Distribution characteristics of soil iron-bound organic carbon in profiles along the elevation in Poyang Lake wetland.

The mineral protection of iron oxides is considered as one of the key mechanisms for the long-term stability of soil organic carbon. To investigate the distribution patterns and regulatory mechanisms of soil iron-bound organic carbon (Fe-OC) at the soil profile scale, we collected soil samples of high beach, middle beach, low beach and mud flat along the elevation gradient in Nanji Wetland National Nature Reserve of Poyang Lake, and measured soil basic physicochemical properties, the different soil iron fractions, and the Fe-OC content in the 0-100 cm profiles. The results showed that the average contents of Fe-OC at different elevations was (0.9±0.1) g·kg^-1,accounting for 11.6%±1.4% of total soil organic carbon (<i>f</i>_Fe-OC). Along the elevation gradient, both Fe-OC at depths of 0-10 cm and 50-00 cm, as well as <i>f</i>_Fe-OC across the entire 0-100 cm depth, showed a trend of first increase and then decline. However, there was no significant difference in Fe-OC content at depths of 10-50 cm across different elevations. The average soil carbon-iron molar ratio (OC/Fe) in the 0-100 cm profiles at different elevations was 0.1±0.0, and the OC/Fe in all soil layers was less than 1, indicating that adsorption was the primary mode of iron-carbon coupling. Fe-OC was significantly positively correlated with ferrous iron, amorphous iron oxide, complexed iron, soil organic carbon, total nitrogen, total phosphorus, soil moisture, ammonium nitrogen, nitrate nitrogen, and available phosphorus, but negatively correlated with soil bulk density and soil pH. The structural equation modelling showed that elevation regulates Fe-OC by driving the transformation between soil redox state and iron oxides morphology via changing soil moisture, pH, and soil organic carbon.