Delimiting soil chemistry thresholds for nickel hyperaccumulator plants in Sabah (Malaysia)
Résumé
Nickel hyperaccumulator plants have been the
focus of considerable research because of their unique
ecophysiological characteristics that can be exploited in
phytomining technology. Comparatively little research has
focussed on the soil chemistry of tropical nickel hyperaccumulator
plants to date. This study aimed to elucidate
whether the soil chemistry associated with nickel hyperaccumulator
plants has distinctive characteristics that could
be indicative of specific edaphic requirements. The soil
chemistry associated with 18 different nickel hyperaccumulator
plant species occurring in Sabah (Malaysia) was
compared with local ultramafic soils where nickel hyperaccumulator
plants were absent. The results showed that
nickel hyperaccumulators in the study area were restricted
to circum-neutral soils with relatively high phytoavailable
calcium, magnesium and nickel concentrations. There
appeared to be a ‘threshold response’ for the presence of
nickel hyperaccumulator plants at[20 lg g-1 carboxylicextractable
nickel or [630 lg g-1 total nickel, and [pH
6.3 thereby delimiting their edaphic range. Two (not
mutually exclusive) hypotheses were proposed to explain
nickel hyperaccumulation on these soils: (1)
hyperaccumulators excrete large amounts of root exudates
thereby increasing nickel phytoavailability through intense
rhizosphere mineral weathering; and (2) hyperaccumulators
have extremely high nickel uptake efficiency thereby
severely depleting nickel and stimulating re-supply of Ni
from diffusion from labile Ni pools. It was concluded that
since there was an association with soils with highly labile
nickel pools, the available evidence primarily supports
hypothesis (2).