The cycling of nitrogen in agroecosystems is influenced by the amount and quality of organic matter applied to soils. The N-mineralization of15N-labelled ryegrass (Lolium perenneL.) was significantly slightly higher in a soil with long-term application of a high amount of farmyard manure (FYM) than in soils receiving either mineral fertilizer, a low amount of FYM or no manure. However, in general the decomposition of ryegrass in the selected soils varied only slightly. The microbial decomposition processes differed somewhat in the soil manured with a low amount of FYM from those receiving other manure treatments.