While litter traits have been used for decades to predict decomposition rates through the Leaf Economics
Spectrum (LES) acquisitive to conservative trait lens, litter trait and litter mixture effects on soil carbon (C)
priming effects (PE) is less known. To assess whether the LES can predict soil C PE during the decomposition of
litter mixtures from complex ecosystems, a 99-day incubation experiment was conducted. The experiment
involved soil and leaf litter from three Canadian riparian land uses (grasslands, deciduous agroforests, and
coniferous agroforests), incubated as single plant species, and as plant species mixtures in their natural proportions.
Natural abundance δ13C-isotopic analysis was used to assess the proportion of CO2 derived from leaf
litter versus soil C mineralization at initial (0–21 days, stage I) and later (22–99 days, stage II) stages of
decomposition. Single species positions on the LES mirrored litter-C mineralization dynamics, yet PE emissions
remained harder to predict with LES traits. Nonetheless, higher PE emissions relative to litter-C mineralization
were observed with litter from LES species expressing conservative traits compared to LES species expressing
acquisitive traits. Leaf litter mixtures from both agroforests, expressing high leaf litter trait functional dissimilarity,
resulted in synergistic, non-additive effects on soil PE. These findings contribute to our understanding of
the effects of leaf litter functional traits and trait mixing on soil PE, highlighting that mixed-species litter
decomposition can lead to non-additive effects on soil C dynamics in forested riparian ecosystems.