Organic Carbon in Bulk Soil and Water Stable Aggregate Sizes

Assessing the content of total SOC by an elemental analyzer (Interscience EA1108) is the simplest and most direct measurement to evaluate the effect of soil management practices and cropping systems on SOC accumulation or decomposition. However, an estimate of the total amount of organic carbon (TOC) in the bulk soil does not inform on the mechanism of SOM incorporation, but it is only an indirect evaluation of both SOM dynamics and stabilization process.

A large number of chemical or physical fractionation methods have been applied to SOM studies in order to quantify organic matter pools with different OC turnover time and dynamics. The separation of various SOC fractions, based on both SOM physical protection and biochemical recalcitrance, was generally achieved by fractionation of soil particle sizes or aggregates (Gregorich et al. 1989; Angers et al. 1995; Puget et al. 2000; Six et al. 2004), density flotation of aggregates and organic mineral complexes (Christensen 1992; Gregorich et al. 1997; Janzen et al. 1992), chemical separation (Spaccini et al. 2000; Derenne and Largeau 2001; Helfrich et al. 2007), and a combination of physical and chemical methods (Spaccini et al. 2001; Leifeld and Kogel-Knabner 2005; QuerĂ³a et al. 2006). Although each technique may provide information to clarify SOM accumulation processes, most of the applied methodologies show different specificities according to soil and procedure. Thus, no common protocol has been yet established for univocal or unambiguous determination of SOM pools and fractions (Smith et al. 2002; von Lutzow et al. 2007).

In the field experiments of the MESCOSAGR project, we focused on the determination of OC content in both bulk soils and water-stable aggregate sizes. Moreover, we appraised the molecular composition of humic substances which were annually extracted from soil after each crop cycle during the three experimental years (see Sect. 4.5).

The TOC content and its distribution among soil aggregates showed the short-and medium-term effects of various field treatments on OM incorporation in soil and its interactions with soil particles. On the other hand, the structural characterization of humic substances extracted from soil indicated the effect of field experiments on the molecular composition of the most stable soil organic components, and the potential of different management for a long-term stabilization of SOM.

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