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Clonal variation in heavy metal accumulation and biomass production in a poplar coppice culture. II. Vertical distribution and phytoextraction potential
Environmental Pollution 133 (2005) 541–551 www.elsevier.com/locate/envpol
Clonal variation in heavy metal accumulation and biomass production in a poplar coppice culture. II. Vertical distribution and phytoextraction potential I. Laureysensa,*, L. De Temmermanb, T. Hastira, M. Van Gyselc, R. Ceulemansa a
University of Antwerp, Campus Drie Eiken, Department of Biology, Universiteitsplein 1, B-2610 Wilrijk, Belgium b Veterinary and Agrochemical Research Centre (VAR), Leuvensesteenweg 17, B-3080 Tervuren, Belgium c University of Antwerp, Campus Drie Eiken, Department of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 5 March 2004; accepted 25 June 2004
Poplar shows potential for phytoextraction of Al, Cd and Zn on slightly contaminated soils. Abstract Short rotation coppice cultures (SRC) are intensively managed, high-density plantations of multi-shoot trees. In April 1996, an SRC field trial with 17 different poplar clones was established in Boom (Belgium) on a former waste disposal site. In December 1996 and January 2001, all shoots were cut back to a height of 5 cm to create a coppice culture. For six clones, wood and bark were sampled at the bottom, middle and top of a shoot in August and November 2002. No significant height effect of metal concentration was found, but for wood, metal concentrations generally increased toward the top of the shoot in August, and decreased toward the top of the shoot in November. Phytoextraction potential of a clone was primarily determined by metal concentration and by biomass production. Shoot size and number of shoots per stool were less important, as a high biomass production could be achieved by producing a few large shoots or many smaller shoots. Clone Fritzi Pauley accumulated 1.4 kg haÿ1 of Al over two years; Wolterson and Balsam Spire showed a relatively high accumulation of Cd and Zn, i.e. averaging, respectively 47 and 57 g haÿ1 for Cd and 2.4 and 2.0 kg haÿ1 for Zn over two years. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Poplar (Populus spp.); SRC; Phytoremediation; Waste disposal site; Heavy metals
1. Introduction Industrial processes, mining, and other human activities have resulted in considerable contamination of soils with heavy metals and other pollutants (Clijsters et al., 2000). Several studies have shown the potential of willow for site reclamation and partial decontamination, as several species and clones of the genus Salix take up
* Corresponding author. Tel.: C32 3 820 22 89; fax: C32 3 820 22 71. E-mail address: [email protected] (I. Laureysens). 0269-7491/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.06.013
relatively high levels of heavy metals (Riddell-Black, 1994; Watson et al., 1999; Aronsson and Perttu, 2001; Pulford et al., 2002). The same holds true for poplar. Poplars (or cottonwoods) are being used throughout North America to clean up sites that contain e.g. heavy metals, pesticides, and landfill leachates. Poplars are well suited for phytoremediation because they can remove contaminants in several ways, including degrading them, confining them, or by acting as filters or traps (Isebrands and Karnosky, 2001). Poplar and willow are often grown in short rotation coppice cultures (SRC), i.e. intensively managed plantations for rotations shorter than 15 years (Dickmann and Stuart, 1983;
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Macpherson, 1995). Plant material is selected for high biomass production, high growth vigor, and disease resistance. Cultural management includes site preparation, high planting density, and coppicing (Dickmann and Stuart, 1983; Macpherson, 1995; Ledin and Willebrand, 1996). Coppicing refers to the cutting of a tree at the base of its trunk, resulting in the emergence of new shoots from the stump and/or roots (Blake, 1983). A coppice regime not only makes replanting of trees unnecessary for several rotations, but also results in a much higher biomass yield for several species (Sennerby-Forsse et al., 1992; Macpherson, 1995). Studies on phytoextraction have mainly focused on metal hyperaccumulating plants, as they accumulate 100–1000-fold the levels normally accumulated in plants, with no adverse effects on their growth (Reeves et al., 1999). However, hyperaccumulators are usually small with slow growth, and they have no economic value (Glass, 2000). In comparison with hyperaccumulators, trees tend to take up relatively small amounts of heavy metals, but they provide economic return of contaminated land through the production of biomass. Moreover, SRC has many additional ecological benefits, e.g. a positive impact on biodiversity, nutrient capture and carbon sequestration (Gordon, 1975; Perttu, 1995). Wood from SRC has traditionally been seen as a resource for the paper and pulp industry. But, in light of the greenhouse effect and the depletion of fossil fuels, SRC is now seen as a source of energy, because of the possibility of carbon sequestration and the substitution of fossil fuels. Furthermore, SRC on polluted land may reduce dust-blow, leaching and run-off of contaminated water (Watson et al., 1999; Isebrands and Karnosky, 2001). Both biomass production and metal concentration should be taken into account when assessing the phytoextraction potential of a species or clone. Many studies have shown ‘‘toxic’’ metals to accumulate primarily in the root system; relatively high metal concentrations have also been found in leaves and bark (Rachwal et al., 1992; Landberg and Greger, 1996; Pulford et al., 2001; Thiry et al., 2002). As the amount of bark of a stem depends on its diameter, the shoot diameter distribution and population dynamics of the species or clone might also be considered. Large clonal variations in the number of shoots per stool and in the diameter distribution have already been demonstrated in poplar SRC (Laureysens et al., 2003). To determine the metal concentration in a shoot, samples are usually taken at one specific height, or at random on the stem. However, several studies have shown a pronounced vertical distribution of heavy metals within the shoot of e.g. Salix spp. (Sander and Ericsson, 1998), Fagus sylvatica L. (Glavac et al., 1990; Luyssaert et al., 2001), and Pinus sylvestris L. (Thiry et al., 2002). To our knowledge, no such information is
available for poplar. Therefore, the aims of this study were to study clonal variation in vertical distribution of heavy metals within a shoot, and to quantify the phytoextraction potential of six poplar clones in a short rotation coppice culture on a ‘slightly contaminated soil’. In an accompanying paper we have shown a significant clonal variation in metal accumulation. Al, Cd and Zn were most efficiently taken up, and were primarily accumulated in leaves and bark (Laureysens et al., 2004a).
2. Materials and methods 2.1. Experimental set-up and plant material In April 1996, a short rotation coppice plantation was established in an industrial zone of Boom near Antwerp, Belgium (51 05#N, 04 22#E) on an old household waste disposal site. In the 1970s, the waste was covered with a 2 m thick layer of sand, clay and mixed rubble. The soil was characterised by a bulk density between 1.221 g cmÿ3 and 1.621 g cmÿ3 (heavy clay-loam soil), and a pH between 7.3 and 8.1. The upper soil horizons contained between 0.8% and 1.8% organic matter. The nutrient and mineral reserves were very high in comparison with forest soils, but moderate in comparison with agricultural soils (Laureysens et al., 2004b). The site is situated at 5 m above sea level and has a temperate climate, with a mean temperature of 11 C and a mean annual precipitation of 847 mm. Prior to planting, the area was levelled and cleaned of large stones, plastic, metal and other debris. A rotor tiller was used in the early spring of 1996 for final pre-planting soil preparation. Seventeen poplar (Populus) clones, belonging to different species and interspecific hybrids were planted as 25 cm long dormant, unrooted cuttings, after being soaked in water for 24 h. Cuttings were planted manually to a depth of 20 cm, leaving one or two buds above the soil surface. They were planted in a doublerow design with alternating inter-row distances of 0.75 m and 1.5 m, and a spacing of 0.9 m between cuttings within the rows, accommodating an overall planting density of 10 000 cuttings per ha. A randomised block design was used with 17 clones ! 3 replicate plots according to a protocol prescribed by the British Forestry Commission (Armstrong, 1997). Individual plot size was 9 m ! 11.5 m, containing 10 rows of 10 trees each. In the centre of each plot, 36 trees (6 ! 6) were used as assessment trees to avoid border effects (Zavitkovski, 1981). Six clones, characterised by a relatively high biomass production, were studied for their phytoremediation capacity, i.e. P. trichocarpa T. & G. ! P. balsamifera L. (T ! B) clone Balsam Spire; P. trichocarpa ! P. deltoides Marshall (T ! D) clone
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Hazendans; P. trichocarpa (T) clones Fritzi Pauley and Trichobel; P. deltoides ! P. nigra L. (D ! N) clone Gaver; and native P. nigra (N) clone Wolterson. Metal concentrations in Balsam Spire and Trichobel were not measured in August. 2.2. Management regime In 1996, the plantation was irrigated once during April and May to promote an optimal establishment. In December 1996, all trees were cut back to a height of ca. 5 cm above soil level to create a coppice culture, i.e. a stool composed of a stump with its shoots. All trees produced between two and ten shoots per stump. The cuttings that did not survive the first growing year were replaced in the spring of 1997 with new 25 cm long cuttings (40 cm for the clones with a mortality rate higher than 10%). In January 2001, i.e. after a first rotation cycle of four years, all trees were cut back again at ca. 5 cm above soil level, resulting in stumps with eight to 19 shoots on average (Laureysens et al., 2003). Mechanical weeding was done frequently during the 1996, 1997 and 2001 growing seasons. In February 1997, a 5 cm thick layer of mulch was applied to reduce weed growth and increase the acidity of the soil. In June 1996, June 1997 and April 2001, weeds in between the trees were treated with a mixture of glyphosate (at 3.2 kg haÿ1) and oxadiazon (at 9.0 kg haÿ1), as the mechanical weed control was not effective. These herbicides were applied using a spraying device with a hood-covered nozzle to minimise impact on the trees. During the 1998 and 1999 growing seasons, the weed vegetation was mechanically cut to ground level with a trimmer. No fertilisation or irrigation was applied after the establishment of the experiment. 2.3. Sampling procedure In August 2002, nine randomly chosen shoots were harvested in one replicate per clone. Shoot diameter (d ) was measured at 22 cm above soil level (Pontailler et al., 1997) with a digital calliper (Mitutoyo, type CD-15DC, U.K.). A piece of stem of 15 cm was sampled at a height of, respectively 40% (bottom), 60% (middle) and 80% (top) of total stem height; bark and wood were separated. Per clone, nine samples were analysed, i.e. three pooled sampled ! three height levels. A pooled sample was composed of three stems. After leaf fall, in November 2002, the same procedure was repeated in another replicate plot per clone. Bark and wood were dried in a forced air oven at 60 C to constant mass, and ground with a cutting mill (IKA-Werke GMBH & CO.KG, Germany). The remaining stems and branches of the sampled shoots were dried at 75 C to constant mass, and dry weight was determined. In the third
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replicate plot of the six clones, 10 randomly chosen shoots were harvested in March 2003, and total stem weight was determined after drying in a forced air over at 75 C to constant mass.
2.4. Analytical methods Bark and wood samples of August were analysed with energy-dispersive X-ray fluorescence (ED-XRF). Homogenized, ground and dry material was pressed to a pellet with a diameter of 3.1 cm at a pressure of 15 Mg cmÿ2. Each pellet was weighed and its dimensions were measured with a digital ruler. Analyses were carried out with a TN Spectrace 5000 (TN Spectrace, Texas, USA) equipped with a 17.5 W air-cooled side window Rh-anode X-ray tube. The take-of angle of the anode is 20 and the tube is sealed with a 127 mm Be window. Tube voltages can be varied between 6 and 50 kV and tube currents up to 0.35 mA. The source sample and sample detector distance are, respectively 9.14 and 2.86 cm. The average incident and emerging angle are 45 . X-rays are detected with a Si (Li) detector of 30 mm2 with a resolution of w160 eV (FWHM at MnKa). The detector surface is covered with a 12.7 mm Be window. We placed a thin Rh filter (50 mm) and a collimator with a diameter of 2 mm in front of the X-ray tube. All measurements were carried out in vacuum and the current was varied to keep the dead time well below 15%. All spectra were recorded for 1000 s applying a tube filament current of 0.01–0.02 mA and a tube voltage of 45 kV. The spectra were evaluated using a smooth filter background correction with 24 iterations and the scattered peaks were fitted using the peak model as described by Van Gysel et al. (2003a,b). Total content of calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), and zinc (Zn) was determined. Bark and wood samples of November were destructed using a dry-ashing procedure (Hoenig, 2001). Homogenized, ground and dry bark and wood material (0.5 g) was ashed at 450 C during 5 h in a muffle furnace. After cooling, 0.5 ml distilled water and 0.1 ml nitric acid (HNO3) were added, followed by a second ashing at 450 C during 1 h. The ash was then treated with 0.5 ml distilled water, 1 ml HNO3 and 0.5 ml hydrofluoric acid (HF), and evaporated to dryness on a sand bath. This procedure was repeated two times after cooling. The remaining ash was dissolved in 1 ml HNO3 and distilled water. Total content of aluminium (Al), arsenic (As), calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), nickel (Ni), lead (Pb), vanadium (V) and zinc (Zn) was determined with inductively coupled plasma optical emission spectrometry (ICP-AES) and/or inductively coupled plasma mass spectrometry (ICP-MS).
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For all analyses, several standard reference materials were used to test the accuracy of the quantification procedure. 2.5. Data analyses Per stem piece (15 cm), the proportion (%) of bark dry mass of total biomass (Zwood C bark dry mass) was calculated. Per shoot, this proportion was averaged over the three height levels and scaled up to the whole shoot, i.e. as a proportion of total shoot biomass. Bark dry mass of other (not harvested) shoots was estimated using an allometric power equation between shoot diameter (d ) and shoot bark dry mass (DMb), i.e. DMb Z ad b, with a and b as regression coefficients (Table 1). Wood dry mass was estimated as total shoot biomass minus bark dry mass. Metal content per stool (MCS) was calculated as the sum of the metal content of all shoots. Metal concentration at the three height levels was averaged to calculate metal content per shoot for wood and bark separately. Metal content per plot (MCP) was calculated as the sum of all MCS of a plot. Total biomass of shoots not harvested was estimated using an allometric power equation between shoot diameter at 22 cm (d ) and total shoot biomass (DM), i.e. DM Z a#d b#, with a# and b# as regression coefficients. When performing regressions, slopes and intercepts were tested for differences among clones by analysis of covariance. The effects of clone and height on metal concentrations were tested with a two-way analysis of variance (ANOVA). The design was a randomised block design with clone, height, and clone ! height interaction as fixed factors, and plot as a random factor. The Tukey–Kramer adjustment was used to control the maximum experimentwise error rate. All statistical analyses were performed in SAS (SA System 6.12, SAS Institute Inc., Cary, NC) using the mixed procedure (Littell et al., 1996) and plot as a replicate. Satterthwaite’s procedure was applied to obtain the denominator degrees of freedom. Prior to analysis, concentration data were tested for normality using the Shapiro–Wilk statistic ( proc univariate in SAS), and outliers were left out of the analyses. A Wilcoxon signed Table 1 Regression coefficients a and b of allometric power equations between the diameter (mm) of a two-year old shoot and its bark dry mass (g) for six poplar clones in a short rotation coppice culture (n Z 9) Balsam Spire Fritzi Pauley Gaver Hazendans Trichobel Wolterson
a
b
R2
0.205 0.174 0.125 0.239 0.155 0.179
1.716 1.824 1.898 1.699 1.875 1.681
0.825 0.914 0.914 0.829 0.925 0.859
rank test was used to test for differences between the August and November concentrations. This test was performed with the npar1way procedure in SAS (SAS Institute Inc., 1990). Correlations among metal content, wood and bark dry mass, total biomass, number of shoots and shoot diameter were tested using Spearman rank correlation tests performed with StatMost 2.50 (DataMost Corporation, Salt Lake City, USA), with correlations considered significant at the P ! 0.05 level.
3. Results 3.1. Mid versus late season Metal concentrations in wood were generally lower than those in bark (Tables 2 and 3, Figs. 1 and 2). In wood, Fe and Zn concentrations were significantly higher in November, while Cu and Mn concentrations were significantly higher in August. No significant seasonal variation was found for K and Ca. In bark, K and Ca showed a significantly higher concentration in November, while for Mn, Cu and Zn the reverse was found. Other metals could not be compared between August and November, as these were not determined in August. 3.2. Vertical distribution In Fig. 1, concentrations of macronutrients Ca, Fe, K, Mg and Zn are shown for clones Fritzi Pauley, Gaver, Hazendans and Wolterson in both August and November. A significant height effect was found for only a few elements and clones. Nevertheless, in wood a general trend was observed for all metals in August, i.e. concentrations of all metals generally increased toward the top of the shoot. The increase was most pronounced for Ca, i.e. a difference of 44% between the bottom (40% of total shoot height) and top (80% of total shoot height) of shoots. In November, the concentrations of Ca, K, Mg and Zn in the wood generally decreased toward the top of the shoot; for Fe, the reverse was found. However, the trend was less consistent for all clones than in August (Fig. 1). For bark, no obvious trend was found. The highest concentration was often found at mid-level (60% of total shoot height) (Fig. 2). In Table 2, wood concentrations of elements Al, Cd, Co, Cu, Mn, Na, Ni, Pb and V in November at three height levels are shown for clones Fritzi Pauley, Gaver, Hazendans and Wolterson. As for the macronutrients, only a few elements and clones showed a significant height effect. In wood, concentrations of Al, Cd, Co and Cu generally increased toward the top of the shoot; for Mn, Na, Ni and V no general trend was observed; for Pb, the highest concentration was generally found at
Table 2 Mean metal concentration (standard error) in wood of two-year old poplar shoots at a relative height (RH) of 40%, 60% and 80% of total shoot height RH (%)
Al
As
Cd
Co
Cu
Fritzey Pauley
40 60 80
81.2 (16) 81.3 (15) 108.1 (11)
0.135 (0.050) 0.093 (0.036) 0.151 (0.008)
0.55 (0.06) 0.64 (0.02) 0.92 (0.04)
0.015 (0.007) 0.030 (0.019) 0.025 (0.005)
3.1 (0.5) 3.4 (0.2) 6.1 (0.7)
Gaver
40 60 80
10.4 (1) 10.5 (4) 11.8 (2)
0.104 (0.026) 0.144 (0.012) 0.085 (0.016)
1.64 (0.21) 1.76 (0.16) 1.78 (0.69)
0.029 (0.008) 0.029 (0.004) 0.036 (0.006)
Hazendans
40 60 80
– – –
0.277 (0.057) 0.352 (0.057) 0.461 (0.047)
0.56 (0.05) 0.64 (0.06) 0.88 (0.10)
Wolterson
40 60 80
25.0 (7) 34.4 (17) 41.6 (10)
0.063 (0.032) 0.095 (0.049) 0.080 (0.031)
1.65 (0.27) 2.09 (0.08) 2.73 (0.17)
Mn
Na
Ni
Pb
V
7.9 (2.9) 5.3 (2.7) 5.8 (2.1)
73 (23) 69 (43) 63 (23)
0.97 (0.60) 0.40 (0.06) 0.41 (0.03)
0.26 (0.08) 0.27 (0.06) 0.27 (0.04)
0.01 (0) 0.01 (0) 0.01 (0)
5.1 (0.3) 5.4 (0.5) 5.1 (2.4)
4.3 (1.3) 4.3 (1.4) 5.1 (1.9)
25 (6) 29 (2) 26 (11)
0.98 (0.22) 1.19 (0.26) 0.91 (0.10)
0.29 (0.08) 0.54 (0.20) 0.20 (0.09)
0.02 (0.01) 0.01 (0) 0.01 (0)
0.062 (0.003) 0.071 (0.023) 0.143 (0.004)
4.6 (0.4) 7.1 (0.2) 14.1 (0.1)
16.0 (6.3) 13.6 (5.8) 25.6 (7.3)
– – –
0.96 (0.12) 0.89 (0.22) 1.25 (0.07)
0.68 (0.19) 0.94 (0.34) 0.90 (0.20)
0.26 (0.01) 0.21 (0.10) 0.35 (0.01)
0.024 (0.010) 0.035 (0.006) 0.058 (0.006)
3.3 (0.8) 4.9 (0.6) 7.7 (0.5)
13.0 (6.0) 15.9 (2.0) 23.8 (3.2)
126 (97) 167 (71) 267 (27)
1.34 (0.85) 0.91 (0.22) 1.12 (0.21)
0.16 (0.03) 0.12 (0.02) 0.32 (0.16)
0.01 (0) 0.01 (0) 0.01 (0)
Shoots were harvested in November 2002. All concentrations were expressed as mg gÿ1.
Table 3 Mean metal concentration (standard error) in bark of two-year old poplar shoots at a relative height (RH) of 40%, 60% and 80% of total shoot height Clone
RH (%)
Al
As
Cd
Co
Fritzey Pauley
40 60 80
106 (24) 110 (6) 116 (14)
0.171 (0.030) 0.145 (0.022) 0.216 (0.032)
1.8 (1.8) 1.7 (1.7) 1.6 (1.6)
0.13 (0) 0.13 (0.01) 0.13 (0.01)
5.0 (0.1) 5.4 (0.4) 6.7 (0.2)
Gaver
40 60 80
21 (9) 29 (2) 14 (2)
0.196 (0.030) 0.145 (0.006) 0.141 (0.011)
8.9 (0.5) 6.9 (0.1) 6.0 (0.2)
0.23 (0.01) 0.18 (0.01) 0.19 (0.02)
Hazendans
40 60 80
0.534 (0.090) 0.535 (0.034) 0.328 (0.035)
1.8 (0.1) 1.8 (0.2) 1.3 (0.3)
Wolterson
40 60 80
0.130 (0.054) 0.177 (0.064) 0.108 (0.050)
5.7 (0.4) 5.2 (0.3) 5.2 (0.4)
– – – 22 (5) 42 (10) 23 (6)
Cu
Mn
Na
Ni
Pb
V
3 (1) 9 (4) 9 (3)
40 (7) 72 (19) 103 (36)
0.87 (0.04) 0.85 (0.16) 0.82 (0.04)
0.70 (0.08) 0.63 (0.09) 0.53 (0.19)
0.07 (0) 0.06 (0.01) 0.05 (0.02)
8.0 (0.3) 9.4 (1.1) 9.0 (0.3)
33 (5) 37 (11) 29 (8)
242 (39) 258 (29) 143 (62)
2.19 (0.42) 2.00 (0.30) 1.46 (0.12)
0.71 (0.64) 0.05 (0.03) 0.77 (0.75)
0.11 (0.01) 0.06 (0) 0.05 (0.01)
0.41 (0.03) 0.37 (0.03) 0.26 (0.07)
6.3 (0.1) 11.3 (1.6) 13.5 (0.8)
17 (6) 17 (3) 5 (3)
– – –
1.34 (0.16) 1.26 (0.04) 1.12 (0.06)
1.04 (0.20) 1.23 (0.11) 0.74 (0.13)
0.47 (0.06) 0.50 (0.02) 0.42 (0.01)
0.13 (0.02) 0.12 (0.01) 0.18 (0.04)
6.4 (0.3) 6.9 (0) 7.9 (0.3)
14 (6) 13 (8) 21 (12)
83 (61) 126 (87) 91 (58)
1.61 (0.28) 1.23 (0.02) 1.92 (0.32)
2.40 (0.30) 2.55 (0.13) 1.46 (0.30)
0.16 (0.03) 0.16 (0.01) 0.07 (0.01)
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Clone
Shoots were harvested in November 2002. All concentrations were expressed as mg gÿ1. 545
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I. Laureysens et al. / Environmental Pollution 133 (2005) 541–551 Hazendans
Relative height (%)
Wolterson 80
80
60
60
40
40
0
0
Fritzi Pauley
Gaver
Relative height (%)
Potassium Calcium Iron Zinc Magnesium
80
80
60
60
40
40
0
0
Concentration (µg g-1)
Concentration (µg g-1)
Fig. 1. Mean metal concentration in wood of two-year old poplar shoots of clones Wolterson, Hazendans, Gaver and Fritzi Pauley at three height levels (40%, 60% and 80% of total shoot height). Concentrations in both August (solid line) and November (dashed line) are shown. Error bars present the standard error.
mid-level (Table 2). In bark, no general trend was observed; highest or lowest concentrations were often found at mid-level (Table 3). 3.3. Metal content per stool Total wood and bark dry mass per stool were calculated as the sum of estimated wood and bark dry mass of all shoots of a stool. Bark dry mass per shoot was estimated using an allometric power equation between shoot diameter and bark dry mass. Allometric
equations differed significantly among clones, i.e. clones Trichobel, Fritzi Pauley and Gaver had a relatively higher proportion of bark than clones Balsam Spire and Wolterson (Table 1). Total wood and bark metal content per stool were compared among clones for elements Al, Cd and Zn, as we have previously shown that these three metals are most efficiently taken up (Laureysens et al., 2004a). Clone Fritzi Pauley showed the highest Al accumulation, while Wolterson and Balsam Spire showed the highest Cd and Zn uptake. For these three metals, metal
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Relative height (%)
Wolterson
Hazendans
80
80
60
60
40
40
0
0
Relative height (%)
Gaver
Potassium Calcium Iron Zinc Magnesium
Fritzi Pauley
80
80
60
60
40
40
0
0
Concentration (µg g-1)
Concentration (µg g-1)
Fig. 2. Mean metal concentration in bark of two-year old poplar shoots of clones Wolterson, Hazendans, Gaver and Fritzi Pauley at three height levels (40%, 60% and 80% of total shoot height). Concentrations in both August (solid line) and November (dashed line) are shown. Error bars present the standard error.
content per stool was significantly positively correlated with wood and bark dry mass, stool biomass, number of shoots per stool and mean shoot diameter; shoot diameter was significantly negatively correlated with the number of shoots per stool. The number of shoots per stool differed significantly among clones (Table 4). Clones Hazendans and Fritzi Pauley showed a rapid elimination of small shoots, with 1–12 relatively large shoots; shoot elimination for clones Balsam Spire and Wolterson was much slower, leaving stools with up to 34 shoots per stool. Both wood and bark dry mass were significantly positively correlated with the number of shoots per stool and with shoot diameter. Clone Fritzi
Pauley showed the highest mean Al content per stool, i.e. 158 mg; Wolterson and Balsam Spire showed the highest accumulation of Cd (respectively 5.1 and 6.4 mg on average per stool) and Zn (respectively 264 and 221 mg on average per stool) (Table 5). 3.4. Phytoextraction potential In contrast to metal content per stool, metal content per plot (MCP) represents the real phytoextraction potential (Table 5), as it includes stool mortality. Nevertheless, no significant correlation between MCP and stool mortality was found for Al and Cd, although
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Table 4 Mean number of stools per plot, mean number of shoots per stool, mean shoot diameter (mm), mean wood and bark dry mass (DM) per stool (g) and total above ground woody biomass (Mg haÿ1) for two-year old poplar shoots of six clones
Balsam Spire Fritzi Pauley Gaver Hazendans Trichobel Wolterson
Nr. stools
Nr. shoots per stool
Shoot diameter
Wood DM per stool
Bark DM per stool
Total biomass (Mg haÿ1)
32 32 29 31 32 33
14.0 3.5 8.5 4.6 5.9 14.6
11.67 27.53 17.46 20.8 18.14 14.01
1289 1463 1198 694 1100 1705
267 235 273 200 303 269
13.9 15.0 11.4 7.8 12.9 18.1
(2) (1) (3) (1) (2) (1)
(1.6) (0.6) (1.2) (0.9) (0.8) (1.3)
(1.24) (4.39) (2.43) (3.11) (2.53) (0.63)
(159) (191) (61) (125) (277) (111)
(28) (16) (32) (35) (72) (6)
(2.1) (2.2) (0.5) (1.5) (3.8) (0.8)
Standard errors are shown in brackets.
plot biomass production was significantly correlated with stool mortality (r Z ÿ0.566). Clones Wolterson, Fritzi Pauley and Balsam Spire showed the highest above ground woody biomass production (wood C bark), averaging 18, 15 and 14 Mg haÿ1, respectively after two years (Table 4). Fritzi Pauley showed the highest Al accumulation over two years, i.e. 1.4 kg haÿ1 (Table 5). For Al, MCP was significantly correlated with wood dry mass (r Z 0.513) and woody biomass production (r Z 0.587); no significant correlation was found with bark dry mass, number of shoots or mean shoot diameter. Clones Wolterson and Balsam Spire showed the highest accumulation of Cd, i.e. 47 and 57 g haÿ1, respectively, and of Zn, i.e. 2.4 and 2.0 kg haÿ1 (Table 5). For Cd, MCP was significantly correlated with wood dry mass (r Z 0.472), woody biomass production (r Z 0.503), and number of shoots per stool (r Z 0.719) and per plot (r Z 0.706). For Zn, MCP was significantly correlated with wood dry mass (r Z 0.746), woody biomass production (r Z 0.806), stool mortality (r Z ÿ0.543), and number of shoots per stool (r Z 0.492) and per plot (r Z 0.526).
4. Discussion Stem wood contains the xylem vessels, transporting water and nutrients from the roots to the stem and foliage. The inner bark includes the living phloem, which transports photosynthates and transformed nutrients to growing stem parts and to the roots. When the living phloem dies, dead outer bark is produced (Raven
et al., 1992). In our study, we did not separate inner and outer bark, so we can only distinguish xylem (wood) and phloem with periderm (bark). Metals are directly transferred between xylem and phloem through transfer cells, and indirectly via the foliage (Raven et al., 1992). However, the mobility within the plant is not high for all metals. Metals with high mobility are e.g. K, Mg, Na, Mg and Zn; metals with low mobility are e.g. Ca, Fe, B and Cu. The xylem elements act as exchange columns, adsorbing ions like Ca. Alkali elements as K are mainly present as soluble salts and are less strongly associated with the organic compounds of the wood (Ferguson and Bollard, 1976). Therefore, gradients should be more pronounced for metals with low mobility, which was confirmed by our results, i.e. the difference in concentration between bottom and top of a shoot was highest for Ca and Fe. In our study, no clear height profile was found in bark. In wood, gradients were usually more pronounced in August. In August, metal concentration generally increased towards the top of the shoot, probably reflecting the gradient in the xylem sap. The same height profile was found for Ca, Mg and Mn in the xylem sap of, respectively beech and Chinese gooseberry vines in March–May (Ferguson and Eiseman, 1981; Glavac et al., 1990). Sander and Ericsson (1998) found the same trend for metals (e.g. Ca, Cu and Ni) in bark and wood of two-year old willow shoots in March– April. In pine, Thiry et al. (2002) found no gradient in wood, but K concentration increased with height in the outer bark. Ca, on the other hand, decreased with height in the inner bark. A decrease with height was also found for Ca, Mg and Mn in November for beech (Glavac
Table 5 Mean metal content (standard error) per stool and per hectare for six poplar clones in a short rotation coppice culture, representing the phytoextraction potential
Balsam Spire Fritzi Pauley Gaver Hazendans Trichobel Wolterson
Al (mg stoolÿ1)
Cd (mg stoolÿ1)
Zn (mg stoolÿ1)
Al (kg haÿ1)
Cd ( g haÿ1)
Zn (kg haÿ1)
98 158 19 37 60 65
6.4 1.4 5.5 1.7 1.3 5.1
221 157 74 107 107 264
0.87 1.39 0.15 0.32 0.55 0.59
57 13 43 16 12 47
2.0 1.4 0.6 0.9 1.0 2.4
(12) (19) (1) (6) (14) (4)
(0.7) (0.2) (0.8) (0.9) (0.3) (0.2)
The two-year old shoots were harvested in November 2002.
(26) (19) (6) (19) (26) (8)
(0.13) (0.20) (0.01) (0.06) (0.16) (0.02)
(9) (2) (10) (9) (3) (2)
(0.3) (0.2) (0) (0.2) (0.3) (0.1)
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et al., 1990), comparable to our study on poplar in November. Besides a seasonal variation in vertical distribution, a significant seasonal variation in metal concentration was found for K, Ca, Mn, Fe, Cu and Zn. However, metal concentrations should not be compared between August and November, because they were measured using different analytical techniques, i.e. XRF in August and ICP-MS/ICP-AES in November. In addition, concentrations in both seasons were measured on different plots. It has already been shown that the distribution pattern of every metal within a plant is regulated physiologically, but that the absolute concentration varies with soil and site characteristics (Momoshima et al., 1995). As short rotation coppice shoots are usually harvested during winter, metal concentrations of bark and wood should be considered in the dormant period. In addition, their dry mass is needed to determine the phytoextraction potential of a clone. Metal concentrations are often higher in bark (Riddell-Black et al., 1997; Pulford et al., 2001; Thiry et al., 2002), but the biomass proportion of bark is relatively small and progressively decreases as a shoot grows larger. In this study, the relative proportion of bark of total shoot dry mass was significantly different among clones. In addition, we have previously shown significant clonal differences in the number and size of shoots per stool (Laureysens et al., 2003). Consequently, mean bark dry mass per stool differed significantly among clones. At stool level, metal content was significantly correlated with bark dry mass, and bark dry mass was significantly correlated with the number of shoots per stool and their diameter. However, number of shoots per stool and shoot diameter were significantly negatively correlated. Clone Fritzi Pauley with a few large shoots per stool showed high Al accumulation, while clones Wolterson and Balsam Spire with a large number of smaller shoots showed high Cd and Zn accumulation. All three clones showed similar biomass production, but contrasting shoot size frequency distributions. In November, clone Fritzi Pauley showed a mean Al concentration in wood of 90 mg gÿ1, while for clones Wolterson and Balsam Spire wood Al concentration averaged 34 and 64 mg gÿ1, respectively. For Cd, the concentration averaged 2.2 and 3.3 mg gÿ1 in clones Wolterson and Balsam Spire, respectively; for Zn, the concentration averaged 147 and 144 mg gÿ1 in clones Wolterson and Balsam Spire, respectively. Clone Fritzi Pauley had a mean Cd and Zn concentration of, respectively 0.7 and 92 mg gÿ1. Furthermore, metal content per plot was significantly correlated with wood dry mass and total biomass production, but not with bark dry mass. For Cd and Zn, a significant correlation between metal content per plot and number of shoots was found, because clones Wolterson and Balsam Spire had the highest Cd and Zn
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concentration and accumulation. These results suggest that selection and improvement of poplar clones for phytoextraction should focus on biomass production, stool survival and metal concentration; population dynamics should not be taken into account. After two years, clone Fritzi Pauley showed the highest accumulation of Al, i.e. averaging 1.4 kg haÿ1, while clones Wolterson and Balsam Spire showed phytoextraction potential for Cd and Zn, i.e. averaging, respectively 47 and 57 kg haÿ1 for Cd and 2.4 and 2.0 kg haÿ1 for Zn. Several studies have shown that clones with a high uptake of a combination of several metals have not yet been identified (Riddell-Black, 1994; Pulford et al., 2001). This is probably due to the antagonistic properties of several metals. Likewise, hyperaccumulators accumulate only one or a limited number of metals. However, the uptake by these plants is much higher in comparison with poplar or willow. Therefore, Ernst (1996) suggested using these short rotation coppice cultures on slightly contaminated soils. The trees will take up part of the heavy metals and will additionally stabilise the soil, reducing metal leaching and dust-blow. The combination of wood for energy production with phytoremediation will make both economically more feasible. Metals in the biomass remain in the ashes or are filtered to avoid translocation of the heavy metal pollutants to the atmosphere (Punshon and Dickinson, 1997). In this study, we examined dry mass partitioning between wood and bark in the stem of a shoot; partitioning to the branches was not taken into account. The proportion of bark in the branches is relatively high, and poplar clones differ significantly in their branch production (Ceulemans et al., 1990; Gielen et al., 2002). Further research should also focus on branches to figure out whether ‘branchy’ clones accumulate more metals. In addition, we did not study root accumulation, although many studies have shown that most metals accumulate in the roots (Kabata-Pendias and Pendias, 1984; Glavac et al., 1990; Alloway, 1995; Landberg and Greger, 1996). This would imply that for phytoremediation purposes roots need to be ploughed up after the last rotation cycle rather than be rotovated and left in the soil. Root metal concentrations and possible clonal differences could be the objects of further research. We have, however, shown that poplar SRC offer possibilities for phytoremediation of slightly contaminated soils.
Acknowledgements This study is being supported by a research contract with the Province of Antwerp. All plant materials were kindly provided by the Institute for Forestry and Game Management (Geraardsbergen) and by the Forest Research, Forestry Commission (UK). The project has
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been carried out in close co-operation with Eta-com B., supplying the grounds and part of the infrastructure, and with the logistic support of the city council of Boom. We gratefully acknowledge P. Aronsson, N. Dickinson, I.D. Pulford, and D. Riddell-Black for useful suggestions on sampling protocol and analyses; C. Lemmens and A. Pellis for help with sampling and harvesting; as well as H. Baeten, Dr. M. Hoenig, and R. Paraschiv for help with heavy metal analysis.
References Alloway, B.J., 1995. Heavy Metals in Soils. Chapman & Hall, Glasgow, UK. Armstrong, A., 1997. The United Kingdom network of experiments on site/yield relationships for short rotation coppice, Forestry Commission Research Information Note 294. Forestry Commission, Edinburgh. Aronsson, P., Perttu, K., 2001. Willow vegetation filters for wastewater treatment and soil remediation combined with biomass production. The Forestry Chronicle 77, 293–299. Blake, T.J., 1983. Coppice systems for short-rotation intensive forestry: the influence of cultural, seasonal and plant factors. Australian Forest Research 13, 279–291. Ceulemans, R., Stettler, R.F., Hinckley, T.M., Isebrands, J.G., Heilman, P.E., 1990. Crown architecture of Populus clones as determined by branch orientation and branch characteristics. Tree Physiology 7, 157–167. Clijsters, H., Vangronsveld, J., van der Lelie, N., Colpaert, J., 2000. Ecological aspects of phytostabilization of heavy metal contaminated soils. In: Ceulemans, R., Bogaert, J., Deckmyn, G., Nijs, I. (Eds.), Topics in Ecology. Structure and Function in Plants and Ecosystems. University of Antwerp, UIA, Wilrijk, pp. 299–306. Dickmann, D.I., Stuart, K.W., 1983. The Culture of Poplars in Eastern North America. Michigan State University Publications. Michigan State University, East Lansing, Michigan. Ernst, W.H.O., 1996. Bioavailability of heavy metals and decontamination of soils by plants. Applied Geochemistry 11, 163–167. Ferguson, I.B., Bollard, E.G., 1976. The movement of calcium in woody stems. Annals of Botany 40, 1057–1065. Ferguson, A.R., Eiseman, J.A., 1981. Xylem sap from Actinidia chinensis: gradients in sap composition. Annals of Botany 48, 75–80. Gielen, B., Calfapietra, C., Claus, A., Sabatti, M., Ceulemans, R., 2002. Crown architecture of Populus spp. is differentially modified by freeair CO2 enrichment (POPFACE). New Phytologist 153, 91–99. Glass, D.J., 2000. Economic potential of phytoremediation. In: Raskin, I., Ensley, B.D. (Eds.), Phytoremediation of Toxic Metals: Using Plants to Clean up the Environment. John Wiley & Sons, Inc., New York, USA, pp. 15–31. Glavac, V., Koenies, H., Ebben, U., 1990. Seasonal variation of calcium, magnesium, potassium, and manganese contents in xylem sap of beech (Fagus sylvatica L.) in a 35-year-old limestone beech forest stand. Trees 4, 75–80. Gordon, J.C., 1975. The productive potential of woody plants. Iowa State Journal of Research 49, 267–274. Hoenig, M., 2001. Preparation steps in environmental trace element analysis – facts and traps. Talanta 54, 1021–1038. Isebrands, J.G., Karnosky, D.F., 2001. Environmental benefits of poplar culture. In: Dickmann, D.I., Isebrands, J.G., Eckenwalder, J.E., Richardson, J. (Eds.), Poplar Culture in North America. NRC Research Press, National Research Council of Canada, Ottawa, Canada, pp. 207–218.
Kabata-Pendias, A., Pendias, K., 1984. Trace Elements in Soils and Plants. CRC Press Inc., Boca Raton, Florida. Landberg, T., Greger, M., 1996. Differences in uptake and tolerance to heavy metals in Salix from unpolluted and polluted areas. Applied Geochemistry 11, 175–180. Laureysens, I., Blust, R., De Temmerman, L., Lemmens, C., Ceulemans, R., 2004a. Clonal variation in heavy metal accumulation and biomass production in a poplar coppice culture: I. Seasonal variation in leaf, wood and bark concentrations. Environmental Pollution 131, 485–494. Laureysens, I., Bogaert, J., Blust, R., Ceulemans, R., 2004b. Biomass production of 17 poplar clones in a short-rotation coppice culture on a waste disposal site and its relation to soil characteristics. Forest Ecology and Management 187, 295–309. Laureysens, I., Deraedt, W., Indeherberge, T., Ceulemans, R., 2003. Population dynamics in a six-year old coppice culture of poplar. I. Clonal differences in stool mortality, shoot dynamics and shoot diameter distribution in relation to biomass production. Biomass and Bioenergy 24, 81–95. Ledin, S., Willebrand, E., 1996. Handbook on How to Grow Short Rotation Forests. Department of Short Rotation Forestry, Swedish University of Agricultural Sciences, Uppsala, Sweden. Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 1996. SAS System for Mixed Models. SAS Institute Inc., Cary, NC, USA. Luyssaert, S., Van Meirvenne, M., Lust, N., 2001. Cadmium variability in leaves of a Salix fragilis: simulation and implications for leaf sampling. Canadian Journal of Forest Research 31, 313–321. Macpherson, G., 1995. Home-grown Energy from Short-rotation Coppice. Farming Press Books, Ipswich, UK. Momoshima, N., Eto, I., Kofuji, H., Takashima, Y., Koike, M., Imaizumi, Y., Harada, T., 1995. Distribution and chemical characteristics of cations in annual rings of Japanese cedar. Journal of Environmental Quality 24, 1141–1149. Perttu, K.L., 1995. Ecological, biological balances and conservation. Biomass and Bioenergy 9, 107–116. Pontailler, J.Y., Ceulemans, R., Guittet, J., Mau, M., 1997. Linear and non-linear functions of volume index to estimate woody biomass in high density young poplar stands. Annales des Sciences Forestie`res 54, 335–345. Pulford, I.D., Riddell-Black, D., Stewart, C., 2002. Heavy metal uptake by willow clones from sewage sludge-treated soil: the potential for phytoremediation. International Journal of Phytoremediation 4, 59–72. Pulford, I.D., Watson, C., McGregor, S.D., 2001. Uptake of chromium by trees: prospects for phytoremediation. Environmental Geochemistry and Health 23, 307–311. Punshon, T., Dickinson, N.M., 1997. Mobilisation of heavy metals using short-rotation coppice. Aspects of Applied Biology 49, 285– 292. Rachwal, L., De Temmerman, L.O., Istas, J.R., 1992. Differences in the accumulation of heavy metals in poplar clones of various susceptibility to air pollution. Arboretum Ko´rnickie 37, 101–111. Raven, P.H., Evert, R.F., Eichhorn, S.E., 1992. Biology of Plants, fifth ed. Worth Publishers, Inc., New York. Reeves, R.D., Baker, A.J.M., Borhidi, A., Berazain, R., 1999. Nickel hyperaccumulation in the serpentine flora of Cuba. Annals of Botany 83, 29–38. Riddell-Black, D., 1994. Heavy metal uptake by fast growing willow species. In: Aronsson, P., Perttu, K. (Eds.), Willow Vegetation Filters for Municipal Wastewaters and Sludges. A Biological Purification System. Department of Ecology and Environmental Research, Section of Short Rotation Forestry, Uppsala, Sweden, pp. 145–151. Riddell-Black, D., Pulford, I.D., Stewart, C., 1997. Clonal variation in heavy metal uptake by willow. Aspects of Applied Biology 49, 327–334.
I. Laureysens et al. / Environmental Pollution 133 (2005) 541–551 Sander, M.-L., Ericsson, T., 1998. Vertical distributions of plant nutrients and heavy metals in Salix viminalis stems and their implications for sampling. Biomass and Bioenergy 14, 57–66. SAS Institute Inc., 1990. SAS/STAT User’s Guide, Version 6, fourth ed. SAS Institute Inc., Cary, NC, USA. Sennerby-Forsse, L., Ferm, A., Kauppi, A., 1992. Coppicing ability and sustainability. In: Mitchell, C.P., Ford-Robertson, J.B., Hinckley, T., Sennerby-Forsse, L. (Eds.), Ecophysiology of Short Rotation Forest Crops. Elsevier Applied Science, Oxford, pp. 146–184. Thiry, Y., Goor, F., Riesen, T., 2002. The true distribution and accumulation of radiocaesium in stem of Scots pine (Pinus sylvestris L.). Journal of Environmental Radioactivity 58, 243–259.
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Van Gysel, M., Lemberge, P., Van Espen, P., 2003a. Description of Compton peaks in energy-dispersive X-ray fluorescence spectra. X-ray Spectrometry 32, 139–147. Van Gysel, M., Lemberge, P., Van Espen, P., 2003b. Implementation of a spectrum fitting procedure using a robust peak model. X-ray Spectrometry 32, 434–441. Watson, C., Pulford, I.D., Riddell-Black, D., 1999. Heavy metal toxicity responses of two willow (Salix) varieties grown hydroponically: development of a tolerance screening test. Environmental Geochemistry and Health 21, 359–364. Zavitkovski, J., 1981. Small plots with unplanted plot border can distort data in biomass production studies. Canadian Journal of Forest Research 11, 9–12.
Clonal variation in heavy metal accumulation and biomass production in a poplar coppice culture. II. Vertical distribution and phytoextraction potential I. Laureysensa,*, L. De Temmermanb, T. Hastira, M. Van Gyselc, R. Ceulemansa a
University of Antwerp, Campus Drie Eiken, Department of Biology, Universiteitsplein 1, B-2610 Wilrijk, Belgium b Veterinary and Agrochemical Research Centre (VAR), Leuvensesteenweg 17, B-3080 Tervuren, Belgium c University of Antwerp, Campus Drie Eiken, Department of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 5 March 2004; accepted 25 June 2004
Poplar shows potential for phytoextraction of Al, Cd and Zn on slightly contaminated soils. Abstract Short rotation coppice cultures (SRC) are intensively managed, high-density plantations of multi-shoot trees. In April 1996, an SRC field trial with 17 different poplar clones was established in Boom (Belgium) on a former waste disposal site. In December 1996 and January 2001, all shoots were cut back to a height of 5 cm to create a coppice culture. For six clones, wood and bark were sampled at the bottom, middle and top of a shoot in August and November 2002. No significant height effect of metal concentration was found, but for wood, metal concentrations generally increased toward the top of the shoot in August, and decreased toward the top of the shoot in November. Phytoextraction potential of a clone was primarily determined by metal concentration and by biomass production. Shoot size and number of shoots per stool were less important, as a high biomass production could be achieved by producing a few large shoots or many smaller shoots. Clone Fritzi Pauley accumulated 1.4 kg haÿ1 of Al over two years; Wolterson and Balsam Spire showed a relatively high accumulation of Cd and Zn, i.e. averaging, respectively 47 and 57 g haÿ1 for Cd and 2.4 and 2.0 kg haÿ1 for Zn over two years. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Poplar (Populus spp.); SRC; Phytoremediation; Waste disposal site; Heavy metals
1. Introduction Industrial processes, mining, and other human activities have resulted in considerable contamination of soils with heavy metals and other pollutants (Clijsters et al., 2000). Several studies have shown the potential of willow for site reclamation and partial decontamination, as several species and clones of the genus Salix take up
* Corresponding author. Tel.: C32 3 820 22 89; fax: C32 3 820 22 71. E-mail address: [email protected] (I. Laureysens). 0269-7491/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2004.06.013
relatively high levels of heavy metals (Riddell-Black, 1994; Watson et al., 1999; Aronsson and Perttu, 2001; Pulford et al., 2002). The same holds true for poplar. Poplars (or cottonwoods) are being used throughout North America to clean up sites that contain e.g. heavy metals, pesticides, and landfill leachates. Poplars are well suited for phytoremediation because they can remove contaminants in several ways, including degrading them, confining them, or by acting as filters or traps (Isebrands and Karnosky, 2001). Poplar and willow are often grown in short rotation coppice cultures (SRC), i.e. intensively managed plantations for rotations shorter than 15 years (Dickmann and Stuart, 1983;
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Macpherson, 1995). Plant material is selected for high biomass production, high growth vigor, and disease resistance. Cultural management includes site preparation, high planting density, and coppicing (Dickmann and Stuart, 1983; Macpherson, 1995; Ledin and Willebrand, 1996). Coppicing refers to the cutting of a tree at the base of its trunk, resulting in the emergence of new shoots from the stump and/or roots (Blake, 1983). A coppice regime not only makes replanting of trees unnecessary for several rotations, but also results in a much higher biomass yield for several species (Sennerby-Forsse et al., 1992; Macpherson, 1995). Studies on phytoextraction have mainly focused on metal hyperaccumulating plants, as they accumulate 100–1000-fold the levels normally accumulated in plants, with no adverse effects on their growth (Reeves et al., 1999). However, hyperaccumulators are usually small with slow growth, and they have no economic value (Glass, 2000). In comparison with hyperaccumulators, trees tend to take up relatively small amounts of heavy metals, but they provide economic return of contaminated land through the production of biomass. Moreover, SRC has many additional ecological benefits, e.g. a positive impact on biodiversity, nutrient capture and carbon sequestration (Gordon, 1975; Perttu, 1995). Wood from SRC has traditionally been seen as a resource for the paper and pulp industry. But, in light of the greenhouse effect and the depletion of fossil fuels, SRC is now seen as a source of energy, because of the possibility of carbon sequestration and the substitution of fossil fuels. Furthermore, SRC on polluted land may reduce dust-blow, leaching and run-off of contaminated water (Watson et al., 1999; Isebrands and Karnosky, 2001). Both biomass production and metal concentration should be taken into account when assessing the phytoextraction potential of a species or clone. Many studies have shown ‘‘toxic’’ metals to accumulate primarily in the root system; relatively high metal concentrations have also been found in leaves and bark (Rachwal et al., 1992; Landberg and Greger, 1996; Pulford et al., 2001; Thiry et al., 2002). As the amount of bark of a stem depends on its diameter, the shoot diameter distribution and population dynamics of the species or clone might also be considered. Large clonal variations in the number of shoots per stool and in the diameter distribution have already been demonstrated in poplar SRC (Laureysens et al., 2003). To determine the metal concentration in a shoot, samples are usually taken at one specific height, or at random on the stem. However, several studies have shown a pronounced vertical distribution of heavy metals within the shoot of e.g. Salix spp. (Sander and Ericsson, 1998), Fagus sylvatica L. (Glavac et al., 1990; Luyssaert et al., 2001), and Pinus sylvestris L. (Thiry et al., 2002). To our knowledge, no such information is
available for poplar. Therefore, the aims of this study were to study clonal variation in vertical distribution of heavy metals within a shoot, and to quantify the phytoextraction potential of six poplar clones in a short rotation coppice culture on a ‘slightly contaminated soil’. In an accompanying paper we have shown a significant clonal variation in metal accumulation. Al, Cd and Zn were most efficiently taken up, and were primarily accumulated in leaves and bark (Laureysens et al., 2004a).
2. Materials and methods 2.1. Experimental set-up and plant material In April 1996, a short rotation coppice plantation was established in an industrial zone of Boom near Antwerp, Belgium (51 05#N, 04 22#E) on an old household waste disposal site. In the 1970s, the waste was covered with a 2 m thick layer of sand, clay and mixed rubble. The soil was characterised by a bulk density between 1.221 g cmÿ3 and 1.621 g cmÿ3 (heavy clay-loam soil), and a pH between 7.3 and 8.1. The upper soil horizons contained between 0.8% and 1.8% organic matter. The nutrient and mineral reserves were very high in comparison with forest soils, but moderate in comparison with agricultural soils (Laureysens et al., 2004b). The site is situated at 5 m above sea level and has a temperate climate, with a mean temperature of 11 C and a mean annual precipitation of 847 mm. Prior to planting, the area was levelled and cleaned of large stones, plastic, metal and other debris. A rotor tiller was used in the early spring of 1996 for final pre-planting soil preparation. Seventeen poplar (Populus) clones, belonging to different species and interspecific hybrids were planted as 25 cm long dormant, unrooted cuttings, after being soaked in water for 24 h. Cuttings were planted manually to a depth of 20 cm, leaving one or two buds above the soil surface. They were planted in a doublerow design with alternating inter-row distances of 0.75 m and 1.5 m, and a spacing of 0.9 m between cuttings within the rows, accommodating an overall planting density of 10 000 cuttings per ha. A randomised block design was used with 17 clones ! 3 replicate plots according to a protocol prescribed by the British Forestry Commission (Armstrong, 1997). Individual plot size was 9 m ! 11.5 m, containing 10 rows of 10 trees each. In the centre of each plot, 36 trees (6 ! 6) were used as assessment trees to avoid border effects (Zavitkovski, 1981). Six clones, characterised by a relatively high biomass production, were studied for their phytoremediation capacity, i.e. P. trichocarpa T. & G. ! P. balsamifera L. (T ! B) clone Balsam Spire; P. trichocarpa ! P. deltoides Marshall (T ! D) clone
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Hazendans; P. trichocarpa (T) clones Fritzi Pauley and Trichobel; P. deltoides ! P. nigra L. (D ! N) clone Gaver; and native P. nigra (N) clone Wolterson. Metal concentrations in Balsam Spire and Trichobel were not measured in August. 2.2. Management regime In 1996, the plantation was irrigated once during April and May to promote an optimal establishment. In December 1996, all trees were cut back to a height of ca. 5 cm above soil level to create a coppice culture, i.e. a stool composed of a stump with its shoots. All trees produced between two and ten shoots per stump. The cuttings that did not survive the first growing year were replaced in the spring of 1997 with new 25 cm long cuttings (40 cm for the clones with a mortality rate higher than 10%). In January 2001, i.e. after a first rotation cycle of four years, all trees were cut back again at ca. 5 cm above soil level, resulting in stumps with eight to 19 shoots on average (Laureysens et al., 2003). Mechanical weeding was done frequently during the 1996, 1997 and 2001 growing seasons. In February 1997, a 5 cm thick layer of mulch was applied to reduce weed growth and increase the acidity of the soil. In June 1996, June 1997 and April 2001, weeds in between the trees were treated with a mixture of glyphosate (at 3.2 kg haÿ1) and oxadiazon (at 9.0 kg haÿ1), as the mechanical weed control was not effective. These herbicides were applied using a spraying device with a hood-covered nozzle to minimise impact on the trees. During the 1998 and 1999 growing seasons, the weed vegetation was mechanically cut to ground level with a trimmer. No fertilisation or irrigation was applied after the establishment of the experiment. 2.3. Sampling procedure In August 2002, nine randomly chosen shoots were harvested in one replicate per clone. Shoot diameter (d ) was measured at 22 cm above soil level (Pontailler et al., 1997) with a digital calliper (Mitutoyo, type CD-15DC, U.K.). A piece of stem of 15 cm was sampled at a height of, respectively 40% (bottom), 60% (middle) and 80% (top) of total stem height; bark and wood were separated. Per clone, nine samples were analysed, i.e. three pooled sampled ! three height levels. A pooled sample was composed of three stems. After leaf fall, in November 2002, the same procedure was repeated in another replicate plot per clone. Bark and wood were dried in a forced air oven at 60 C to constant mass, and ground with a cutting mill (IKA-Werke GMBH & CO.KG, Germany). The remaining stems and branches of the sampled shoots were dried at 75 C to constant mass, and dry weight was determined. In the third
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replicate plot of the six clones, 10 randomly chosen shoots were harvested in March 2003, and total stem weight was determined after drying in a forced air over at 75 C to constant mass.
2.4. Analytical methods Bark and wood samples of August were analysed with energy-dispersive X-ray fluorescence (ED-XRF). Homogenized, ground and dry material was pressed to a pellet with a diameter of 3.1 cm at a pressure of 15 Mg cmÿ2. Each pellet was weighed and its dimensions were measured with a digital ruler. Analyses were carried out with a TN Spectrace 5000 (TN Spectrace, Texas, USA) equipped with a 17.5 W air-cooled side window Rh-anode X-ray tube. The take-of angle of the anode is 20 and the tube is sealed with a 127 mm Be window. Tube voltages can be varied between 6 and 50 kV and tube currents up to 0.35 mA. The source sample and sample detector distance are, respectively 9.14 and 2.86 cm. The average incident and emerging angle are 45 . X-rays are detected with a Si (Li) detector of 30 mm2 with a resolution of w160 eV (FWHM at MnKa). The detector surface is covered with a 12.7 mm Be window. We placed a thin Rh filter (50 mm) and a collimator with a diameter of 2 mm in front of the X-ray tube. All measurements were carried out in vacuum and the current was varied to keep the dead time well below 15%. All spectra were recorded for 1000 s applying a tube filament current of 0.01–0.02 mA and a tube voltage of 45 kV. The spectra were evaluated using a smooth filter background correction with 24 iterations and the scattered peaks were fitted using the peak model as described by Van Gysel et al. (2003a,b). Total content of calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), and zinc (Zn) was determined. Bark and wood samples of November were destructed using a dry-ashing procedure (Hoenig, 2001). Homogenized, ground and dry bark and wood material (0.5 g) was ashed at 450 C during 5 h in a muffle furnace. After cooling, 0.5 ml distilled water and 0.1 ml nitric acid (HNO3) were added, followed by a second ashing at 450 C during 1 h. The ash was then treated with 0.5 ml distilled water, 1 ml HNO3 and 0.5 ml hydrofluoric acid (HF), and evaporated to dryness on a sand bath. This procedure was repeated two times after cooling. The remaining ash was dissolved in 1 ml HNO3 and distilled water. Total content of aluminium (Al), arsenic (As), calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), nickel (Ni), lead (Pb), vanadium (V) and zinc (Zn) was determined with inductively coupled plasma optical emission spectrometry (ICP-AES) and/or inductively coupled plasma mass spectrometry (ICP-MS).
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For all analyses, several standard reference materials were used to test the accuracy of the quantification procedure. 2.5. Data analyses Per stem piece (15 cm), the proportion (%) of bark dry mass of total biomass (Zwood C bark dry mass) was calculated. Per shoot, this proportion was averaged over the three height levels and scaled up to the whole shoot, i.e. as a proportion of total shoot biomass. Bark dry mass of other (not harvested) shoots was estimated using an allometric power equation between shoot diameter (d ) and shoot bark dry mass (DMb), i.e. DMb Z ad b, with a and b as regression coefficients (Table 1). Wood dry mass was estimated as total shoot biomass minus bark dry mass. Metal content per stool (MCS) was calculated as the sum of the metal content of all shoots. Metal concentration at the three height levels was averaged to calculate metal content per shoot for wood and bark separately. Metal content per plot (MCP) was calculated as the sum of all MCS of a plot. Total biomass of shoots not harvested was estimated using an allometric power equation between shoot diameter at 22 cm (d ) and total shoot biomass (DM), i.e. DM Z a#d b#, with a# and b# as regression coefficients. When performing regressions, slopes and intercepts were tested for differences among clones by analysis of covariance. The effects of clone and height on metal concentrations were tested with a two-way analysis of variance (ANOVA). The design was a randomised block design with clone, height, and clone ! height interaction as fixed factors, and plot as a random factor. The Tukey–Kramer adjustment was used to control the maximum experimentwise error rate. All statistical analyses were performed in SAS (SA System 6.12, SAS Institute Inc., Cary, NC) using the mixed procedure (Littell et al., 1996) and plot as a replicate. Satterthwaite’s procedure was applied to obtain the denominator degrees of freedom. Prior to analysis, concentration data were tested for normality using the Shapiro–Wilk statistic ( proc univariate in SAS), and outliers were left out of the analyses. A Wilcoxon signed Table 1 Regression coefficients a and b of allometric power equations between the diameter (mm) of a two-year old shoot and its bark dry mass (g) for six poplar clones in a short rotation coppice culture (n Z 9) Balsam Spire Fritzi Pauley Gaver Hazendans Trichobel Wolterson
a
b
R2
0.205 0.174 0.125 0.239 0.155 0.179
1.716 1.824 1.898 1.699 1.875 1.681
0.825 0.914 0.914 0.829 0.925 0.859
rank test was used to test for differences between the August and November concentrations. This test was performed with the npar1way procedure in SAS (SAS Institute Inc., 1990). Correlations among metal content, wood and bark dry mass, total biomass, number of shoots and shoot diameter were tested using Spearman rank correlation tests performed with StatMost 2.50 (DataMost Corporation, Salt Lake City, USA), with correlations considered significant at the P ! 0.05 level.
3. Results 3.1. Mid versus late season Metal concentrations in wood were generally lower than those in bark (Tables 2 and 3, Figs. 1 and 2). In wood, Fe and Zn concentrations were significantly higher in November, while Cu and Mn concentrations were significantly higher in August. No significant seasonal variation was found for K and Ca. In bark, K and Ca showed a significantly higher concentration in November, while for Mn, Cu and Zn the reverse was found. Other metals could not be compared between August and November, as these were not determined in August. 3.2. Vertical distribution In Fig. 1, concentrations of macronutrients Ca, Fe, K, Mg and Zn are shown for clones Fritzi Pauley, Gaver, Hazendans and Wolterson in both August and November. A significant height effect was found for only a few elements and clones. Nevertheless, in wood a general trend was observed for all metals in August, i.e. concentrations of all metals generally increased toward the top of the shoot. The increase was most pronounced for Ca, i.e. a difference of 44% between the bottom (40% of total shoot height) and top (80% of total shoot height) of shoots. In November, the concentrations of Ca, K, Mg and Zn in the wood generally decreased toward the top of the shoot; for Fe, the reverse was found. However, the trend was less consistent for all clones than in August (Fig. 1). For bark, no obvious trend was found. The highest concentration was often found at mid-level (60% of total shoot height) (Fig. 2). In Table 2, wood concentrations of elements Al, Cd, Co, Cu, Mn, Na, Ni, Pb and V in November at three height levels are shown for clones Fritzi Pauley, Gaver, Hazendans and Wolterson. As for the macronutrients, only a few elements and clones showed a significant height effect. In wood, concentrations of Al, Cd, Co and Cu generally increased toward the top of the shoot; for Mn, Na, Ni and V no general trend was observed; for Pb, the highest concentration was generally found at
Table 2 Mean metal concentration (standard error) in wood of two-year old poplar shoots at a relative height (RH) of 40%, 60% and 80% of total shoot height RH (%)
Al
As
Cd
Co
Cu
Fritzey Pauley
40 60 80
81.2 (16) 81.3 (15) 108.1 (11)
0.135 (0.050) 0.093 (0.036) 0.151 (0.008)
0.55 (0.06) 0.64 (0.02) 0.92 (0.04)
0.015 (0.007) 0.030 (0.019) 0.025 (0.005)
3.1 (0.5) 3.4 (0.2) 6.1 (0.7)
Gaver
40 60 80
10.4 (1) 10.5 (4) 11.8 (2)
0.104 (0.026) 0.144 (0.012) 0.085 (0.016)
1.64 (0.21) 1.76 (0.16) 1.78 (0.69)
0.029 (0.008) 0.029 (0.004) 0.036 (0.006)
Hazendans
40 60 80
– – –
0.277 (0.057) 0.352 (0.057) 0.461 (0.047)
0.56 (0.05) 0.64 (0.06) 0.88 (0.10)
Wolterson
40 60 80
25.0 (7) 34.4 (17) 41.6 (10)
0.063 (0.032) 0.095 (0.049) 0.080 (0.031)
1.65 (0.27) 2.09 (0.08) 2.73 (0.17)
Mn
Na
Ni
Pb
V
7.9 (2.9) 5.3 (2.7) 5.8 (2.1)
73 (23) 69 (43) 63 (23)
0.97 (0.60) 0.40 (0.06) 0.41 (0.03)
0.26 (0.08) 0.27 (0.06) 0.27 (0.04)
0.01 (0) 0.01 (0) 0.01 (0)
5.1 (0.3) 5.4 (0.5) 5.1 (2.4)
4.3 (1.3) 4.3 (1.4) 5.1 (1.9)
25 (6) 29 (2) 26 (11)
0.98 (0.22) 1.19 (0.26) 0.91 (0.10)
0.29 (0.08) 0.54 (0.20) 0.20 (0.09)
0.02 (0.01) 0.01 (0) 0.01 (0)
0.062 (0.003) 0.071 (0.023) 0.143 (0.004)
4.6 (0.4) 7.1 (0.2) 14.1 (0.1)
16.0 (6.3) 13.6 (5.8) 25.6 (7.3)
– – –
0.96 (0.12) 0.89 (0.22) 1.25 (0.07)
0.68 (0.19) 0.94 (0.34) 0.90 (0.20)
0.26 (0.01) 0.21 (0.10) 0.35 (0.01)
0.024 (0.010) 0.035 (0.006) 0.058 (0.006)
3.3 (0.8) 4.9 (0.6) 7.7 (0.5)
13.0 (6.0) 15.9 (2.0) 23.8 (3.2)
126 (97) 167 (71) 267 (27)
1.34 (0.85) 0.91 (0.22) 1.12 (0.21)
0.16 (0.03) 0.12 (0.02) 0.32 (0.16)
0.01 (0) 0.01 (0) 0.01 (0)
Shoots were harvested in November 2002. All concentrations were expressed as mg gÿ1.
Table 3 Mean metal concentration (standard error) in bark of two-year old poplar shoots at a relative height (RH) of 40%, 60% and 80% of total shoot height Clone
RH (%)
Al
As
Cd
Co
Fritzey Pauley
40 60 80
106 (24) 110 (6) 116 (14)
0.171 (0.030) 0.145 (0.022) 0.216 (0.032)
1.8 (1.8) 1.7 (1.7) 1.6 (1.6)
0.13 (0) 0.13 (0.01) 0.13 (0.01)
5.0 (0.1) 5.4 (0.4) 6.7 (0.2)
Gaver
40 60 80
21 (9) 29 (2) 14 (2)
0.196 (0.030) 0.145 (0.006) 0.141 (0.011)
8.9 (0.5) 6.9 (0.1) 6.0 (0.2)
0.23 (0.01) 0.18 (0.01) 0.19 (0.02)
Hazendans
40 60 80
0.534 (0.090) 0.535 (0.034) 0.328 (0.035)
1.8 (0.1) 1.8 (0.2) 1.3 (0.3)
Wolterson
40 60 80
0.130 (0.054) 0.177 (0.064) 0.108 (0.050)
5.7 (0.4) 5.2 (0.3) 5.2 (0.4)
– – – 22 (5) 42 (10) 23 (6)
Cu
Mn
Na
Ni
Pb
V
3 (1) 9 (4) 9 (3)
40 (7) 72 (19) 103 (36)
0.87 (0.04) 0.85 (0.16) 0.82 (0.04)
0.70 (0.08) 0.63 (0.09) 0.53 (0.19)
0.07 (0) 0.06 (0.01) 0.05 (0.02)
8.0 (0.3) 9.4 (1.1) 9.0 (0.3)
33 (5) 37 (11) 29 (8)
242 (39) 258 (29) 143 (62)
2.19 (0.42) 2.00 (0.30) 1.46 (0.12)
0.71 (0.64) 0.05 (0.03) 0.77 (0.75)
0.11 (0.01) 0.06 (0) 0.05 (0.01)
0.41 (0.03) 0.37 (0.03) 0.26 (0.07)
6.3 (0.1) 11.3 (1.6) 13.5 (0.8)
17 (6) 17 (3) 5 (3)
– – –
1.34 (0.16) 1.26 (0.04) 1.12 (0.06)
1.04 (0.20) 1.23 (0.11) 0.74 (0.13)
0.47 (0.06) 0.50 (0.02) 0.42 (0.01)
0.13 (0.02) 0.12 (0.01) 0.18 (0.04)
6.4 (0.3) 6.9 (0) 7.9 (0.3)
14 (6) 13 (8) 21 (12)
83 (61) 126 (87) 91 (58)
1.61 (0.28) 1.23 (0.02) 1.92 (0.32)
2.40 (0.30) 2.55 (0.13) 1.46 (0.30)
0.16 (0.03) 0.16 (0.01) 0.07 (0.01)
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Clone
Shoots were harvested in November 2002. All concentrations were expressed as mg gÿ1. 545
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I. Laureysens et al. / Environmental Pollution 133 (2005) 541–551 Hazendans
Relative height (%)
Wolterson 80
80
60
60
40
40
0
0
Fritzi Pauley
Gaver
Relative height (%)
Potassium Calcium Iron Zinc Magnesium
80
80
60
60
40
40
0
0
Concentration (µg g-1)
Concentration (µg g-1)
Fig. 1. Mean metal concentration in wood of two-year old poplar shoots of clones Wolterson, Hazendans, Gaver and Fritzi Pauley at three height levels (40%, 60% and 80% of total shoot height). Concentrations in both August (solid line) and November (dashed line) are shown. Error bars present the standard error.
mid-level (Table 2). In bark, no general trend was observed; highest or lowest concentrations were often found at mid-level (Table 3). 3.3. Metal content per stool Total wood and bark dry mass per stool were calculated as the sum of estimated wood and bark dry mass of all shoots of a stool. Bark dry mass per shoot was estimated using an allometric power equation between shoot diameter and bark dry mass. Allometric
equations differed significantly among clones, i.e. clones Trichobel, Fritzi Pauley and Gaver had a relatively higher proportion of bark than clones Balsam Spire and Wolterson (Table 1). Total wood and bark metal content per stool were compared among clones for elements Al, Cd and Zn, as we have previously shown that these three metals are most efficiently taken up (Laureysens et al., 2004a). Clone Fritzi Pauley showed the highest Al accumulation, while Wolterson and Balsam Spire showed the highest Cd and Zn uptake. For these three metals, metal
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Relative height (%)
Wolterson
Hazendans
80
80
60
60
40
40
0
0
Relative height (%)
Gaver
Potassium Calcium Iron Zinc Magnesium
Fritzi Pauley
80
80
60
60
40
40
0
0
Concentration (µg g-1)
Concentration (µg g-1)
Fig. 2. Mean metal concentration in bark of two-year old poplar shoots of clones Wolterson, Hazendans, Gaver and Fritzi Pauley at three height levels (40%, 60% and 80% of total shoot height). Concentrations in both August (solid line) and November (dashed line) are shown. Error bars present the standard error.
content per stool was significantly positively correlated with wood and bark dry mass, stool biomass, number of shoots per stool and mean shoot diameter; shoot diameter was significantly negatively correlated with the number of shoots per stool. The number of shoots per stool differed significantly among clones (Table 4). Clones Hazendans and Fritzi Pauley showed a rapid elimination of small shoots, with 1–12 relatively large shoots; shoot elimination for clones Balsam Spire and Wolterson was much slower, leaving stools with up to 34 shoots per stool. Both wood and bark dry mass were significantly positively correlated with the number of shoots per stool and with shoot diameter. Clone Fritzi
Pauley showed the highest mean Al content per stool, i.e. 158 mg; Wolterson and Balsam Spire showed the highest accumulation of Cd (respectively 5.1 and 6.4 mg on average per stool) and Zn (respectively 264 and 221 mg on average per stool) (Table 5). 3.4. Phytoextraction potential In contrast to metal content per stool, metal content per plot (MCP) represents the real phytoextraction potential (Table 5), as it includes stool mortality. Nevertheless, no significant correlation between MCP and stool mortality was found for Al and Cd, although
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Table 4 Mean number of stools per plot, mean number of shoots per stool, mean shoot diameter (mm), mean wood and bark dry mass (DM) per stool (g) and total above ground woody biomass (Mg haÿ1) for two-year old poplar shoots of six clones
Balsam Spire Fritzi Pauley Gaver Hazendans Trichobel Wolterson
Nr. stools
Nr. shoots per stool
Shoot diameter
Wood DM per stool
Bark DM per stool
Total biomass (Mg haÿ1)
32 32 29 31 32 33
14.0 3.5 8.5 4.6 5.9 14.6
11.67 27.53 17.46 20.8 18.14 14.01
1289 1463 1198 694 1100 1705
267 235 273 200 303 269
13.9 15.0 11.4 7.8 12.9 18.1
(2) (1) (3) (1) (2) (1)
(1.6) (0.6) (1.2) (0.9) (0.8) (1.3)
(1.24) (4.39) (2.43) (3.11) (2.53) (0.63)
(159) (191) (61) (125) (277) (111)
(28) (16) (32) (35) (72) (6)
(2.1) (2.2) (0.5) (1.5) (3.8) (0.8)
Standard errors are shown in brackets.
plot biomass production was significantly correlated with stool mortality (r Z ÿ0.566). Clones Wolterson, Fritzi Pauley and Balsam Spire showed the highest above ground woody biomass production (wood C bark), averaging 18, 15 and 14 Mg haÿ1, respectively after two years (Table 4). Fritzi Pauley showed the highest Al accumulation over two years, i.e. 1.4 kg haÿ1 (Table 5). For Al, MCP was significantly correlated with wood dry mass (r Z 0.513) and woody biomass production (r Z 0.587); no significant correlation was found with bark dry mass, number of shoots or mean shoot diameter. Clones Wolterson and Balsam Spire showed the highest accumulation of Cd, i.e. 47 and 57 g haÿ1, respectively, and of Zn, i.e. 2.4 and 2.0 kg haÿ1 (Table 5). For Cd, MCP was significantly correlated with wood dry mass (r Z 0.472), woody biomass production (r Z 0.503), and number of shoots per stool (r Z 0.719) and per plot (r Z 0.706). For Zn, MCP was significantly correlated with wood dry mass (r Z 0.746), woody biomass production (r Z 0.806), stool mortality (r Z ÿ0.543), and number of shoots per stool (r Z 0.492) and per plot (r Z 0.526).
4. Discussion Stem wood contains the xylem vessels, transporting water and nutrients from the roots to the stem and foliage. The inner bark includes the living phloem, which transports photosynthates and transformed nutrients to growing stem parts and to the roots. When the living phloem dies, dead outer bark is produced (Raven
et al., 1992). In our study, we did not separate inner and outer bark, so we can only distinguish xylem (wood) and phloem with periderm (bark). Metals are directly transferred between xylem and phloem through transfer cells, and indirectly via the foliage (Raven et al., 1992). However, the mobility within the plant is not high for all metals. Metals with high mobility are e.g. K, Mg, Na, Mg and Zn; metals with low mobility are e.g. Ca, Fe, B and Cu. The xylem elements act as exchange columns, adsorbing ions like Ca. Alkali elements as K are mainly present as soluble salts and are less strongly associated with the organic compounds of the wood (Ferguson and Bollard, 1976). Therefore, gradients should be more pronounced for metals with low mobility, which was confirmed by our results, i.e. the difference in concentration between bottom and top of a shoot was highest for Ca and Fe. In our study, no clear height profile was found in bark. In wood, gradients were usually more pronounced in August. In August, metal concentration generally increased towards the top of the shoot, probably reflecting the gradient in the xylem sap. The same height profile was found for Ca, Mg and Mn in the xylem sap of, respectively beech and Chinese gooseberry vines in March–May (Ferguson and Eiseman, 1981; Glavac et al., 1990). Sander and Ericsson (1998) found the same trend for metals (e.g. Ca, Cu and Ni) in bark and wood of two-year old willow shoots in March– April. In pine, Thiry et al. (2002) found no gradient in wood, but K concentration increased with height in the outer bark. Ca, on the other hand, decreased with height in the inner bark. A decrease with height was also found for Ca, Mg and Mn in November for beech (Glavac
Table 5 Mean metal content (standard error) per stool and per hectare for six poplar clones in a short rotation coppice culture, representing the phytoextraction potential
Balsam Spire Fritzi Pauley Gaver Hazendans Trichobel Wolterson
Al (mg stoolÿ1)
Cd (mg stoolÿ1)
Zn (mg stoolÿ1)
Al (kg haÿ1)
Cd ( g haÿ1)
Zn (kg haÿ1)
98 158 19 37 60 65
6.4 1.4 5.5 1.7 1.3 5.1
221 157 74 107 107 264
0.87 1.39 0.15 0.32 0.55 0.59
57 13 43 16 12 47
2.0 1.4 0.6 0.9 1.0 2.4
(12) (19) (1) (6) (14) (4)
(0.7) (0.2) (0.8) (0.9) (0.3) (0.2)
The two-year old shoots were harvested in November 2002.
(26) (19) (6) (19) (26) (8)
(0.13) (0.20) (0.01) (0.06) (0.16) (0.02)
(9) (2) (10) (9) (3) (2)
(0.3) (0.2) (0) (0.2) (0.3) (0.1)
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et al., 1990), comparable to our study on poplar in November. Besides a seasonal variation in vertical distribution, a significant seasonal variation in metal concentration was found for K, Ca, Mn, Fe, Cu and Zn. However, metal concentrations should not be compared between August and November, because they were measured using different analytical techniques, i.e. XRF in August and ICP-MS/ICP-AES in November. In addition, concentrations in both seasons were measured on different plots. It has already been shown that the distribution pattern of every metal within a plant is regulated physiologically, but that the absolute concentration varies with soil and site characteristics (Momoshima et al., 1995). As short rotation coppice shoots are usually harvested during winter, metal concentrations of bark and wood should be considered in the dormant period. In addition, their dry mass is needed to determine the phytoextraction potential of a clone. Metal concentrations are often higher in bark (Riddell-Black et al., 1997; Pulford et al., 2001; Thiry et al., 2002), but the biomass proportion of bark is relatively small and progressively decreases as a shoot grows larger. In this study, the relative proportion of bark of total shoot dry mass was significantly different among clones. In addition, we have previously shown significant clonal differences in the number and size of shoots per stool (Laureysens et al., 2003). Consequently, mean bark dry mass per stool differed significantly among clones. At stool level, metal content was significantly correlated with bark dry mass, and bark dry mass was significantly correlated with the number of shoots per stool and their diameter. However, number of shoots per stool and shoot diameter were significantly negatively correlated. Clone Fritzi Pauley with a few large shoots per stool showed high Al accumulation, while clones Wolterson and Balsam Spire with a large number of smaller shoots showed high Cd and Zn accumulation. All three clones showed similar biomass production, but contrasting shoot size frequency distributions. In November, clone Fritzi Pauley showed a mean Al concentration in wood of 90 mg gÿ1, while for clones Wolterson and Balsam Spire wood Al concentration averaged 34 and 64 mg gÿ1, respectively. For Cd, the concentration averaged 2.2 and 3.3 mg gÿ1 in clones Wolterson and Balsam Spire, respectively; for Zn, the concentration averaged 147 and 144 mg gÿ1 in clones Wolterson and Balsam Spire, respectively. Clone Fritzi Pauley had a mean Cd and Zn concentration of, respectively 0.7 and 92 mg gÿ1. Furthermore, metal content per plot was significantly correlated with wood dry mass and total biomass production, but not with bark dry mass. For Cd and Zn, a significant correlation between metal content per plot and number of shoots was found, because clones Wolterson and Balsam Spire had the highest Cd and Zn
549
concentration and accumulation. These results suggest that selection and improvement of poplar clones for phytoextraction should focus on biomass production, stool survival and metal concentration; population dynamics should not be taken into account. After two years, clone Fritzi Pauley showed the highest accumulation of Al, i.e. averaging 1.4 kg haÿ1, while clones Wolterson and Balsam Spire showed phytoextraction potential for Cd and Zn, i.e. averaging, respectively 47 and 57 kg haÿ1 for Cd and 2.4 and 2.0 kg haÿ1 for Zn. Several studies have shown that clones with a high uptake of a combination of several metals have not yet been identified (Riddell-Black, 1994; Pulford et al., 2001). This is probably due to the antagonistic properties of several metals. Likewise, hyperaccumulators accumulate only one or a limited number of metals. However, the uptake by these plants is much higher in comparison with poplar or willow. Therefore, Ernst (1996) suggested using these short rotation coppice cultures on slightly contaminated soils. The trees will take up part of the heavy metals and will additionally stabilise the soil, reducing metal leaching and dust-blow. The combination of wood for energy production with phytoremediation will make both economically more feasible. Metals in the biomass remain in the ashes or are filtered to avoid translocation of the heavy metal pollutants to the atmosphere (Punshon and Dickinson, 1997). In this study, we examined dry mass partitioning between wood and bark in the stem of a shoot; partitioning to the branches was not taken into account. The proportion of bark in the branches is relatively high, and poplar clones differ significantly in their branch production (Ceulemans et al., 1990; Gielen et al., 2002). Further research should also focus on branches to figure out whether ‘branchy’ clones accumulate more metals. In addition, we did not study root accumulation, although many studies have shown that most metals accumulate in the roots (Kabata-Pendias and Pendias, 1984; Glavac et al., 1990; Alloway, 1995; Landberg and Greger, 1996). This would imply that for phytoremediation purposes roots need to be ploughed up after the last rotation cycle rather than be rotovated and left in the soil. Root metal concentrations and possible clonal differences could be the objects of further research. We have, however, shown that poplar SRC offer possibilities for phytoremediation of slightly contaminated soils.
Acknowledgements This study is being supported by a research contract with the Province of Antwerp. All plant materials were kindly provided by the Institute for Forestry and Game Management (Geraardsbergen) and by the Forest Research, Forestry Commission (UK). The project has
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been carried out in close co-operation with Eta-com B., supplying the grounds and part of the infrastructure, and with the logistic support of the city council of Boom. We gratefully acknowledge P. Aronsson, N. Dickinson, I.D. Pulford, and D. Riddell-Black for useful suggestions on sampling protocol and analyses; C. Lemmens and A. Pellis for help with sampling and harvesting; as well as H. Baeten, Dr. M. Hoenig, and R. Paraschiv for help with heavy metal analysis.
References Alloway, B.J., 1995. Heavy Metals in Soils. Chapman & Hall, Glasgow, UK. Armstrong, A., 1997. The United Kingdom network of experiments on site/yield relationships for short rotation coppice, Forestry Commission Research Information Note 294. Forestry Commission, Edinburgh. Aronsson, P., Perttu, K., 2001. Willow vegetation filters for wastewater treatment and soil remediation combined with biomass production. The Forestry Chronicle 77, 293–299. Blake, T.J., 1983. Coppice systems for short-rotation intensive forestry: the influence of cultural, seasonal and plant factors. Australian Forest Research 13, 279–291. Ceulemans, R., Stettler, R.F., Hinckley, T.M., Isebrands, J.G., Heilman, P.E., 1990. Crown architecture of Populus clones as determined by branch orientation and branch characteristics. Tree Physiology 7, 157–167. Clijsters, H., Vangronsveld, J., van der Lelie, N., Colpaert, J., 2000. Ecological aspects of phytostabilization of heavy metal contaminated soils. In: Ceulemans, R., Bogaert, J., Deckmyn, G., Nijs, I. (Eds.), Topics in Ecology. Structure and Function in Plants and Ecosystems. University of Antwerp, UIA, Wilrijk, pp. 299–306. Dickmann, D.I., Stuart, K.W., 1983. The Culture of Poplars in Eastern North America. Michigan State University Publications. Michigan State University, East Lansing, Michigan. Ernst, W.H.O., 1996. Bioavailability of heavy metals and decontamination of soils by plants. Applied Geochemistry 11, 163–167. Ferguson, I.B., Bollard, E.G., 1976. The movement of calcium in woody stems. Annals of Botany 40, 1057–1065. Ferguson, A.R., Eiseman, J.A., 1981. Xylem sap from Actinidia chinensis: gradients in sap composition. Annals of Botany 48, 75–80. Gielen, B., Calfapietra, C., Claus, A., Sabatti, M., Ceulemans, R., 2002. Crown architecture of Populus spp. is differentially modified by freeair CO2 enrichment (POPFACE). New Phytologist 153, 91–99. Glass, D.J., 2000. Economic potential of phytoremediation. In: Raskin, I., Ensley, B.D. (Eds.), Phytoremediation of Toxic Metals: Using Plants to Clean up the Environment. John Wiley & Sons, Inc., New York, USA, pp. 15–31. Glavac, V., Koenies, H., Ebben, U., 1990. Seasonal variation of calcium, magnesium, potassium, and manganese contents in xylem sap of beech (Fagus sylvatica L.) in a 35-year-old limestone beech forest stand. Trees 4, 75–80. Gordon, J.C., 1975. The productive potential of woody plants. Iowa State Journal of Research 49, 267–274. Hoenig, M., 2001. Preparation steps in environmental trace element analysis – facts and traps. Talanta 54, 1021–1038. Isebrands, J.G., Karnosky, D.F., 2001. Environmental benefits of poplar culture. In: Dickmann, D.I., Isebrands, J.G., Eckenwalder, J.E., Richardson, J. (Eds.), Poplar Culture in North America. NRC Research Press, National Research Council of Canada, Ottawa, Canada, pp. 207–218.
Kabata-Pendias, A., Pendias, K., 1984. Trace Elements in Soils and Plants. CRC Press Inc., Boca Raton, Florida. Landberg, T., Greger, M., 1996. Differences in uptake and tolerance to heavy metals in Salix from unpolluted and polluted areas. Applied Geochemistry 11, 175–180. Laureysens, I., Blust, R., De Temmerman, L., Lemmens, C., Ceulemans, R., 2004a. Clonal variation in heavy metal accumulation and biomass production in a poplar coppice culture: I. Seasonal variation in leaf, wood and bark concentrations. Environmental Pollution 131, 485–494. Laureysens, I., Bogaert, J., Blust, R., Ceulemans, R., 2004b. Biomass production of 17 poplar clones in a short-rotation coppice culture on a waste disposal site and its relation to soil characteristics. Forest Ecology and Management 187, 295–309. Laureysens, I., Deraedt, W., Indeherberge, T., Ceulemans, R., 2003. Population dynamics in a six-year old coppice culture of poplar. I. Clonal differences in stool mortality, shoot dynamics and shoot diameter distribution in relation to biomass production. Biomass and Bioenergy 24, 81–95. Ledin, S., Willebrand, E., 1996. Handbook on How to Grow Short Rotation Forests. Department of Short Rotation Forestry, Swedish University of Agricultural Sciences, Uppsala, Sweden. Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 1996. SAS System for Mixed Models. SAS Institute Inc., Cary, NC, USA. Luyssaert, S., Van Meirvenne, M., Lust, N., 2001. Cadmium variability in leaves of a Salix fragilis: simulation and implications for leaf sampling. Canadian Journal of Forest Research 31, 313–321. Macpherson, G., 1995. Home-grown Energy from Short-rotation Coppice. Farming Press Books, Ipswich, UK. Momoshima, N., Eto, I., Kofuji, H., Takashima, Y., Koike, M., Imaizumi, Y., Harada, T., 1995. Distribution and chemical characteristics of cations in annual rings of Japanese cedar. Journal of Environmental Quality 24, 1141–1149. Perttu, K.L., 1995. Ecological, biological balances and conservation. Biomass and Bioenergy 9, 107–116. Pontailler, J.Y., Ceulemans, R., Guittet, J., Mau, M., 1997. Linear and non-linear functions of volume index to estimate woody biomass in high density young poplar stands. Annales des Sciences Forestie`res 54, 335–345. Pulford, I.D., Riddell-Black, D., Stewart, C., 2002. Heavy metal uptake by willow clones from sewage sludge-treated soil: the potential for phytoremediation. International Journal of Phytoremediation 4, 59–72. Pulford, I.D., Watson, C., McGregor, S.D., 2001. Uptake of chromium by trees: prospects for phytoremediation. Environmental Geochemistry and Health 23, 307–311. Punshon, T., Dickinson, N.M., 1997. Mobilisation of heavy metals using short-rotation coppice. Aspects of Applied Biology 49, 285– 292. Rachwal, L., De Temmerman, L.O., Istas, J.R., 1992. Differences in the accumulation of heavy metals in poplar clones of various susceptibility to air pollution. Arboretum Ko´rnickie 37, 101–111. Raven, P.H., Evert, R.F., Eichhorn, S.E., 1992. Biology of Plants, fifth ed. Worth Publishers, Inc., New York. Reeves, R.D., Baker, A.J.M., Borhidi, A., Berazain, R., 1999. Nickel hyperaccumulation in the serpentine flora of Cuba. Annals of Botany 83, 29–38. Riddell-Black, D., 1994. Heavy metal uptake by fast growing willow species. In: Aronsson, P., Perttu, K. (Eds.), Willow Vegetation Filters for Municipal Wastewaters and Sludges. A Biological Purification System. Department of Ecology and Environmental Research, Section of Short Rotation Forestry, Uppsala, Sweden, pp. 145–151. Riddell-Black, D., Pulford, I.D., Stewart, C., 1997. Clonal variation in heavy metal uptake by willow. Aspects of Applied Biology 49, 327–334.
I. Laureysens et al. / Environmental Pollution 133 (2005) 541–551 Sander, M.-L., Ericsson, T., 1998. Vertical distributions of plant nutrients and heavy metals in Salix viminalis stems and their implications for sampling. Biomass and Bioenergy 14, 57–66. SAS Institute Inc., 1990. SAS/STAT User’s Guide, Version 6, fourth ed. SAS Institute Inc., Cary, NC, USA. Sennerby-Forsse, L., Ferm, A., Kauppi, A., 1992. Coppicing ability and sustainability. In: Mitchell, C.P., Ford-Robertson, J.B., Hinckley, T., Sennerby-Forsse, L. (Eds.), Ecophysiology of Short Rotation Forest Crops. Elsevier Applied Science, Oxford, pp. 146–184. Thiry, Y., Goor, F., Riesen, T., 2002. The true distribution and accumulation of radiocaesium in stem of Scots pine (Pinus sylvestris L.). Journal of Environmental Radioactivity 58, 243–259.
551
Van Gysel, M., Lemberge, P., Van Espen, P., 2003a. Description of Compton peaks in energy-dispersive X-ray fluorescence spectra. X-ray Spectrometry 32, 139–147. Van Gysel, M., Lemberge, P., Van Espen, P., 2003b. Implementation of a spectrum fitting procedure using a robust peak model. X-ray Spectrometry 32, 434–441. Watson, C., Pulford, I.D., Riddell-Black, D., 1999. Heavy metal toxicity responses of two willow (Salix) varieties grown hydroponically: development of a tolerance screening test. Environmental Geochemistry and Health 21, 359–364. Zavitkovski, J., 1981. Small plots with unplanted plot border can distort data in biomass production studies. Canadian Journal of Forest Research 11, 9–12.