Linking root abundance and exudation with soil CO2/O2 fluxes ratio in three crops

Measuring root abundance, root exudation, and exudation chemistry requires destructive and cumbersome measurement protocols. Respiratory CO₂ and O₂ fluxes are simpler to measure and are related to these processes: 1) greater root abundance and exudation of labile compounds are expected to increase respiratory fluxes; 2) the apparent respiratory quotient (ARQ, the CO₂/O₂ fluxes ratio) is primarily derived from the nominal oxidation state of carbon (NOSC) of the respiratory substrate, with high expected ARQ in roots and rhizosphere (fed by exudates) and low ARQ in bulk soil. To test the control of root abundance and exudation on respiration fluxes, we conducted a pot experiment in a greenhouse with three crops (Brassica napus, Helianthus annuus and Panicum miliaceum). We measured CO₂ and O₂ fluxes from soil chambers and from jar incubations of excised roots and soil samples. Additionally, we measured root abundance and exudation rates and characterized exudate composition using untargeted direct-infusion Orbitrap mass spectrometry. Molecular formulas were assigned to derive NOSC and stoichiometric indices (H/C, O/C, N/C and P/C). We found that the species differed in their CO₂ and O₂ fluxes, reflecting their distinct root traits. Higher respiration fluxes were associated with greater root abundance and exudation rates, but a higher exudate N/C ratio was the strongest predictor. While the effect of species on ARQ was insignificant, small and consistent interspecific differences in ARQ were observed. Brassica napus exhibited comparatively high ARQ values, coinciding with exudates with high N/C ratio and NOSC, fast root respiration, and high root abundance. Overall, our results indicate promising links between root exudation and respiratory fluxes, with exudate N/C emerging as a significant factor in flux variability.