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.