Ecosystem type effects on the stabilization of organic matter in soils: Combining size fractionation with sequential chemical extractions

The long-term stabilization of organic matter in mineral soils is widely believed to be mainly driven by its interaction with the mineral matrix rather than by its recalcitrance. This interaction involves several processes, and the relative importance of each one may vary according to the ecosystem type. Here we investigate organo-mineral interactions in soils across different ecosystems (coniferous and broadleaved forests, grasslands, and technosols), widely differing in their soil organic matter (SOM) content, by an approach involving two consecutive steps: (i) a size fractionation by wet sieving after sonication, to isolate the particulate organic matter (POM, > 20 mu m) from the organo-mineral complex (OMC, < 20 mu m), and (ii) a characterization of the OMC by sequential extractions with different chemicals, each one disrupting a specific kind of bond between SOM and active mineral surfaces. The OMC accounted for > 60% of total organic C and > 75% of total N contents. Differences among ecosystems affected the total amount of SOM, but barely its distribution between POM and OMC. The sequential extractions showed that a substantial amount of the organic matter in the OMC (20-30%) was weakly bound to minerals. Other components of the OMC, in contrast, were much less dominant than expected, i.e., the organic matter stabilized by carbonate coating (5-6% of total organic C in the OMC) and by iron oxyhydroxydes (3-6%). By far, the main pool of the OMC was the final residue remaining after all extractions (34-48% of total organic C in the OMC). Because this residue remains unextractable with NaOH after removing all the active mineral components, such unextractability can be due only to its own chemical characteristics. Future research should aim at its in-depth characterization and at elucidating its ecological functions.