The decline in soil organic matter with cropping is a major factor affecting the sustainability of cropping systems. Changes in total C levels are relatively insensitive as a sustainability measure. Oxidation with different strength KMnO4 has been shown to be a more sensitive indicator of change. The relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping, whilst the relative size of the unoxidised fraction increased. Changes in delta(13)C ratio have been used to measure C turnover in systems which include C3 and C4 species.