Pellets from Biogas Digestates: A Substantial Source of N2O Emissions

The field application of pellets from biogas residues resulted in high N2O emissions which could not yet be parametrized through soil drivers. Therefore, the aim of this study was to determine potential N2O production from pellets themselves. N2O and CO2 release from the pure pellet body (in form of intact, crushed or finely ground pellets produced from biogas digestates) were measured during the first seven days after pellet wetting under constant laboratory conditions. Three pellet water contents were examined: 47, 62 and 72% water of the total fresh pellet weight. Additional replicates of similarly wetted intact pellets were used to determine NH4+, NO3− and DOC contents on days 0, 1 and 4 of incubation. Two further treatments of wet intact pellets (62% moisture) were sterilized prior or after moistening to investigate the emissions’ origin. N2O release was found to increase with decreasing pellet size fraction. A maximum of N2O fluxes within all three fractions was determined at 62% moisture, whereas lowest fluxes were measured at 72% moisture. The cumulative N2O emissions over seven days ranged between 1 µg N2O–N g−1 pellet (intact pellets at 72% moisture) and 166 µg N2O–N g−1 pellet (finely ground pellets at 62% moisture). In general, our findings indicate that denitrification was the main factor for N2O emissions, driven by indigenous microbial communities already present in the pellets. The results show that the N2O emissions released by the pellets themselves can explain a major portion of the N2O fluxes measured in situ.