Estimating ammonia emissions from a winter wheat cropland in North China Plain with field experiments and inverse dispersion modeling

A field-scale experiment was conducted in the spring of 2012 at a winter wheat cropland, aiming to quantify ammonia (NH3) emissions from surface fertilization under realistic cultivation conditions. Since the fertilization lasted about 20 days for hundreds of divided plots and three types of fertilizers were used (i.e., urea, ammonium sulfate and compound nitrogen-phosphorous-potassium fertilizer), the heterogeneity was one of the significant characteristics of the cropland NH3 emissions during the experiment, which is a great challenge for the classical micrometeorological methods to calculate NH3 fluxes. Based on continuous measurements of NH3 concentrations at two heights (2.5 m and 8 m) and detailed records of the fertilization plot by plot, an inverse dispersion method was employed to derive the heterogeneous NH3 emissions and the corresponding emission factors (EFs). The EFs derived from this experiment for urea, ammonium sulfate and compound fertilizer were 12.0% ± 3.1%, 8.5% ± 1.6% and 4.5% ± 1.7%, respectively. The EF of urea we obtained was lower than most of other domestic measurements and those used in the NH3 emission inventories in China. Measurements on EFs of ammonium sulfate and compound fertilizer are not available in China. However, the EFs of ammonium sulfate and compound fertilizer we obtained were comparable to those used in NH3 emission inventories of China.