Transport of soil colloids and its relation to biogeochemical cycling of organic carbon under dynamic redox conditions

Abstract

Release and transport of organic carbon (OC), a large portion of which can complexes with mineral colloids, often leads to long-term C sequestration. Despite colloids’ potential importance, the role of mobile colloids in carbon cycling is not well understood. In this study, we conducted experiments using syringe columns with objectives to understand 1) the dynamic formation and disassociation processes of mineral-organic associations (MOAs) and transport of OC in relation to soil colloid mobility, 2) the coupled physical and biogeochemical processes of soil OC under dynamic redox conditions. ☐ Regression analyses of chemical properties (pH, Eh, turbidity, ionic strength) for leachate solutions as functions of released water-dispersible colloids were all linear, indicating clear correlations between colloid release with aerobic and anaerobic conditions. Mobilized C and colloids in leachates were quantified by fractionating samples into two particle-size classes (450-1000 nm, 100-450 nm). Under all redox conditions, we found that a larger amount of OC was contained in the 100-450nm size fraction, and a decreased redox potential resulted in an increasing release of this size fraction. In addition, soils in the columns were fractionated into three particle sizes: 450-1000 nm, 100-450 nm, and 2.3-100 nm, and we found that the 2.3-100 nm fraction accumulated the highest C concentration. Moreover, analyses of C and N stocks and their natural abundance of δ13C, δ15N in different sized aggregates indicated that, as redox potential decreased, the most significant change occurred in the 100-450 nm fraction. Overall, expending of oxygen in soil system enhances the release of colloids, which in turn facilitates C turnover in soil. The 100-450 nm colloidal fraction is the most important size fraction as the potential exchangeable or biogeochemical-reactive fraction due to its high loading capacity of organic carbon and its lability under dynamic redox conditions.