密闭室抽气法监测稻田氨挥发的几个问题

Several Problems about Dynamic Chamber Technique in Determining Ammonia Volatilization from Rice Fields

密闭室抽气法是稻田氨挥发的常用监测方法,但该方法在实施过程中仍存在一些问题,导致不同研究结果之间缺乏可比性,影响稻田氨排放的系统分析与评估。研究了换气频率、抽气时间段、是否串联洗气瓶、抽气室与洗气瓶规格等监测参数以及抽气与自然风对比对氨挥发量的影响。结果表明,氨挥发随换气频率的增加而增加,其增加速度分三个阶段,挥发量与换气次数的对数呈线性相关;尿素快速水解期与水解基本结束后的氨挥发日变化规律不同;直通型、球形多孔型洗气头分别较圆盘多孔型洗气头氨挥发量低25.6%和8.5%;抽气室内径越大,气相高度越低,氨挥发量越低;串联洗气瓶测定的氨挥发仅为单独洗气瓶的88.6%;抽气室内田面水蒸发量随抽气速率增加而增加,抽气与自然放置情况下氨挥发量相近时,后者田面水蒸发量大。建议密闭室抽气法监测稻田氨挥发采用直径15 cm的抽气室,配单独流量计,气相高度5~8 cm,抽气量15~20 L·min^(-1)左右,无需串联洗气瓶,选择圆盘多孔型或直杆多孔型洗气瓶。

【Objective】Ammonia emitted from agricultural fields has led to significant adverse effects on air, soil and water environment. To evaluate the fate of applied nitrogen fertilizers, guide the rational application of nitrogen(N) fertilizers and eliminate the adverse environmental effects caused by ammonia volatilization(AV), it is important to measure AV in field conditions. The dynamic chamber method is commonly used in determining AV from rice fields. However, there are still some drawbacks associated with this method. These limitations lead to the incomparability between different research results and thus affect the systematic analysis and assessment of regional ammonia emissions from farmland. 【Method】In this study, the effects of the air exchange rate, time of the measurement, characteristics of the dynamic chamber, types of air-washing device, and the difference between vacuum pumping and natural wind environment on AV were studied. 【Result】 Results showed that AV increased with the increase of air exchange rate. Overall, there were three stages during the increase of AV which was linearly correlated with the logarithm of the air exchange rate. The diurnal variation of AV was different between the period of rapid hydrolysis of the applied urea and the period after rapid hydrolysis of urea. As for the types of air-washing device, AV values measured with the straight through and spherical porous types were 25.6% and 8.5% lower than that measured with disk porous type, respectively. The larger the inner diameter and the lower the height of the gas phase of the dynamic chamber are, the lower the ammonia volatilization is. Volatilized ammonia absorbed by two gas washing bottles in series is only 88.6% of that absorbed by the single gas washing bottle. The evaporation within the dynamic chamber increased with the air exchange rate. Although the amount of surface water loss within the container which was ventilated via vacuum pumping was similar to that under the natural ventilation environment, AV of the former container is relatively low. This indicates that the process of AV is affected to some extent by the vacuum pump and air-washing system. 【Conclusion】It is obvious that the influencing factors such as air exchange rate, measurement period during the rapid hydrolysis of applied urea, characteristics of the dynamic chamber, and the air-washing device should all be considered in measuring AV. The transparent dynamic chamber(diameter 15 cm) connected with a separate flowmeter and only one gas-washing bottle, containing gas-washing holes on the side of the disk or the end side of the straight rod,airflow rate of 15-20 L·min^(-1), and the gas phase height of 5-8 cm are recommended to be employed in measuring ammonia volatilization from rice fields.