大气环境科学实验室知识库

     本研究调查了我国北方典型历史污灌区土壤汞污染现状，报道了土壤汞含量的空间分布和垂直分布特征。由于中国北方污灌区分布广泛且纬度差异较大，每个污灌区的作物生长情况、气象条件均不一致，原位观测试验不仅难以实现连续多点观测而且观测结果也将不具备可比性。针对原位定点观测试验的缺点，本文采用土壤置换方法，在相同自然条件下采用动力学通量箱法对我国北方多个典型
历史污灌区的农田土壤汞排放通量进行观测，获得了玉米—小麦生育期（225 天）连续的具有可比性的农田土壤汞排放通量数据。同时，对比研究了5 种作物和3个不同种植密度冠层下的农田土壤汞排放通量及其影响因素。通过以上实验，归纳总结了影响不同历史污灌区农田土壤汞排放的关键环境要素和造成不同种植作物和种植密度下农田土壤汞排放的核心影响因素，建立了二阶参数化方程，随后对我国北方历史污灌区农田土壤汞排放通量进行了初步的估算。
      研究结果表明，我国北方历史污灌区农田表层土壤总汞浓度空间分布不均，省会城市附近的浓度高于地级和县级农田汞的浓度，这可能与污灌历史有关；其它层总汞浓度的空间分布与表层的分布特征基本一致。在垂直分布上，土壤总汞的分布特征与土壤有机质的分布特征相一致，相关性分析结果也表明二者呈正相关关系；而与pH 的相关性不明显。各采样点土壤总汞浓度都有从表层到底层逐渐降低的趋势。我国北方历史污灌区农田土壤目前的汞污染情况为：轻度汞污染的样点约占采样总数的23%；中度汞污染的样点约占57%；重度汞污染的样点约占20%；通过区域调查发现，我国北方历史污灌区的土壤汞污染在一些地方仍然比较严重，所有采样点平均土壤总汞浓度为117.2±138.0 ng g-1，明显高于中国土壤背景值（38 ng g-1）。
      土壤置换实验的观测结果表明，4 个污灌区农田和1 个清灌农田的土壤汞排放通量在作物不同生育期均呈现出相似变化规律。污水灌溉通过增加土壤总汞含量，显著地提升了土壤气态汞的排放通量。在玉米—小麦两种作物不同的生育期，土壤汞排放通量与土壤总汞含量均呈线性正相关响应关系，而且农田土壤汞的排放潜力大于森林和城市土壤。玉米季和小麦季土壤汞排放通量的昼夜比值分别为3.94 和3.41，与土壤总汞浓度变化关系不大。根据数据显示，自然条件下土壤汞 排放通量对土壤温升的响应并不是简单的正相关响应关系。在土壤温度在19℃～27℃范围内逐渐升高时，农田土壤排汞通量快速增加。另外，土壤水分的增加促进了历史污灌区农田土壤中汞的释放，这可能与中国北方自然降水条件下，土壤水分长期处于的非饱和状态有关。农田土壤汞排放随环境空气汞含量的增加表现为双峰曲线，这是不同于其他陆地生态系统土壤汞排放的主要特征。污水灌溉能显著提高土壤中汞的逃逸率，使得土壤汞排放补偿点随之升高。
      不同种植作物和种植密度对农田土壤汞排放通量产生显著影响。在暖季，不同种植作物的农田（行距30 cm）土壤汞排放通量（ng m-2 h-1）的平均值排序为：高粱（35.38±21.64）>燕麦(33.22±20.95）>玉米（28.76±17.81）>大豆（22.99±14.96）；所有样地暖季排放通量的平均值均大于冷季小麦（-5.55±16.61）；在暖季，不同种植密度的农田土壤汞排放通量（ng m-2 h-1）平均为：常规玉米（28.76±17.81，行距30 cm）> 密植玉米（23.05±15.90，行距20 cm），在冷季为：稀植小麦（-4.87±16.92，行距40 cm）>常规小麦（-5.55±16.61，行距30 cm）。各试验区农田土壤汞排放通量差异有统计学意义（p<0.05）。在各种环境因子中，不同冠层下的透光率的差异是冠层下环境因子的主要差异，也是造成各冠层下农田土壤汞排放通量不同的核心因素。
      根据以上实验结果，得出影响我国北方历史污灌区不同汞污染农田土壤汞排放的主要环境因子为土壤汞含量，另外，不同冠层下的透光率的差异是冠层下环境因子的主要差异。因此，根据土壤汞排放通量与土壤总汞含量、冠层以上太阳总辐射和播后天数之间的响应关系，建立了二阶模型。随后，我们估算了30 cm行距的玉米—小麦轮农田在整个作物生长年的土壤汞排放通量平均值为5.46 ±21.69 ng·m-2 h-1，年累积土壤排汞量为47.83 ug m-2 a-1。基于在我国北方各采样点估算数据的获得的平均累积汞排放通量275.12 ± 333.51 ug m-2 a-1，初步估算我国北方历史污灌区农田土壤汞排放通量约为9.96 t a1，其占陆地排放估算通量200 t a1 的比例为4.98 %，说明我国北方历史污灌农田在全球土壤汞排放中起着重要的作用，具有污灌历史的汞污染农田土壤仍为大气汞的排放热区。

      This study investigated the status of soil mercury pollution in typical historical sewage irrigated areas in northern China, and reported the spatial and vertical distribution characteristics of soil mercury content. Due to the widespread distribution of sewage irrigated areas in northern China and large latitude differences, crop growth and meteorological conditions in each sewage irrigated area were inconsistent. In-situ observation experiments would not only make it difficult to achieve continuous multipoint observations but the observation results might not be comparable. In view of the shortcomings of the in-situ observation experiment, this paper uses soil replacement method and dynamic flux charmber method under the same natural conditions to observe the farmland soil mercury emission fluxes in several typical historical sewage irrigated areas in northern China. Continuously comparable data on mercury emissions from agricultural soils during the wheat growth period (225 days) were got. At the same time, the flux of mercury in farmland soils and its influencing factors under the canopies of 5 crops and 3 different planting densities were compared and studied. Through the above experiments, the key environmental factors that affect the mercury emissions from farmland soils in different historical sewage irrigated areas and the core factors that cause the mercury emissions from farmland soils under different crops and planting densities were summarized. Second-order parameterized equations were established. Apreliminary estimate of the flux of mercury emissions from farmland soils in historical sewage irrigated areas was made.
      The results show that the spatial distribution of total mercury concentration in the surface soil of historical sewage irrigated area in northern China was uneven, and the soil mercury concentration near the provincial capital city was higher than that in the prefecture level and county-level farmlands, which might be related to the sewage irrigation history; the spatial distribution of total mercury concentration in other layers was basically consistent with the distribution characteristics of the surface soil. In the vertical distribution, the distribution characteristics of total mercury in soil were consistent with the distribution characteristics of soil organic matter, and the correlation analysis results also show that there was a positive correlation between the two, but the correlation with pH was not obvious. The total mercury concentration of soil in each sampling point decreased gradually from the surface layer to the bottom layer. The current situation of mercury pollution in farmland soil of historical sewage irrigated area in northern China was as follows: the sample points of light mercury pollution accounted for 23% of the total number of samples; the sample points of moderate mercury pollution accounted for 57%; the sample points of heavy mercury pollution accounted for 20%; through regional investigation, it was found that the soil mercury pollution in historical sewage irrigated area in northern China was still serious in some places, and the average total soil mercury concentration of all sampling points was 117.2 ± 138.0 ng-1, significantly higher than the background value of Chinese soil (38ng g-1).
      The results of soil replacement experiments showed that the soil mercury emission fluxes of four sewage irrigation areas and one fresh-water irrigation area showed similar changes in different growth periods of crops. By increasing the total mercury content in the soil, sewage irrigation significantly increased the emission flux of gaseous mercury in the soil. At different growth stages of maize and wheat, there were linear positive correlations between soil mercury emission flux and soil total mercury content,and the potential of mercury emission from farmland soil was greater than that from forest and urban soil. The diurnal ratios of soil mercury emission fluxes in corn and wheat seasons were 3.94 and 3.41, respectively, which had little relationship with the total mercury concentration. According to the data, the response of soil mercury emission flux to soil temperature rise under natural conditions was not a simple positive correlation. When the soil temperature gradually increased in the range of 19℃ ~ 27℃, the soil mercury flux increased rapidly. In addition, the increase of soil water promoted the release of mercury in the farmland soil of historical sewage irrigated area, which may be related to the long-term unsaturated state of soil water under the natural precipitation in northern China. With the increase of mercury content in the air, the soil mercury emission in farmland shows a double peak curve, which is different from other terrestrial ecosystems. Sewage irrigation coulde significantly improve the emission rate of mercury in the soil, which makes the compensation point of mercury emission increase.
        Different planting crops and planting density had significant effect on the mercury emission flux of farmland soil. In the warm season, the average value of soil mercury emission flux (ng m-2 h-1) of farmland with different crops (row spacing of 30 cm) was as follows: broomcorn (35.38 ± 21.64) > oats (33.22 ± 20.95) > corn (28.76 ± 17.81) >soybean (22.99 ± 14.96); the average value of soil mercury emission flux in the warm season was higher than that of wheat (- 5.55 ± 16.61). The average mercury emission flux (ng m-2 h-1) was: conventional maize (28.76 ± 17.81, row spacing 30 cm) > Close Planting Maize (23.05 ± 15.90, row spacing 20 cm). In the cold season, it was: sparse planting wheat (- 4.87 ± 16.92, row spacing 40 cm) > conventional wheat (- 5.55 ±16.61, row spacing 30 cm). There was significant difference in mercury emission flux of farmland soil in each experimental area (P < 0.05). Among all environmental factors,the difference of light transmittance under different canopy was the main difference of environmental factors under canopy, and it was also the core factor that causes the different mercury emission flux of farmland soils under different canopies.
      Based on the above experimental results, we concluded that the main environmental factor affecting the mercury emission of different mercury polluted farmland soils in the northern historical sewage irrigated area was the soil mercury content; in addition, the difference of light transmittance under different canopies was the main difference of environmental factors under the canopies. Therefore, a secondorder model was established based on the response of soil mercury emission flux to soil total mercury content, solar radiation above canopy and days after sowing. Then, we estimated the average soil mercury emission flux of 30 cm row spacing corn-wheat rotation farmland in the whole crop growth year as 5.46 ± 21.69 ng m-2 h-1, and the annual cumulative soil mercury emission was 47.83 ug m-2 a-1. Based on the average cumulative mercury emission flux of 275.12 ± 333.51ug m-2 a-1 obtained from the estimated data of each sampling point in the north of China, it was preliminarily estimated that the soil mercury emission flux of the historical sewage irrigated area in the northern China was about 9.96 t a-1, which accounts for 4.98% of the estimated land mercury emission flux of 200 t a-1, indicating that the historical sewage irrigation area in the northern China played an important role in the global soil mercury emission. The farmland soils with sewage irrigation history were still the hot area of atmospheric mercury emission.