Globally, the constrained stoichiometry of living organisms is responsible for the coupling of carbon (C) and nitrogen (N) in living organic matter, with afterlife effects in dead organic matter. This coupling has long been thought to foster trade-offs among several key functions of ecosystems, such as soil fertility and C sequestration. However, while there is evidence for a general coupling of C and N cycling in ecosystems, there are many ways in which these cycles also diverge, both temporally and spatially, under the influence of multiple biotic and abiotic drivers. Here, focussing on the role of plant residues in feeding and steering the C and N cycles in soil, we examine how the C:N stoichiometry of two types of litter (leaf and root) of 12 herbaceous species varies during decomposition and how these elements end up as stabilized (mineral-associated organic matter, MAOM) or more bioavailable forms of C and N (particulate organic matter, POM). Irrespective of the initial chemistry or litter type of all 24 plant residues, the C:N stoichiometry of all materials converged along the decomposition process. Consequently, there were major differences in the C:N stoichiometry of compounds released during decomposition, from low C:N early on to very high C:N at later stages, indicating a major temporal decoupling of C and N cycling during decomposition processes. Overall, the decomposition process of our 24 litters led to major differences in the fate of C and N in soils. As expected, a trade-off was observed between the litter ability to increase soil N availability versus soil C storage (POM-C and MAOM-C), with more recalcitrant litters (high [lignins], low leachate [N] and [C]) favouring soil C storage over N availability. This pattern was strongly driven by the contrasting biochemistry of leaf versus root litters, suggesting that leaf and root litters are highly complementary in the way they contribute to these two fundamental functions of ecosystems, leaves playing a major role in soil N availability, whereas roots contribute disproportionately to soil C storage. This trade-off remained strong even when correcting for the shear effect of higher C versus N content in initial litter, indicating that the type of biochemical compounds contained in litter is as important as the relative proportion of C versus N elements. Orthogonal to this trade-off, substantial variation in the ability of plant litters to improve both soil N availability and C storage was nonetheless visible among both leaves and roots, suggesting interesting perspectives in agroecosystems for selecting and growing versatile plant species capable of influencing positively several ecosystem functions.