Who’s on first? Part I: Influence of plant growth on C association with fresh soil minerals

Abstract:

Mineral surfaces provide sites for carbon stabilization in soils, protecting soil organic matter (SOM) from microbial degradation. SOM distributed across mineral surfaces is expected to be patchy and certain minerals undergo re-mineralization under dynamic soil conditions, such that soil minerals surfaces can range from fresh to thickly-coated with SOM. Our research investigates the intersection of microbiology and geochemistry, and aims to build a mechanistic understanding of plant-derived carbon (C) association with mineral surfaces and the factors that determine SOM fate in soil. Plants are the primary source of C in soil, with roots exuding low-molecular weight compounds during growth and contributing more complex litter compounds at senescence. We grew the annual grass, Avena barbata, (wild oat) in a 99 atom% ¹³CO₂ atmosphere in soil microcosms incubated with three mineral types representing a spectrum of reactivity and surface area: quartz, kaolinite, and ferrihydrite. These minerals, isolated in mesh bags to exclude roots but not microorganisms, were extracted and analyzed for total C and ¹³C at multiple plant growth stages. At plant senescence, the quartz had the least mineral-bound C (0.40 mg-g^-1) and ferrihydrite the most (0.78 mg-g^-1). Ferrihydrite and kaolinite also accumulated more plant-derived C (3.0 and 3.1% ¹³C, respectively). The experiment was repeated with partially digested ¹³C-labled root litter to simulate litter decomposition during plant senescence. Thus, we are able evaluate contributions derived from living and dead root materials on soil minerals using FTIR and ¹³C-NMR. We find that mineral-associated C bears a distinct microbial signature, with soil microbes not only transforming SOM prior to mineral association, but also populating mineral surfaces over time. Our research shows that both soil mineralogy and the chemical character of plant-derived compounds are important controls of mineral protection of SOM.