A Case Study on the Effect of Urea Application on the Annual Budget of Carbon Gases Emission from a Paddy Field

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The CH4 and CO2 emissions from the paddy field without straw incorporation were measured through a year to consider their effects on global warming and carbon cycle in a paddy field. Three rates of urea application were determined to examine the effect of changes in rice biomass on the carbon budget: 120, 80, and 40kg N/ha. During the growing period, CH4 flux of the flooded field was measured by a closed chamber method. The amount of carbon fixed by rice plants was estimated on the basis of the dry weight and the carbon content. During the fallow period, CO2 flux was measured by the chambermethod.With increases in urea application rate, CH4 emission decreased and the carbon fixed by rice plants increased. The CO2 fluxes at all rates of urea application changed similarly because the absolute amounts of incorporated rice root were small. The CH4 contributed to the total CO2- equivalent emission from the soil by 73-77% with considering the GWPs. The carbon-gas emission from the soil was mostly determined by CO2 through the year, and was estimated at 87-147g C/m2.Consequently, increases in urea application rate were effective for a single-year mitigation both of carbon gases emission and carbon gases loss from the soil. However, the carbon gases loss appeared to be positive at all rates of urea application. This result indicates a necessity of organic matter application to sustain soil carbon storage. The overall consideration is necessary to balance greenhouse gases emission with soil carbon storage.