Carbon (C) addition through crop residues (residue-C), C dioxide emission (CO 2-C) and the soil C stock (soil-C) are components of the C cycle in the soil-plant-atmosphere system. This 18-year study was conducted to identify agricultural practices that could potentially increase C retention in the soil and lessen global warming trends. The three C pools (residue-C, CO 2-C and soil-C) under different tillage systems (CT-conventional tillage and NT-no tillage) and cropping systems (O/M-oat ( Avena strigosa [ Avena nuda])/maize ( Zea mays) and V/M-vetch ( Vicia sativa)/maize) were evaluated and the CO 2-C/[residue-C+soil-C] quotient was proposed as C retention index (CRI), where low values indicate a high capacity of the management system to keep C in the soil. The CO 2-C emissions were measured for 17 months (between November 2002 and March 2004), sampling of aboveground residues of cover crops and harvest indexes of maize were used to quantify C addition by cropping systems, and soil-C stocks (0-0.2 m) were evaluated in 2003. The soil temperature (0.05 m) and gravimetric water content (0-0.05, 0.05-0.1 and 0.1-0.2 m) were also monitored from May 2003 onwards. In comparison to 1985, the C balance was negative in the soil under CT (-0.31 t ha -1 year -1 for O/M and -0.10 t ha -1 year -1 for V/M). On the other hand, the C balance was positive in NT soil, but only under V/M (+0.15 t ha -1 year -1) due to the greater C addition by crop residues. The CO 2-C emission was related to the soil temperature (r>0.85). The total CO 2-C emission varied from 3.6 to 4.0 t ha -1 and was not affected by the soil management systems. However, the CRI allowed a clear discrimination of the soil management systems to keep C in the soil. The C retention potential increased in the following order: CT O/M
