Many studies have shown that reduced tillage systems such as shallow till (ST) and no till (NT) can be efficient strategies to increase soil organic carbon (SOC) compared to inversion tillage (IT), with a mean rate of sequestration of 100 to 200 kg C ha-1 per year (Lal, 2004; IPCC, 2007). However, several recent meta-analyses (e.g. Angers et al, 2008; Luo et al, 2010; Virto et al, 2011) add controversy, showing that C sequestration can vary from significantly positive to negative values. Long term experiments (LTE) are invaluable tools to bring reliable conclusions on these effects. We examined one of the oldest LTEs in the world, located at Boigneville in Northern France. Three tillage systems have been differentiated for 40 years, in a maize–wheat rotation: NT, ST tilled down to 5-8 cm depth and IT tilled down to 25 cm by mouldboard ploughing. SOC and SON (nitrogen) contents were measured every 4 years since 1970. Bulk density was measured in some years before 1990 and regularly from 2000 onwards. SOC stocks were calculated on the basis of constant soil mass. The first results obtained show that SOC increased slightly versus time in all treatments. The increase was faster in NT and ST than IT during the first 10-15 years, but slower (even negative) during the last 30-40 years. After 40 years, SOC distribution in soil profile was very different between treatments, suggesting different turnover times. These results will be thoroughly examined as well as the uncertainty on SOC estimates.