Summary Particulate organic matter fractions (POM), defined as sand-sized organic separates in soils, are known to be labile organic components with a rapid turnover. Recently, POM fractions were identified to be metal-enriched in both metal-contaminated and uncontaminated soils. However, mechanisms for such metal-enrichment are poorly understood, because of the paucity of information on the chemical properties of POM. The aim of this study was to quantify the reactivity of POM towards Cu and to show a POM-size effect on this reactivity. POM was isolated from soils with different organic amendment managements: straw (S), conifer compost (CC), and non-amended (NA). Two POM size fractions were isolated by density-fractionation in water: 50-200 mum and 200-2000 mum. These fractions were studied for their metal contents, acid-base properties and affinity toward Cu. The buffer capacity and Cu affinity were modeled by FITEQL 4.0 software and compared between the two POM size fractions. Each POM size fraction provided a buffer capacity due to the presence of reactive sites, the greatest being for the 50-200 mum POM fractions. A signature of organic inputs as seen by the buffer capacities was observed for the 50-200 mum but not for the 200-2000 mum POM fractions. But Cu affinity was comparable between the coarse and fine POM fractions and no significant differences were found between NA, S and CC samples. We checked the hypothesis that decreasing POM size due to degradation processes generates more reactive surface sites. Results confirmed that soil POM plays a key role as a metal sink, due to its chemical properties.