Comparison of alternative chemical density and physical methods for isolating soil organic matter fractions in high carbon soils

Soils beneath Portulacaria afra (spekboom) within the evergreen Albany Subtropical Thicket of South Africa contain unusually high soil organic carbon concentrations despite the biome's semi-arid climate. Fractionation of soil organic matter (SOM) into particulate organic matter (POM) and mineral-associated organic matter (MAOM) provide insights into the mechanisms of soil carbon (C) accumulation. Conventionally, sodium polytungstate (SPT) is used in density fractionation, but its high-cost limits sample numbers. Cheaper, reliable alternatives to SPT are therefore required to understand the C dynamics of spekboom and other high-C soils. Particle size (PS) sieving has been proposed as cheaper alternative to density fractionation, but its application in spekboom soils is unknown. This study compared SPT with PS fractionation and density fractionation using cheaper, highly soluble salts: sodium iodide (NaI), potassium iodide (KI), and calcium chloride (CaCl 2 ). The relationship between permanganate oxidizable carbon (POXC) and POM-C was also explored. High C (>5 %) topsoils with a range of POM:MAOM ratios from the spekboom thicket, a pine forest, a wetland, and a grassland were evaluated. Results demonstrated that fractionation methods significantly affected ( p < 0.05) fractions in each soil. NaI showed similar performance to SPT in SOM fractionation followed by KI. The PS method used in this study compared poorly to SPT, underestimating POM-C (34–39 %) and overestimating MAOM-C (39–51 %) in high POM soils (spekboom and pine forest soils) with the opposite effect in lower POM soils (wetland and grassland). The CaCl 2 method was also not ideal due to salt entrainment. Some soils did not conform to the expected linear relationship between POM-C and POXC demonstrating that POXC method is not always suitable. Thus, NaI is proposed as a cheaper alternative to replace SPT in density fractionation of spekboom soils. Findings from this study have significant implications in appropriate method selection for SOM fractionation of high C soils. • NaI, KI, CaCl 2 and particle size fractionation methods were compared with SPT. • NaI method was proposed as cheaper alternative to replace SPT. • Particle size may underestimate POM and overestimates MAOM in some high POM soils. • Particle size cannot differentiate soils with different MAOM/POM proportions. • POXC may not always be reliable in predicting labile C (POM–C).