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Soil respiration (Rs) is the largest carbon (C) flux from terrestrial ecosystems to the atmosphere and is of great significance to the global C budget. An increasing number of studies have assessed Rs through in situ observations and model estimates over the last decades, but the sources and pathways of soil carbon dioxide (CO2) are not fully understood, and great uncertainty remains in Rs partitioning of soil CO2 sources. Here, we compiled 236 paired observations that measured soil CO2 fluxes after concurrently removal of living roots (and rhizosphere), litter, and both roots and litter in plant input manipulation experiments conducted at 14 forest sites to partition root + rhizosphere (Rr), litter (Rl) and soil organic matter-derived microbial respiration (Rm) in total soil CO2 flux. We found that Rr, Rl and Rm accounted for 20.1%, 21.8% and 62.7% of the total Rs, respectively. Mean annual precipitation (MAP) was the most important factor driving Rr/Rs, Rl/Rs and Rm/Rs, and MAP was positively correlated with Rr/Rs and Rl/Rs but negatively correlated with Rm/Rs, suggesting a significant climatic control over the proportions of Rs components. Across all sites, the proportions of Rr/Rs and Rl/Rs increased but Rm/Rs decreased with the increase in soil CO2 flux, suggesting that the proportions of root- and litter-derived soil CO2 are generally higher in the tropics than in cold temperate and boreal forests. More accurate partitioning of Rr, Rl and Rm to elucidate different sources and pathways of soil CO2 flux will provide important insights for the global Rs assessment and terrestrial C budget.

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