Biogas Digester Hydraulic Retention Time Affects Oxygen Consumption Patterns and Greenhouse Gas Emissions after Application of Digestate to Soil

Knowledge about environmental impacts associated with the application of anaerobic digestion residue to agricultural land is of interest owing to the rapid proliferation of biogas plants worldwide. However, virtually no information exists concerning how soil-emitted N2O is affected by the feedstock hydraulic retention time (HRT) in the biogas digester. Here, the O-2 planar optode technique was used to visualize soil O-2 dynamics following the surface application of digestates of the codigestion of pig slurry and agro-industrial waste. We also used N2O isotopomer analysis of soil-emitted N2O to determine the N2O production pathways, i. e., nitrification or denitrification. Two-dimensional images of soil O-2 indicated that anoxic and hypoxic conditions developed at 2.0-and 1.5-cm soil depth for soil amended with the digestate produced with 15-d (PO15) and 30-d (PO30) retention time, respectively. Total N2O emissions were significantly lower for PO15 than PO30 due to the greater expansion of the anoxic zone, which enhanced N2O reduction via complete denitrification. However, cumulative CO2 emissions were not significantly different between PO15 and PO30 for the entire incubation period. During incubation, N2O emissions came from both nitrification and denitrification in amended soils. Increasing the HRT of the biogas digester appears to induce significant N2O emissions, but it is unlikely to affect the N2O production pathways after application to soil.