The liquid (LS) and solid fraction (SS) of a biogas slurry from dedicated crops, the composted solid fraction (CSS) and a municipal solid waste compost (MSWC) were compared in a soil incubation at 200 mg N kg (1), to assess CO2 emissions and potential C and N mineralization. Products were also compared for nitrogen apparent recovery fraction (ARF) in a pot trial with Italian ryegrass. LS showed the highest C mineralization (63.6%), soil mineral N (>100 mg kg (-1)), and ARF (50.3%). SS showed 21.6% C mineralization, slight N immobilization (23.6 mg kg (-1)) and 7.3% ARF. In CSS, a 5.1% C mineralization in soil added to 26.3% C loss during composting, resulting in 31.4% overall C loss. Moreover, composting SS to CSS curbed the emission from 4210 to 1100 mg CO2 kg (1) soil, still double than the reference MSWC (507 mg CO2 kg (-1) soil). Despite high mineralization of supplied carbon, LS emitted less CO2 than SS: 936 mg CO2 kg (1) soil. It appears, therefore, that LS acts as a source of easily available nitrogen, while SS plays the role of an amendment with some limitations due to soil N immobilization. CSS mitigates N immobilization, but the composting process determines relevant CO2 losses to the atmosphere. (C) 2011 Elsevier Ltd. All rights reserved.