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 (ARQsa), and in headspace air from incubations of bulk-soil (ARQbs) and tree stem-tissues (ARQts for fresh tissues; ARQts24 after 24-h storage) in 10 measurement campaigns over 15 months in a Mediterranean oak forest. Mean (range) values were: ARQsa = 0.76 (0.60-0.92), ARQbs = 0.75 (0.53-0.90), ARQts = 0.39 (0.19-0.70), and ARQts24 = 0.68 (0.42-1.08). Both ARQts and ARQts24 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 ARQts) can explain the results. The mean ARQbs (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 ARQbs to 0.37, at the most, but estimated to have small effect under typical respiration rates. ARQsa was usually higher than ARQbs 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.

Forests exchange CO and O with the atmosphere at similar molar ratios. Correspondingly, the apparent respiratory quotient (CO/O flux ratio, ARQ) is expected to be ≈1 given the stoichiometry of organic substrates in soils and plants. However, measured ARQ values often deviate from ≈1, and it is still unclear how CO and O fluxes are balanced among ecosystem components, and what are the sources of ARQ variability. Here we measured ARQ of soil pore space air (ARQ), and in headspace air from incubations of bulk-soil (ARQ), tree stem-cores (ARQ) and roots in 10 measurement campaigns over 15 months in a Mediterranean oak forest. Mean (range) values were: ARQ = 0.76 (0.60-0.92), ARQ = 0.75 (0.53-0.90), and ARQ = 0.39 (0.19-0.70). As expected, ARQ was usually higher than ARQ and lower than the ARQ of incubated roots (range of 0.73-0.96). Variability in ARQ was correlated with soil moisture parameters. Temperature positively correlated with ARQ and ARQoutside the growing season. Abiotic O uptake by Fe was demonstrated to reduce ARQ, but this effect would be significant under field conditions only if respiration rates are very low. We hypothesize that low measured ARQ values likely result from selective decomposition of reduced compounds and physical protection of oxidized compounds. ARQ, measured at two stem positions, was lower than expected from oxidation of any possible substrate, indicating partial retention of respired C. The overall ARQ <1 reveals an imbalance of stem-soil CO and O fluxes that is unexpected at the ecosystem level.