Effects of dairy processing sludge and derived biochar on greenhouse gas emissions from Danish and Irish soils

Globally, to ensure food security bio-based fertilizers must replace a percentage of chemical fertilizers. Such replacement must be deemed sustainable from agronomic and greenhouse gas (GHG) emission perspectives. For agronomic performance several controlled protocols are in place but not for testing GHG emissions. Herein, a pre-screening tool is presented to examine GHG emissions from bio-waste as fertilizers. The various treatments examined are as follows: soil with added mineral nitrogen (N, 140 kg N ha^-1) fertilizer (MF), the same amount of MF combined with dairy processing sludge (DS), sludge-derived biochar produced at 450 °C (BC450) and 700 °C (BC700) and untreated control (CK). These treatments were combined with Danish (sandy loam) or Irish (clay loam) soils, with carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) emissions and soil inorganic-N contents measured on selected days. During the incubation, biochar mitigated N₂O emissions by regulating denitrification. BC450 reduced N₂O emissions from Danish soil by 95.5% and BC700 by 97.7% compared to emissions with the sludge application, and for Irish soil, the N₂O reductions were 93.6% and 32.3%, respectively. For both soils, biochar reduced CO₂ emissions by 50% as compared to the sludge. The lower N₂O reduction potential of BC700 for Irish soil could be due to the high soil organic carbon and clay content and pyrolysis temperature. For the same reasons emissions of N₂O and CO₂ from Irish soil were significantly higher than from Danish soil. The temporal variation in N₂O emissions was correlated with soil inorganic-N contents. The CH₄ emissions across treatments were not significantly different. This study developed a simple and cost-effective pre-screening method to evaluate the GHG emission potential of new bio-waste before its field application and guide the development of national emission inventories, towards achieving the goals of circular economy and the European Green Deal.