Clay mineralogy determines the importance of biological versus abiotic processes for macroaggregate formation and stabilization

Plant residues, living roots and microbial activity play an important role in aggregate formation and the stabilization of soil organic carbon (SOC), but their impact might differ among soils with different clay mineralogy. We investigated the effect of these organic agents on aggregation and SOC during a 76-day incubation of 2-mm sieved soil from an illitic Kastanozem and a kaolinitic Ferralsol, subjected to the following treatments: (i) control (no residue input or plant growth), (ii) residue input, (iii) living plants, and (iv) residue input and living plants. After 46 and 76 days, aggregate size distribution, aggregate-associated SOC and microbial-C were measured. In both soils, microbial-C was less in the control than in the residue and/or plant treatments. After 46 days, new large macroaggregates (> 2000 mu m) were formed in the control treatment of the kaolinitic soil, but not of the illitic soil. Control macroaggregates in the kaolinitic soil were formed out of silt and clay particles without accumulating C. Residue input and plant growth had a greater positive effect on macroaggregate formation in the illitic than in the kaolinitic soil. A stronger relation was found between microbial-C and amount of large macroaggregates in the illitic than in the kaolinitic soil. We conclude that kaolinitic soils can rapidly form macroaggregates independent of biological processes due to physical or electrostatic interactions between the 1:1 clay minerals and oxides. However, biological processes led to stronger organic bonds between the illite compared with the kaolinite clay, resulting in more macroaggregates with long-term stability in the illitic than in the kaolinitic soil.