We are working to understand the role of the soil microbial community in forming and releasing zinc (Zn) and cadmium (Cd) sulfides in a soil naturally very high in these metals. Hotspots of high Zn cause extreme phytotoxicity in horticultural fields in the Elba region. Seasonal wetting and draining exacerbate these problems. Willows grown as hedgerows are tolerant of and take up high concentrations of these metals. Willows and their associated microbial communities could prove useful in phytoremediation studies at this and other sites contaminated with heavy metals.

Naturally occurring peat soils in western New York contain anomalously high concentrations of Zn, Cd, and sulfur (S). In some cases, total soil Zn has been reported near 88,000 mg/kg, and zones of Zn phytotoxicity are frequently reported throughout the area. Under anoxic conditions, metal sulfides are relatively insoluble and immobile and therefore pose little threat to the biota of the system. However, when these agricultural peat fields are drained for planting every spring, thus aerating the system, the metal sulfides become oxidized and release sulfates along with free Zn and Cd into the soil solution in bioavailable forms. High metal concentrations in the soil solution can cause phytotoxicity in specific areas and, if leaching occurs, can contaminate downstream surface waters. Although previous research on this site to date suggests indirectly that microbial activity is responsible for this key process, no studies have focused on the soil microbial sulfur oxidation pathway presumably responsible. This latter subject is the focus of our study.

The study site has been mapped explicitly with a geographical position system and sampled along a grid to interpolate concentrations of zinc, cadmium, sulfur, copper, pH, water content, soil organic matter, and microbial community composition in relation to these factors. Data clearly show a modifying effect of the willow rhizosphere and exacerbating effects associated with seasonal draining. Microbial populations in the rhizosphere of willows in bordering hedgerows are being examined for their roles in modulating Zn and Cd uptake in willow trees.

Understanding the role of willow rhizosphere microbial populations in forming metal complexes that aid in `detoxifying` high metal concentrations in the rhizosphere will aid in developing phytoremediation strategies for sites contaminated with heavy metals. The longer-term impacts of this project will be in informing practitioners about the potential for phytoremediation at these and other affected sites. The willow species we are examining has a high potential for being commercialized to address soil remediation via plant uptake mechanisms and suitable rhizosphere microbial communities.

National Science Foundation

Penn State University