A landscape shaped by toxicity
In the Monaragala district of Sri Lanka's Uva Province, the ground itself presents a hostile environment for most plant life. This region hosts ultramafic rock outcrops, particularly serpentinite, which weather into soils with a uniquely challenging chemical composition. These soils are naturally low in essential plant nutrients like calcium, nitrogen, and phosphorus but contain high concentrations of heavy metals such as nickel (Ni), chromium (Cr), and cobalt (Co).
The ultramafic sites, including Ginigalpelessa and Indikolapelessa, are located along the tectonic boundary between Sri Lanka's Highland and Vijayan Complexes. The soil here can contain nickel concentrations between 6,000 and 10,000 mg/kg, far exceeding the typical soil range of 5 to 500 mg/kg. This harsh chemical environment leads to stunted growth in most trees and results in low overall plant diversity. Yet, this forbidding environment is precisely what has driven the evolution of a specialized and scientifically valuable flora.
The hyperaccumulators
Certain plants have developed the remarkable ability to thrive in these toxic soils by absorbing and storing massive quantities of heavy metals in their tissues, a process known as hyperaccumulation. To qualify as a nickel hyperaccumulator, a plant must store more than 1,000 mg of nickel per kg of its dry tissue. Research in the Monaragala area and other Sri Lankan serpentinite deposits has identified several such species.
The grass Apluda mutica is a dominant species in some areas and a notable nickel accumulator, with concentrations measured between 900 and 2,800 mg/kg. Other identified hyperaccumulators include Hybanthus enneaspermus, Evolvulus alsinoides, and Crotalaria verrucosa. The bio-ore produced by incinerating Crotalaria verrucosa can contain as much as 7,279 mg/kg of nickel.
This unique biological trait is used for phytomining, an experimental green technology. The process involves cultivating hyperaccumulator plants on low-grade ore deposits, harvesting the metal-rich biomass, and incinerating it to produce a "bio-ore" from which the metal can be extracted. This method is low-cost, solar-powered alternative to conventional mining for recovering valuable metals like nickel from otherwise uneconomical deposits.