Tropical forests are highly-diverse ecosystems that contain many plants with a wide distribution of above- and below-ground traits. This ecological setting creates the opportunity for a diversity of belowground interactions in the rhizosphere where plant roots, mycorrhizal fungi, and soil microbes participate in the decomposition of soil organic matter and the uptake and assimilation of the carbon and nutrients released in this process. Here, we experimentally explored how rhizosphere interactions influenced belowground carbon dynamics including the activity of bacterial and fungal community members, providing insight to the possible mechanisms that drive these relationships. We constructed rhizosphere manipulation mesocosms that excluded roots, mycorrhizae, or all rhizosphere components (roots and mycorrhizal fungi). We installed mesocosms beneath trees in a lowland tropical forest at La Selva Biological Station in Puerto Viejo de Sarapiqui, Costa Rica. We added a 13C-labeled starch or leaf substrate to the mesocosms to explore the contingency of rhizosphere influence on the complexity of carbon inputs and to trace these C substrates into the microbial community. After 9 months of in situ incubation, we harvested the mesocosms and, among other measurements, performed 13C PLFA on soil samples from each mesocosm to assess whether the rhizosphere manipulation impacted the microbial assimilation of each substrate.
Results/Conclusions
We found that the presence of roots was negatively related to the incorporation of labeled carbon from the leaf substrates into microbial biomass (p < 0.001) and had no effect on the assimilation of 13C-starch substrates (p = 0.25). We did not detect an effect of the presence of mycorrhizal fungi on microbial assimilation of labeled leaf substrates (p = 0.70) or labeled starch substrates (p = 0.83). These results may indicate microbial preference for the simple sugars released by roots as root exudates, whereby microbes consume these sources of carbon instead of the more complex leaf substrates when available. In the absence of roots and the exudates that they produce, the microbial community begins processing more complex sources of carbon. By highlighting the relationship between roots and microbial decomposition, this research will help prioritize the incorporation of various components of the rhizosphere into the models that represent carbon dynamics.