New method for the estimation of nitrous oxide emission rates from an agricultural watershed

Abstract

We developed a new and improved method, the ‘high-emission-incorporation (HEI) method’, for estimating soil nitrous oxide (N2O) emission rates at a watershed level based on nitrogen (N) input (consisting of fertilizer, manure, slurry and excreta N) and N surplus (calculated by subtracting the amount of crop yield and consumed N from the N input) of different sites in a livestock farm located in a watershed. The main characteristic of this method is the inclusion of extremely high N2O emission rates, ‘outlier’, which are normally excluded from estimation. High N2O emission rates were estimated using the regression model obtained from the measured N2O values and the amounts of N surplus; normal N2O emission rates were estimated using the regression model obtained from the measured values and the amount of N input. The probability of occurrence of a high flux was used to incorporate calculated high and normal N2O emissions into one. The annual N2O emission rate from the livestock farm in the watershed (467 ha), estimated using the HEI method, was 1156 ± 147 kg N year−1 over a 5-year period. The annual N2O emission rates calculated using the site-specific emission factor (EF = 0.0789) and the emission factor of the Intergovernmental Panel on Climate Change (EF = 0.01) were 1838 ± 585 kg N year−1 and 673 (522–1103) kg N year−1, respectively. The estimated value using the measure-and-multiply method, in which each land-use area is multiplied by the representative emission rate for each land-use type, was 964 (509–1610) kg N year−1. The N2O emission rates estimated by our newly developed method were consistent with the values calculated by the measure-and-multiply method and offered improvement over this measure because the new measure can also predict future N2O emission rates from the watershed.