The Effects of Ectomycorrhizae on Soil Organic Carbon in a California Oak Woodland.

The presence of tree roots and symbiotic mycorrhizal fungi is recognized to have a substantial impact on carbon (C) dynamics in soils.  This study examines the quantity and quality of exudates from the ecto-mycorrhizal fungi of blue oak and grey pine seedlings in a California oak woodland.  Hyphal in-growth cores (40 μm mesh) containing field soil were inserted into the soil surrounding the seedlings.  The hyphal cores were compared to cores that allowed the penetration of both roots and hyphae (2 mm mesh), in order to examine the C input from mycorrhizal structures.  Blue oak and grey pine seedlings surrounding the cores were labeled with ¹³CO₂ and extracted seven days later for analysis.  Water extractable dissolved organic carbon (DOC), soil organic C and microbial biomass were similar for all core types and seedling species.  There was a strong correlation between the amount of labeled DO¹³C found in the in-growth cores and the presence of hyphae.  The quality of the DOC was determined by DAX-8 fractionation and UV-vis 254.  Both analyses showed that the composition of the DOC in the hyphal in-growth cores contained a higher percentage of aromatic functional groups.  Aromatic compounds are known to sorb strongly to Al and Fe oxides and clays, leading to the formation of stable soil organic carbon.  These results suggest that hyphae have an important role in the production of exudates and that the presence of hyphae could play an important role in long-term soil organic carbon content.