Decomposition of Soil Organic Matter As Affected By Clay Types, Pedogenic Oxides and Plant Residue Addition Rates.

Carbon sequestration in soil is known to be affected by the presence of various clay minerals and pedogenic oxides. However, little information is available on the relative effects of these minerals on soil carbon stabilization.

In order to assess the effect of different clay minerals on the cumulative respiration of soil under various plant residue addition rates, an incubation experiment was carried out at 24^o C using clays extracted from soils dominated by kaolinite/illite, smectite and allophane. Wheat residue was added at the rate of 0, 1.5, 3, 6, 12 and 24 g kg ^-1 to sand amended with 30% of the respective clay (w/w). The residue addition corresponded to 0, 0.63, 1.26, 2.53, 5.06 and 10.12 g C kg^-1soil. Microbial biomass carbon was measured at the end of the experiment using a 24 h chloroform fumigation method.

After 36 days of incubation, the cumulative respiration per g C added was the highest with 24 g kg^-1 wheat residue addition rate in kaolinitic/illitic clay and the lowest with 1.5 g kg^-1residue addition rate in allophanic clay. Clay types had no significant effect on cumulative respiration at the lowest residue addition rates, but a strong effect at higher residue addition rates. Compared to kaolinite/illitic and smectite clay, allophanic clay reduced the cumulative respiration to a greater extent. This could be explained by the higher surface area and stronger binding sites of allophanic clay compared to other clays, which protected organic C from decomposition through the formation of strong clay-organic complexes.

In a similar experiment, wheat straw was applied at the rate of 20 gm kg^-1 (8.46 g C kg^-1) to the sand amended with the above clays at a rate of 7.5 and 40% (w/w). Removal of iron oxides from the minerals strongly increased the decomposition of wheat residue at both clay concentrations 7.5 % and 40 % clay (w/w). The increase in cumulative amount of C mineralised due to oxide removal from 7.5 % clay (w/w) concentration samples of kaolinitic/illitic, smectitic & allophanic clay types was 7.44 , 11.81 and 18.06 % respectively. It could be concluded that carbon stabilization in soil depends not only on the types of clay minerals and their concentration, but also on the pedogenic oxides associated with those minerals.