Nitrous Oxide Emissions and Its Influencing Factors from an Agricultural Headwater Ditch During a Maize Season in the Hilly Area of Central Sichuan Basin
农田周边的排水沟渠不仅是农田养分迁移的重要通道,也是氮转化非常活跃的场所和潜在的氧化亚氮(N_2O)排放源.本研究以川中丘陵区农田源头沟渠为对象,在6~9月的玉米季(雨季),采用静态箱-气相色谱法对其N_2O排放开展原位观测.结果表明,在整个观测期有自然植被覆盖的沟渠生态系统(V)N_2O累积排放量(以N计)为0.43 kg·hm~(-2),而无自然植被覆盖的对照处理(NV,代表沟渠中的沉积物-水界面系统)则为0.07 kg·hm~(-2).该沟渠生态系统的N_2O平均排放通量[14.7 mug·(m~2·h)~(-1)]已达到本地区玉米农田直接排放的水平,表明玉米季中农田源头沟渠是不容忽视的N_2O源.川中丘陵区雨季丰富的降雨径流携带大量农田硝态氮进入沟渠,促进N_2O产生和排放,此外,植物的存在可大幅度提高农田沟渠的N_2O间接排放系数(V:0.05% vs. NV:0.01%).由于本研究的N_2O间接排放系数远低于2006年IPCC建议的缺省值(0.25%),如果仅采用IPCC缺省值来估算本地区沟渠生态系统的N_2O排放,可能导致较大误差.未来研究中应对原位观测多予以重视,为进一步修正其缺省值提供理论依据.
英文摘要:
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 (N_2O) emission. In this study, N_2O 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 N_2O 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 N_2O flux from the agricultural headwater ditch ecosystem [14.7 mug·(m~2·h)~(-1)] was comparable to the magnitude of direct N_2O flux from the maize cropland in this region. Nitrate concentration in the overlying water could be the main factor governing N_2O emission from the ditch, especially in the rainy season. The existence of vegetation increased the indirect N_2O emission factor (V: 0.05% vs. NV: 0.01%). The indirect N_2O 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 N_2O emissions. More attention should be paid to in situ measurements to reduce uncertainty of N_2O estimation calculated by only applying the default value for emission factor from the IPCC.