Pairwise O2 and CO2 in soil studies: using the apparent respiration quotient to partition soil respiration components

Soil CO₂ efflux is the sum of heterotrophic and autotrophic respiration. However, measuring these two fluxes separately in intact forest soils, in high spatiotemporal resolution, is challenging and costly. The apparent respiratory quotient (ARQ), defined as the ratio of CO₂ efflux to O₂ influx, is primarily determined by the stoichiometry of the respiratory substrate and exhibits the potential to differentiate between the components of soil CO₂ efflux. Empirical studies have demonstrated that ARQ is approximately 1 for root respiration, which is associated with carbohydrate metabolism, and 0.7-0.8 for mineral soil respiration, which is associated with decomposition of soil organic matter. In order to explore the use of ARQ in respiration partitioning, we combined a novel continuous measurement of ARQ from soil chambers with laboratory incubations in a temperate pine forest. To test the ability of ARQ to partition soil respiration sources, three different approaches were applied: Firstly, root abundance was controlled by placing chambers along a root density gradient (close and far from tree stems) and by no-root control chambers. Secondly, chamber ARQ was compared with ARQ of the individual soil components. Thirdly, the results were compared to respiration partitioning evaluated by radiocarbon (Δ¹⁴C) measurements. ARQ of individual soil components was highest in roots (0.96 ± 0.01, mean ± standard error), intermediate in litter (0.88 ± 0.03), and lowest in soils from the organic layer (0.83 ± 0.04) and mineral layer (0.83 ± 0.06). The mean ARQ in the soil chambers was 0.91 ± 0.08, and was usually within the range of the individual soil components. This suggests that root respiration contributed 62% to total respiration. Chamber ARQ was higher than the no-roots chambers (0.74 ± 0.05) and higher at closer to the tree stems (0.96 ± 0.07 vs. 0.84 ± 0.09 far from the stems), suggesting that root respiration percentage from total respiration was 72% near the stems and 21% far from the stems. For comparison, according to Δ¹⁴C, root respiration’s contribution was 55% and 14% for close and far from the stems, respectively. On a diurnal timescale, soil CO₂ efflux was synchronized with air temperature, while ARQ exhibited an out-of-phase relationship with air temperature, with higher values recorded during the night than daytime (0.97 ± 0.06 vs. 0.85 ± 0.10, respectively). The elevated nocturnal ARQ may be attributed to greater temperature sensitivity in mineral soil respiration than in root respiration, reduced ARQ during daylight hours due to transport of root-respired CO₂ in the xylem stream, or increased root oxidation with elevated ARQ during night. Overall, our primary results indicated that ARQ is a cost-effective approach to disentangle respiratory sources at seasonal and diurnal scales.