The Concept of Humification

It has been increasingly proved that simple, mainly alkyl, recalcitrant organic compounds deriving from both plant residue decomposition and microbial resyn-thesis are progressively incorporated into the most stable SOM fractions (Piccolo 1996; Lichtfouse 1998). Piccolo (1996) proposed that hydrophobic humic components in soil protect easily degradable compounds. He postulated that incorporation of polar molecules in associations of hydrophobic components may contribute to prevent an otherwise rapid microbial degradation of hydrophilic molecules and enhance their persistence in soil. This hypothesis is in accordance with the model of humic superstructures, by which humic molecules self-assemble into hydrophobic or hydrophilic domains according to their reciprocal affinity.

Based on this, the concept of humification must be revised or corrected. Humification should not be longer intended as increased polymerization of soil organic compounds, as previously assumed, but as a progressive accumulation of hydro-phobic and recalcitrant relatively small humic molecules in superstructures (Piccolo 1996; Lorenz et al. 2007). However, the heterogeneity of humic molecules in soil leads to the formation of mixed supramolecular structures. It was shown that the stable soil humic superstructures still contained hydrophilic molecules incapsulated in the hydrophobic domains, thereby being protected from biological degradation (Xu and Hatcher 2002; Piccolo et al. 2005a; Spaccini et al. 2006). This phenomenon was interpreted as a mechanism of hydrophobic protection by which labile hydrophilic molecules are included in hydrophobic humic superstructures and preserved from mineralization. It could be thus assumed that tightly bound humic associations containing mainly resistant alkyl remains of vegetative tissues may incorporate, by a random self-organizing process, also a few hydrophilic molecules or associated clusters of them.

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