京郊典型设施蔬菜地土壤N_2O排放特征

利用静态暗箱-气相色谱法对北京郊区设施蔬菜地典型种植模式(番茄-白菜-生菜)下土壤N2O排放特征进行了周年(2012年2月22日—2013年2月23日)观测,探讨了不同处理下(即不施氮肥处理(CK)、农民习惯施肥处理(FP)、减氮优化施肥处理(OPT)和减氮优化施肥+硝化抑制剂处理(OPT+DCD))N2O排放特征及土壤温度、土壤湿度、土壤无机氮含量对土壤N2O排放的影响。结果表明:每次施肥+灌溉之后设施蔬菜地会出现明显的N2O排放高峰,持续时间一般为3—5 d。不同处理N2O排放通量变化范围在-0.21—14.26 mg N2O m-2h-1,平均排放通量0.03—0.36 mg N2O m-2h-1。整个蔬菜生长季各处理N2O排放与土壤孔隙含水率(WFPS)均表现出极显著的正相关关系(P0.01);不施氮处理5 cm深度土壤温度与N2O排放通量呈现显著的正相关关系(P0.05);各处理N2O排放与土壤表层硝态氮含量具有较一致变化趋势。不同处理下N2O年度排放总量差异显著,依次顺序为FP((20.66±0.91)kg N/hm2)OPT((12.79±1.33)kg N/hm2)OPT+DCD((8.03±0.37)kg N/hm2)。与FP处理相比,OPT处理和OPT+DCD处理N2O年排放总量分别减少了38.09%和61.13%。各处理N2O排放系数介于0.36%—0.77%,低于IPCC 1.0%的推荐值。在目前的管理措施下,合理减少施氮量和添加硝化抑制剂是减少设施蔬菜地N2O排放量的有效途径。

Characteristics of nitrous oxide emissions from typical greenhouse vegetable fields in Beijing suburbs

Nitrous oxide (N₂O) has been recognized as one of the most important trace gases in the atmosphere that causes global warming and stratospheric ozone depletion. Nitrogen (N) fertilizer is considered as the primary source of N₂O emissions from agricultural soils. As a large agricultural country, China consumes the greatest amount of synthetic N fertilizer which accounts for 30% of the world consumptions. Therefore, quantifying N₂O emissions from agricultural soils and seeking suitable mitigation measures have become a relatively hot issue in international global climate change studies. However, the task has proved to be uneasy because N₂O production and emission processes are very complex and are influenced by a number of soil and environmental variables, interacting soil water and N processes, crop uptake and management practices. Especially the N₂O emissions from the greenhouse vegetable systems are more complex because the system obtain relatively higher inputs of fertilizer (e.g., N fertilizer application rate can be as high as 1,500 kg N/hm²), more water irrigation and cultivation disturbance. This paper reported a field experiment with intensive measurements of N₂O fluxes from a greenhouse vegetable system with varied management treatments in Fangshan District located in the western suburbs of Beijing, China. N₂O fluxes in conjunction with the main environmental drivers (i.e., soil temperature, soil moisture, soil NO₃^--N and soil NH₄⁺-N) were observed from Feb. 2012 to Feb. 2013. Four treatments, i.e., the control treatment (CK), the farmers' practice treatment (FP), the optimized fertilization treatment (OPT), and the OPT treatment with nitrification inhibitor amendment treatment (OPT+DCD), were implemented during the experimental period to test the impacts of fertilization on N₂O fluxes from the agroecosystem. The CK had no fertilizer applied; FP consisted of 2,470 kg N/hm² with 1,270 and 1,200 kg N/hm² from synthetic fertilizer and manure, respectively; the OPT reduced synthetic fertilizer rate to 573 kg N/hm². The field was planted with tomato, cabbage and lettuce rotation during the two experimental years. The results indicated that large amount of N₂O emissions were observed in the spring and summer periods when the soil had relatively high temperatures and moisture. N₂O emission peaks were measured following each event of fertilization or irrigation. The high peaks usually lasted for 3-5 days. During the experimental period, N₂O emission rates ranged from -0.21-14.26 mg N₂O m^-2 h^-1 with daily means ranging from 0.03-0.36 mg N₂O m^-2 h^-1. The annual cumulative N₂O emissions ranged from 1.69-20.66 kg N hm^-2, based on which the annual emission factor was calculated to be 0.36%-0.77% of the fertilizer N. Compared to the FP treatment, the OPT and OPT+DCD treatments both significantly reduced the annual N₂O emissions by 38.09% and 61.3%(P < 0.05), respectively. The N₂O emissions during the tomato, lettuce, cabbage and fallow periods accounted for 60.65%, 26.32%, 10% and 3.3% of annual cumulative emissions, respectively. The fertilizer-induced N₂O emissions varied across the N application rates, the crop growing periods and the management treatments. N₂O fluxes were positively related to the soil water filled pore space (WFPS) when the WFPS values varied between 48.88%-79.88%(P < 0.05). There was no consistent correlation between N₂O fluxes and the soil temperature at soil depth of 5 cm. Higher soil available nitrogen, especially nitrate, contributed higher N₂O emissions. In conclusion, alternative management practices such as reduced fertilizer application rate and amendment of DCD could effectively reduce N₂O emissions from the greenhouse vegetable field which usually emitted more N₂O than other croplands in China.