Biochar and its combination with inorganic or organic amendment on growth, uptake and accumulation of cadmium on lettuce

Biochar as an environmentally friendly soil amendment, has attracted widespread attention and in combination with other amendments is considered to be more conducive to crop growth, soil fertility, and crop safety production. However, a comprehensive understanding of the differences among different biochars, biochar with inorganic amendment, and biochar with organic amendment with respect to soil health, plant growth, and Cd bioavailability or phytoavailability is necessary. Therefore, a sixty-day pot experiment was conducted to investigate the effects of five biochars derived from rice husk (RH), pig manure (PM), sewage sludge (SS), corn straw (CS), and wood chip (WC) and their respective combination with lime or compost on cadmium (Cd) immobilization and adsorption as well as physicochemical features and enzyme activities in contaminated soil. The results revealed that the sole application of PM, SS, and CS biochars had a positive impact on lettuce growth and photosynthetic pigments. In addition, mixing PM or SS biochar with compost further promoted lettuce growth due to higher soil fertility and enzyme activities. However, as compared to unamended soil, CS biochar-compost combination, RH, PM, SS, and CS biochar-lime combination highly reduced lettuce plant height and fresh weight. Furthermore, the application of amendments with biochar-lime mixtures greatly reduced Cd bioavailability. X-ray diffraction analysis and Fourier Transform Infrared spectra revealed that Cd immobilization mainly involved complexation and co-precipitation on mineral surfaces as well as hydroxyl or carboxyl functional groups in amendments. In comparison to a single application and biochar-lime combination, the biochar-compost combinations i.e RH, PM, SS, and WC were more effective in lowering Cd translocation and bioaccumulation in lettuce.