Biochar effects on uptake of cadmium and lead by wheat in relation to annual precipitation: a 3-year field study

Biochar has been widely studied for its ability to reduce plant uptake of heavy metals by lowering metal bioavailabilities through adsorption and pH-driven fixation reactions. However, the long-term effect of biochar on heavy metal bioavailabilities in alkaline soils under natural redox condition is rarely studied. Here, we report a study examining the effects of biochar on bioavailability and partitioning of cadmium (Cd) and lead (Pb) among different soil fractions over 3 years in a field study with wheat (Triticum aestivum L.). Plots were established on two similar soils having low and high levels of contamination, both of which were amended in the first year with wheat straw biochar at 0, 20, and 40 t ha^-1. Precipitation patterns varied greatly over the study period, with 2014 having record drought, which was followed by 2 years having extreme flooding events. Results showed a significant increase in grain yield and reductions in Cd and Pb concentrations in wheat grain in the biochar-amended soils in 2014. In contrast, bioavailable (exchangeable) heavy metal concentrations and plant uptake of Cd and Pb were significantly higher in the subsequent very wet years in 2015 and 2016, where the effects of biochar were much more variable and had an overall lesser effect on reducing heavy metal uptake. The results suggest that fluctuations in soil pH and redox caused by periodic drought and flood cycles strongly drive metal cycling through mobilization and immobilization of metals associated with different mineral phases. Under these conditions, biochar may have reduced efficacy for reducing heavy metal uptake in wheat.