Microscale investigations of soil heterogeneity: Impacts on zinc retention and uptake in zinc-contaminated soils

Metal contaminants in soils can persist for millennia, causing lasting negative impacts on local ecosystems. Long-term contaminant bioavailability is related to soil pH and to the strength and stability of solid-phase associations. We combined physical density separation with synchrotron-based microspectroscopy to reduce solid-phase complexity and to study Zn speciation in field-contaminated soils. We also investigated Zn uptake in two Zn-hyperaccumulating ecotypes of Noccaea caerulescens (Ganges and Prayon). Soils were either moderately contaminated (500- 800 mg Zn kg^-1 via contaminated biosolids application) or grossly enriched (26,000 mg Zn kg^-1 via geogenic enrichment). Soils were separated using sodium polytungstate into three fractions: light fraction (LF) (< 1.6 g cm^-3), medium fraction (MF) (1.6-2.8 g cm^-3), and heavy fraction (HF) (> 2.8 g cm^-3). Approximately 45% of the total Zn was associated with MF in biosolids-contaminated soils. From these data, we infer redistribution to the MF after biosolids application because Zn in biosolids is principally associated with HF and LF. Our results suggest that increasing proportions of HF-associated Zn in soils may be related to greater relative Zn removal by Zn hyperaccumulating plants. Using density fractions enabled assessment of Zn speciation on a microscale despite incomplete fractionation. Analyzing both density fractions and whole soils revealed certain phases (e.g., ZnS, Zn coprecipitated with Fe oxides) that were not obvious in all analyses, indicating multiple views of the same soils enable a more complete understanding of Zn speciation.