Difficulties in using soil-based methods to assess plant availability of potentially toxic elements in biochars and their feedstocks.

The use of biochars in agriculture to improve soil function and carbon sequestration is expected to increase into the future. We aimed to identify the most suitable chemical extractants for the risk assessment of potentially toxic element (PTE) availability in biochars produced from a range of feedstocks, and to investigate the changes in PTE extractability that occur as a result of feedstock pyrolysis using five common extraction methods. We evaluated these methods with regard to their ability to predict PTE phytoavailability in four different biochars against metal uptake by wheat. No single extractant significantly correlated well with ≥ 4 PTEs from the 10 examined, highlighting that the availability and binding mechanism of individual PTEs differed by biochar type. Commonly used PTE extraction methods need to be reviewed for use with biochars, and that some biochars may be able to stabilise PTEs, reducing risks of contamination upon land application.