Early- and later-stage priming effects induced by spruce root fractions are regulated by substrate availability, stoichiometry and C input

•Soluble fraction addition caused an immediate and short-lasting priming effect (PE)•Insoluble fraction addition induced a gradual and long-lasting PE.•Substrate availability controls the early-stage PE via substrate-derived labile C.•Substrate C:N ratio and C input regulate the later-stage PE via C use efficiency.•High substrate availability and C:N ratio correspond to a small net C accumulation.