Mycorrhizal fungi generally enhance decomposition through the priming effect rather than reduce it via the Gadgil effect. Analyzing 43 studies, mycorrhizal fungi increased substrate decomposition by 6.7%, with variations influenced by substrate C:N ratio and absolute latitude. The type of mycorrhizal fungi did not significantly affect this outcome. These findings suggest a global tendency for mycorrhizal fungi to promote decomposition, potentially impacting soil carbon storage.