Page 161 - Plant-Based Remediation Processes

152 T. Vamerali et al.

Brassicaceae, a botanical family which should be exploited in phytoremediation as
it also includes several hyperaccumulators (Kra ¨mer 2010). The biomass reached by
radish in our waste was obviously poor when compared with the uncontaminated
reference. Its productivity was similar to that of the small-biomass yielding
hyperaccumulator T. caerulescens (Anderson et al. 1999), but it is expected to
have greater efficiency in pluri-contaminated sites. Indeed, fodder radish has been
used to good effect in our past experiments in a pluri-contaminated agricultural soil
near Milan, showing better shoot and root growth than controls and better
phytoextraction than other Brassicaceae, such as Indian mustard, oilseed rape
(B. napus L. var. oleifera D.C.) and garden cress (Lepidium sativum L.) (Mosca
et al. 2004). The behaviour of fodder radish below-ground was favourable; its
RLD in the 0.3-m layer being similar to that of controls. There was no significant
‘species tillage’ interaction and, unfortunately, much of the root length was
confined to the top 0.1-m; root distribution was similar between tillages in sun-
flower and alfalfa, whereas fodder radish and Italian ryegrass positively moved a
greater fraction of length (~15 %) downwards (below 0.1 m depth) with ripping.
At this point, the question which arose was: which are the criteria allowing
selection of efficient herbaceous species in pyrite? A partial answer comes from the
morphological features of roots. For instance, better translocation of metals from
2
roots to shoots were related to high specific root length (R ¼ 0.21), whereas the
maximum root length (Italian ryegrass), although correlated with shoot metal
2
concentrations (R ¼ 54 %), is probably an invalid criterion when considered
alone. The greater relative RLD and above-ground productivity with respect to
the reference soil, as occurred in fodder radish, seem to provide better all-round
criteria. The response of woody species was also variable in terms of growth and
metal accumulation. Compared with controls, the wood production of white poplar
was much less affected than that of black poplar ( 65 % vs. 91 %), with small
differences between tillages (Fig. 8.5). In general, As and Pb had not accumulated
in twigs, Cu was low and Co was just above the detection limit (Table 8.2). Our tree
species confirmed initial observations on spontaneous Salicaceae, only Zn having
interesting levels, especially after ripping, i.e., 94 vs. 75 mg kg 1 of ploughing
(means). We detected a negative correlation between biomass and concentrations of
metals, so that white poplar yielded the best biomass and European aspen the
highest metal concentrations. Also for trees, the expected removals were basically
poor, although more precise phytoextraction balances depend on plant density of
short-rotation coppices and rotation cycle.
When working on pyrite improvement, the first requirement is undoubtedly
increased yields and, secondly, increased metal concentrations and translocation.
Substantial productivity gains can be achieved with abundant fertilisation
(Marchiol et al. 2007) and small plant sizes suggest that plant density could be
increased, at least for wide-spaced crops like sunflower. Grasses could not tolerate
frequent cutting in the contaminated area because of insufficient growth after the
first harvest, as in Italian ryegrass and alfalfa. Undoubtedly, the latter was disad-
vantaged by the absence of root nodulation with Rhizobium and by particularly high
shoot As, like Salicaceae, which lacked mycorrhization (data not shown).