It is known, that in sand fractions of soil are concentrated mainly residues of soil fauna and vegetation of the middle stage of decomposition, which determines the value of high C/N ratio [Greenland, Ford, 1964; Chichester, 1968; Turchenek, Oades, 1979; Lowe, Hinds, 1983; Tissien, Stewart, 1983; Pollution et al. 1984; Cartroux, Schnitzer, 1987 et al.]. It is most susceptible to biological degradation as the part of the labile soil organic matter.
As Zenon mentioned, organic matter in this size fraction is relatively fresh and only partially decayed (labile) material often represented as particulate organic matter (POM). The sand fraction also has low surface area and little to no aggregation. Clays, especially those with high surface area, adsorb decaying organics, making them less accessible to microbes and decay. The silt and clay fractions also tend to form aggregates around decaying organic particles, which renders physical protection against decay.
Dear Zenon and FM Hons. As pointed out the adsoption capacity of clays is important in relation to formation of aggregates and physical protection important delayed organic matter decay. Look to Tisdale Oades original concepts.
Higher soil organic matter will result as a function of both clay content but also the Calcium content of soils. This is well demonstrated in soil literature.
Since organic matter and clay both have overall net negative charge left alone they will repel each other making the soil organic open to decay. Calcium ion puts these together by the electrostatic interaction to both Clay and Soil organic matter.
Attractive agglomeration renders the materials less labile to decay because of reduced sites for decay. Working on a sandy soil to accentuate clay and silt plus optimizing Calcium levels should result in upping the Carbon sequestration ceiling.
Moreover, the acidified subsoils which are high in acidity and low in Calcium is another ideal field of C sequestration opportunities.
For sandy soils the use of fine silt and clay sediments along with optimized Calcium will also be useful.
The problems of increasing soil organic matter is often much more of unlimited decay function rather than inability to produce the raw soil organic material. In terms of delaying rampant decay fine sediments and Calcium will be useful in this effort. This will allow for greater ability to counteract global greenhouse gases in the atmosphere when appreciated and applied.
I join with Dr.Ghafoor to compliment Dr.Hepperly and Dr.Hons for their very good responses.Aggregation and physical protection of organic matter facilitated by calcium plays great role in carbon sequestration. as mentioned by Dr.Hepperly. As I understand the role of clay or clay plus silt aided by divalent cations may become prominent with humified organic matter.Plant residues and particulate carbon are referred to as labile carbon.Particulate carbon has size range of 0.053-2 mm which overlaps with fine sand and sand sizes.Well aggregated fine textured soils may offer some sort of physical protection to labile carbon compared to non aggregated sandy soils .Adsorption and linkages through Ca can also help to protect partially decomposed organic matter in clay or clay plus silt dominant soils.It often mentioned that the turn over time in soil of labile carbon is
Relatively higher labile organic matter in sand fraction is due to mostly less surface area than clay fraction.
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