CARBON AND NITROGEN MINERALIZATION IN AN ACID SOIL FERTILIZED WITH COMPOSTED URBAN REFUSES

The C- and N-mineralization kinetics of a Cambisol over granite fertilized with four composted urban refuses (one of them amended with CaCO3 in the composting process) as well as the degradation kinetics of the wastes themselves were studied. The C-mineralization was determined by incubation of the samples for 42 days at 28-degrees-C in a thermostat bath and measurement of the CO2 evolved from the samples. The N-mineralization was performed by aerobic incubation in an incubator at the same temperature and for the same time as in the case of C. Addition of the composts to the soil significantly increased the C-mineralization rate. About 30% of the organic C of the composts (only 16% in the amended compost) was mineralized after 6 weeks incubation. Most C mineralization took place in the first 3 weeks for only about 3% of the organic C (0.9% in the amended compost) was mineralized between weeks 3 and 6. The amended compost was the only one that stimulated the soil N-mineralization rate. The other composts provoked N-immobilization (or denitrification) during the incubation and although an increase of the net inorganic-N production was detected after week 2, the net N-mineralization rate was almost nil at the end of the incubation. Net nitrification predominated over net ammonification in the soil with or whitout addition of composts. The non-amended compost behaviours seem to show that they had not undergone enough stabilization; as for the amended composted refuse its singular behaviour could be due to its high carbonate content.