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 mineralization in a sewage sludge–cotton waste mixture at different stages of the composting process performed by the Rutgers static pile system. Three composting samples were selected: initial mixture (I); the end of the 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 mineralization fitted a combined first- and zero-order kinetic for I and a first-order kinetic for E and M. Potentially mineralizable C was very similar in E and M, whereas a mineralization rate constant (K) lower than 0.033 d−1 may be taken as characteristic of a mature compost. N was immobilized 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 immobilization in the soil. Potentially mineralizable N, calculated from the first-order kinetic function, was low, increasing from E (5.45%) to M (9.12%), the opposite occurring with the rate constant, leading to very close values of mineralization rate (No×K). An experiment with perennial ryegrass (Lolium perenne) in which the same composting samples were applied to the soil, confirmed the results of N mineralization. Addition of I to soil caused an initial deficiency of N in plants due to N immobilization and competition between plants and microorganisms for inorganic N. However, further re-mineralization of immobilized N provided similar N to plants as that in the unfertilized soil. The initial waste mixture may require a long safety period after addition to soil before sowing, and it would release inorganic N in the soil later in the cropping season. The compost which had not undergone a maturation phase had similar nutritional N value as the mineral fertilizer, but the greatest N fertilizer efficiency was found in the mature compost, 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.