Aggregate dispersion for soil organic matter fractionation: a case study from Québec, Canada

Soil organic matter fractionation is often used in studies aiming at better understanding soil organic matter (SOM) dynamics. Particle-size fractionation using a 53 µm threshold is a simple and effective method to isolate SOM pools with contrasting properties. Although numerous methods of aggregate dispersion exist, little is known about their efficacy to completely disperse aggregates. With the goal of developing a standardized method for Québec, this study compared three widely used aggregate dispersal methods on 30 soils from across the province, covering a wide range of textures and organic carbon content. The three tested methods were (1) ultrasonication at 450 J mL −1 (US450), (2) shaking with glass beads in water (H 2 O), and (3) shaking with glass beads in a sodium hexametaphosphate solution (SHMP). Although the mass and carbon recovery were high for all three methods, the US450 method showed a higher mass of the >53 µm fraction and a lower mass of the <53 µm fraction compared to the other two methods, which would reflect an incomplete dispersion. This hypothesis was supported by scanning electron microscopy images. The carbon and nitrogen concentrations in both fractions were also lower for the US450 method in the >53 µm fraction for heavy-textured soils and higher in the <53 µm fraction for all soils. Since no significant differences were observed regarding fractions composition indicators between the H 2 O and SHMP methods, the H 2 O method was identified as the most appropriate fractionation method, as it is simple and does not require the use of chemical products.