Preservation of Soil Iron-Bound Organic Carbon in a Karst Ditch Wetland: A Case Study in Caohai Lake, China

The correlation between soil organic carbon (OC) and iron oxides is recognized as a significant mechanism for the long-term preservation of organic carbon. However, the impact of various phases of iron oxides on OC stabilization in karst ditch wetlands remains unclear. This study was conducted at Caohai National Nature Reserve, the largest karst plateau freshwater lake in China. Forty ditch samples of 0~20 cm soil layer were collected for low-frequency mass magnetic susceptibility (χ_LF), frequency magnetic susceptibility (χfd%), total iron (Fe_t), dithionite-extractable iron (Fe_p), oxalate-extractable iron (Fe_o), organically complexed iron (Fe_p), OC and iron-bound organic carbon (Fe–OC). Soil χfd% ranged from 6.70 to 14.61%, and χfd%, Fe_t, Fe_d, and crystalline iron (Fe_d – Fe_o) were positively correlated with χ_LF. The alteration of χ_LF is primarily influenced by the pedogenic formation of ultrafine ferrimagnets. Molar ratios of Fe–OC/Fe_d ranged from 0.50 to 2.28, with Fe–OC accounting for 9.25±3.84 % of the bulk OC. The fraction of Fe–OC in OC strongly depended on the molar ratios of C/N, Fe_d, and crystalline iron. At the 0.05 significance level, OC showed significant positive correlations with Fe–OC/Fe_d, Fe_o, and Fe_p, but negatively correlations with Fe_d and crystalline iron. The presence of the complexation of Fe_p with OC might promote the accumulation of OC in karst ditch wetlands. Further research should investigate the turnover times of the Fe_p complexation with organic carbon and its molecular compositions from microbial utilization and decomposition in the ditch wetlands.