Soil mineral assemblage influences the abundance and mean residence time of soil organic matter both directly, through sorption reactions, and indirectly, through influences on microbial communities. Though organo-mineral interactions are at the heart of soil organic matter cycling, current models mostly lack parameters describing specific mineral assemblages or phases, and treat the mineral-bound pool as a single homogenous entity with a uniform response to changes in climatic conditions. We used pyrolysis GC/MS in combination with stable isotopes and radiocarbon abundance to examine mineral-bound soil organic matter fractions from a lithosequence of forest soils. Results suggest that different mineral assemblages tend to be associated with soil organics of specific molecular composition, and that these unique suites of organo-mineral complexes differ in mean residence time. We propose that mineralogy influences the composition of the mineral-bound soil organic matter pool through the direct influence of mineral surface chemistry on organo-mineral bond type and strength in combination with the indirect influence of soil acidity on microbial community composition. The composition of the mineral-bound pool of soil organic matter is therefore partially dictated by a combination of compound availability and sorption affinity, with compound availability controlled in part by microbial community composition. Furthermore, results are suggestive of a preferential sorption of N-containing moieties in Fe-rich soils. These bonds appear to be highly stable and confer extended mean residence times.
Heckman, Katherine; Throckmorton, Heather; Horwath, William; Swanston, Christopher; Rasmussen, Craig. 2018. Variation in the Molecular Structure and Radiocarbon Abundance of Mineral-Associated Organic Matter across a Lithosequence of Forest Soils. Soil Systems. 2(2): 36-. 12 p. https://doi.org/10.3390/soilsystems2020036.