Redox-driven changes in water-dispersible colloids and their role in carbon cycling in hydromorphic soils

Redox fluctuations in hydromorphic soils can influence ecosystem functions by altering the cycling of organic carbon (OC) and other elements in both the aqueous and solid phases throughout the soil profile. Most studies focusing on the mobility of dissolved OC in rice paddy soils have often disregarded the contribution of colloidal particles. We provide a detailed chemical and size-related characterization of water-dispersible soil colloids and their depth distribution in two soil profiles under long-term temperate paddy and non-paddy management, by asymmetric flow field-flow fractionation. Anoxic conditions enhanced colloid dispersion with a preferential release of the finer colloid fractions (5 and 4-fold increase in the < 30 nm and 30–240 nm fractions, respectively). The mobility of OC-rich, iron (Fe) and aluminium (hydr)oxide and aluminosilicate colloids along the soil profile was probably responsible for their depletion in the topsoil and a corresponding accumulation in the deeper illuvial horizons compared to the non-paddy soil. However, the release, percolation and subsequent reoxidation of Fe2+ was also shown to be a plausible mechanism leading to the formation of fine colloids in the subsoil. Redox-driven changes in colloid distribution were also linked to the differences in OC and pedogenetic Fe stocks in these two agro-ecosystems.