Effects of reclamation of paddy fields on soil iron-bound organic carbon in Minjiang River estuarine wetland

Iron oxide bound organic carbon is the main pathway for long-term stability of organic carbon. However, study of its mechanism remains weak. To understand the impact of estuarine wetland reclamation of paddy field on soil iron-carbon binding characteristics, we measured the soil iron-bound organic carbon (Fe-OC) and its related indicators in the natural reed (Phragmite australis) wetland and paddy field reclamation in Minjiang River estuary, Fujian Province. Results show that the wetland reclamation significantly affected the soil oxidation and reduction condition, and the redox process significantly affected the transformation of iron (Fe) phase in soil. After the wetland reclamation, the content of bivalent iron Fe(Ⅱ), trivalent iron Fe(Ⅲ), active total iron (HCl-Fe_t), and Fe(Ⅲ)/Fe(Ⅱ) in the soil significantly decreased by 24.68%, 52.56%, 51.45%, and 35.68%, respectively (P<0.05). The content of free Fe oxide (Fe_d) and amorphous iron (Fe_o) in the soil significantly decreased by 21.64% and 29.24%, respectively (P<0.05), but the content of complex iron (Fe_p) increased. In addition, the wetland reclamation significantly affected the soil carbon retention, and the content of Fe-OC and soil organic carbon (SOC) in the soil significantly decreased by 39.03% and 18.42% after the reclamation (P<0.05). In both reed wetland and paddy field, soil Fe-OC was combined dominantly through adsorption. The contribution rate of paddy field soil Fe-OC to SOC (f_Fe-OC) was significantly higher than that of reed wetland (P<0.05). Finally. there were significant positive correlations (P<0.01) between soil TN, water content, conductivity, Fe, SOC, dissolved organic carbon, and Fe-OC. This study provided scientific guidance for wetland restoration and increasing soil carbon sequestration.