We compared the release of C and N from untreated and anaerobically digested plant biomasses and animal manures. Based on losses of C during the plant biomass (feed) passage in cattle, during anaerobic digestion (AD), and during incubation with soil, the retention of C in soil was estimated. When related to the amount of C originally present in the feed, the digestion by cattle and/or AD did not significantly affect the longer-term retention of C in soil whereas the contribution to the labile soil C pool differed. Digestion by cattle and AD increased the release of inorganic N but with no simple relationship between N release and C retention. The increase in N availability due to AD was equivalent to 10-35% of total N in slurry, but after AD of plant biomass and faeces the N availability increased significantly more.