Potential nitrogen mineralization, plant utilization efficiency and soil CO2 emissions following the addition of anaerobic digested slurries

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 Nkg1, 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 kg1), and ARF (50.3%). SS showed 21.6% C mineralization, slight N immobilization (23.6 mg kg1) 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 kg1 soil, still double than the reference MSWC (507 mg CO2 kg1 soil). Despite high mineralization of supplied carbon, LS emitted less CO2 than SS: 936 mg CO2 kg1 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