Delimiting soil chemistry thresholds for nickel hyperaccumulator plants in Sabah (Malaysia)

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 μg g^?1 carboxylic-extractable nickel or >630 μg 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).