The CO₂/O₂ fluxes ratio (ARQ) measured in soils and plants contains valuable information about the respiratory-substrate stoichiometry and biotic and abiotic non-respiratory processes reacting with the gases. For meaningful use in biogeochemical studies it is necessary to resolve the substrate and processes effects. In addition, unique ARQ signatures can be used to weight contributions to soil respiration. We investigated these uses by measurements in soil pore space air (ARQ_sa), and in headspace air from incubations of bulk-soil (ARQ_bs) and tree stem-tissues (ARQ_ts for fresh tissues; ARQ_ts24 after 24-h storage) in 10 measurement campaigns over 15 months in a Mediterranean oak forest. Mean (range) values were: ARQ_sa = 0.76 (0.60-0.92), ARQ_bs = 0.75 (0.53-0.90), ARQ_ts = 0.39 (0.19-0.70), and ARQ_ts24 = 0.68 (0.42-1.08). Both ARQ_ts and ARQ_ts24 were below 1.0, the value expected for carbohydrate respiration in plants. Involvement of non-respiratory processes like non-phototrophic CO₂ re-fixation and wound-response O₂ uptake (for ARQ_ts) can explain the results. The mean ARQ_bs (0.75) probably represents the stoichiometry of the respiratory substrate, which is lower than expected using bulk soil organic matter (SOM) stoichiometry (~0.95), suggesting a labile, less oxidized, SOM pool contributes more to respiration fluxes. Abiotic O₂ uptake by Fe²⁺ was demonstrated to reduce ARQ_bs to 0.37, at the most, but estimated to have small effect under typical respiration rates. ARQ_sa was usually higher than ARQ_bs and lower than root ARQ (which, when measured, ranged from 0.73-0.96), demonstrating the potential of ARQ to partition the autotrophic and heterotrophic sources of soil respiration.