Warming effects on root morphological and physiological traits: The potential consequences on soil C dynamics as altered root exudation

Root and rhizosphere processes contribute significantly to soil carbon (C) fluxes, yet mechanism by which they do are not well understood. One of the contributing factors to this uncertainty is the lack of understanding about the role of root-derived labile C inputs in influencing soil organic matter (SOM) decomposition. We conducted an experiment to compare the pure impacts of two coniferous species through roots on the soil CO2 efflux and examine the species' response to experimental warming using infrared heaters. Warming markedly increased exudation rates I (μg C cm−1 root length h−1) and II (μg C cm−2 root area h−1) in the two species plots; however, the Picea asperata species had significantly higher root exudation rates than that of the Abies faxoniana species, regardless of warming treatment. The differences in the root morphological and physiological traits between the two species could be responsible for this variation in exudation and response to experimental warming. The P. asperata plots had significantly higher soil respiration rates (2.36 μmol m−2 s−1 on average) relative to the A. faxoniana plots (2.02 μmol m−2 s−1 on average). Similarly, the temperature sensitivity of SOM decomposition (Q10) was 1.19 times higher in the P. asperata plots than the A. faxoniana plots. The magnitude and direction of warming effects on the soil CO2 efflux varied considerably with tree species. The warming marginally increased the mean soil respiration by 5.3% in the P. asperata plots and significantly decreased the mean soil respiration by 10.8% in the A. faxoniana plots over the 4-year period. Our results collectively provide robust evidence that tree species can differ in their effects on shaping Q10 and controlling the soil CO2 efflux via root exudation, thereby implying altered patterns of soil C cycle between tree species in response to global warming. This calls for incorporating root-derived C inputs in controlling the microbial regulation of SOM decomposition in climate-carbon models to better predict soil C dynamics under global environmental change.