Sodium hexametaphosphate and ultrasonication − Comparing chemical and physical dispersion methods for soil organic matter fractionation

Abstract

Soil organic matter (SOM) fractionation into particulate organic matter (POM) and mineral-associated organic matter (MAOM) is widely applied to investigate soil organic carbon (SOC) stabilization and turnover, yet the choice of dispersion method strongly influences fractionation outcomes. This study tested a chemical dispersion approach using sodium hexametaphosphate (Na-HMP) and compared it with mechanical ultrasonication at 450 J mL −1 (US450), previously shown to be effective. Method performance was assessed based on mass and organic carbon (OC) recovery, reproducibility, and the quality of fraction separation, the latter assessed using natural 13 C abundance in soils transitioning from C 3 to C 4 vegetation. Both methods achieved high mass and OC recoveries (>90 %) with low variability between replicates (median coefficient of variation (CV) < 5 %), indicating that either dispersion approach can be applied reproducibly. However, significant differences in fraction allocation were observed: Na-HMP yielded higher mass and OC contributions in the POM fraction at several sites, whereas US450 consistently promoted a more MAOM-dominated distribution. The proportion of C 4 -derived carbon (C) differed strongly between POM and MAOM for both methods; however, the capacity to resolve C pools based on δ 13 C signatures was significantly higher under Na-HMP, attributed to Na-HMP weakening organo-mineral associations and mobilizing mineral-bound OC. While this reflects chemical redistribution of organic matter (OM) between fractions rather than a strictly physical size-based separation, it nonetheless sharpened the isotopic contrast between POM and MAOM. Overall, US450 is recommended for size-based separation and aggregate disruption, whereas Na-HMP is preferred for maximizing isotopic contrast and isolating pools with distinct turnover rates. The chemical mobilization of OM observed under Na-HMP suggests that mineral-associated OC may be sensitive to ionic disturbances, a pattern that warrants further investigation in the context of salt-affected or sodic soils.