Particulate organic matter (POM) is operationally defined as 50-2000 um-sized organic particles in soils. The quantity and quality of this labile organic constituent were shown to be strongly influenced by soil management, but the use of POM as an index for soil quality is still a matter of discussion. Recently, POM was identified as a metal-enriched organic fraction in both metal-contaminated and uncontaminated soils. The mechanisms for such metal-enrichment remain poorly understood, as there is little information on the chemical properties of POM and the fate of metal-associated POM during biodegradation. Abiotic as well as biotic processes may be involved. In this work we report both lab and in situ investigations of various polluted or unpolluted soils, aiming to assess the intrinsic - abiotic-reactivity of soil POM fractions and to follow the status of metals in POM during biodegradation. In the lab various POM size fractions were extracted by means of physical fractionation. EDTA was used to extract metals from the isolated organic fractions and metal concentrations were determined by AAS. Chemical reactivity of each POM fractions was quantified by potentiometric titrations. In situ, we collected undisturbed samples for sub-microscopic studies. The nature, morphology and microfabrics of soil constituents were studied on thin sections and elemental mapping was performed by u-Xray fluorescence on decaying POM. In spite of their relatively low abundance in soil (< 5% of total bulk soil weight), the POM fractions often contain a significant proportion of the total metal stock of the soils, i.e. around 10-20% for Zn and Pb, and up to 20-30% for Cu and Cd. The results show that total metal concentrations increase with decreasing POM size, while metal extractability decreases. Each POM-size fraction provides a buffer capacity due to the presence of reactive sites but affinity for copper was found comparable between the coarse and fine POM fractions. Finer POM fractions were found however to exhibit the highest contents in reactive sites per g of organic carbon. This suggests that with decreasing POM size, mainly due to decomposition, new reactive surface sites are generated. The micromorphological study reveals the occurrence of black opaque parts in decaying large POM fragments, which coincide with hot spots of heavy metals. Combined with microscopic observations, the analytical results suggest that metals are immobilized as organo-metallic complexes on very small POM-derived organic fragments. Such a process would explain the increasing organic C and metal contents frequently observed in the < 20 um fractions of metal contaminated soils, and suggest that small POM-derived organic particles may favour long-term metal sequestration in soils. These findings show that POM is undoubtedly an interacting medium and suggest a role of fine POM fractions as a metal sink in soils and of coarser POM fractions as a metal source.