Elemental composition of willow short rotation crops biomass depending on variety and harvest cycle

Highlights

• The study showed a significant effect of the factors on the elemental composition.

• Extending the harvest cycle resulted in lower element concentrations.

• Biomass of the UWM 200 clone had the best quality parameters.

• K, P, Mg, Zn and Cr had the highest number of significant correlations.

Abstract

The use of woody biomass in thermochemical conversion is still the most popular method of obtaining bioenergy from this material. Elemental composition has a significant effect on biomass quality and parameters of the burning process. The aim of this study was to determine the content of selected macroelements (phosphorus, potassium, magnesium, calcium, sodium and sulphur), microelements (boron, copper, iron, manganese and zinc) and trace elements (cadmium, chromium, nickel and lead) in willow biomass of two different varieties and three clones, harvested in annual, biennial and triennial cycles in north-eastern Poland.

The study found significant variability between varieties/clones and harvest cycles. Biomass of the UWM 200 clone of Salix alba in triennial harvest rotation had the best parameters as a potential fuel source; it was found to contain the significantly smallest concentrations of P, K, Na, S (1.48, 5.45, 0.08, 0.35 g kg^-1 DM, respectively) and Fe, Cr, Pb (208.85, 4.91, 0.83 mg

kg^-1, respectively ). The content of all of the elements under study decreased significantly with an extended harvest cycle, and the differences, when comparing the annual to the triennial cycle, ranged from -9% to -29% for macroelements, from -7% to -48% for microelements and from -11% to -20% for trace elements.

Keywords

Biomass, inorganic elements, trace elements, SRWC

1. Introduction

Wood biomass has been used by humans as fuel since prehistoric times and it accounts for 6% of all energy sources globally [1]. It is estimated that more than 2 billion people in developing countries depend on wood for heating and cooking, which shows that wood energy is mostly used on a non-commercial basis [2]. However, since resources of the main energy feedstock, such as coal and oil, are becoming exhausted and because of the global policy of reducing CO2 emission, woody biomass has been increasingly often consumed in energy generation for industry, where it can be used traditionally to produce heat and electricity by combustion, gasification, but also – owing to its rich chemical composition – it has enormous potential in the chemical industry and biorefineries [3, 4].

One source of biomass are lignocellulosic plants, such as willow, grown in short rotation (short rotation crops, short rotation woody crops or short rotation forestry) [5] depending on the harvest cycle, usually 1-7 years [6]. It has been shown in numerous studies of willow plantation productivity that extending the harvest cycle increases the yielding potential and growth in 3- or 4-year harvest cycles is among the optimum solutions [7-9].

Woody biomass is used mainly in burning processes [10]. One of the greatest advantages of biomass is the possibility of co-firing with coal in combustion installations, originally designed only for coal. Co-firing biomass and coal has technical, economic and environmental advantages over the other options of biomass use [10]. However, it must be emphasized that biomass provides much less energy per unit than good quality coal [11, 12], but the environmental benefits of co-firing (i.e. reducing CO₂ emissions) still makes this source of energy very attractive. However, it should be stressed that co-combustion of biomass with coal is one of the worst methods of its use as fuel, especially when the process is conducted in installations designed for burning coal, which lowers the effectiveness of using the energy contained in biomass. Furthermore, when it comes to the content of ash as a product of incomplete combustion, the biomass of SRWC can contain several times less ash than biomass of straw or grass [13] and up to a dozen times less compared to coal [14].

However, major ash-forming elements like Al, Ca, Fe, Mg, K, Na, P, Si, Ti; volatile minor elements like As, Cd, Hg, Pb, Zn and partly or non-volatile elements like Ba, Co, Cr, Cu, Mo, Mn and V are the cause of ash melting, corrosion, aerosol emissions and they are of crucial importance in the choice of an ash utilisation method [15]. Ash from biomass burning can be a valuable agent used to improve soil properties in growing many plant species and - when combined with other fertilisers - they can have a beneficial effect on the productivity of crops [16, 17]. However, the more macro-, micro- and trace elements in biomass, the more ash remains from burning and, consequently, less energy can be obtained from a unit of biomass.

Compared to coal, biomass can contain higher concentrations of Mn, K, P, Ca, Mg, Na and the elemental composition of biomass can be affected by such factors as plant species, geographic location, age of the plants and fertilization and pesticides used in cultivation [18]. It is a well-known fact that extending the harvest cycle reduces the content of chemical elements in woody biomass per mass unit, which is associated with a higher content of the elements in bark than in wood and the bark/wood ration increases with extending the harvest cycle because of the growing diameter of shoots [19, 20]. Variability of the content of chemical elements in biomass of different willow clones has been observed in earlier studies [19, 21]. It has been shown that a large biomass yield of relatively high energy value was obtained from the varieties under study [22].

However, it is important to know the elemental concentration in high-yielding varieties because plant varieties of the best biomass quality parameters should be used for SRWC plantations whose biomass will be used in thermal conversion, which will help to reduce the negative impact on combustion furnaces and power boilers. Therefore, considering the variability of the chemical composition, demonstrated by other researchers, which depends on such factors as variety/clone, local habitat conditions and harvest cycle, it is important to seek a combination of these factors which will prove optimal for growing willow as an energy crop.

Therefore, the aim of this study was to determine the content of selected macroelements (P, K, Mg, Ca, Na, S), microelements (B, Cu, Fe, Mn, Zn) and trace elements (Cd, Cr, Ni, Pb) in willow biomass of two different varieties and three clones harvested in annual, biennial and triennial cycles in north-eastern Poland.

2. Materials and methods

2.1. Description of the experiment

The study was based on an exact, two-factorial experiment with willow coppice, located in the north of Poland in the Kwidzyn Lowlands in the village of Obory (53°43′ N, 18°53′ E), conducted from 2003 to 2006. The experiment was located on humus alluvial soil, classified as—Mollic Fluvisols, which is the optimal soil for willow cultivation. No mineral fertilisers were applied in the year when the experiment was established. The following doses of fertilisers were sown manually in the subsequent years of the experiment N – 90 kg ha⁻¹, P–18 kg ha⁻¹, K–66 kg ha⁻¹.

The first factor examined in the experiment were two willow varieties and three clones, all of which were cultivated by University of Warmia and Mazury in Olsztyn: klony UWM 200, and UWM 095 (obydwa z gatunku Salix alba L.), odmiany Tur, Turbo and klon UWM 046 (wszytskie z gatunku Salix viminalis L.). The other factor were three harvest cycles: every year, every two years and every three years. The detailed information about setting up and running experiment is provided in a previous research article by Stolarski et al. [22].

2.2. Sampling procedure

One and two-year old plants were harvested after the 2005 vegetation period and three-year old plants after the 2006 vegetation period.

Willows were harvested manually with a blade trimmer powered by a combustion engine. During the harvest, representative plant stems were collected from each plot to determine chemical characteristics.

2.3. Laboratory analyses

Potassium, calcium and sodium were assayed by flame photometry. Samples were mineralised in H₂SO₄ and, subsequently, the intensity of radiation was measured which was emitted by ions of a given element formed when the solution was sprayed in the photometer flame (C. Zeiss Jena). The content of phosphorus and boron was determined by colorimetry on a Specol colorimeter. In order to determine the phosphorus content, samples were mineralised in H₂SO₄, followed by measurement of the deposit colour formed after the reacting mixture (HNO₃ + NH₄VO₃ + (NH₄)6Mo₇O₂₄) was added. In order to determine the content of boron, samples were mineralised in a muffle furnace at 500°C, the post-mineralisation residue was dissolved in HNO₃ and diantrimide was then added and the colour intensity was measured. Sulphur was assayed by nephelometry on a Specol colorimeter. Samples were mineralised in a muffle furnace at 500°C, the post-mineralisation residue was dissolved in HNO₃, BaCl₂ was added and the turbidity was measured. Magnesium, copper, iron, manganese, nickel, lead, cadmium and chromium content were measured by atomic absorption spectrometry (AAS). For the magnesium assay, samples were mineralised in H₂SO₄, whereas samples for the other assays were mineralised in a mixture of acids: HNO₃ and HClO₄ (4:1). Subsequently, absorption of radiation by ions was measured when each solution was sprayed in the flame.

The analyses were carried out in three replications. The concentrations of all elements in the study are presented on dry matter.

2.4. Statistical analysis

The results of the tests were analysed statistically using STATISTICA 9.1 PL. The mean arithmetic values, Pearson correlation coefficients and standard deviation of the examined features were calculated. Homogeneous groups for the examined characteristics were determined by means of Tukey’s HSD multiple comparison test with the significance level set at P<0.05.

3. Results and discussion

3.1. Macroelements in willow biomass

The content of macroelements in willow biomass was significantly differentiated by varieties and clones as well as by harvest cycles (Table 1; Fig. 1; Fig. 2). The UWM 200 clone had the lowest mean content of P and K (1.76 and 6.99 g kg^-1 DM, respectively) (Fig. 1), but the K content in an annual harvest cycle was significantly lower in the Tur variety (Table 1). Furthermore, the highest content of K, Mg and S was found in UWM 095 (9.86; 3.07 and 0.54 g kg^-1 DM, respectively), which also contained the lowest concentration of Ca (4.93 g kg^-1 DM) of all the varieties and clones under study (Fig. 1).

The Tur and Turbo varieties had similar properties in regard to the mean content of Ca and Mg (the same homogeneous groups) and the differences in their concentrations of Na and S (0.01 and 0.06 g kg^-1 DM, respectively) were small, but statistically significant. The lowest S content was found in the biennial harvest cycle in the Tur variety and in the triennial cycle in the UWM 200 clone compared to the other harvest cycles and the other varieties (Table 1). Furthermore, the UWM 046 clone had the highest content of P in each harvest cycle (2.85 g kg^-1 DM, mean) and the content of the other macroelements in it was similar to the mean level for all the varieties/clones and harvest cycles (Fig. 1; Fig. 2).

The sequence of mean macroelements content (in order of abundance) was as follows: K–Ca–Mg–P–S–Na (Fig. 2). In general, the content of macroelements P, K, Ca Mg and S in willow biomass decreased significantly with extended harvest cycles (triennial-to-annual differences: -27%; -26%; -29%; -9% and -12%, respectively). However, significant differences for Mg and S were observed only between the biennial and annual cycles (Fig. 2). A tendency was observed in other studies of the chemical composition of different willow clones with a decreasing proportion of these elements in an extended harvest cycle, but the differences were smaller for P and K (by 5 and 8 p.p. [percentage points], respectively), whereas they were slightly larger for Mg and S (by 6 and 1 p.p., respectively) than in this study [19].

A comparison of the mean content of macroelements in biomass harvested in a triennial cycle showed that the contents of Ca and Na were comparable, but that of P, K and Mg were significantly higher (by 5.54; 2.80 and 1.43 g kg^-1, respectively) than in the study conducted by Tharakan et al. [21]. Furthermore, willow biomass analysed in a study by Liu et al. [19] contained by 0.20 g kg^-1 on average more S, comparable concentrations of Na and much less of the other macroelements.

The considerable discrepancies between the biomass chemical composition in various studies could be caused by the sampling procedures [23], since representative sections of shoots were used for analysis in [21], whereas whole shoots were analysed in [19] and in the current study. Apart from chlorine, S, K and Na in biofuels play a major role in deposit formation and corrosion mechanisms [24]. Solid biomass should contain less than 1.0, less than 2.0 and less than 7.0 g kg^-1 DM of S, Na and K, respectively, to ensure unproblematic combustion [25]. The S or Na content did not exceed these levels in any of the varieties, irrespective of the harvest cycle (Table 1). However, the content of K was lower than 7.0 g kg^-1 DM only in the UWM 200 clone and in the Turbo variety in the 2- and 3-year harvest cycle (Table 1). Therefore, in terms of the macroelement content, the UWM 200 clone and the Turbo variety had the best quality parameters as a potential source of fuel for the combustion process.

Table 1

Figure 1

Figure 2

3.2. Microelements in willow biomass

The content of microelements in willow biomass was significantly differentiated by varieties, clones and by harvest cycles (Table 2; Fig. 3; Fig. 4). Fe was found to dominate in biomass of all varieties and clones; its mean content was the highest in the UWM 095 clone and in the Turbo variety and it was the lowest in the UWM 200 clone (Fig. 3). However, although the content of Fe in the UWM 200 clone was among the highest in the annual and biennial harvest cycle (homogeneous group "a"), the mean content was affected by a considerably different concentration in the triennial cycle (Table 2).

The mean content of Mn in all varieties ranged from 100 to 150 mg kg^-1 DM (Fig. 3). Its highest concentration was found in biomass of UWM 095; it is noteworthy that the standard deviation was so high because, as with the content of Fe in UWM 200, there was a considerable difference between the triennial cycle, when the Mn content was the lowest, and the other cycles, when it was the highest (Table 2). Furthermore, for Zn, the UWM 046 clone contained the highest concentrations of this element irrespective of the harvest cycle (Fig. 3; Table 2).

The mean concentrations of B and Cu were not as diversified as those of the other microelements (Fig. 3), it is noteworthy that the content of B in UWM 200 and of Cu in UWM 046 in the annual harvest cycle was considerably lower than in the other varieties and clones (Table 2). The sequence of mean microelements content (in order of abundance) was as follows: Fe–Mn–Zn–Cu–B (Fig. 4). As with the content of macroelements, the content of microelements in willow biomass decreased with extended harvest cycles (triennial to annual differences: -29%; -7%; -13%; -48% and -40%, respectively for B, Cu, Fe, Mn and Zn), but significant differences for Cu were only observed between the biennial and annual cycles (Fig. 4).

The contents of Fe, Cu and Mn were particularly high compared to other studies. The mean content of iron in 3-year willow shoots as determined in the study by Liu et al. [19] did not exceed 40 mg kg^-1 DM, which is nearly eight times less than the mean level found in the current study (Fig. 4; Table 2). The differences for Cu and Mn were also large (more than three times and nearly twice less, respectively) and only the content of Zn did not differ significantly between studies [19], with its mean content in this study found to be smaller in the triennial harvest cycle (by ca. 1.2 mg kg^-1 DM).

Compared to forest residues, SRWC biomass in the triennial harvest cycle contained much less Cu, Mn and Zn [26]. Moreover, when converted to weight (% DM), the content of Zn in biomass of none of the varieties or clones in any of the harvest cycle exceeded a level which guaranteed unproblematic combustion, i.e. 0.08% [25]. However, typical values for willow SRC (except Zn) adopted by European Committee for Standardization and cited by Biedermann and Obernberger [24], are much smaller than those found in this study. On the other hand, there is a deficiency of knowledge about critical values of Fe, Cu and Mn for unproblematic combustion, so it cannot be ascertained without doubt whether an above-average content of these elements in willow biomass could cause considerable issues in combustion installations. A high concentration of iron could be a particular cause for concern, but compared to the average worldwide concentration of this element in coal, the willow biomass analysed in this study contained nearly 43 times less Fe [27]. To sum up, the best parameters in terms of mean content of microelements among the willow varieties and clones under study were found in the biomass of the UWM 200 clone.

Table 2

Fig. 3

Fig. 4

3.3. Trace elements in willow biomass ash

As with macro- and microelements, the content of trace elements in willow biomass was significantly differentiated by varieties and clones as well as by harvest cycles (Table 3; Fig. 5; Fig. 6). Nickel and chromium dominated among such elements in all the clones and varieties and their highest mean content was found in the UWM 046 clone (Fig. 5). Although the mean content of Cd in this clone was the lowest, the highest concentrations of cadmium were found in the triennial cycle compared to other varieties (Table 3). Similar mean concentrations of Cd were found in the Turbo variety (the same homogeneous group); this variety was also found to contain the lowest mean concentrations of Pb, but (as in the previous case) the content of Pb in the triennial cycle was the highest (Table 3). The sequence of mean trace elements content (in order of abundance) was as follows: Ni–Cr–Pb–Cd (Fig. 6).

In general, a decreasing mean content of the elements under study with an extended harvest cycle was also confirmed for trace elements (triennial to annual differences: -15%; -11%; -19% and -20%, respectively for Ni, Cr, Pb, Cd (Fig. 6). Compared to the other studies, only the Cd content was significantly lower, whereas the concentrations of the other trace elements were up to 20 times higher [15, 19]. However, compared to coal, the content of each of the trace elements in willow biomass as determined in this study was lower, which is particularly manifest for Pb [27]. As earlier studies have shown, trace elements can be accumulated in biomass through water, soil, pesticides, fertilizers and additives [28], so it is difficult to ascertain which factor caused the concentration to be higher than ordinary. Since trace elements in biomass have high volatilisation potential [15, 28], it can be claimed that of all of the clones and varieties under study, the biomass of the UWM 200 clone has the best mean parameters.

Table 3

Fig. 5

Fig. 6

3.4. Correlation between elements

The strongest correlations among macroelements were found for potassium, for which significant correlations were not found only with calcium, sodium and cadmium (Table 4). The amount of K in willow biomass correlated the most strongly with the content of Mg, P and S (0.77; 0.67 and 0.67, respectively). Correlations of Mg and P with K were also the strongest, whereas S was correlated the most strongly with Zn, although the difference was small (0.01). Tharakan et al. [21] also demonstrated significant correlations between potassium and other macroelements. The correlations between these elements can be attributed to the fact that macroelements are the key plant nutrients and uptake of one stimulates the uptake of the others [29]. Of the microelements, manganese and zinc were the most strongly correlated (each with 10 significant correlations with other elements). The content of manganese was the most strongly correlated with the content of cadmium, while the content of zinc was the most strongly correlated with the content of nickel (0.58 and 0.80, respectively). Among the trace elements, the largest number of significant correlations were found between the content of chromium and that of the other elements. This element was also found to be the most strongly correlated with iron. However, the strongest positive correlation among trace elements (0.80) was found between nickel and zinc.

Table 4

4. Conclusions

This study has shown that the contents of macro- and micro- and trace elements in willow biomass are significantly differentiated by variety, clone and harvest cycle. The optimum parameters of willow biomass intended for thermochemical conversion can be obtained in 3-year harvest cycles. The content of all the elements under study decreased significantly with an extended harvest cycle and the differences (when comparing the annual to the triennial cycle) ranged from -9% to -29% for macroelements, from -7% to -48% for microelements and from -11% to -20% for trace elements.

Considering biomass harvested in the three-year rotation, the best parameters were found in the UWM 200 clone, whose biomass contained the significantly lowest concentrations of P, K, Na, S, Fe, Cr and Pb. On the other hand, the worst parameters of biomass as a potential combustion fuel were demonstrated in the UWM 046 clone, whose biomass contained the significantly highest concentrations of Cd, Cr, Ni, Zn, Fe, B, P, Ca and Na. The findings have shown that when biomass quality is concerned, the UWM 200 clone can be recommended as good fuel for generating electricity and heat, although it must be emphasised that the clone biomass (as in the other cases) contained high concentrations of iron, manganese and copper - much more than in studies conducted by other authors.

In view of the above results, the dynamically changing environment and the resulting variability in the factors affecting the elemental composition of willow biomass, further studies are necessary for confirmation.

Acknowledgements

This work has been financed by the Department of Plant Breeding and Seed Production statutory sources.

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Figure caption

Fig. 1. Mean content of macroelements depending on varieties/clones (bars represent standard deviation; letters indicate homogeneous groups)

Fig. 2. Mean content of macroelements in willow biomass depending on harvest cycle (bars represent standard deviation; letters indicate homogeneous groups)

Fig. 3. Mean content of microelements depending on varieties/clones (bars represent standard deviation; letters indicate homogeneous groups)

Fig.4. Mean content of microelements in willow biomass depending on harvest cycle (bars represent standard deviation; letters indicate homogeneous groups)

Fig. 5. Mean content of trace elements depending on varieties/clones (bars represent standard deviation; letters indicate homogeneous groups)

Fig. 6. Mean content of trace elements in willow biomass depending on harvest cycle (bars represent standard deviation; letters indicate homogeneous groups)

Table 1

The content of macroelements in willow biomass depending on cultivar/clone and harvest cycle

Harvest cycle	Variety/Clone	P	K	Mg	Ca	Na	S
Annual		g kg^-1DM
	UWM 200	2.24±0.03 d	9.38±0.04 c	2.61±0.09 b	7.12±0.78 b	0.10±0.01 d	0.61±0.01 a
	UWM 095	2.91±0.07 b	12.12±0.01 a	3.33±0.02 a	5.63±0.97 d	0.09±0.00 d	0.57±0.02 b
	Tur	2.33±0.07 d	7.87±0.00 f	2.23±0.03 d	9.14±0.14 a	0.18±0.00 a	0.42±0.01 c
	Turbo	2.54±0.01 c	8.39±0.01 e	2.45±0.04 c	6.94±0.07 b	0.15±0.01 b	0.47±0.01 c
	UWM 046	3.31±0.05 a	10.05±0.05 b	3.33±0.02 a	6.20±0.42 c	0.13±0.01 c	0.54±0.01 b
	UWM 200	1.55±0.02 f	6.16±0.02 j	2.39±0.13 c	5.45±0.02 d	0.10±0.01 d	0.46±0.01 c
	UWM 095	2.26±0.01 d	8.65±0.01 d	2.90±0.02 b	4.47±0.36 e	0.11±0.01 d	0.53±0.01 b
Biennial	Tur	2.27±0.03 d	7.49±0.02 g	2.37±0.35 c	7.03±0.35 b	0.16±0.01 b	0.39±0.01 d
	Turbo	2.26±0.01 d	6.73±0.07 h	2.39±0.14 c	7.86±0.14 b	0.15±0.00 b	0.50±0.01 b
	UWM 046	2.85±0.05 b	8.77±0.03 d	2.70±0.02 b	5.35±0.12 d	0.12±0.01 c	0.42±0.01 c
	UWM 200	1.48±0.02 f	5.45±0.02 k	2.52±0.05 c	5.17±0.69 d	0.08±0.01 d	0.35±0.01 d
	UWM 095	1.58±0.05 f	8.80±0.13 d	2.97±0.05 b	4.69±0.53 e	0.09±0.00 d	0.54±0.01 b
Triennial	Tur	1.98±0.05 e	7.28±0.05 g	2.38±0.03 c	4.37±0.08 e	0.14±0.02 b	0.47±0.02 c
	Turbo	2.37±0.04 d	6.53±0.13 i	2.14±0.00 d	5.07±0.21 d	0.13±0.00 c	0.48±0.01 c
	UWM 046	2.40±0.01 d	7.43±0.04 g	2.68±0.03 b	5.52±0.20 d	0.14±0.01 b	0.46±0.03 c

Mean  standard deviation; a,b,c..., homogenous groups interaction AB

Table 2

The content of microelements in willow biomass depending on cultivar/clone and harvest cycle

Harvest cycle	Cultivar	B	Cu	Fe	Mn	Zn
Annual		mg kg^-1 DM
	UWM 200	2.00±0.20 h	14.15±0.05 b	345.85±0.15 a	160.75±0.25 d	68.40±0.20 g
	UWM 095	6.94±0.06 b	15.65±0.35 a	347.67±15.32 a	202.90±2.10 a	87.48±0.53 d
	Tur	5.61±0.06 c	13.56±0.24 c	365.00±11.65 a	150.60±3.00 f	89.74±0.27 c
	Turbo	3.01±0.05 g	15.25±0.25 a	320.93±14.06 b	154.50±1.50 e	99.35±0.45 b
	UWM 046	7.67±0.12 a	11.85±0.05 e	334.93±8.62 b	145.80±0.20 g	127.15±0.15 a
	UWM 200	3.51±0.05 f	12.30±0.10 d	355.80±0.20 a	156.75±0.25 e	74.10±0.10 f
	UWM 095	3.93±0.03 e	14.80±0.10 b	304.70±1.30 c	196.30±0.70 b	58.20±0.60 i
Biennial	Tur	4.68±0.10 d	12.05±0.25 e	283.90±0.10 c	131.05±0.05 h	85.25±0.05 e
	Turbo	3.58±0.03 f	12.75±0.05 d	325.20±6.40 b	181.00±0.90 c	86.99±0.02 d
	UWM 046	3.60±0.09 f	14.20±0.20 b	257.90±0.10 d	112.10±0.10 i	73.70±0.10 f
	UWM 200	3.51±0.01 f	13.40±0.10 c	208.85±0.15 e	105.85±0.85 j	53.35±0.25 j
	UWM 095	3.22±0.00 g	12.77±0.13 d	325.20±6.40 b	41.40±0.80 n	49.69±0.64 k
Triennial	Tur	3.49±0.01 f	14.45±0.15 b	280.80±0.20 c	92.30±0.30 l	67.15±0.25 h
	Turbo	3.83±0.06 e	12.60±0.03 d	346.65±5.65 a	79.10±0.10 m	44.85±0.02 i
	UWM 046	3.91±0.09 e	12.15±0.05 d	355.85±0.85 a	101.45±1.55 k	69.05±0.15 g

Mean  standard deviation; a,b,c..., homogenous groups interaction AB

Table 3

The content of trace elements in willow biomass depending on cultivar/clone and harvest cycle

Harvest cycle	Cultivar	Cd	Cr	Ni	Pb
Annual		mg kg^-1 DM
	UWM 200	0.16±0.01 a	10.24±0.04 b	10.65±0.13 b	2.28±0.04 a
	UWM 095	0.15±0.01 a	9.96±0.38 b	10.62±0.31 b	1.67±0.10 c
	Tur	0.15±0.01 a	11.08±0.52 a	11.00±0.32 a	1.75±0.05 c
	Turbo	0.16±0.01 a	10.12±0.19 b	11.81±0.05 a	1.03±0.02 d
	UWM 046	0.15±0.00 a	10.13±0.30 b	11.63±0.06 a	1.64±0.16 c
	UWM 200	0.15±0.00 a	10.37±0.03 b	9.05±0.04 c	2.00±0.02 b
	UWM 095	0.14±0.00 b	9.92±0.01 b	8.72±0.01 d	1.03±0.02 d
Biennial	Tur	0.14±0.01 b	6.78±0.02 d	11.21±0.01 a	1.48±0.03 c
	Turbo	0.16±0.01 a	10.06±0.20 b	11.62±0.03 a	1.01±0.04 d
	UWM 046	0.14±0.01 b	10.46±0.04 b	11.14±0.01 a	1.05±0.03 d
	UWM 200	0.10±0.01 c	4.91±0.04 e	9.32±0.03 c	0.83±0.16 e
	UWM 095	0.09±0.01 d	8.59±0.05 c	9.39±0.09 c	1.18±0.03 d
Triennial	Tur	0.12±0.02 c	8.16±0.05 c	9.81±0.03 c	1.14±0.03 d
	Turbo	0.14±0.00 b	8.73±0.17 c	8.32±0.01 d	1.45±0.05 c
	UWM 046	0.17±0.01 a	10.21±0.09 b	10.78±0.66 b	1.34±0.11 d

Mean  standard deviation; a,b,c..., homogenous groups interaction AB

Table 4

The simple correlation coefficients for the elements under study

	P	K	Mg	Ca	Na	S	B	Cu	Fe	Mn	Zn	Cd	Cr	Ni	Pb
P	1.00	0.67*	0.44*	0.23	0.35*	0.30*	0.62*	0.16	0.25	0.37*	0.68*	0.56*	0.49*	0.59*	0.13
K	0.67*	1.00	0.77*	0.03	-0.16	0.67*	0.51*	0.46*	0.31*	0.38*	0.43*	0.21	0.44*	0.35*	0.29*
Mg	0.44*	0.77*	1.00	0.30*	-0.46*	0.53*	0.53*	0.18	0.06	0.22	0.31*	-0.05	0.17	0.15	0.04
Ca	0.23	0.03	-0.30*	1.00	0.61*	-0.08	0.16	-0.10	0.33*	0.39*	0.53*	0.47*	0.31*	0.63*	0.33*
Na	0.35*	-0.16	-0.46*	0.61*	1.00	-0.31*	0.18	-0.18	0.25	0.08	0.47*	0.47*	0.29	0.55*	0.00
S	0.30*	0.67*	0.53*	-0.08	-0.31*	1.00	0.07	0.26	0.55*	0.31*	0.11	0.25	0.50*	0.01	0.43*
B	0.62*	0.51*	0.53*	0.16	0.18	0.07	1.00	-0.10	0.23	0.31*	0.63*	0.14	0.15	0.28	0.18
Cu	0.16	0.46*	0.18	-0.10	-0.18	0.26	-0.10	1.00	-0.14	0.40*	-0.04	0.03	0.17	0.04	-0.16
Fe	0.25	0.31*	0.06	0.33*	0.25	0.55*	0.23	-0.14	1.00	0.24	0.25	0.55*	0.73*	0.09	0.65*
Mn	0.37*	0.38*	0.22	0.39*	0.08	0.31*	0.31*	0.40*	0.24	1.00	0.49*	0.58*	0.43*	0.32*	0.26
Zn	0.68*	0.43*	0.31*	0.53*	0.47*	0.11	0.63*	-0.04	0.25	0.49*	1.00	0.48*	0.40*	0.80*	0.22
Cd	0.56*	0.21	-0.05	0.47*	0.47*	0.25	0.14	0.03	0.55*	0.58*	0.48*	1.00	0.66*	0.49*	0.38*
Cr	0.49*	0.44*	0.17	0.31*	0.29	0.50*	0.15	0.17	0.73*	0.43*	0.40*	0.66*	1.00	0.32*	0.41*
Ni	0.59*	0.35*	0.15	0.63*	0.55*	0.01	0.28	0.04	0.09	0.32*	0.80*	0.49*	0.32*	1.00	0.05
Pb	0.13	0.29*	0.04	0.33*	0.00	0.43*	0.18	-0.16	0.65*	0.26	0.22	0.38*	0.41*	0.05	1.00

* correlation coefficients significant at the level of ≤ 0.05