A comparison of Tier 1, 2, and 3 methods for quantifying nitrous oxide emissions from soils amended with biosolids

Municipal biosolids are a nitrogen (N)-rich agricultural fertilizer which may emit nitrous oxide (N₂O) after rainfall events. Due to sparse empirical data, there is a lack of biosolids-specific N₂O emission factors to determine how land-applied biosolids contribute to the national greenhouse gas inventory. This study estimated N₂O emissions from biosolids-amended land in Canada using Tier 1, Tier 2 (Canadian), and Tier 3 (Denitrification and Decomposition model [DNDC]) methodologies recommended by the Intergovernmental Panel on Climate Change (IPCC). Field data was from replicated plots at 8 site-years between 2017 and 2019 in the provinces of Quebec, Nova Scotia and Alberta, Canada, representing three distinct ecozones. Municipal biosolids were the major N source for the crop, applied as mesophilic anaerobically digested biosolids, composted biosolids, or alkaline-stabilized biosolids alone or combined with an equal amount of urea-N fertilizer to meet the crop N requirements. Fluxes of N₂O were measured during the growing season with manual chambers and compared to N₂O emissions estimated using the IPCC methods. In all site-years, the mean emission of N₂O in the growing season was greater with digested biosolids than other biosolids sources or urea fertilizer alone. The emissions of N₂O in the growing season were similar with composted or alkaline-stabilized biosolids, and no greater than the unfertilized control. The best estimates of N₂O emissions, relative to measured values, were with the Tier 3 > adapted Tier 2 with biosolids-specific correction factors > standard Tier 2 = Tier 1 methods of the IPCC, according to the root mean square error statistic. The Tier 3 IPCC method was the best estimator of N₂O emissions in the Canadian ecozones evaluated in this study. These results will be used to improve methods for estimating N₂O emissions from agricultural soils amended with biosolids and to generate more accurate GHG inventories.