SOIL ORGANIC-CARBON CHANGES IN CRACKING CLAY SOILS UNDER COTTON PRODUCTION AS STUDIED BY CARBON FRACTIONATION

This study examined soil carbon levels across a wide range of cracking clay soils used for growing cotton in Australia by using a simple car bon fractionation procedure. The soils studied included reference and cropped sites. The procedure employed determines soil carbon fractions based on their ease of oxidation to obtain the labile (C-L) and non-l abile (C-NL) carbon components. Based on the total carbon (C-T): labil e carbon (C-L): and non-labile carbon (C-NL) of a cropped soil relativ e to a reference soil, various monitoring indices were developed. It w as found that cultivation has generally led to a decrease in the organ ic carbon status of the soils. The effect of cultivation was found to be more pronounced in the C-L and the carbon management index (CMI) th an in the C-T and C-NL The changes in the ratio of C-L to C-NL as a re sult of cultivation have been variable. The CMI has generally declined during cultivation, and since the CMI has incorporated the changes ta king place in C-T, C-L, and C-NL, the use of this index can provide ve ry useful results in monitoring of organic matter status of soils.