Influence of Clay Mineralogy on the Stability of Coprecipitated Dissolved Organic Carbon with Ferric Iron.

Stabilization of organic carbon (C) through its interaction with clay and iron is an important process that decreases organic C decomposition and increases soil C sequestration. However, the stability of these copreciptates in soil and the extent these flocs resist microbial decomposition is poorly understood. The objective of this study was to understand the nature of these interactions among dissolved organic C (DOC), iron and clay, and the reduction and transformation of C associated with these flocs or organic matter assemblages. We conducted a laboratory incubation experiment using Kaolinite and Montmorillonite clays with or without iron (3) addition to study the characteristics and decomposability of the organic matter assemblages. Using CO₂ production over 30 days as an index of bioavailability, we found that the stability of coprecipitated organic C was higher in the floc coprecipitated with Montmorillonite than with Kaolinite when no iron existed. Meanwhile, lower CO₂ production was found in the floc containing higher iron content. Throughout the experiment, cumulative respiration was 1-5 fold higher for Montmorillonite, 1-9 fold higher for Kaolinite and 3-9 fold higher for floc and DOC solution compared to the control. Adding ¹³C enriched ryegrass residues into the same treatments as described above, showed that ryegrass accelerated C mineralization in the organic mater assemblages. As the incubation progressed, the fraction of CO₂ produced from the floc decreased. The addition of ryegrass had no effect on the cumulative CO₂ production from floc. These results indicated that clay mineralogy, as well as iron, play an important role in soil C mineralization and soil organic matter stabilization.