Glued forever: incomplete dispersion hampers POM-MAOM fractionation in soils with high SOC and metal (hydr)oxide content

• Soil dispersion with sodium hexametaphosphate (SHMP) was effective for most soils. • Soil organic carbon and metal (hydr)oxides hamper soil dispersion in SHMP. • Soil texture influenced completeness of dispersion. • Residual moisture content of 40 °C dried soils can indicate incomplete dispersion. • Incomplete dispersion was poorly predicted in sodium and/or magnesium rich soils. Fractionation of soil organic carbon (SOC) into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) has contributed to our understanding of soil carbon cycling. Size fractionation following soil dispersion with sodium hexametaphosphate (SHMP) is a common method to separate POC (>50 µm) from MAOC (<50 µm). Complete dispersion of aggregates is essential to avoid overestimation of the POC and underestimation of the MAOC pools. We investigated how soil properties, particularly the contents of SOC, nanocrystalline aluminium (Al) and iron (Fe) (hydr)oxides, and exchangeable cations, affect soil dispersion with 5 g L −1 SHMP using a set of 38 widely diverse soils. Completeness of dispersion was evaluated by assessing the content of fine particles (<50 µm) present in the coarse fraction (>50 µm) after size fractionation, using laser diffraction analysis. Multiple linear regression revealed that dispersion was less complete with increasing contents of SOC and Al and Fe (hydr)oxides, and also depended on soil texture. On average, incomplete dispersion led to 3.6 g MAOC kg −1 soil (range: 0–42 g kg −1 ) erroneously ending up in the coarse fraction, corresponding with a 33.4% (range 0–217%) overestimation of POC and a 7.8% (range: 0–53%) underestimation of MAOC. The residual moisture content of 40 °C dried soils emerged as a cost-efficient and effective indicator for the risk of incomplete dispersion, except in soils high in exchangeable Na and/or Mg. Checking and reporting completeness of dispersion after size fractionation should be common practice when studying soil carbon pools. This study provides methodological insights that can guide improvements in SOC fractionation accuracy. Future refinements should focus on optimizing dispersion techniques while carefully balancing the risk of POM fragmentation.