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

Van Nguyen, Quan, Jensen, Lars Stoumann, Bol, Roland, Wu, Di, Triolo, Jin Mi, Vazifehkhoran, Ali Heidarzadeh, Bruun, Sander
Journal of environmental quality 2017 v.46 no.5 pp. 1114-1122
agricultural land, anaerobic conditions, anaerobic digestion, biogas, carbon dioxide, denitrification, environmental impact, feedstocks, greenhouse gas emissions, nitrification, nitrous oxide, oxygen, oxygen consumption, pig manure, soil amendments, soil depth, soil treatment
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 O₂ planar optode technique was used to visualize soil O₂ 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 O₂ 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.