1 Inherent organic compounds in biochar – their content, composition and potential toxic effects Wolfram Buss a , Ondřej Mašek a *, Margaret Graham b , Dominik Wüst c a UK Biochar Research Centre, School of Geosciences, University of Edinburgh, Crew Building, Alexander Crum Brown Road, Edinburgh, EH9 3FF, UK b School of Geosciences, University of Edinburgh, Crew Building, Alexander Crum Brown Road, Edinburgh, EH9 3FF, UK C Institute of Agricultural Engineering, Conversion Technology and Life Cycle Assessment of Renewable Resources, University of Hohenheim, Garbenstrasse 9, 70599 Stuttgart, Germany *Correspondence: Ondřej Mašek, tel. 0131 6505095, fax +44 131 662 0478, e-mail: ondrej.masek@ed.ac.uk Abstract Pyrolysis liquids consist of thermal degradation products of biomass in various stages of its decomposition. Therefore, if biochar gets affected by re-condensed pyrolysis liquids it is likely to contain a huge variety of organic compounds. In this study the chemical composition of such compounds associated with two contaminated, high-volatile organic compound (VOC) biochars were investigated and compared with those for a low-VOC biochar. The water-soluble organic compounds with the highest concentrations in the two high-VOC biochars were acetic, formic, butyric and propionic acids; methanol, phenol, o-, m- and p-cresol, and 2,4-dimethylphenol, all with concentrations over 100 µg g - 1 . The concentrations of 16 US EPA PAHs determined by 36 h toluene extractions were 6.09 µg g -1 for the low-VOC biochar. For high-VOC biochar the total concentrations were 53.42 µg g -1 and 27.89 µg g -1 , while concentrations of water-soluble PAHs ranged from 1.5 - 2 µg g -1 . Despite the concentrations of PAHs exceeding biochar guideline values, it was concluded that, for these particular biochars, the biggest concern for application to soil would be the co- occurrence of VOCs such as low molecular weight (LMW) organic acids and phenols, as these can be highly mobile and have a high potential to cause phytotoxic effects. Therefore, based on results of this study we strongly suggest for VOCs to be included among criteria for assessment of biochar quality.