The distribution and relative losses of soil organic carbon fractions in aggregate size fractions from cracking clay soils (Vertisols) under cotton production

The distribution and losses of organic carbon fractions in various aggregate sizes from cracking clay soils were studied to understand some factors associated with losses of soil organic carbon under cultivation. Five pairs of samples from cropped and adjacent uncropped (reference) soils were collected from 5 of the main cotton-growing regions of Australia. Five aggregate sizes were separated from each of these soils (<50 µm, 50-150 µm, 150-250 µm, 250-450 µm, and 450-500 µm). On each of these aggregate fractions, measurements were made for total carbon (CT), labile carbon by ease of oxidation (CL), d 13 C, total light fraction (LF), carbon content of light fraction (C%-LF), and the proportion of soil carbon in the light fraction (LF-C) calculated. CT and CL were found to increase with a decrease in aggregate size, whereas LF was found to decrease with a decrease in aggregate size. Losses of both CT and CL as a result of cultivation were higher in larger aggregates than in smaller aggregates. The δ13C of both the whole soil and the LF was higher in the cropped soil than in the reference soil. It was concluded that most of the organic matter present in the cracking clay soils used for cotton production is highly decomposed, and most of it is concentrated in the microaggregates of the soil.