Greenhouse gas emissions from digestate in soil

Purpose Biogas residues, digestates, contain valuable nutrients and are therefore suitable as agricultural fertilizers. However, the application of fertilizers, including digestates, can enhance greenhouse gas (GHG) emissions. In this study different processes and post-treatments of digestates were analyzed with respect to triggered GHG emissions in soil. Methods In an incubation experiment, GHG emissions from two contrasting soils (chernozem and sandy soil) were compared after the application of digestate products sampled from the process chain of a food waste biogas plant: raw substrate, digestate (with and without bentonite addition), digestates after separation of liquid and solid phase and composted solid digestate. In addition, the solid digestate was sampled at another plant. Results The plant, where the solid digestate originated from, and the soil type influenced nitrous oxide (N2O) emissions significantly over the 38-day experiment. Composting lowered N2O emissions after soil application, whereas bentonite addition did not have a significant effect. High peaks of N2O emissions were observed during the first days after application of acidified, liquid fraction of digestate. N2O emissions were strongly correlated to initial ammonium (NH4+) content. Conclusion Fertilization with dewatered digestate (both fractions) increased N2O emission, especially when applied to soils high in nutrients and organic matter.