Assessment of direct carbon dioxide emission factor from urea fertilizer in temperate upland soil during warm and cold cropping season

Urea [(NH2)2CO] is synthesized by combining ammonia (NH3) and carbon dioxide (CO2). The consumption of atmospheric CO2 for urea production was considered carbon (C) removal by the Industrial Processes and Product Use sector. Based on this evaluation, the Intergovernmental Panel on Climate Change proposed a default value (0.2 kg C per kg urea) for the CO2 emission factor from urea, which is equivalent to the mass percent of C in urea. However, all the C in urea might not be discharged to the atmosphere during the cropping season because some bicarbonates can be released from agricultural fields. To estimate the direct CO2 emission factor, different levels of urea were applied to cultivate red pepper and garlic during the warm and cold seasons, respectively, and CO2 emission rates were monitored during the cropping seasons. The seasonal CO2 fluxes increased significantly with increasing urea application levels, reaching maximum values of 268-273 and 233-250 kg N ha−1 for red pepper and garlic, respectively, and clearly decreased thereafter. Calculated by linear regressions between urea application levels and seasonal 13CO2 fluxes originating from 13C-urea, the direct CO2 emission rates from urea were projected to be 0.062-0.063 and 0.050-0.052 kg C per kg urea for red pepper and garlic cultivation, respectively. Thus, we suggest revising the current IPCC default value (0.2 kg C per kg urea) for the CO2 emission factor from urea to be approximately 0.06 and 0.05 kg C per kg urea for warm and cold cropping seasons, respectively, in temperate upland soils.