WHO'S ON FIRST? CARBON ASSOCIATION AND BACTERIAL AND FUNGAL COLONIZATION OF FRESH SOIL MINERALS IN THE RHIZOSPHERE (Invited Presentation)

Plants are the primary source of C in soil, with roots depositing low-molecular weight compounds during growth and more complex litter compounds at senescence. While the association of minerals with soil organic matter (SOM) is thought to be a critical mode of soil C protection, the mechanisms controlling this relationship are poorly understood. In the rhizosphere, microbes have a disproportionate impact on SOM cycling. However, direct rhizosphere interactions of microbes with soil minerals and SOM are not well characterized. We studied SOM-mineral interactions and the colonization of fresh minerals by soil microbes, asking: (1) How do mineralogy and rhizosphere effects impact SOM-mineral associations? (2) What factors determine the microbial communities that colonize fresh minerals? We followed the fate of ¹³C-labeled plant-derived C in Avena barbata (wild oat) California grassland soil microcosms, incubated with three minerals representing a spectrum of structure and reactivity: quartz, kaolinite, and ferrihydrite-coated quartz. The C deposited on these minerals was characterized and microbial communities colonizing the minerals were sequenced. At plant senescence, quartz had the least mineral-bound C and ferrihydrite had the most. FTIR and ¹³C-NMR analysis of the chemical characteristics of mineral-associated SOM show compositional differences between the three mineral types. We also found significant differences in the bacterial and fungal communities associated with different minerals. Ferrihydrite and quartz were associated with a higher relative abundance of arbuscular mycorrhial fungi, and NanoSIMS imaging of these minerals suggests that fungal hyphae moved C directly from roots to mineral surfaces. Additionally, on the minerals, we found significant enrichment of microbial groups with potential for N-fixation and fast growth. Our findings suggest that: (1) While mineral reactivity enhances SOM association, the presence of even relatively non-reactive surfaces promotes SOM accumulation (2) Microbial colonization of fresh minerals differs depending on mineralogy. Growing roots impact organic C interactions with minerals in the rhizosphere, resulting in distinct microbe-SOM-mineral associations and different chemical characteristics of SOM-mineral interactions.