Carbon mineralization from organic wastes at different composting stages during their incubation with soil

The decomposition of seven different organic waste mixtures prepared with sewage sludges, animal manures, city refuse and industrial and plant residues, was studied during their aerobic incubation with soil. The waste mixtures were composted by the Rutgers static pile system, and four samples of each mixture were collected at various composting stages: the initial mixture, and samples taken during the thermophilic phase, at the end of the active phase and after maturation. These samples were added to a calcareous silt loam soil at a rate of 200 mg per 10 g soil, and the CO2-C evolution was determined during 70 days of aerobic incubation at 28 degrees C. Carbon mineralization decreased as the composting time lengthened. The lowest values of C mineralization were found for the mature samples, and only a compost which had not attained an advanced degree of maturation gave results higher than 25% of TOC. Carbon mineralization followed a combined first- and zero-order kinetic model in most of the samples, suggesting that the organic C of the composting wastes was made up of two organic pools of differing degrees of stability. However, the differences in the slow C mineralization pool at the end of the active phase and after maturation were very small, indicating that the organic matter at both stages was of a similar microbial stability. Comparing the C mineralization which takes place in soil and during composting, it can be concluded that composting is the best way of obtaining maximum C stabilization, which is an important factor in soil conservation and reclamation. (C) 1998 Elsevier Science B.V. All rights reserved.