Erosion and deposition regulate soil carbon by mediating Fe-Carbon fixation mode in a typical catchment in the black soil region of Northeastern China

Soil carbon is critical to soil quality and the global carbon cycle, while Fe is highly sensitive to redox processes and can affect soil carbon stocks by sequestrating and releasing carbon. Soil erosion and deposition can cause the spatial heterogeneity of soil properties along landscapes and alter the redox environment, thus further influence carbon fixation by Fe in situ. Carbon, especially Fe-oxide-bound carbon (OC-Fe) were poorly studied in different erosion intensities. Here, we investigated OC-Fe, soil organic carbon (SOC), and inorganic carbon (SIC) contents at erosion and deposition sites in a typical catchment of Northeast China, and evaluated the responses of oxygen diffusion state (DCo), Fe oxides (Fep, Fed) and valence state of Fe (Fe(II), Fe(III)) to erosion and deposition. We have also quantified factors affecting Fe and carbon to elucidate the drivers of Fe-mediated carbon stock variations induced by erosion. Results showed that with a higher erosion intensity, a greater redistribution was emerged on OC-Fe and SIC in the 0–10 cm soil layer of study catchment. Compared to deposition sites, OC-Fe and SIC increase by 63.32 % and 11.41 times at erosion sites, respectively. However, SOC increased 18.56 % at deposition sites. In the 10–20 cm soil layer, where erosion forces were more pronounced, the binding mode of Fe oxides to carbon changed from co-precipitation to adsorption. The presence of Fe(II) promoted OC-Fe formation. Structural equation modelling（SEM）constructed by the varied particle composition with erosion and deposition explained 79 % of the variation in carbon stock in 0–20 cm soil layer. This study concluded that subsoil carbon stocks (>10 cm) are mainly affected by the application rate of exogenous carbon and variations of its mineral content. Erosion-induced variations of carbon sequestration patterns have continuous effects on farmland carbon turnover capacity.