Phytoextraction of Trace Metals: Principles and Applications - Environmental Sustainability: Role of Green Technologies - page 210

plant growth. Biofuels are considered as a key to
reducing reliance on foreign oil, lowering green-
house gas emissions, and meeting rural develop-
ment goals by developing local and sustainable
energy sources. However, the political and public
support for biofuels has been undermined by
concerns related to food security because the
conversion of croplands to produce biofuels may
cause food shortages and associated increase in
food prices (Koh and Ghazoul 2008 ).
Utilisation of poor-quality soils and contami-
nated land can extend the area available to grow
energy crops, and it can avoid competition
between energy crops and food products. Poplar
( Populus spp.) and willow ( Salix spp.) have been
demonstrated to be the most successful tree crops
that can be grown on contaminated land for bio-
mass production and phytoextraction of TEs
(Pulford and Watson 2003 ). They are fast to
propagate, have many and deep roots, achieve
high annual biomass production, take up large
quantities of water, and generally possess high
tolerance to trace metals (Cd, Cu, Zn, Pb) (Granel
et al. 2002 ; Hu et al. 2013 ; Maxted et al. 2007 ;
Meers et al. 2007 ; Mirck et al. 2005 ; Vervaeke
et al. 2003 ). Salix and Populus spp. have an effec-
tive nutrient uptake and high evapotranspiration
rate and a pronounced clone-specifi c capacity for
heavy metal uptake. Success of Salix spp. (wil-
low) as phytoextracting plants depends on its bio-
mass production, metal accumulation capacity,
and the site of metal accumulation in the plant.
Willow is usually grown in short-rotation coppice
(SRC) systems because plants have the ability to
resprout after harvest. This characteristic makes
willow very suitable to phytoextraction because
the frequency and number of harvests will trigger
higher metal removal. The estimated economic
lifespan of a short-rotation willow coppice stand
is 20-25 years, with 6-7 harvests (the time frame
from planting to fi rst harvest is typically 4 years).
Research on the environmental sustainability
of willow production began in the mid-1980s by
various groups in the USA and Europe (Gomes
2012 ; Rowe et al. 2009 ; Šyc et al. 2012 ; Volk
et al. 2006 ). The willow cropping system utilises
agricultural practices that are familiar to farmers,
and after establishment, it is a relatively low-

input crop with winter harvests, thus having a
limiting effect on other farming operations.
Willow biomass production systems involve
intensive site preparation to control weeds,
double-row mechanical planting of high density
(15,300 plants ha −1 ), nitrogen inputs at the begin-
ning of each rotation, and 3-4-year rotations. It
has been demonstrated that the use of cover crops
during the establishment phase of willow planta-
tions and mechanical control of the cover crops
(such as rolling, undercutting, or partial rototill-
ing) will reduce the risk of erosion during tree
establishment and will allow tree plantations on
sloping farmland.
A common critique to the sustainability of
willow SRC systems is the creation of 'biologi-
cal deserts' across the landscape due to the
monoculture of willow. However, long-term
research on the above- and below-ground biodi-
versity in willow plantations has shown positive
effects on avian biodiversity comparable to nat-
ural habitats including shrubland and succes-
sional habitats (abandoned fi elds, second-growth
forests, regenerating clear-cuts) (Volk et al.
2006 ). Šyc et al. ( 2012 ) have suggested inter-
cropping of fast-growing species such as willow
and poplar with hyperaccumulators to increase
the intake of metals for phytoremediation and to
contribute to increases in biomass production
and positive effects on biodiversity. The feasi-
bility of intercropping hyperaccumulators with
SRC needs to be studied in relation to impedi-
ment of mechanical operations for harvest and
other agronomic practices (fertilisation and
weed control).
It is generally expected that SRC will have
higher water demand than arable crops due to the
higher growth rates, high transpiration rates, and
longer seasonal growth. In some European coun-
tries (i.e. UK), government guidelines require the
plantation of SRC in areas where annual rainfall
is at least 600 mm year −1 (Rowe at al. 2009 ). One
environmental advantage of the high transpira-
tion rate of willow is that the amount of water
removed from the soil by the transpiration stream
can decrease the downward fl ow through the soil
and can reduce leaching losses. In addition, the
perennial nature of SRC, their extensive root