Agricultural headwater ditches, the important locations for retaining and transforming nitrogen derived from the adjacent farmlands, have been regarded as the potential source of nitrous oxide (N2O) emission. In this study, N2O emissions from a typical agricultural ditch in the Hilly Area of Central Sichuan Basin, were observed by closed static chamber-GC technique during the maize season (rainy season), from June to the end of September. During the study period, cumulative N2O emissions in the sediment-water interface without vegetation (NV) and the ditch ecosystem with natural vegetation (V) were 0.07 and 0.43 kg·hm-2, respectively. Meanwhile, the mean N2O flux from the agricultural headwater ditch ecosystem[14.7 μg·(m2·h)-1] was comparable to the magnitude of direct N2O flux from the maize cropland in this region. Nitrate concentration in the overlying water could be the main factor governing N2O emission from the ditch, especially in the rainy season. The existence of vegetation increased the indirect N2O emission factor (V:0.05% vs.NV:0.01%). The indirect N2O emission factor from this field investigation was much lower than the default value proposed by the Intergovernmental Panel on Climate Change (IPCC, 2006) for assessing indirect agricultural N2O emissions. More attention should be paid to in situ measurements to reduce uncertainty of N2O estimation calculated by only applying the default value for emission factor from the IPCC.
Keywords: N2O; ditch ecosystem; indirect N2O emission factor; nitrate; nitrogen cycling; rain.