DYNAMIC TRANSFORMATION OF SEWAGE-SLUDGE AND FARMYARD MANURE COMPONENTS .1. CONTENT OF HUMIC SUBSTANCES AND MINERALIZATION OF ORGANIC-CARBONAND NITROGEN IN INCUBATED SOILS

An incubation experiment was conducted with farmyard manure (FYM) and two municipal sewage sludges (SS): one from Russia (SSR) and one from Sweden (SSS) using a sandy loam soil at 25 degrees C for 6 weeks. The objectives were to investigate mineralisation of organic C and N and t ransformations of humic substances (HS). Cumulative C mineralisation w as about 0.8-1% of total C and cumulative N mineralised was about 3-7% of the pre-incubation total N contents in the treatments. The organic materials contained 1.2-1.5% humic acids (HA) and 0.8-1.4% fulvic aci ds (FA). incubating the soils with manures did not markedly influence soil humus composition, indicating that SS-borne organic C was not con verted into HS during the short-term incubation. SSR was more resistan t to decomposition than SSS, although both SS were less susceptible th an FYM to changes in all parameters.