Sodium iodide (NaI) and sodium polytungstate (SPT) solutions are being indistinctively used in density fractionation procedures of soil organic matter (SOM). This study aimed to evaluate these two solutions, adjusted to a density of 1.8 kg L-1 (NaI-1.8 and SPT-1.8), and particularly of SPT adjusted to densities of 2.0 (SPT-2.0) and 2.2 kg L-1 (SPT-2.2), with regard to their capacity of recovering the light fraction (LF) of SOM. A 20 g soil sample of the 0-5 cm layer of an Acrisol (PVd) and of a Ferralsol (LVd) was dispersed by sonication at 250 J mL-1 and 450 J mL-1, respectively for each soil, in 80 mL NaI or SPT solution. After centrifugation (2.000 g, 90 min), the LF in the supernatant was recovered on a fiberglass filter, dried and analyzed for C content determination (TOC dry combustion) and chemical structure characterization (13C nuclear magnetic resonance spectroscopy). The SPT-1.8 increased LF-C recovery by 152 % in the PVd and 166 % in the LVd, compared to NaI-1.8. Besides, the higher SPT-2.0 density increased LF-C recovery by 57 % in both soils as compared to SPT-1.8. Furthermore, SPT-2.2 recovered 21 % and 5 % more LF-C than SPT-2.0, in the PVd and LVd, respectively, but this additional recovery was not as pronounced as when density was changed from 1.8 to 2.0 kg L-1. Based on data of Fe content in LF and C and Fe content in clay fraction, we estimated that the contamination of clay-C in the LF using SPT solutions varied 1.9-3.5 % in the LVd and 6.8-10.4 % in the PVd. Theses values were considered low compared to the high increments obtained in LF-C recovery. The increases in both alkyl-C/O-alkyl-C ratio and carbonyl-C abundance when NaI was substituted by SPT and when SPT densities increased, suggested that more decomposed LF material was recovered, which was more evident in the LVd. Based on our results, we suggest using SPT solution adjusted to a density of 2.0 kg L-1 in studies focusing on densimetric fractionation of SOM.
densimetric fractionation; sodium iodide; particulate organic matter
