Physical Analysis Of Soil Organic Matter Fractions

The physical fractionation of SOM by density and particle size yields fractions with distinctive chemical characteristics and turnover rates. Studies have shown that cultivation depletes soil of POM and mineral-associated fractions (Cambardella and Elliott, 1992; Tiessen and Stewart, 1983) immediately (Grandy and Robertson, 2006). Flotation in dense liquids such as cesium chloride, bromoform, sodium polytungstate, or silica gel with specific gravities ranging from 1.4 to 2.0 gcm~3 yields the light fraction of SOM. The light fraction is considered to be POM characterized by it no longer being recognizable as plant residues, but not yet transformed into humic substances. The light fraction is both free in soil and protected in soil by entombment within aggregates. Separating soil particles by sieving or ultrasonification into specific aggregate size fractions followed by light fraction separation yields light fraction of varying turnover rates and capacity to influence nutrient cycling.

The light fraction and POM is 5 to 15% of the total C of cultivated soils and up to 25% or more in forest A horizons. The light fraction has been identified as a critical determinant of the cycling of nutrients and C. The depletion of light fraction reduces the capacity of soil to supply nutrients through mineralization processes. Particle size analysis of soil is useful to follow the transformation and stabilization of C and N from the addition of tracers or natural abundance isotopes such as 13C from switches in plant photosynthetic types such as the C4 ^ C3 species (Balesdent and Mariotti, 1987). Using these techniques, as well as 14C enrichment studies and carbon dating, many studies have shown that about 20% of the total soil C turns over in less than 5 years, with the majority being light fraction and POM. The accumulation of POM and mineral-associated C in microaggregates residing within macroaggregates can be used as an early indication of long-term C sequestration as a result of changes in soil management such as tillage (Six et al., 2002).

FIGURE 12.17 Idealized structure of humic acid showing high aliphatic content (adapted from Schulten and Schnitzer, 1993) showing physicochemical interactions with a clay mineral. Organomineral interactions M denotes various cations, such as iron and calcium.
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