Estimating greenhouse gas emissions from soil following liquid manure applications using a unit response curve method

Quantitative information is critical in policy making related to the roles of agriculture in greenhouse gas (GHG) emissions. A Unit Response (UR) curve method was developed in this study for modeling GHG emissions from soil after liquid manure applications. The emission sources (soils and liquid manures) are conceptualized as a set of linear cascaded chambers with equal storage-release coefficients, or two sets of cascaded chambers in parallel, each set having equal storage-release coefficients. The model is based on a two-parameter gamma distribution. Three parameters in this model denote the number of cascaded chambers, the storage-release coefficient, and the multiplier (referring to the total net emissions) added to the gamma distribution function. These parameters can be expressed as functions of site-specific background fluxes without applications of manure/fertilizer. The method was assessed with emissions data from five fields in Washington State. The results showed that at the WSU and Lynden sites, the average excess CH₄ emissions due to manure applications were 0.39 and 0.17 kg CH₄–C ha^? 1, respectively; the average excess CO₂ emissions were 216.50 and 25.20 kg CO₂–C ha^? 1, respectively; and the average excess N₂O were 0.37 and 0.03 kg N₂O–N ha^? 1, respectively. The UR method may fill the gaps between field measurements, simple emission factor (EF) method, and complex process-oriented models. This method has the potential to be used for estimating additional GHG emissions due to manure/fertilizer applications.