Effect of bamboo and rice straw biochars on the bioavailability of Cd, Cu, Pb and Zn to Sedum plumbizincicola

Abstract

Soil contamination with heavy metals has become a global concern because of its adverse effects on ecosystem health and food security. Soil amendments including biochar can reduce the bioavailability of heavy metals in contaminated soils and reduce their risk of entering the food chain. A pot experiment was conducted to investigate the effects of biochars derived from bamboo and rice straw on bioavailability and plant growth in a sandy loam paddy soil naturally co-contaminated with Cd, Cu, Pb and Zn. The soil was moderately acidic (pH=5.7) and low in organic carbon content (8.7gkg−1). Bamboo and rice straw biochars, pyrolyzed at temperatures≥500°C and with two mesh sizes (< 0.25mm and<1mm), were applied at three rates (0, 1% and 5%, w/w). A metal-tolerant plant, Sedum plumbizincicola X. H. Guo et S. B. Zhou sp. nov. was used in the plant growth experiment to examine the bioavailability of these metals. The addition of biochars to soil significantly (p<0.05) increased the above-ground biomass of S. plumbizincicola. By the end of the experiment, soils amended with biochar had pH values significantly (p<0.05) higher, this effect being more accentuated at the high biochar dose and small particle size. The solubility of Cd, Cu, Pb, and Zn as measured by Toxicity Characteristic Leaching Procedure (TCLP) was significantly lower (p<0.05) in the biochar-amended soils than in the control soil. This was paralleled by significant reductions in Cd, Cu, Pb and Zn accumulated in the above-plant biomass of amended soils. Rice straw biochar reduced the concentration of Cu and Pb in the shoots by 46 and 71%, while bamboo biochar reduced concentration of Cd in the shoot by 49%. Finer biochar was more effective on reducing the concentrations of Zn in shoot than the coarse ones, while particle size had no effect on the concentrations of Cd, Cu and Pb in the shoot of S. plumbizincicola. In conclusion, the influence of biochar on heavy metal bioavailability varied not only with the feedstock and application rate of biochars, but also with the metal species. Therefore, biochar should be carefully designed to maximize the reduction of the bioavailability of a given heavy metal in soil.