Effects of nutrient supply and soil cd con

Effects of nutrient supply and soil cd con

Eects of nutrient supply and soil cadmium concentration on

cadmium removal by willow Erika Klang-Westina; ∗, Kurth Perttub

a Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O Box 7014, SE-750 07 Uppsala, Sweden

b Department of Short Rotation Forestry, Swedish University of Agricultural Sciences, P.O Box 7016, SE-750 07 Uppsala, Sweden

Received 1 November 2001; received in revised form 10 May 2002; accepted 23 May 2002 Abstract

This investigation studied the eect of an increased biomass production as a result of fertilization and an elevated Cd concentration in the topsoil on concentration and amount of Cd in two clones of Salix (81090 and 78183) The experiment was conducted over a three year period using 200-dm3 lysimeters 5lled with clay soil A liquid fertilizer containing all essential macro- and micronutrients in balanced proportions by weight was applied at two rates according to growth The lower rate corresponded to 0, 20 and 20 kg N ha−1 during years 1, 2 and 3, respectively, while the higher rate was 30, 60 and 60 kg N ha−1for the same period The Cd levels in the topsoil were an initial content of 0:3 mg Cd (kg dw soil)−1 and 0:6 mg Cd (kg dw soil)−1after addition of CdSO4

Biomass production increased signi5cantly due to fertilization In general, this increase in biomass resulted in a higher

Cd amount in the stem However, the magnitude was small and only statistically signi5cant in some cases, mainly because increased biomass also resulted in a lowered Cd concentration due to an eect of biological dilution Addition of Cd to the

topsoil resulted in higher Cd concentrations and total Cd amounts (concentration×biomass) in the Salix plants In most cases

the increase in total stem Cd amount was 40–80% of the increase in soil Cd concentration, although a directly proportional increase was observed occasionally Clone 81090 had higher concentrations and total amounts of Cd in the stems than clone

78183, while clone 78183 produced more stem biomass The leaves had the highest Cd concentrations, but the total amounts

of Cd were largest in the stems

? 2002 Elsevier Science Ltd All rights reserved

Keywords: Biomass production; Fertilization; Salix; Clone; Cd

1 Introduction

During the 20th Century, arable land in Sweden

has been subjected to anthropogenic input of Cd,

mainly via phosphorus fertilizers and deposition [1]

Calculations by Andersson [2] indicate a 33% increase

∗Corresponding author Tel.: (0)18-672888; fax:

+46-(0)18-672795.

E-mail address: erika.klang.westin@mv.slu.se

(E Klang-Westin).

in average Cd concentrations in the topsoil be-tween 1900 and 1990, based on the levels around

1900 Furthermore, Eriksson [3] found that 5 –10% of an annual harvest of Swedish win-ter wheat had Cd concentrations near or above the limit value for cereals (0:1 mg kg−1) pro-posed by the CODEX committee [4] Elevated Cd concentrations were also found in spring wheat, potatoes and carrots [5]

In the past decade plantations of willow, consist-ing mainly of Salix viminalis L and S dasyclados

0961-9534/02/$ - see front matter ? 2002 Elsevier Science Ltd All rights reserved.

PII: S 0961-9534(02)00068-5

Wimm., have been established on arable land in

Sweden The stem biomass produced has mainly

been used as a biofuel in municipal district heating

plants Today around 16,000 ha are cropped with

Salix, which corresponds to approximately 0.5% of

the total agricultural land in Sweden Several studies

have shown that Salix accumulates high levels of Cd

[6–10] Therefore, the role of Salix as a potential

phytoextractor to remove Cd from moderately

con-taminated soils at stem harvest has been discussed

In relation to other species known to accumulate Cd,

Salix can be de5ned as a high accumulator rather than

a hyperaccumulator of Cd According to the de5nition

by Baker et al [11] hyperaccumulators accumulate

¿ 0:01% Cd in leaf dry mass and may have the metal

evenly distributed throughout the plant Examples of

hyperaccumulators of Cd are Thlaspi caerulescens

and Alyssum murale within the Brassicaceae family

In contrast to these more eIcient species, Salix has a

high biomass production, making it possible to have

a pro5table production of biofuel (see above) at the

same time as the soil is being restored A dierence in

Cd accumulation (uptake and translocation) between

genotypes of Salix has also been demonstrated [7,12]

The choice of clone will therefore also be of

impor-tance for the phytoextraction eect of a Salix stand

The mechanisms that regulate the Cd uptake in

plants are still not known Plant uptake of Cd at low

so-lution concentrations has been reported in reviews by

Grant et al [13] and Greger [14] to be either passive,

metabolic or partially metabolic and partially passive

and may be in competition with the uptake system for

essential trace elements An incorporation of Cd into

the stems of Salix in direct proportion to the biomass

production would imply that Cd uptake is dependent

on the Kuxes of water and mineral nutrients through

the plant Even if the uptake of Cd is not directly

proportional to biomass production, the Cd

incorpo-rated into the stems will still increase with increased

stem yield as long as the increase in biomass is larger

than the decrease in Cd concentration Factors such

as temperature, light and Kow of water through the

plant inKuence growth and may therefore also aect

the Cd uptake However, Perttu et al [15] concluded

that the factors mentioned do not aect the Cd uptake

in Salix The lack of knowledge regarding the

fac-tors that determine the Cd concentration and

mecha-nisms behind Cd uptake in Salix makes it diIcult to

predict the eect of dierent management practices, e.g fertilisation, on the removal of Cd at stem harvest This study was undertaken to investigate the ef-fect of dierent nutrient supplies and soil Cd con-centrations on Cd concon-centrations in stems, leaves and roots in two dierent clones of Salix The hypothesis was that an increased biomass production induced by fertilisation would increase the Cd content in the stems and hence the removal of Cd at stem harvest

2 Materials and methods The experimental area is situated in Uppsala in the east-central Sweden (lat 59◦49N, long 17◦40E,

15 m a.s.) The experiment was carried out in closed lysimeters made from plastic containers (volume ap-prox 200 dm3 and depth 0:9 m) [16] Soil columns consisting of approximately 0:25 m topsoil and 0:50 m subsoil were built up in the lysimeters (Table 1) us-ing an arable clay soil (Eutric Cambisol) collected in the vicinity of Uppsala A drainage pipe covered with sand (approx 0:15 m) was put in the bottom of each lysimeter At the beginning of May 1997, one unrooted

cutting (weight 13±0:5 g) of willow (Salix viminalis

L or Salix dasyclados Wimm.) was planted in each lysimeter The lysimeters were covered with a lid, with

a hole for the shoots The experiment was conducted over three growing seasons, from 1997 to 1999 Treatments consisted of two clones (81090 of Salix dasyclados and 78183 of Salix viminalis), two nutri-ent levels (based on the N-supply), two soil Cd con-centrations and two harvest occasions (2 and 3 years old) Nutrient level 1 (N1) corresponded to appli-cation of 0, 20 and 20 kg N ha−1 during years 1, 2 and 3, respectively Corresponding amounts of N for nutrient level 2 (N2) were 30, 60 and 60 kg N ha−1

Table 1

pH, total Cd concentration (7M HNO3 ), exchangeable Cd (NH4NO3) and carbon content in topsoil and subsoil used in the lysimeters

Soil type pH Total Cd Exchangeable Cd C

(g kg−1) (g kg−1) (%)

0 10 20 30 40 50 60

0 70 140 210 280 350 420

Season 1997 Season 1998 Season 1999

Season 1997 Season 1998 Season 1999

Fig 1 Accumulated irrigation and fertilization for nutrient levels N1 and N2 during the growing seasons 1997 (solid line), 1998 (dotted line), 1999 (dashed line).

The other macronutrients were supplied in relation

to N (see below) The two soil Cd concentrations in

the topsoil were 0:3 mg Cd (kg DW)−1, which was

the concentration of the parent material and 0:6 mg

Cd (kg DW)−1 The increased Cd concentration was

achieved by adding 0:3 mg Cd (kg DW)−1 in the

form of CdSO4 to the topsoil The subsoil had the

same Cd concentration in both treatments (0:23 mg

Cd kg dw−1) Each treatment had two replicates The

lysimeters were installed in the ground in groups of 8

(32 lysimeters in total) Plants of Salix were grown

around the lysimeters to simulate a stand structure

and to eliminate edge eects The spacing between

plants inside and outside the lysimeters was such

that the area for each plant was 0:5 m2 For practical

reasons, the four combinations of clones and nutrient

levels were distributed so that clone and nutrient level

were the same for all experimental units within each

group Soil Cd treatments were randomized within

each group of lysimeters

Irrigation was performed daily from late May until the beginning of October each year with a comput-erized drip irrigation system (Fig 1) The plants received the same amount of water irrespective of nutrient level, in order to reduce the number of exper-imental factors and also to simulate 5eld conditions

A liquid fertilizer (Blomstra, Cederroth International) containing all essential macro- and micronutrients in the following proportions (by weight); 100 N, 20 P,

84 K, 6 Ca, 8 Mg, 8 S, 0.3 Fe, 0.4 Mn, 0.2 B, 0.06

Zn, 0.03 Cu and 0.0008 Mo was applied with the drip irrigation system according to a sigmoid growth curve (cf [17]) (Fig 1) Climate data were obtained from the Ultuna meteorological station close to the 5eld The soil water potential in the lysimeters was controlled by TDR measurements during the season Excess water collected in the drainage pipe at the bottom of each lysimeter was occasionally pumped out Samples of this water were taken for Cd analy-ses and they showed that almost no Cd had leached

out In order to prevent the roots penetrating into

the sand at the bottom, a nylon straining-cloth

(80 m, Bewatex AB) enclosed the topsoil and

subsoil

In November 1998, when the plants were 2 years old

(2-Au98), the 5rst harvest was carried out in half of

the lysimeters In the lysimeters without added Cd, the

roots were destructively sampled During the winter

1998–1999, the remaining plants were unfortunately

severely damaged by 5eld-mice and these also had

to be harvested in early spring 1999 (2-Sp99) The

plants coppiced in spring were allowed to resprout and

were harvested again in autumn 1999 (1-Au99) The

roots were destructively sampled after harvest in all

lysimeters harvested in autumn 1999 The shoots were

cut at about 5 cm from where they were attached to

the cutting (stem base)

During the second and third growing seasons, shed

leaves were collected (from middle of July until all

leaves had fallen) from the plants later being harvested

in the same year A net was mounted surrounding each

plant and held open above the canopy by a sti

circu-lar wire To record plant nutrient status, mature leaves

that had not yet abscised were also collected from

ev-ery plant at the beginning of September in the second

and third growing season In order to cover the

con-centration gradient along the shoot, these leaves were

taken randomly within three sections of the

above-ground stool (unit of roots and shoots originating from

the same cutting) The levels separating each section

were set by dividing the tallest shoot into three Each

section was analyzed separately

The collected shed leaves and the non-abscised

leaves sampled for nutrient analysis were dried

(70◦C), weighed and ground on a Thomas–Wiley

laboratory mill (mesh size 2 mm) and on a Retsch

knife mill (mesh size 0:2 mm), respectively

Subsam-ples from the leaf material were wet ashed (heating

block 150◦

C) in a mixture of 10 ml conc HNO3 and

1 ml conc HClO4 The acids were evaporated

un-til 0.5 ml of perchloric residue remained, then this

was diluted with H2O to a 5nal volume of 35 ml

The extracts were analyzed for Cd on a JY-70 Plus

ICP Emission Spectrometer In addition, the extracts

from the non-abscised leaves were analyzed for the

macro elements P, K, Ca and Mg on ICP (see above)

and subsamples from the same leaf material were

also analyzed for N (Carlo Erba NA 1500 elemental

analyzer) Stems, roots and cuttings were oven dried

at 70◦C to constant weight, weighed and ground on

a Thomas-Wiley laboratory mill (mesh size 2 mm) and analyzed for Cd as described above The roots were clipped before grinding Soil samples were air dried (30–40◦

C), ground to pass through a 2 mm sieve and analyzed for total Cd, exchangeable Cd, organic carbon (C) and pH Total Cd (Cd–HNO3) was analyzed after extraction with 7 M nitric acid (110◦

C, 2h) [18] Exchangeable Cd was estimated

by extracting the soils with 1:0 M NH4NO3 (Cd–

NH4NO3) Total carbon content was analyzed on

an elemental analyzer (LECO CHN-932) and pH was measured in H2O (soil:water ratio 1:5) Water samples were 5ltered (0:2 m) and 1% by volume

of conc HNO3 was added for conservation Anal-yses of Cd on the water samples and soil extracts were performed by means of atomic absorption spec-trophotometry using the graphite furnace technique (Zeeman 4110 ZL)

Because of the unplanned harvest in spring of the third growing season, each harvest was treated separately in a 3-factorial design Statistical analy-ses (ANOVA) were performed with the programme Systat 10.0 (SPSS Inc)

3 Results 3.1 Weather conditions The summer of 1997 was warmer than normal for Swedish conditions (Fig 2) In spite of this, the potential evaporation (Penman) did not exceed pre-cipitation by very much and the accumulated precip-itation during May–September was quite high (Fig 2) The growing season in 1998 was cooler than nor-mal and accumulated precipitation was again quite high (Fig 2) During the growing season of 1999, the temperature was once again higher than normal for the area concerned However, the accumulated pre-cipitation was very low and well below accumulated potential evaporation

3.2 Plant nutrient status

In the second growing season (1998), the leaf N concentration was around 22 mg N (g DW)−1and N

M J J A S O M J J A S O

0 10 20 30 40

0 10 20 30 40

0 10 20 30 40

0 10 20 30 40

0 10 20 30 40

0 10 20 30 40

Acc precip 330 Mean air temp 14.5

Mean air temp 12.5

Mean air temp 14.5

Acc pot evapo 416

Acc precip 329 Acc pot evapo 338

Acc precip 163 Acc pot evapo 447

1997

1998

1999

1997

1998

1999

Fig 2 Daily precipitation (solid line), potential evaporation (dotted line) and mean air temperature for the growing seasons 1997–1999.

In each graph, values for accumulated precipitation (Acc precip.), accumulated potential evaporation (Acc pot.evapo.) and mean air temperature (Mean air temp.) for each growing season (May–October) are given.

Table 2

Means of treatment eects ± SD for N, P, K, Ca, Mg concentrations in non-abscised leaves sampled and analyzed from three sections

within the shoot in autumn 1998 and autumn 1999

Nutrient level 1 22±3 a 19±1 a 5:6±1:1 a 8:1±0:6 a 16±1 a 17±2 a 15±4 a 20±4 a 1:9±0:3 a 3:3±0:3 a

2 22±3 a 23±4 b 4:1±1:0 b 6:3±1:4 b 15±1 a 16±1 a 13±3 b 18±4 b 1:7±0:2 b 2:7±0:4 b

Soil Cd conc 0 22±3 a 22±4 a 4:8±1:5 a 6:6±1:6 a 15±1 a 17±2 a 14±3 a 18±4 a 1:9±0:3 a 2:9±0:5 a

1 22±2 a 19±2 a 4:9±1:1 a 7:8±0:9 b 15±1 a 16±1 a 14±4 a 19±4 a 1:9±0:3 a 3:1±0:3 a

Clone 81090 19±1 a 20±5 a 4:5±0:9 a 7:0±1:3 a 15±1 a 17±2 a 17±1 a 22±2 a 2:1±0:2 a 3:2±0:3 a

78183 24±1 b 22±2 a 5:2±1:5 a 7:4±1:6 a 16±1 a 15±1 b 11±2 b 15±2 b 1:6±0:2 b 2:8±0:4 b

Means within columns followed by dierent letters are dierent at p 6 0:05 when comparing levels within the same treatment and harvest occasion.

concentrations were not signi5cantly (p ¡ 0:05)

af-fected by nutrient level (Table 2) During the third

growing season (1999) when the plants had resprouted

after coppicing, the plants at the high nutrient level

(N2) had signi5cantly (p ¡ 0:05) higher leaf N

con-centrations (23 mg N (g DW)−1) than the plants at the

low nutrient level (N1) (19 mg N (g DW)−1) Leaf

concentrations of the macronutrients P, Ca and Mg

were signi5cantly higher at nutrient level N1

com-pared to nutrient level N2 (p ¡ 0:05), independent of

sampling occasion, while leaf concentration of K was not inKuenced by nutrient level (Table 2)

Clone 81090 had higher leaf concentrations of Ca and Mg than clone 78183, a trend which was also true for K during the third growing season Leaf N concentration was highest for clone 78183 during the second growing season, while leaf concentration of P did not dier between clones Soil Cd concentration did not have any pronounced eects on leaf nutrient concentration

0 150 300 450 600 750 900

-1 )

0 5 10 15 20

-1 )

0.0 0.5 1.0 1.5 2.0 2.5 3.0

-1 )

Clone 81090 Clone 78183

2-Au98 2-Sp99 1-Au99 2-Au98 2-Sp99 1-Au99

(a)

(b)

(c)

Fig 3 Mean values ± SD (n = 2) for stem biomass (DW), concentration (dry weight basis) and total amount of Cd in the stem for each

clone (81090 and 78183), nutrient level (N1 and N2) and soil Cd concentration (Cd0 and Cd1) The plants are 2-years old, harvested in autumn 1998 (2-Au98) and spring 1999 (2-Sp99), and 1-year old sprouts coppiced in autumn 1999 (1-Au99).

3.3 E4ect of nutrient level and soil Cd

concentration on biomass and Cd content of stems

Stem biomass production signi5cantly (p ¡ 0:05)

increased with a higher nutrient supply,

indepen-dent of harvest occasion (Fig 3a) The increase in

mean stem biomass between nutrient levels 1 and 2

amounted to 60–80% (Tables 3a–3c) Figure 3b also

shows that stem Cd concentration was aected by

nutrient level In the stems harvested in autumn 1998

(2-Au98) and spring 1999 (2-Sp99), the Cd

concen-tration was signi5cantly (p ¡ 0:05) higher at the low

nutrient level (N1) than at the high nutrient level (N2)

(Tables 3a and 3b) The same tendency was found in the resprouting 1 yr old stems (1-Au99), but it was not statistically signi5cant (Fig 3band Table 3c) The

to-tal amount (concentration × biomass) of Cd in stems

tended to be slightly higher at the higher nutrient level (N2) than at the low nutrient level (N1), but the dif-ference was only statistically signi5cant (p ¡ 0:05) in the case of the resprouting 1-year old stems (1-Au99) (Fig 3c and Tables 3a–3c) The explanation for the weak eect on amounts of Cd in the stems is the opposing and very consistent relationship between stem biomass production and stem Cd concentration (Fig 3a and b) The eects of nutrient levels N1 and

Table 3a

Stem biomass, Cd concentration, Cd amount in stems harvested in autumn 1998 (2-Au98) as inKuenced by clone, nutrient level and soil

Cd concentration

Analysis of variance

p-values ( = 0:05) Main e4ects

Interactive e4ects

Data for biomass and Cd amount were log-transformed prior to the statistical analyses.

N2 on stem biomass, stem Cd concentration and total

stem Cd amount for each soil Cd concentration (Cd0

and Cd1) and for both clones in Fig 3, remains the

same regardless of soil Cd concentration

Increased levels of Cd in the soil did not inKuence

stem biomass production (Tables 3a–3c) However,

higher soil Cd concentrations signi5cantly raised the

concentration and total amount of Cd in the stems

(Tables 3a–3c) The concentration was 1.4–2.2 times

higher at soil Cd concentration 1 compared to soil Cd

concentration 0 (Fig 3b) The corresponding total Cd

amount in the stems at Cd1 was 1.3–2.1 times the Cd0

level (Fig 3c) In the resprouting 1-year-old stems,

the increase in stem Cd concentration tended to be

somewhat higher than the increase in total Cd amount

3.4 Di4erences in stem growth and Cd content

between clones

Clone 78183 produced signi5cantly more stem

biomass than clone 81090 when harvested after two

growing seasons (2-Au98 and 2-Sp99) (Tables 3a and 3b) In the resprouting plants harvested in autumn

1999 (1-Au99) there were no dierences in stem biomass between clones Clone 81090 had higher stem Cd concentrations than clone 78183, indepen-dent of harvest occasion The total amount of Cd

in the stems was also larger in clone 81090, with the exception of the stems harvested in spring 1999 Clone 81090 yielded a higher root biomass and had

a larger amount of Cd in the roots than clone 78183, independent of harvest occasion (Fig 4)

3.5 Comparison between plant compartments The treatment eects on biomass, Cd concentra-tion and amounts of Cd in leaves and roots followed more or less the same pattern as those in the stems (Fig 4) A comparison of Cd concentrations in the dif-ferent plant compartments showed that the leaves had the highest concentrations independent of harvest oc-casion, nutrient level, clone and soil Cd concentration

Table 3b

Stem biomass and Cd concentration and total Cd amount in stems harvested in spring 1998 (2-Sp99) as inKuenced by clone, nutrient level and Cd concentration in the soil

Analysis of variance

p-values ( = 0:05) Main e4ects

Interactive e4ects

Data for biomass and Cd concentration werelog-transformed prior to the statistical analyses.

(Fig 4) There were no major dierences in Cd

con-centration between the other plant parts (stems, roots

and cuttings) In the 2-year old plants, the amount of

Cd was largest in the stems and lowest in the cuttings,

while leaves and roots were intermediate in this

re-spect In the resprouting 1-year old plants, Cd amounts

in the roots were relatively larger in comparison to

the amounts in the stem For clone 81090, which had

a larger root biomass than clone 78183, this meant

that the total root Cd amount was larger than that

of the stem

4 Discussion

An enhanced nutrient supply resulted in

signi5-cantly higher stem biomass production In general, this

gave rise to lower stem Cd concentrations compared

to when the nutrient supply, and hence biomass

pro-duction, was lower This eect of enhanced growth on

stem Cd concentrations is commonly referred to as a biological dilution eect Biological dilution has also been reported for heavy metals in other plant species investigated elsewhere For example Singh et al [19] saw a suppressed Cd uptake in lettuce when the N level was high (¿ 150 mg N kg−1added to the soil) which could partly be explained by a dilution eect Furthermore, Jones et al [20] observed a substantial increase in the concentration of lead in the shoots of plants whose growth rate was slow due to a nutrient de5ciency Eects of dilution on the concentration

of Cd have also been demonstrated in ryegrass plants growing at dierent rates due to the age of the plants [21]

The opposing and very consistent trends in stem biomass production and stem Cd concentration re-sulted in insigni5cant or small positive eects on the total amount of Cd in the stems (Fig 3c) This indicates that the incorporation of Cd into the stems

is governed by processes which are independent of

Table 3c

Stem biomass and concentration and total amount of Cd in stems harvested in autumn 1999 (1-Au99) as inKuenced by clone, nutrient level and Cd concentration in the soil

Analysis of variance

p-values ( = 0:05) Main e4ects

Interactive e4ects

Data for biomass and Cd amount were log-transformed prior to the statistical analyses.

biomass production Plants of Salix whose growth

rate is slow because of nutritional constraints are

therefore likely to have elevated concentrations of Cd

This seems to be valid also for P, Ca and Mg, but not

for N and K In this context it should be mentioned

that the leaf concentrations of macronutrients in the

present study were in almost the same range as those

presented in some other investigations for young,

fertilized, high-yielding stands sampled in

mid-summer [22]

Stem biomass production is, however, not only

determined by the supply of nutrients Lindroth and

Cienciala [23] concluded that water availability is a

critical factor for growth of Salix in Sweden During

some periods of the growing season, water

avail-ability will probably be the most limiting factor for

growth It is therefore likely that the plants will grow

more slowly than can be expected from available N

during some periods of growth In the present

inves-tigation the plants received the same amount of water

irrespective of nutrient level, in order to reduce the

number of experimental factors and also to simulate 5eld conditions

As pointed out earlier, when comparing the amount

of Cd in the stems at the two nutrient levels the same pattern could be distinguished regardless of harvest occasion However, in the 2-year old plants (2-Au98 and 2-Sp99) the eect of nutrient level on total stem

Cd amount was insigni5cant This was not the case

in the coppiced 1-year old plants (1-Au99), where the amount of Cd was signi5cantly higher at the higher nutrient level compared to the lower nutrient level Similar results were observed in a study with Salix conducted in a climate chamber for two growing sea-sons and where the plants were supplied with N in accordance with growth at two dierent rates [15] In contrast to the present study, the dierence between the high and low nutrient levels was more pronounced and the plants were kept well watered at both nutri-ent levels, while the growth medium was solely a clay mineral (vermiculite) The somewhat diering results between the 1- and 2-year old plants described in the

0 10 20 30

Stem Cutting Root Clone 81090

0 10 20

-3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5

-3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5

2-Au98

2-Au99

L S C R

2-Au98

2-Au99

Fig 4 Mean values (n = 2) for Cd concentration (dry weight basis) and total Cd amount per plant in leaves (L), stems (S), cuttings (C) and roots (R) for each clone (81090 and 78183), nutrient level (N1 and N2) and soil Cd concentration (Cd0 and Cd1) The plants are 2-years old harvested in autumn 1998 (2-Au98) and 1-year old sprouts coppiced in autumn 1999 (1-Au99) For the plants harvested in autumn 1998 (2-Au98), data for the cuttings and roots are missing.

present investigation might be a consequence of

cop-picing According to Bollmark [24], coppicing may,

for example, result in changes in growth rate for

dif-ferent plant parts depending on nutrient level and also

changes in mobilization and translocation of nutrients

and carbohydrates, which in turn may aect the

con-centration and amount of Cd in the stems Other

expla-nations for the dierences between the 2- and 1-year

old plants in this study might be changed weather

con-ditions between the second and the third growing

sea-son, but also the higher water availability in the 1-year

old plants as they received the same amount of water

as before coppicing

An increased soil Cd concentration in the topsoil

in the current investigation increased the Cd

concen-tration and the total Cd amount in the plants,

demon-strating the ability of Salix to take up more Cd from

more contaminated soils In some cases, the increase

in total stem Cd amount tended to be almost directly

proportional to the increase in soil Cd concentration

However, more often the increase in stem Cd amount

was between 40 and 80% of the increase in Cd content

of the topsoil The reason that the increase in stem Cd

concentration is less than the increase in the topsoil

Cd concentration may be that a signi5cant proportion

of Cd in the stems is taken up from the subsoil Clone 81090 had higher stem Cd concentrations compared to clone 78183 The reason might be clone speci5c or a consequence of the higher stem biomass production of clone 78183 As pointed out earlier, var-ious clones dier in their ability to take up Cd and

to transport Cd up to the shoot [7] Clones also dier

in their distribution of Cd between stems and leaves, which has been seen by Perttu et al [15] The choice

of clone will therefore be important for the removal of

Cd at stem harvest Both clones in this investigation had an intermediate transport of Cd up to the shoot

In general, the leaves had higher Cd concentrations than the stems, a trend also recorded by Riddel-Black [25] However, the total amount of Cd is larger in the stems, at least if harvest is performed after two or more growing seasons

5 Conclusions

• Increased fertilization in this experiment

consis-tently resulted in increased biomass production,