Cleansing soils high in toxic metals using shrub willows
CALS Impact Statement
Salix (willow) species have been documented to possess an unusual capability to extract certain heavy metals, particularly cadmium and zinc, from soil and bioconcentrate them. We are investigating several aspects of this ability in the field and greenhouse. Specifically, we are measuring the potential for zinc and cadmium accumulation and metal tolerance of a number of native and non-native shrub willow species grown in hydroponic systems. Once we determine the genetic potential of these willows for toxic metal accumulation, we will field-test the potential of shrub willows to extract soil cadmium and zinc from a well characterized, metal-contaminated field site. The high sulfur concentration in willow leaf tissue that we have measured at some sites is also of interest because of its potential toxicity to ungulate animals. Therefore, we are studying the extent to which shrub willow is able to concentrate sulfur from soils to determine whether sulfur uptake is related to heavy metal uptake.
Soil contamination by toxic heavy metals, while well documented in inner-city and industrial areas, is a widespread and growing problem in agricultural regions as well. Agricultural practices have historically increased the soil burden of heavy metals through numerous amendments (phosphate fertilizers containing cadmium, high-copper fungicidal sprays, insecticides containing lead and arsenic, animal manures enriched in copper and zinc, and sewage sludges high in numerous toxic metals). Phytoremediation (using plants to extract toxins from soils) is a low-cost, low-impact approach to soil cleanup that can reverse the gradual buildup of toxic metals in soils.
We have measured the cadmium, zinc, and sulfur concentrations in willow growing in zinc- contaminated and uncontaminated soils as well as under the greenhouse-controlled conditions of hydroponics, and have determined the potential of 14 different cultivars of willow to extract zinc, cadmium, and sulfur from soils. Several of these show excellent potential, both because of their vigorous growth and high yield potential, but also because of their relative tolerance to high concentrations of zinc and cadmium in their leaves without significant growth reduction.
The development of phytoremediation technology depends on field-testing of plant species with potential to extract toxins from soil. Our research is providing at least one commercial enterprise that supplies plant materials for this purpose with data that will allow them to better assess the real value of this technology. Our initial tests show two superior cultivars well suited to the purpose of phytoremediation.