Effects of biochar on heavy metal bioavailability and uptake by tobacco (Nicotiana tabacum) in two soils

Biochar, as a soil conditioner, not only improves soil fertility but also changes the bioavailability of heavy metals in soil-crop systems, which may lead to certain environmental issues. To investigate the effect of biochar on the bioavailability of heavy metals (Cd, Cu, Ni, and Pb) in soils, a two-year field experiment with five biochar rates (0, 5, 15, 20, and 40 Mg ha⁻¹) was conducted in yellow-brown soil (pH 5.32) and yellow soil (pH 6.39) in southwestern China with tobacco as an example crop. The results showed that, in the acidic soil, available Cd content increased with biochar rate when the rate exceeded 15 Mg ha⁻¹, whereas in neutral yellow soil, the Cd content increased significantly only at the rate of 40 Mg ha⁻¹. Biochar application did not cause any significant increase in Cd content or accumulation in the crop. The soil available Cu, Ni, and Pb contents decreased with increasing biochar rate, and the decrement varied with soil pH and time after biochar application. Cu, Ni and Pb contents and their accumulations in tobacco decreased with higher biochar application rate. The decreases in soil available Cu, Ni and Pb contents may result from immobilization of these heavy metals by electrostatic attraction, ion exchange, surface precipitation, complexation, while the increase in soil available Cd content resulted from high Cd content in feedstock of the biochar. The biochar derived from tobacco stem contained 7.63 mg kg⁻¹ Cd, which added 6.20, 30.0, 93.0, 124, and 248 ug kg⁻¹ to the top 10 cm soils for the application rate of 1, 5, 15, 20, and 40 Mg ha⁻¹, respectively. It is important to identify heavy metal contamination in biochar before it is used as a soil amendment. In conclusion, biochar application can decrease soil available and plant total Cu, Ni, and Pb content, and the degrees of decrease varies with soil pH and biochar application rate. Meanwhile, we should be careful about the possible heavy metal pollution from feedstock of biochar.