the samples: moisture, higher heating value (HHV), lower heating value (LHV), ash content, volatile matter and fixed carbon, bulk density and concentrations of C, H, S, N, Cl. Each analysis was performed in three replications. Bulk density was determined by weighing chips in a 0.8 m3 container with a known weight. Next, the moisture was determined with the oven-dry method. For this aim, a sample of biomass was dried at 105 ◦ C to a constant weight (PN (Polish Standard) 80/G-04511). Next, the biomass was ground and the HHV was determined with the dynamic method using an IKA C 2000 calorimeter (IKA Werke Gmbh&Co. KG) in accordance with the PN-81/G-04513 standard. The LHV of the biomass was calculated according to Kopetz et al. (2007). Volatile matter, fixed carbon and ash content were determined at 550 ◦ C with an ELTRA TGA-THERMOSTEP automatic thermogravimetric analyzer (PN-ISO 562). The content of nitrogen was analyzed with the Kjeldahl method using a K-435 unit and B-324 BÜCHI distiller (BÜCHI Labortechnik AG), while the content of chlorine was determined using an Eschka mixture.
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0.0000 0.0000 0.0000
16.12 ± 0.28a 3.28 ± 0.41c 11.29 ± 0.61b 2.53 ± 0.28d 9.70 ± 6.87a 6.91 ± 4.70b 13.70 ± 2.62a 2.91 ± 0.51b 8.31 ± 5.87
0.0000 0.0000 0.0000
0.84a 1.22c 1.83b 0.85d 20.62a 14.10b 7.86a 1.54b 17.60 ± ± ± ± ± ± ± ± ± 48.36 9.84 33.86 7.60 29.10 20.73 41.11 8.72 24.92
0.0000 0.0000 0.0000
1.73a 2.33c 3.64b 1.73c 39.16a 27.80b 13.60a 2.68b 33.62 ± ± ± ± ± ± ± ± ± 92.23 19.05 67.27 15.50 55.64 41.39 79.75 17.27 48.51
0.0008 0.3027 0.9902
0.55 0.36 0.13 0.19 0.44b 0.18a 0.44 0.18 0.52 ± ± ± ± ± ± ± ± ± 1.64 1.83 2.42 2.60 1.73 2.51 2.03 2.22 2.12
0.0009 0.0000 0.9570
5.11 2.01 4.03 0.13 11.18a 10.82b 5.87a 416b 11.31 ± ± ± ± ± ± ± ± ± 40.38 20.56 32.81 13.19 30.47 23.00 36.59 16.88 26.73
0.0000 0.0000 0.9593
0.18 0.25 0.34 0.14 1.89a 1.89b 0.84a 0.82b 1.98 ± ± ± ± ± ± ± ± ± 8.07 4.55 6.56 3.06 6.31 4.81 7.32 3.80 5.56
P-value Variety (A) Soil (B) AB
Mean for experiment
Mean for soils
Mean for varieties
Turbo
±Standard deviation; a, b, c ... homogeneous groups; Significant at P < 0.05.
0.0152 0.0011 1.0000
± ± ± ± ± ± ± ± ±
1.78 2.52 1.78 2.52 3.19b 3.19a 2.33b 2.86a 3.47 0.0152 0.0011 1.0000
Number of plants (plants ha−1 )
Loam soil Loamy sand soil Loam soil Loamy sand soil UWM 043 Turbo Loam soil Loamy sand soil
The bulk density and moisture of biomass differed significantly between the cultivar (P = 0.0000) and soil type (P = 0.0007; and P = 0.0002 respectively). No significant effect was observed between cultivar and soil interactions (P = 0.7708; and P = 0.1691 respectively) (Table 2). The bulk density of fresh chips of UWM 043 cultivar was significantly higher (321.28 kg m−3 ) than from the Turbo cultivar (Table 4). The average moisture of the biomass of three-year-old willow plants was 49.14%, ranging from 47.96% for the UWM 043 cultivar to 50.31% for the Turbo cultivar. The higher heating value of willow biomass was significantly higher in the Turbo cultivar, where it accounted for an average of 19.61 MJ kg−1 of dry matter (P = 0.0014). The biomass obtained from the light soil was characterized by significantly higher HHV compared to the biomass obtained from the loam soil site (P = 0.0000).
Parameter
3.3. Evaluation of the biomass quality
Table 2 Plant density, biometric traits and yield of three-year-old willow stands.
The fresh biomass productivity and dry wood yield differed significantly between the variety (P = 0.0000), soil type (P = 0.0000) and between their interactions (P = 0.0000) (Table 2). The willow dry biomass obtained from the UWM 043 grown in three-year rotation in the field experiment on the commercial plantation was high (9.70 Mg ha−1 year−1 on average), ranging from 3.28 Mg ha−1 year−1 on loamy sand soil to 16.12 Mg ha−1 year−1 on loam soil. In turn, the cultivar Turbo yielded on average 40.4% less, although on loam soil it produced an average dry biomass of over 11 Mg ha−1 year−1 d.m. Our analyses demonstrated that the dry biomass was positively correlated with the number of plants (0.77), their height (0.96) and diameter of shoots (0.94) (Table 3).
Mortality (%)
3.2. Biomass productivity
2.78 7.41 5.86 10.49 5.09 8.18 4.32 8.95 6.64
Plant height (m)
Shoot diameter (mm)
Number of shoots
Fresh biomass production (Mg ha−1 )
The number of three-year-old willow plants was on average 16 806 plants ha−1 , thus being 6.64% lower than the initial number (Table 2). Significantly higher plant mortality was recorded for the Turbo cultivar than for the UWM 043 (P = 0.0152). A similar dependence for this trait was found on both sandy loam soil as well as loam soil (P = 0.0011). The average height of three-year-old plants in the experiment was 5.56 m. The plants of the UWM 043 cultivar were significantly higher (6.31 m) than those of Turbo cultivar (4.81 m) (P = 0.0000). The plants growing on loam soil were on average nearly twice as tall as the plants on loamy sand soil (P = 0.0000). The UWM 043 cultivar grown on loam soil yielded plants which – at the height of 8.07 m – were 5 m taller than Turbo plants grown on loamy sand soil. The cultivar Turbo produced significantly more shoots than the UWM 043 cultivar (P = 0.0008), whereas the average diameter of shoots from the UWM 043 was 30.47 mm, being 7.5 mm larger than in the case of Turbo. The plants grown on loam soil produced significantly thicker shoots than the plants grown on loamy sand soil (P = 0.0000). In conclusion, the UWM 043 cultivar produced fewer shoots, but the shoots of this cultivar had a larger diameter and larger height, which had a positive effect on the greater yield of this cultivar compared to the Turbo cultivar.
321 454 321 454 575a 575b 420a 514b 625
3.1. Plant mortality and biometric traits
± ± ± ± ± ± ± ± ±
Dry matter yield (Mg ha−1 )
3. Results
UWM 043
All statistical analysis were performed using STATISTICA 9.0 software package (StatSoft Inc.). A multiple SNK (Student NewmanKeuls) test at a significance level alpha = 0.05 was applied in order to analyse homogenous groups for the studied features of the biomass.
17500 16667 16944 16111 17083 16528 17222 16389 16806
Dry matter yield (Mg ha−1 year−1 )
2.4. Statistical analysis
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Table 3 The correlation coefficients for the analysed traits. Number of plants
Number of shoots
Plant height
Shoot diameter
Fresh biomass
Dry biomass
Biomass energy value
Number of plants Number of shoots Plant height Shoot diameter Fresh biomass Dry biomass Biomass energy value
1.00 −0.40 0.81* 0.75* 0.76* 0.77* 0.77*
-0.40 1.00 −0.48 −0.36 −0.35 −0.37 −0.37
0.81* −0.48 1.00 0.95* 0.96* 0.96* 0.96*
0.75* −0.36 0.95* 1.00 0.94* 0.94* 0.94*
0.76* −0.35 0.96* 0.94* 1.00 1.00* 1.00*
0.77* −0.37 0.96* 0.94* 1.00* 1.00 1.00*
0.77* −0.37 0.96* 0.94* 1.00* 1.00* 1.00
*
Correlation coefficients significant at the level of ≤0.05.
Table 4 Thermophysical properties and chemical composition of the biomass obtained from three-year-old willow stems. Bulk density (kg m−3 )
Moisture content (%)
HHV (MJ kg−1 d.m.)
LHV (MJ kg−1 )
Ash (% d.m.)
Fixed carbon (% d.m.)
Volatile matter (% d.m.)
C (% d.m.)
H (% d.m.)
N (% d.m.)
S (% d.m.)
Cl (% d.m.)
Loam soil Loamy sand soil Loam soil Loamy sand soil UWM 043
310.00 ± 2.00 332.55 ± 9.45
47.56 ± 0.34 48.36 ± 0.42
19.38 ± 0.02c 19.66 ± 0.04b
9.00 ± 0.07 8.97 ± 0.12
1.23 ± 0.01d 1.56 ± 0.00b
19.19 ± 0.07c 20.06 ± 0.02b
78.88 ± 0.06a 78.14 ± 0.01c
49.98 ± 0.13 50.11 ± 0.02
5.98 ± 0.01 5.89 ± 0.02
0.50 ± 0.00b 0.40 ± 0.01d
0.024 ± 0.001 0.023 ± 0.001
0.006 ± 0.000 0.008 ± 0.001
262.50 ± 4.50 287.78 ± 11.48
49.66 ± 0.17 50.96 ± 0.02
19.40 ± 0.03c 19.83 ± 0.04a
8.55 ± 0.05 8.48 ± 0.02
1.41 ± 0.01c 1.68 ± 0.02a
20.01 ± 0.25b 21.42 ± 0.05a
78.71 ± 0.03b 76.62 ± 0.06d
49.77 ± 0.12 50.14 ± 0.26
5.92 ± 0.00 5.96 ± 0.09
0.62 ± 0.02a 0.47 ± 0.00c
0.029 ± 0.001 0.025 ± 0.001
0.008 ± 0.001 0.010 ± 0.000
321.28 ± 13.78a 47.96 ± 0.56b
19.52 ± 0.16b
8.98 ± 0.09a
1.39 ± 0.18b
19.62 ± 0.48b
78.51 ± 0.41a
50.04 ± 0.11
5.93 ± 0.05
0.45 ± 0.06b
0.023 ± 0.001b
0.007 ± 0.001b
Turbo Loam soil
275.14 ± 15.89b 50.31 ± 0.072a 286.25 ± 26.20b 48.61 ± 0.18b
19.61 ± 0.24a 19.39 ± 0.02b
8.52 ± 0.05b 8.78 ± 0.25
1.55 ± 0.15a 1.32 ± 0.10b
20.71 ± 0.79a 19.60 ± 0.48b
77.67 ± 1.14b 78.80 ± 0.10a
49.96 ± 0.27 49.88 ± 0.16b
5.94 ± 0.06 5.95 ± 0.03
0.54 ± 0.08a 0.56 ± 0.06a
0.027 ± 0.002a 0.026 ± 0.003a
0.009 ± 0.001a 0.007 ± 0.001b
310.16 ± 26.26a 49.66 ± 1.45a
19.74 ± 0.10a
8.73 ± 0.28
1.62 ± 0.07a
20.74 ± 0.75a
77.38 ± 0.83b
50.12 ± 0.17a
5.92 ± 0.07
0.44 ± 0.04b
0.024 ± 0.002b
0.009 ± 0.001a
298.21 ± 27.96 0.0000 0.0007 0.7708
19.56 ± 0.20 0.0014 0.0000 0.0053
8.75 ± 0.25 0.0000 0.2598 0.6509
1.47 ± 0.18 0.0000 0.0000 0.0035
20.17 ± 0.84 0.0000 0.0000 0.0084
78.09 ± 0.93 0.0000 0.0000 0.0000
50.00 ± 0.20 0.3768 0.0267 0.2193
5.93 ± 0.05 0.8313 0.3792 0.0517
0.50 ± 0.08 0.0000 0.0000 0.0025
0.025 ± 0.003 0.0002 0.0019 0.0693
0.008 ± 0.002 0.0012 0.0012 1.0000
Parameter UWM 043
Turbo
Mean for varieties Mean for soils
Loamy sand soil Mean for experiment Variety (A) Soil (B) AB
49.14 ± 1.37 0.0000 0.0002 0.1691
±Standard deviation; a, b, c... homogeneous groups; Significant at P < 0.05.
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Parameter
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However, when the moisture was taken into account, it was shown that the LHV of biomass from both sites was at the same level (P = 0.2598). In turn, the UWM 043 cultivar had a significantly higher LHV (8.98 MJ kg−1 ) than the Turbo cultivar (P = 0.0000). The ash content, fixed carbon and volatile matter in biomass differed significantly between the cultivar (P = 0.0000), soil type (P = 0.0000) and their interactions (Table 4). The biomass of the UWM 043 contained significantly less ash (1.39% d.m.) compared to the Turbo. The content of fixed carbon ranged from 19.19 to 21.42% d.m., whereas the content of volatile matter was between 76.62 and 78.88% d.m. The content of carbon determined in threeyear-old willow wood was on average 50.04% d.m. in the UWM 043 cultivar and 49.96% d.m. in Turbo (P = 0.3768). Among the analyzed soil sites, significantly more carbon was found in the biomass obtained from the plants grown on loamy sand (P = 0.0267). The content of hydrogen in the willow biomass was not differentiated by the experimental factors (5.93% d.m., on average). In contrast, the content of nitrogen, sulphur and chlorine was significantly higher in the biomass of cultivar Turbo. The concentrations of sulphur and chlorine in the willow biomass determined in this experiment were low, i.e. 0.025% d.m. and 0.008% d.m., respectively. 3.4. Biomass energy value The energy value of the biomass was significantly differentiated by the willow variety (P = 0.0000), soil type (P = 0.0000) and between their interactions (P = 0.0000) (Fig. 3). The highest biomass energy value was found for UWM 043 cultivar grown on loam soil (276.7 GJ ha−1 year−1 ). The amount of energy in the cultivar Turbo grown on the same site was 85 GJ ha−1 year−1 lower. The significantly lowest value of this feature was found in the Turbo cultivar cultivated in loamy sand soil (only 43.8 GJ ha−1 year−1 ). With an average energy value of coal equal to 25 GJ Mg−1 , it can be concluded that the energy contained in the biomass of cultivars UWM 043 and Turbo plants grown on loam soil correspond to 11.1 and 7.7 Mg of coal, respectively. 4. Discussion Dry biomass productivity of SRWC willow cultivated in a three-year rotation cycle varied significantly, depending on the cultivar and soil conditions. Out of the two cultivars bred at the UWM, the UWM 043 was more productive. The studies carried out in Poland showed that the willow yield from short rotation plantations tested in field trials under optimal conditions reached up to 30 Mg ha−1 year−1 d.m. (Stolarski et al., 2008; Stolarski et al., 2011b). The average experimental yields, in turn, were between 10 and 12 Mg ha−1 year−1 d.m. (Tworkowski et al., 2006). However, yields from large-scale plantations were, in general, significantly lower than those obtained from large (70–300 ha) plantations tested by the UWM researchers, and ranged from 4 to 10 Mg ha−1 year−1 d.m. (Tworkowski et al., 2010). The reasons why yields from large-scale plantations were low were the difficulties in selecting and preparing the land area, mistakes made while setting up plantations, planting randomly chosen willow clones, ineffective weed control and inadequate fertilization. Searle and Malins (2014) also state that yields from field trials are much higher than from large scale plantations, due to the edge effect, easier and more frequent crop management, etc. On large-scale plantations, yields are also lower due to biomass losses, difficulties in crop management and harvesting inefficiency. For example, in Sweden, experimental plantations gave high willow yields, which were not successfully repeated on largescale commercial plantations (Melin and Larsson, 2005). The highest average yields (12 Mg ha−1 year−1 d.m.) were achieved
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on non-irrigated commercial plantations of willow harvested in three-year rotation (Melin and Larsson, 2005). This relationship has been verified by the data obtained from 1512 plantations kept from 1986 to 2000, where the average willow yields were just 2.67 Mg ha−1 year−1 d.m. and the highest ones reached 20.54 Mg ha−1 year−1 d.m. (Mola-Yudego, 2011). Finally, typical yields produced by Salix viminalis plants on commercial plantations in the United Kingdom are from 6 to 11 Mg ha−1 year−1 d.m. (Bullard et al., 2002; Powlson et al., 2005). In a study conducted in Denmark with eight willow cultivars grown on five different soils with different methods of crop management and levels of fertilization in a three-year harvest rotation, the willow yield levels were highly differentiated and ranged from 2.9 to 10.2 Mg ha−1 year−1 d.m. (Larsen et al., 2014). In another study, conducted in the USA, 18 cultivars of willow grown on two plots (three-year harvest cycle) were shown to have significantly different yields, ranging from 3.54 to 13.58 Mg ha−1 year−1 d.m. (Serapiglia et al., 2013). A high yield of 4 willow clones (average 14.1 Mg ha−1 year−1 d.m.) grown on very good soil and fertilized with mineral fertilizers was obtained in a four-year harvest cycle in another study by Stolarski et al., 2013. In Poland, the most productive is the cultivation of willow on soils abundant in nutrients, where the groundwater table is high (down to 2 m). On a large-scale plantation of willow set up on alluvial soil in the Vistula river valley, where different willow cultivars were grown on a well-managed plantation and harvested in short (2- or 3-year) cycles, the average yield was 20 Mg ha−1 year−1 d.m. (Tworkowski et al., 2010). In the authors’ own study, the growth and yield of willow plants varied significantly according to cultivar and the soil conditions. Particularly large differences were observed during long periods without precipitation. A large water deficit occurred at a loamy sand soil site due to the absence of groundwater capillarity between the lower layer of sand and the upper layer of loamy sand in the soil profile. Roots of willow plants had therefore much worse conditions for the uptake of water than those of plants growing on loam soil site, where the permeation of water through soil capillaries to the rhizosphere was constant. It has been demonstrated that the cultivar, type of soil and water access are significant yield-forming factors in willow growth, which was confirmed by other authors (Bungart and Hüttl, 2001; Labrecque and Teodorescu, 2003; Fischer et al., 2005; Tworkowski et al., 2006). Sevel et al. (2012) also found that willow yield was significantly differentiated by clones and soil types, indicating that different clones may be better suited for different soil types. They found that that dry matter production ranged from 5.2 Mg ha−1 year−1 d.m. for Inger cultivated on the sandy soil to 8.8 Mg ha−1 year−1 d.m. for Tora cultivated on the organic soil. Cultivars Inger, Sven, Tora and Tordis cultivated on sandy soil yielded at the same level, whereas Inger and Tora cultivated on organic soil produced significantly higher yields than Sven and Tordis. The literature generally recommends harvesting SRWC willow every three years, justifying this recommendation not only by higher productivity versus shorter rotation systems but also by the good quality of the harvested biomass as fuel (Kopp et al., 1997; Labrecque et al., 1997; Adler et al., 2005; Labrecque and Teodorescu, 2003; Smaliukas et al., 2007). Biomass to be used as solid fuel should be characterized by a low content of moisture, ash and alkaline elements, but have a high energy value. For processes of biochemical conversion of biomass to liquid fuels, the ratio of cellulose to lignin and hemicelluloses content is also important (McKendry, 2002). In the authors’ own experiment, the UWM 043 cultivar was characterized by a higher LHV of biomass and a higher specific density compared to Turbo. This variety also had significantly less ash, nitrogen, sulphur and chlorine than the Turbo cultivar, which makes it a prospective choice as an energy crop. One reason for a better quality of the UWM 043 biomass was the fact that it produced
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Fig. 3. The energy value of SRWC biomass. Significant at P < 0.05; P-value: Variety (A) – 0.0000; Soil (B) – 0.0000; AB – 0.0000
fewer shoots, which were larger in diameter than shoots produced by Turbo. Adler et al. (2005) found that thinner shoots contain more bark than thicker shoots and the bark has much more N, P, K, Mg, Ca, Cd, Pb, Co and Zn than wood. This feature affects the combustion process and can accelerate corrosion of furnaces. The content of ash in willow biomass is directly dependent on the biomass content of alkaline elements. The lower their content in fuel, the less ash is generated during its combustion (Tharakan et al., 2003). In the authors’ previous studies, a significant effect of the cultivar and willow clone on the content of ash and elements was found (Stolarski, ˙ 2009; Krzyzaniak et al., 2015).
5. Conclusions The biometric traits, yield and quality of biomass produced by Salix spp. varied significantly according to both the cultivar and the soil site. This experiment showed that willow Salix viminalis grown in three-year cycle on good soils (in this case, loam) under the conditions present in north-eastern Poland can produce plants over 8 m tall with shoots over 4 cm in diameter. In turn, production of Salix spp. on much poorer soils (in this study, loamy sand) resulted in lower average plant height and shoot diameter. These factors directly influence willow yield. Under optimal conditions, on a commercial plantation of willow in north-eastern Poland, established on heavy soil in a three-year rotation system, it is possible to obtain up to 16.12 Mg ha−1 year−1 of dry biomass. The UWM 043 cultivar yielded significantly higher than the Turbo cultivar. The biomass produced by the UWM 043 proved to be more valuable fuel owing to a higher bulk density, lower moisture content, higher LHV and lower content of nitrogen, sulphur and chlorine. The present research has indicated that studies on the selection of willow cultivars or clones with respect to soil conditions should be continued, because this factor has a significant influence on yield volumes, energy value and biomass quality.
Acknowledgments The authors wish to acknowledge the financial support for the studies provided by project grant N R12 0065 10, from the National Centre for Research and Development.
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