Labile and Stable Organic Carbon Fractions in Water Stable Aggregates and Their Contribution to Aggregate Stability in Paddy Soils

Evidence has suggested that either labile organic carbon (OC) or stable OC play a role in improving aggregate stability. Therefore, this study determined the OC fractions in water stable aggregates (WSA) and their contribution to the formation of the WSA in paddy soils, on the Central Plain of Thailand. Analysis of the OC fractions in the WSA was determined using wet oxidation with hydrogen peroxide (H₂O₂). The chemical composition of the organic compounds in the WSA was investigated using Fourier transform mid-infrared (FT-IR) spectroscopy. The results showed that the WSA content of the studied soils significantly increased with increasing organic and inorganic cementing/flocculating agents, such as soil organic carbon, clay, polyvalent cations, and sesquioxides. The labile OC and stable OC contents in the WSA also significantly increased with increased WSA content, suggesting the physical protection of OC fractions against microbial decomposition. The FT-IR analysis revealed that labile OC in the WSA, both before and after wet oxidation with H₂O₂, was dominated by polysaccharides, supporting the physical protection of labile OC by the WSA. Paddy soils containing higher organic and inorganic cementing/flocculating agents had higher stable OC, such as hydrophobic aromatic compounds, in the WSA, compared to paddy soils containing lower organic and inorganic cementing/flocculating agents. In turn, the WSA content of the soils in this study significantly increased with increases in the hydrophobic aromatic compounds in the WSA, suggesting the important role of stable hydrophobic organic compounds in enhancing the formation and stabilization of soil aggregates in these paddy soils.