Is Clay-Bound Soil Organic Carbon Vulnerable to Priming?.

Increasing evidence suggests that the rhizosphere priming effect may be an important phenomenon in which stimulated mineralization of native soil organic carbon (C) occurs due to increased exudation of organic compounds from roots to soil. Clay particles are capable of adsorbing soil organic C to their surfaces, which can render the adsorbed compounds unavailable for microbial consumption. We investigated whether this clay-bound soil organic C may become vulnerable to priming due to root exudation under a reverse-isotope-labelling laboratory incubation in order to partition mineralization of three C sources: added root-exudate C, clay-bound C, and native organic C of bulk soil. Reverse isotope labeling involves two parallel incubations, both of which receive the same two C-source additions, root exudates and clay-bound C. However, in each incubation, only one of the two C sources is ¹³C-labeled. Clays were extracted from two different soil types from the same white fir forest ecosystem with contrasting mineral assemblages and mixed with a solution of organic acids, amino acids, and carbohydrates in order to adsorb C to the clay surfaces. These manipulated clays were subsequently added to soil and incubated with repeated inputs of simulated root exudates in order to quantify clay-bound C and bulk soil C priming. We found root exudate input stimulated a 26.8 ± 6.6% increase in mineralization of clay-bound C. There were no significant differences in substrate-derived CO₂-C among the treatments, nor with native soil organic matter-derived CO₂-C. Clay-bound C is a significant form of relatively stable organic C. Our findings suggest this stable soil C may become susceptible to stimulated mineralization due to increased root exudate input to soil.