Sustainable application of biochar to soil requires careful consideration of sorbed mobile compounds that can be released into the environment and their relationship with bulk chemical properties. Seven biochars were produced by pyrolysis of corn stalk from 350 to 650 °C, and residual volatile organic compounds (VOCs) were investigated by head space solid-phase microextraction and gas chromatography–mass spectrometry. Over 80 compounds were detected in poorly carbonized biochars, including thermal degradation products of lignin (2-methoxyphenols), cellulose and hemicellulose (1,4:3,6-dianhydro-β-d-glucopyranose, C1–C3 furans and furanones, C1–C2 cyclopentenones), lipids (aromatic and C2–C8 aliphatic acids), and proteins (aromatic nitriles, amides). The presence of potentially harmful compounds, such as benzene, toluene, ethylbenzene, xylenes, phenols, volatile fatty acids, and polycyclic aromatic hydrocarbons, highlights the importance of controlling the biochar production process. Statistically significa...