Conversion of pastoral land to arable cropping can result in severe soil degradation due to loss of soil organic matter. Identifying management practices that maintain organic matter under cropping is a priority area of research in New Zealand. A field experiment was established in spring 2000 in the Canterbury Plains (the major arable farming region in New Zealand) to quantify the effects of tillage practices on soil organic matter levels. The site, which had been under pasture for > 10 years, contained 84 t C/ha in the upper 25 cm layer. The experiment included three tillage treatments: intensive tillage (ploughing to ~20 cm, followed by secondary cultivation); minimum tillage (shallow cultivation to ~10 cm); and no tillage. The protocols used to determine soil C included adjustment for tillage-induced changes in bulk density. One approach involved use of the equivalent mass method whereby the amount of C in a constant soil mass (3500 t/ha = 25 cm sampling depth under pasture) was estimated. A second approach involved adjusting sampling depth based on changes in plot surface elevation, determined from a level monitoring survey. In general, estimates of C change were similar for the two measurement protocols. Loss of C varied between tillage treatments. For example, a large quantity of C was lost during the first cropping season under intensive tillage (11% decline), whereas there was very little change in C under no tillage.