Density fractionation of organic matter in dolomite-derived soils

Dolomite (CaMg(CO₃)₂) constitutes half of the global carbonates. Thus, many calcareous soils have been developing rather from dolomitic rocks than from calcite (CaCO₃)-dominated limestone. We developed a physical fractionation procedure based on three fractionation steps, using sonication with subsequent density fractionation to separate soil organic matter (SOM) from dolomite-derived soil constituents. The method avoids acidic pretreatment for destruction of carbonates but aims at separating out carbonate minerals according to density. The fractionation was tested on three soils developed on dolostone parent material (alluvial gravel and solid rock), differing in organic-C (OC) and inorganic-C (IC) concentrations and degree of carbonate weathering. Soil samples were suspended and centrifuged in Na-polytungstate (SPT) solutions of increasing density, resulting in five different fractions: two light fractions < 1.8 gcm^-3 (> 20 μm and < 20 μm), rich in OC and free of carbonate, and two organomineral fractions (1.8-2.4 gcm^-3 and 2.4-2.6 gcm^-3), containing 66-145 mg g^-1 and 16-29 mg g^-1 OC. The organomineral fractions consist of residual clay from carbonate weathering such as clay minerals and iron oxides associated with SOM. The fifth fraction (> 2.6 gcm^-3) was dominated by dolomite (85%-95%). The density separation yielded fractions differing in mineral compositions, as well as in SOM, indicated by soil-type-specific OC distributions and decreasing OC : N ratios with increasing density of fractions. The presented method is applicable to a wide range of dolomitic and most likely to all other calcareous soils.