Distribution of Organic Carbon Fractions in Soil Aggregates and Their Contribution to Soil Aggregate Formation of Paddy Soils

Different soil organic matter fractions play different roles in governing soil aggregation. Therefore, this study determined the distribution of labile organic carbon (OC) and stable OC fractions in soil aggregates and their contribution to the soil aggregation of paddy soils in the Central Plain of Thailand. Quantitative analysis of the OC fractions in soil aggregates was determined using wet oxidation method and their changes in the chemical composition of organic compounds were investigated using FTIR spectroscopy. The results showed that the large macroaggregate (LMA: 2–8 mm) content of studied soils generally increased with increasing soil organic carbon, clay, polyvalent cations and aluminum (Al) oxides contents. Generally, both the labile OC and stable OC fractions increased with increasing aggregate size classes, suggesting a hierarchical order of soil aggregation and physical protection of organic matter against microbial decomposition. The labile OC fraction in all aggregate size classes was positively correlated with the content of LMA (r = 0.54*–0.77**), suggesting that the labile OC fraction represented a major organic cementing agent that contributed to LMA formation. FTIR analysis revealed that the labile OC fraction mainly consisted of plant-derived and microbial-derived polysaccharides. Therefore, paddy soils with higher organic cementing agent (labile OC fraction) and inorganic cementing/flocculating agents may provide a stronger intimate association of these transient binding agents, particularly the microbial-derived polysaccharides with clay mineral surfaces and metal cations/Al oxides. In turn, the greater amounts of stabilized, microbial-derived polysaccharides also efficiently promoted the formation of LMA from a combination of microaggregates.