Short-term biochar-induced increase in soil CO2 release is both biotically and abiotically mediated

► Biochar induces a short-term burst in soil respiration. ► Biochar alters soil physical properties but this does not change soil CO2 release. ► Both dissolved inorganic and organic C released from the biochar cause the observed increase in CO2 release. ► Biochar significantly reduces the breakdown of native soil organic matter (i.e. negative priming). ► The short-term burst in soil CO2 efflux is small relative to the C sequestered in soil and biochar.