Since 1995, there are in INIA-Uruguay different crop rotation systems to evaluate physical and economically the evolution of onion production and the soil properties (physical, chemical and biological) to maintain environment and natural resources. The objective of the present work is to know the evolution of the organic carbon (Org. C) content in soil after ten years of vegetable crop rotation. The continuous vegetable system is: onion - hen manure in the summer; onion - sweet corn (fallow buried); onion – green manure (moha); onion – bean production. In all sequences nitrogen response is evaluated (0, 80, and 120 kg of N.ha-1) on onion crop. Sample soil at 0.20 m depth was taken after onion harvest during 10 years, once a year, to measure the organic carbon content. Experimental design is split plot with three replications. Results of two years (1995 and 2004) are analyzed to study the stability of organic carbon in soil in all crop onion rotation. Sequence of onion crop with hen manure without urea (0 kg of N.ha-1) lost significantly less organic carbon in soil (42 kg) when compared with the other sequences and levels of nitrogen incorporated (900 kg in average of Org. C lost). Among the others sequence did not have significant difference. In addition to this, onion - hen manure sequence yielded significantly more than the other sequences in the 10 years of experiment. This results show that organic carbon content in soil allowed compare the different sequences and to know which is the best in order to maintain good conditions to produce maintaining natural resources through the time.