农田N_2O通量测定方法分析

ANALYSIS ON METHODS OF FLUX MEASUREMENT OF NITROUS OXIDE FROM A CROP FIELD

采用空气动力学方法、波文比 /能量平衡法及密闭箱法结合气相色谱分析对农田N2 O通量进行了测定。在夏天和秋天观测前的 1～ 2周 ,148和 10 9kgNhm-2 肥料被分别施入夏玉米田和冬小麦田。实验表明 :空气动力学法与波文比 /能量平衡法测定的N2 O通量在冬小麦田较为接近 ,在夏玉米田则相差较大。夏玉米冠层上方温湿梯度的细微变化可导致能量平衡法计算的N2 O通量发生较大离散 ,从而与空气动力学法的计算结果产生较大偏差。观测期间 ,微气象法和密闭箱法测定的农田N2 O日平均通量在夏玉米田分别为 18± 37和 8± 5ngNm-2 ·s-1;在冬小麦田分别为 43± 2 1和 6± 1ngNm-2 ·s-1。微气象法测定的农田N2 O通量高于密闭箱法的观测值 ,由此推测作物冠层可释放N2O ,其机制可能是土壤中生成的N2 O被作物的蒸腾液流输送到大气中。微气象法和密闭箱法测定的N2 O通量间的差异在冬小麦田大于夏玉米田 ,表明冬小麦释放N2 O的量大于夏玉米。这可能是冬小麦根系分布较夏玉米深的缘故。采用密闭箱法观测时 ,箱内气温发生变化 ,半小时内最高可增加10℃ ,因地温变化小 ,其对土壤中N2 O的生成并无显著影响。由于密闭箱法观测忽略或低估了植物释放的N2 O ,且箱内的边界层阻力可能高于箱外 ,其测定的地表N2 O通量偏小。

Aerodynamic technique, Bowen ratio/energy balance method (BREB) and close chamber method combined with gas chromatography (GC) analysis were used to measure nitrous oxide (N 2 O) fluxes in a crop field. One or two weeks before observations in summer and autumn, nitrogen was applied to a summer maize field and a winter wheat field at 148 and 109kgNhm -2 respectively. N 2O fluxes measured by aerodynamic technique and BREB method were similar in winter wheat field,but there was a large difference between them in summer maize field mainly because a few changes in gradients of temperature and humidity above the crop canopy will lead to great errors in N 2 O fluxes measured by BREB method. Daily N 2 O fluxes measured by micrometeorological and chamber approaches were 18±37 and 8±5ngNm -2 ·s \|1 in summer maize field and 43±21 and 6±1ngNm \|2 ·s -1 in winter wheat field respectively. N 2 O fluxes measured by micrometeorological method were higher than those measured by chamber method. It is suggested that N 2O could be released from crops. The possible way is that N 2O generated in the soil is conveyed to the atmosphere by the transpiration stream. The differences between N 2O fluxes measured by two methods were big in winter wheat field and small in summer maize field. It is implied that the amounts of N 2O released from winter wheat canopy were larger than those from maize canopy mainly because the roots of winter wheat grew deeper than maize roots did. Chamber method has been studied in the experiments. The results showed that the air temperature inside a 0 25m 2 chamber on the surface in a crop field could be increased by 10℃ at most during half hour closure period. The small change in soil temperature has little effect on N 2 O production in the soil. N 2O fluxes released from plant may be neglected or underestimated in chamber based measurements. The boundary layer resistance inside the chamber is probably higher than that outside the chamber. It may result in small magnitudes of N 2 O fluxes measured by chamber method.