Decomposition of soil organic matter (SOM) may either increase or decrease after fresh organic inputs, the phenomena which are termed as “priming effect”. Crop residues and labile C additions can prime SOM decomposition, but it is not known how labile C inputs affect SOM in the presence of decaying plant residues varying in quality (e.g. from previous crops, a common situation in arable soils). We used a dual 13C/14C isotopic labelling to partition soil CO2 efflux and microbial biomass for three C sources: labile C (glucose), partly decomposed wheat residues (leaves and roots) and SOM. 14C-labelled glucose was added to the soil after 30 days of pre-incubation with 13C-labelled residues (separately leaves or roots). After glucose addition, the leaf residue decomposition rate declined by up to 65%, while roots remained unaffected. Despite the differences between residue decomposition rates, the quantity of primed SOM remained similar between leaf and root residue treatments after the addition of glucose. Glucose alone caused cumulative positive SOM priming of 193 μg C g−1 soil over 90 days, corresponding to 60% of SOM decomposition without addition. Addition of glucose to soil together with partly decomposed plant residues induced up to 45% higher SOM priming than single residues priming effect (∼250 μg C g−1). Remarkably, this priming effect induced by glucose and residues was only due to intensive SOM decomposition during the first 18 days. On the subsequent period (after 18 days of glucose), decline in SOM priming and increase in residue decomposition indicate a shift in microbial activity i.e. from active-to slow-growing microbes. Glucose addition strongly increased the proportion of microbial biomass from SOM but decreased the proportion from residue C, suggesting a preferential use of SOM over plant residues following glucose exhaustion. These results are consistent with the view that labile C inputs induce SOM priming and suggest for the first time, that labile C controls the intensity and decomposition rate of both SOM and decaying plant residues. Concluding, irrespective of the quality of partly decomposed residues, input of labile C (e.g. through rhizodeposition) has overall an additive effect in increasing decomposition of SOM. Such studies of interactions between pools and identification of three C sources were only possible by the application of an innovative dual 13C/14C labelling approach.