Anaerobic digestion (AD) delivers renewable energy but generates large digestate streams that remain difficult to manage. Pyrolysis can transform solid digestate into biochar, yet how digestate origin controls biochar properties and integrated use with liquid digestate (LD) remains insufficiently understood. This study pyrolysed agri-residue, food waste, and sewage sludge digestates at 650 °C, characterized the resulting biochars, and evaluated biochar-only, LD-only, and biochar + LD treatments in a short-term greenhouse bioassay in a soil–plant system. Feedstock origin strongly shaped biochar composition and functional potential. Agri-residue digestate biochar was carbon-rich (63.20%) and enriched in N, K, and Mg; food waste digestate biochar showed the strongest alkalinity and salinity signature (pH 11.94; EC 2.26 dS m−1), whereas sewage sludge digestate biochar had the highest yield (52.99%), ash content (77.91%), surface area (30.40 m2 g−1), total P and trace-element loading. In the greenhouse bioassay, LD was the dominant driver of short-term physiological performance and biomass production, and biochar co-application modified the response in a feedstock-dependent manner. Relative to LD alone, agri-residue digestate biochar + LD significantly increased fresh biomass by approximately 29%, whereas food waste and sewage sludge digestate biochars provided limited growth benefit. These results demonstrated that digestate-derived biochars are not interchangeable amendments and should be selected according to feedstock-specific carbon, mineral, alkalinity, salinity and contaminant-risk profiles for integrated solid–liquid digestate valorisation.