Our purpose was to determine whether or not aerobic and anaerobic waste treatment differ in the conservation of waste C. The stability of waste C was evaluated by determining mass losses of C during aerobic and anaerobic waste treatment and CO 2 evolution during decomposition in soil using materials of the same origin. During waste treatment, mass losses of C were highest in the aerobic treatment and composting and lowest during anaerobic waste treatment. Following their application to soil, the amount of CO 2C evolved from wastes was highest from anaerobically-treated material, intermediate from non-decomposed material and lowest from aerobically-treated and composted waste. Measured in terms of the decomposition of waste C during treatment and in the soil, the effect on the C stabilization efficiencies of the various waste treatment was ranked as follows: aerobically-treated and composted > non-decomposed > anaerobically-treated. It was pointed out that the larger N recirculation through anaerobically-treated wastes may be of equal or greater importance for the maintenance of soil organic matter contents in agricultural soils compared with the higher stabilization of waste C against decomposition. On the other hand, for reclaiming degraded soils, aerobically-treated (composted) wastes are better than anaerobically-degraded wastes.