Residues of bioenergy production chains as soil amendments: immediate and temporal phytotoxicity.

The current shift towards bioenergy production increases streams of bioenergy rest-products (RPs), which are likely to end-up as soil amendments. However, their impact on soil remains unclear. In this study we evaluated crop phytotoxicity of 15 RPs from common bioenergy chains (biogas, biodiesel, bioethanol and pyrolysis). The RPs were mixed into a sandy soil and the seedling root and shoot elongation of lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), and wheat (Triticum aestivum L.) were measured. Immediate phytotoxic effects were observed with biodiesel and bioethanol RPs (root elongation reduced to 14-60% for the three crops; P<0.05). However, phytotoxicity was no longer significant after seven days. Digestates had no phytotoxic effect whereas biochars ranged from beneficial to detrimental depending on the original feedstock and temperature of pyrolysis. Biochar amendment alleviated phytotoxicity of bioethanol by-products for wheat and radish. Phytotoxicity assessment is critical for successful soil amendment with bioenergy RPs.