Root Carbon Interaction with Soil Minerals Is Dynamic, Leaving a Legacy of Microbially Derived Residues

Minerals preserve the oldest, most persistent soil carbon, and mineral characteristics appear to play a critical role in the formation of soil organic matter (SOM) associations. To test the hypothesis that roots, and differences in carbon source and microbial communities, influence mineral SOM associations over short timescales, we incubated permeable mineral bags in soil microcosms with and without plants, inside a (CO2)-C-13 labeling chamber. Mineral bags contained quartz, ferrihydrite, kaolinite, or soil minerals isolated via density separation. Using C-13-nuclear magnetic resonance, Fourier transform ion cyclotron resonance mass spectrometry, and lipidomics, we traced carbon deposition onto minerals, characterizing total carbon, C-13 enrichment, and SOM chemistry over three growth stages of Avena barbata. Carbon accumulation was rapid and mineral-dependent but slowed with time; the accumulated amount was not significantly affected by root presence. However, plant roots strongly shaped the chemistry of mineral-associated SOM. Minerals incubated in a plant rhizosphere were associated with a more diverse array of compounds (with different functional groups.carbonyl, aromatics, carbohydrates, and lipids) than minerals incubated in an unplanted bulk soil control. We also found that many of the lipids that sorbed to minerals were microbially derived, including many fungal lipids. Together, our data suggest that diverse rhizosphere-derived compounds may represent a transient fraction of mineral SOM, rapidly exchanging with mineral surfaces.

Automated summary

Minerals play a critical role in preserving soil carbon and forming soil organic matter associations. Roots, carbon sources, and microbial communities influence the formation of these associations, with minerals in plant rhizospheres being associated with a more diverse array of compounds.