ESTIMATION AND ATTRIBUTION OF NITROUS OXIDE EMISSIONS FOLLOWING SUBSURFACE APPLICATION OF ANIMAL MANURE: A REVIEW

Field studies demonstrate increased ammonia (NH3) abatement efficiencies (up to 90%) of subsurface application and incorporation of animal manures compared to conventional surface application methods. However, a portion of the nitrogen (N) conserved may come at the expense of increased nitrous oxide (N2O) emissions produced during denitrification and nitrification processes in the soil. Research of N2O emissions subsequent to subsurface manure application is limited (particularly in the U.S.) and results are variable. Cumulative N2O emissions typically range from 0.1% to 3% of the total N applied following manure injection; however, only a few studies indicate a statistical difference in N2O emissions when compared to surface application methods. Circumstances attributed to increased emissions vary among studies and include the concentration of readily metabolizable water-soluble carbon (C) in manure slurry compared to background levels in the soil, soil moisture conditions pre- and post-subsurface application that drive nitrification and denitrification processes, localized N form and oxygen (O-2) concentration at the injection site, and injection depth, which can dictate the length of the diffusion path of N2O to the atmosphere. Research also draws correlations of N2O emission rates with subsurface application method, atmospheric conditions during and subsequent to application, and solids separation and dilution of manure slurry. Results thus far suggest that increased N2O emission is not an inevitable consequence of NH3 abatement strategies and should not dissuade subsurface manure application practices. Furthermore, potential reductions in indirect N2O emissions may be realized with subsurface application methods when compared to high NH3 volatilization and N deposition associated with surface application methods. Considering these tradeoffs and the ongoing debate on the role of anthropogenic greenhouse gases such as N2O and their role in climate change warrants more investigation of the factors that contribute to N2O emissions. This review synthesizes the available research on N2O emissions following subsurface application of animal manure to both grassland and arable land, compares results from different application techniques, and examines the conditions and circumstances that lead to N2O emissions.