Heavy metal phytoavailability in a contaminated soil of northeastern Oklahoma as affected by biochar amendment

High concentrations of heavy metals (HM) in soils have negative impacts on plants, human health, and the environmental quality. The purpose of this study was to evaluate the effects of biochars on the bioaccessibility of Zn, Pb, and Cd in a contaminated soil in the Tar Creek area of NE Oklahoma, as well as on the growth and uptake of these elements by perennial ryegrass (Lolium perenne). Biochars were produced from switchgrass (SGB) and poultry litter (PLB) feedstocks at 700 °C and applied to the soil at 0.0, 0.5, 1.0, 2.0, and 4.0% (w/w), with three replications. Regardless of the feedstock, both soil organic carbon (SOC or OC) and pH increased as the rates of biochars increased, which significantly decreased the HM bioaccessibility (p < 0.01). The Zn and Cd extracted by DTPA were highly correlated (p < 0.0001) with their concentration in ryegrass shoots and roots. Except for some significant positive correlations (p < 0.05), HM concentrations in ryegrass shoots and roots were not correlated with their biomass (p > 0.05). Both bioconcentration factor (BCF) and transfer factor decreased as the rates of biochars applied increased, especially for Pb and Cd (p < 0.01). Our results suggest it is beneficial to use biochars at Tar Creek as a soil amendment to reduce HM bioaccessibility and metal uptake by ryegrass.