INFLUENCE OF SEWAGE-SLUDGE COMPOST STABILITY AND MATURITY ON CARBON AND NITROGEN MINERALIZATION IN SOIL

Incubation experiments with soil were made to study C and N mineraliza tion in a sewage sludge-cotton waste mixture at different stages of th e composting process performed by the Rutgers static pile system. Thre e composting samples were selected: initial mixture (I); the end of th e active phase (E); and the mature compost (M). The CO2-C evolved (as % of added C) from the composting samples after 70 d decreased in the order: I (62.3%) > E (22.3%) > M (19.8%). The course of C mineralizati on fitted a combined first-and zero-order kinetic for I and a first-or der kinetic for E and M. Potentially mineralizable C was very similar in E and M, whereas a mineralization rate constant (K) lower than 0.03 3 d(-1) may be taken as characteristic of a mature compost. N was immo bilized in soil treated with sample I, although it only lasted for 3 d in soil treated with sample E, followed by a mineralization phase. In soil treated with sample M the phase of N mineralization started the first day, which means that immature compost leads to an initial N imm obilization in the soil. Potentially mineralizable N, calculated from the first-order kinetic function, was low, increasing from E (5.45%) t o M (9.12%), the opposite occurring with the rate constant, leading to very close values of mineralization rate (N(o)xK). An experiment with perennial ryegrass (Lolium perenne) in which the same composting samp les were applied to the soil, confirmed the results of N mineralizatio n. Addition of I to soil caused an initial deficiency of N in plants d ue to N immobilization and competition between plants and microorganis ms for inorganic N. However, further re-mineralization of immobilized N provided similar N to plants as that in the unfertilized soil. The i nitial waste mixture may require a long safety period after addition t o soil before sowing, and it would release inorganic N in the soil lat er in the cropping season. The compost which had not undergone a matur ation phase had similar nutritional N value as the mineral fertilizer, but the greatest N fertilizer efficiency was found in the mature comp ost, because of its high concentration of NO3-N and to a lesser extent also to its mineralizable N. The mature compost can even be added to the soil when the crop is growing. (C) 1998 Elsevier Science Ltd. All rights reserved.