EVALUATION OF MATURITY AND STABILITY OF PRUNING WASTE COMPOST AND THEIR EFFECT ON CARBON AND NITROGEN MINERALIZATION IN SOIL.

To study the maturity and stability degree of pruning waste, four compost samples originated from pruning waste, leaves, and grass clippings were collected each from a different pilot pile at different stage of the composting process: initial nondecomposed material (C1); 2 months old at the end of the bio-oxidative stage (C2); 7 months old during the maturation phase (C3); and 12 months old at the end of the maturation phase (C4). These samples were added to a clay soil and the CO2 evolution and inorganic N (NO3^-N and NH4^+-N) were measured during 56 days of aerobic incubation. Results from the evaluation of compost maturity and stability showed that dehydrogenase activity (DH-ase) and the CO2 evolution could be used as good indicators of the pruning waste composting process. In contrast, humification parameters data from the organic matter fractionation did not contribute to the assessment of compost maturity. Neither the cation exchange capacity nor the germination index showed a clear tendency during the composting time, suggesting that these parameters are not suitable for evaluating the degree of compost maturity and stability of this material.Regardless of compost maturity and stability, C mineralization occurred in two phases: a first rapid phase (corresponding to the decomposition of the most labile products by microorganisms) and a second, slower phase, during which the most resistant organic products mineralized. During the first stage, the model was fitted to a first-order equation, whereas in the second phase the model was a zero-order equation for all samples except the soil amended with the initial material (S + C1), where the best fit corresponded to a first-order equation. Similar results obtained for samples C3 and C4 showed that organic matter had similar microbial stability at both stages and the composting process could be shortened by 5 months without any problem for soil application. Due to N immobilization observed during the initial stages of decomposition, the addition of nondecomposed material (C1) or of compost sampled at the end of the bio-oxidative phase (C2) may cause a deficiency of N in plants due to the N immobilization.