The role of organo-clay associations in limiting organic matter decay: Insights from the Dajiuhu peat soil, central China

The mineral-bound organic matter (OM) in soils is of great importance for the global carbon cycle. Long-standing debate exists regarding the mechanisms of soil organic matter (SOM) preservation and, especially, the role of organo-clay mineral associations. Investigation of such associations is rarely undertaken in peat soils. In this study, we examine organo-clay relationships in the Dajiuhu peat soil in Hubei Province, central China, based on molecular markers, which allows for understanding of rates of OM degradation and the preservation of OM associated with clay minerals. We analyzed the major and trace element chemistry, clay mineralogy, and lipid compositions of both the coarse- and clay-sized fractions of the soil. The uniformity of trace element and REE distribution patterns in the soil (except for Layer 2) suggests that the source materials were well mixed. Chemical index of alteration (CIA) values of 70-85 indicate moderate to strong weathering processes. A significant correlation between Corg:P and TOC may indicate that redox conditions were a significant control on OM preservation. The clay fraction of Dajiuhu peatland soils also has played an important role in preserving OM, especially in anoxic environments. Fourier transform infrared spectrophotometry (FTIR) and X-ray diffraction (XRD) analysis show that OM is absorbed mainly on the edges of illite grains rather than in interlayer sites. The amphoteric margins of illite crystals allow them to more effectively bind organic compounds. The high Rl/h ratio, high carbon preference index (CPI), and high ratio of unsaturated-to-saturated fatty acids (C18:2/C18) in the clay-sized fraction provide further biomarker evidence that illite efficiently protects OM from biodegradation, thus enhancing the OM content of soils.