Unprotected, Physically Protected and Chemically Protected Organic Matter Pools in Organically Amended Soils.

Organic amendments, such as solid manure (SM) and crop residues (CR), have long been used as a means to enhance or at least to maintain soil organic matter (OM). However, much remains to be learned about the underlying mechanisms by which the OM added with these amendments stabilizes in the soil. In this work, soil samples were collected from long-term experimental plots under a corn (Zea mays L.)–winter wheat (Triticum aestivum L.) rotation either unamended or yearly amended with SM or CR for 44 years at rates of 7.5 Mg ha^-1 after corn crops or 6.0 Mg ha^-1 after wheat crops. Dissolved OM, mineral-free particulate OM located outside aggregates (not protected from decomposition by physical and chemical mechanisms), mineral-free OM occluded within both macroaggregates and microaggregates (weakly and strongly protected by physical mechanisms, respectively), and OM in intimate association with soil minerals (protected by chemical mechanisms) were isolated from soil samples by a new physical fractionation method. The whole soils and OM fractions were analyzed for organic C. With respect to the unamended soil, SM and CR addition increased soil organic C by 35 and 11%, respectively. The mineral-associated OM fraction explained by far most of the increase in total organic C both in SM- and CR-amended soils (up to 60% for SM and 72% for CR). These results provide evidence, at least in the OM-depleted agricultural soils examined here, that primary organo-mineral complexes constitute the most relevant pool for long-term OM stabilization and therefore C sequestration in organically amended soils.