Biosolids-Derived Trace Metals in Soils and Their Potential Bioavailability in New Zealand Pasture Soils.

Beneficial use of municipal biosolids is an environmentally desirable alternative to disposal, as these materials can meet crop needs for macronutrients and build soil organic matter.  Undesirable outcomes include the potential accumulation of trace metals which may subsequently become bioavailable.  Five soil types that represent common pasture lands in New Zealand were used to prepare undisturbed soil monoliths for use in a lysimeter experiment.  Biosolids from three municipalities were applied each year for five years while soil concentrations were monitored in the surface layer.  After five years, surface soils were collected to determine the bioavailability of metals to spring wheat (T. aestivum).  Compared to the control, biosolids significantly increased total soil concentrations for Cr, Cu, Ni, Pb, and Zn.  Wheat shoot concentrations increased for Cd, Cu, and Zn.  Mild salt (CaNO₃) extracts performed better predicting metals uptake by wheat compared to the total soil concentration.  Diffusive gradients in thin films (DGT) were used and compared to other soil extracts and soil pH as potential predictors of bioavailability.  DGT theory accounts for dissolved metals as well as the pool of bound metals that could be dissolved as the soil solution concentration is depleted by plant roots, and thus represents an “effective concentration” of both solution and bound metals.  In general, the mass taken up by wheat represents less than half of the mass of metal measured in the soil solution.   For the duration of the bioassay, it appears that plants do not significantly stress the solution concentrations, and it appears the effective concentration might not extend the bioavailability prediction compared to simple soil extractions.    Despite significant increases in total soil concentration of Cr, Ni, and Pb, none of these metals appeared in increased concentrations in wheat shoots.