How biased is size fractionation derived mineral-associated organic carbon?

• Fine POC contamination in size-fractionated MAOC is relevant for C-rich sandy soils. • Size and density fractionated agricultural soils equally, except for C-rich sandy soils. • Qualitatively, fine POC was more similar to MAOC than to coarse POC. • For majority of temperate agricultural soils, size-fractionation is a valid method. Mineral-associated organic carbon (MAOC) is regarded as the more persistent soil organic carbon (SOC) as opposed to the more fast-cycling particulate organic carbon (POC). Both fractions are typically isolated using physical fractionation based on size (wet sieving) or density (flotation in heavy solution). Size fractionation is faster and cheaper, and concepts about organo-mineral interactions are centered around the reactive silt + clay-sized soil fraction. However, with wet-sieving, there is a chance that fine (f)POC is also quantified as MAOC. A systematic quantification of such contamination and a comprehensive comparison of the two physical fractionation methods is missing. We selected 40 archived agricultural topsoils forming an SOC x texture gradient matrix to evaluate fPOC contents and characteristics by an additional density fractionation step. Subsequently, we quantitatively compared size and density-derived fractions from a total of 140 samples from the same archive. With on average 2.2 ± 4.2 % of total SOC, the contribution of fPOC to MAOC was very low. The isolated fPOC fraction also resembled MAOC in its physico-chemical properties with abundance of mineral particles, narrow C:N ratio and being relatively enriched in 13 C. Most OC-rich sandy soils had outlying high fPOC contents, leading to overestimations of MAOC by up to 212 %. Accordingly, size and density fractionation were very similar for a wide range of soils, while for 22 OC-rich sandy soils (> 800 g kg −1 sand, >20 g kg −1 SOC) size fractionation strongly overestimated MAOC. Simple physical fractionation cannot be conducted without certain (cross)-contaminations, while their magnitude appears to be minor given the multiple sources of error in organic matter fractionation. However, POC-rich sandy soils should not be fractionated by size (alone).