Considerable debate exists around the relative roles of aboveground versus belowground plant inputs in supplying carbon to the soil organic carbon (SOC) pool. It is critical to resolve the importance of these pathways in order to accurately model the terrestrial C cycle and predict its responsive to global change. Yet, few empirical studies have tested actual mechanistic differences that underpin aboveground versus belowground plant C pathways, independent of the quantity and quality of C flowing through them. We examined three hypothesized features that may affect the proportion of above- vs. below-ground plant C inputs stabilized in the mineral horizon of the soil: 1) the physical distance travelled, 2) the proximity to the dense rhizosphere microbial community, 3) delivery to the soil as pulsed versus continuous inputs. We inserted a fixed quantity of the common plant compound glucose (99 atom% 13C labelled) into soil microcosms, through aboveground and belowground entry points, proximately and distally from an artificial root, and as pulsed versus continuous input.
Results/Conclusions
We recovered 62.1% more C stabilized in microcosms where C was entering belowground versus aboveground, largely due to the physical barrier of the organic horizon. We found 52.5% more C stabilized from high-volume pulses (indicative of leaf litter leachate) than from low-volume continuous inputs (simulating root exudates), due to greater overall spread/coverage of the input. We recovered 57.6% more total stable SOC in inputs that entered distally from the root (i.e. ‘bulk soil inputs) than proximately to the root (‘rhizosphere inputs’), though a much higher concentration of SOC and of microbial biomass C in rhizosphere soil. Overall, these results help define specific attributes that may permit the above- or below-ground pathway to dominate C supply to the soil within different contexts.