Fertilizing potential and CO2 emissions following the utilization of fresh and composted food-waste anaerobic digestates

Wet and dry-batch anaerobic digestion, and composting are common technologies in food waste (FW) management, resulting in different outputs. However, the effects of composting on carbon dioxide (CO2) emissions, nitrogen (N) and phosphorus (P) fertilizing capacity in view of closing nutrient cycle are still poorly investigated. In this work, two FW anaerobic digestates from the wet (D1) and dry-batch process (D2), and their respective composts (C1 and C2) were tested in a soil incubation (84 days at 25 °C) to assess CO2 emissions in comparison with a mixed (animal slurry/energy crop) digestate (BD) and a reference municipal solid waste compost (MSWC). The same products were also tested for the relative P efficiency (RPE) in soil, in comparison with a chemical-P source (30 mg P kg−1). Lastly, the apparent recovery fraction of N (N-ARF) from the five organic products was determined in a pot test with ryegrass (84 days; 300 kg available N ha−1), compared to a chemical fertilizer (NPK). Composting strongly reduced net-CO2 emissions compared to the two digestates (625 vs. 2850 mg CO2 kg−1 soil). Oppositely, composting very modestly influenced RPE that ranged around 100–90% in D1 and C1, and ≈30% in D2 and C2. Moreover, composting did not significantly reduce N-ARF that ranked in descending order as follows: NPK (77.5%) > D1 = BD (17.7%) ≥ C1 (14.7%) > MSWC (3.6%) > D2 (1.2%) > C2 (−3.1%). Composting was shown a reliable strategy for FW digestate management, as it reduces potential CO2 emission without affecting these products' N- and P-fertilizing capacity.