The Effect of Soil Bacteria and Perlite on Plant Growth and Soil Properties in Metal Contaminated Samples

In order to achieve remediation of contaminated substrates, phyto-extraction in pot experiments utilizing lettuce seedlings as universal accumulator plants was investigated. As test substrates, mine tailings from Shiheung and Okdong mines in Korea (particularly high in Pb, Zn, Cu, and Cd), as well as samples from historic mining site at Oberzeiring in Austria (particularly high in Pb, Sb and As) were used, and compared with adjacent farmland soils. After 21 days of growth in the test substrate, the lettuce plants were harvested, and the adjacent soils parted in bulk and root soils. Special soil bacteria, adapted to high Cd levels (Exiguobacter sp.) and capable of adsorbing large amounts of cadmium from solution, as well as perlite (Samson Perlite Inc.) were added to the test substrates before plant growth. Speciation changes in the solids were investigated by sequential leaching, utilizing neutral MgCl₂ (exchangeable), 0.16 M acetic acid, hydroxylamine pH 2, oxalate pH 3, H₂O₂ oxidation, and reflux with aqua regia. Plant growth induced differentiation between root and bulk soils, the differences were more pronounced for the non-contaminated controls. The iron-hydroxide phase increased about 30%, and also the amount of iron-hydroxide bound Be, Cd, Co, Cu, Mg, Mo, Sb and V concentrations, coming mainly from less mobile fractions. The Mn hydroxide phase, however (hydroxylamine), remained rather constant. After plant growth, the root soils were significantly lower in available P, and significantly higher in available Ca, Mn, and Na than the corresponding bulk soils. Addition of Cd-adapted soil bacteria led to a severe decrease of plant yield, but metal uptake changed in both directions. Exchangeable P in both root and bulk soil decreased, and Be, Co, Cr, Fe, K, Li, Mg, Mn, Ni, and Sr in the residual organic fraction increased. This can be interpreted as competition for nutrients, dissolution of residuals by bacterial action, and adsorption to a tightly bound biomass. Addition of perlite hardly affected the plant yield, and again metal uptake changed in both directions, but led to a decrease of siderophilic elements in the Fe- and Mn hydroxides of the bulk soil. In the root soil, perlite addition above all decreased available K, P and As, with respect to the untreated samples. Bacteria addition to perlite treated soils shifted some elements from weak acid mobile towards less available fractions.