Characterizing the Influence of Plant Growth on Carbon Association with Fresh Soil Minerals

The zone of root influence (rhizosphere) is a dynamic soil environment with active carbon release, highly active and phylogentically distinct microbial communities, and unique edaphic properties (i.e. redox, pH). We hypothesize that mineral-SOM associations in the rhizosphere differ from those in bulk soil, possibly altering the persistance and fluxes of this carbon. We investigated how SOM first associates with soil minerals during growth of Avena barbata, a Mediterranean annual grass. We grew A. barbata with 99 atom% 13CO2 and tracked 13C-labeled photosynthate into soil where three minerals types were incubated: quartz, kaolinite, and ferrihydrite. Our work suggests these different mineral types recruit different amounts of mineral-associated carbon and microbial community composition (16S and ITS with Illumina MiSeq). Here, we have focused on the micron scale to characterize the composition and source of mineral-associated SOM. Using scanning transmission X-ray microscopy (STXM), we defined major classes of C compounds, and overlaid those data with characterization of Fe mineral speciation. In the same regions of interest, we used correlated nano-scale secondary ion mass-spectrometry (NanoSIMS) analyses to assess the isotopic enrichment of these C compounds. Based on the degree of 13C-labeling in each feature, we can determine whether the initial source of the OM was plant-derived, bulk soil, or mixed. For a more bulk-scale perspective, we also conducted sequential extractions of SOM associated with Fe and Al phases to look at the bonding chemistry of these compounds. We found that minerals in the rhizosphere had unique SOM compositions, notably, rhizosphere-incubated minerals were more enriched in aromatic compounds. The source of these aromatics is likely from the bulk soil and not from the plants, suggesting re-mobilization of SOM during plant growth. Our work also shows the importance of considering the microbial communities that associate with soil minerals, as C degraders, C mobilizers, and as C sources (i.e. "zombie C").