The fate of glucose, a low molecular weight compound of root exudates, in the belowground foodweb of forests and pastures

Increasing evidence suggests that much of belowground, heterotrophic activity in terrestrial ecosystems is fueled by inputs of low molecular weight carbon compounds (LMWCCs). Root exudation (rhizodeposition) is a primary source of these inputs and will likely increase with rising atmospheric CO2. Yet the fates of these compounds belowground, as well as the environmental factors that influence them, are relatively unexplored. Using stable isotopes we track the fate of one dominant LMWCC, glucose, in three pasture and three forest sites located in South Carolina, USA. We resolve glucose-derived C in CO2, dissolved and soil organic C (DOC, SOC), microbial biomass, and microarthropods (Collembola, oribatid and mesostigmatid mites). After 72 h, the greatest proportions of glucose-C are in microbial biomass and SOC, followed by CO2, DOC, and microarthropods. Within this short time frame, glucose-C propagates through the foodweb to the highest trophic level, predatory mesostigmatid mites. The biomass of these predators is the only variable that explains the relative partitioning across sites of glucose-C, with higher biomass associated with reduced partitioning of glucose-C to respiration and hence greater retention belowground. Our results suggest that LMWCCs entering belowground systems may propagate through soil foodwebs rapidly, and that their partitioning belowground may potentially be determined by higher trophic levels.