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Sorption Temperature and and the Stability of Iron-Bound Soil Organic Matter

The preservation of soil organic matter (SOM) is an important control on the global cycling of carbon. Long-term preservation of SOM has important implications on soil fertility and climate regulation. Minerals, such as iron oxides, can react with SOM and serve as a preservation mechanism for SOM. Globally, iron oxide-SOM interactions form a “rusty carbon sink” which protects up to 22% of organic carbon in marine sediments. Climate changes, such as warming, may alter the size or efficacy of the “rusty carbon sink.” The effects of temperature, SOM composition, and mineral particle size on the formation and stability of iron oxide-SOM associations were investigated through batch sorption experiments, incubation experiments, and thermal analyses. The sorption extent of humic acid (HA) to microphase hematite was greater than that of fulvic acid (FA). The sorption extent for both HA and FA was found to be independent of temperature. The incubation and thermal analysis of microphase hematite-bound SOM suggested that HA is more biologically stable and less exergonic upon decomposition than FA, but there were no relationships with stability and sorption temperature. When normalized to specific surface area, the sorption extent of HA to nanophase hematite had a greater sorption extent than microphase hematite, but the sorption extent of nanophase hematite was also found to be temperature-independent. These results suggest that the size and efficacy of the “rusty carbon sink” may remain unchanged with warming climates. Furthermore, these results highlight (1) the importance of indirect temperature effects such as increased weathering and precipitation reactions which can alter the particle size distribution of soils and sediments and (2) SOM composition over direct sorption temperature in understanding future SOM dynamics.