Dynamic Release and Transport of Colloids and Colloidal Organic Carbon in a Seasonally Saturated Wetland

Wetlands play an important role in the global carbon cycle because their dynamic redox conditions control soil carbon retention, transformation, and release. Reduction in wetlands induces iron dissolution, pH shift and hence promotes release and mobilization of mineral colloids and colloid-associated-organic carbon (OC). The mobilized colloids and colloidal OC transport from wetlands to streams or rivers through both surface runoff and subsurface groundwater flow, therefore they serve as important carbon sources and support biological functions in the downstream aquatic ecosystems. However, due to their small size, i.e. 1-1000 nm, low concentration, and their highly sensitive nature to exposure of oxygen, the role of mobile colloids and colloidal OC in wetland carbon cycles have not been identified in most previous studies. In this study, we collected soil water from wetland inlet to outlet at different depths and monitored changes in water table, OC, colloid concentrations, and shifting in solution chemistry, i.e. pH, electrical conductivity (EC), redox potential (Eh), Fe(II) concentrations. Results demonstrated that concentrations of colloids and OC in the soil water from shallow wells significantly changed with fluctuating water table due to evapotranspiration and storm event and were strongly related to pH and Fe(II) concentrations. On the contrary, EC largely influenced subsurface colloid and OC concentrations but the pH effect was limited. Additionally, colloid and OC concentrations consistently increased from wetland inlet to outlet at both shallow and deep wells, suggesting colloids and OC exported through both surface and subsurface flow paths. These findings have implications for colloidal release and colloidal-facilitated-transport of carbon in redox and hydrologic dynamic environments.