Characterization of Organic Matter in Density-Size Fractions of Soil Aggregates By DRIFT Spectroscopy Combined with Sodium Hypochlorite Oxidation.

Soil organic matter (OM) is decomposed and associated with mineral components within aggregate structures. The objective of this study was to characterize the relatively unstable OM in density-size fractions of macro- and microaggregates separated from two types of soils (Udifluvent and Melanudand) using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy combined with oxidation with sodium hypochlorite (NaClO). The soil samples were separated into four fractions of water-stable aggregates (>1000 μm, 250-1000μm, 53-250μm, <53μm). The aggregate fractions were further separated by density-size fractionation with a sodium polytungstate solution (1.6 g cm^-3) into free particulate OM (fPOM), occluded particulate OM (oPOM), heavy particulate OM (hPOM; >53 μm) and mineral-associated OM (MAOM; <53μm) fractions. The fPOM and oPOM were present primarily in the macroaggregates (>250 μm). The density-size fractions were treated with 6% NaClO solution (pH 8.0), and the NaClO-oxidizable C and N were calculated as the differences between the amounts before and after the treatment with NaClO. The C/N ratio of NaClO-oxidizable OM was relatively high in the fPOM and oPOM fractions and relatively low in the MAOM fraction. For the hPOM fraction, it varied with the size of aggregates, being highest in the small macroaggregate (250-1000 μm) and lowest in the microaggregate (53-250μm). The infrared spectra were obtained by subtracting the DRIFT spectra after the oxidation with NaClO from those before the oxidation. Infrared spectroscopy showed that the NaClO-oxidizable OM in the fPOM fraction consisted mainly of aliphatic compounds, proteineous materials, lignin and polysaccharides, whereas that in the oPOM fraction was characterized by high contents of aromatic compounds and lignin. The NaClO-oxidizable OM in the hPOM and MAOM fractions contained less lignin and more proteinous materials than that in the fPOM and oPOM fractions. For the andic soil, the hPOM and MAOM fractions were characterized by rich in aromatic compounds.