Characteristics Of Mercury Release From Rice-wheat/rape Rotation System And Its Influencing Factors

As a global pollutant, mercury is receiving intensive scrutiny due to its special physical characteristics. It occurs in various physical and chemical forms, likely volatilization and long range transportation in the environment. In recent years, the concentration of mercury is increasing in the atmosphere due to the elevated emission of anthropogenic activities, such as combustion of fossil fuels, roasting and smelting of ores etc. Most of atmospheric Hg in ambient air, above 90%, is made up of elemental mercury (Hg⁰), while the remainders are divalent Hg compounds, either in the gaseous phase or bound to particles, and methylmercury. The Hg⁰ (g) has a relatively long residence time of 0.5～2 years in the atmosphere, and which can transportation for a long distance along the global atmosphere cycling and to pose a global pollution. Researches of mercury pollution associated with human health risk had been considerable concerned in past years. In recent years, the hotspot issues of mercury pollution were mostly turned to the aspect of understanding the laws of mercury cycling and transformation in the atmosphere on global scale. So, it is important to discern the resources of mercury emission and measure or estimate the amounts correctly of mercury exchange.The atmospheric loading of mercury is come from natural and anthropogenic sources. Emission inventories indicate that the contribution of mercury emission from natural sources is about 50% within the total amount of emission from global surface. Many researches involve to mercury emission from environmental surface in recent years, which include evasion from active volcano, earth heat-radiating, forest-fire, soil/water surface, transpiration from foliage of vegetation and urban surface etc, have provided lots of references to assessment of annual inventories of atmospheric mercury budget on global scale. However, there are some drawbacks with error and uncertainty in these estimations due to the restriction of limited spatial-temporal scale. Average inventories of mercury emission from natural sources has been estimated ranges between 2500 and 19325 t yr^-1, and the contribution from anthropogenic sources ranges between 910 and 110001 yr^-1. Researches on biogeochemical cycle of mercury in recent years were mostly focused on assessment of mercury release flux on environment surface, and an average rate of Hg evasion, 1.5 ng m^-2h^-1, was taken to be the normal standard rate. Nevertheless, the field studies lately exhibited that the foregoing rate would lead to lower estimate the contribution from natural sources of mercury emission. Therefore, it is important to distinguish and determine the characteristics of various natural surfaces, and that is for the purpose of improvement estimation of mercury budget exactly and fully understanding global biogeochemical cycling of mercury.At present, for study mercury emission from natural sources, dynamic flux chamber (DFC) technique has been adopted widely, which was first used by Schroeder et al. (1989) and Xiao et al. (1991) for measurement mercury exchange flux over different environmental interfaces (e.g. soil/water-air). Through the studies for last decade, people have realized preliminarily many important aspects of mercury exchange flux from various natural surfaces, but there are scarcely reports that involved with the mercury exchange characteristics in the interfaces of periodicity dry-wet alternating environment (e.g. water level fluctuating zone of reservoir, paddy-upland rotation farm land). Especially, there are lack of informations about the aspect of the seasonal varies characteristics of mercury release and climatic influencing factors in the special environment.Paddy-upland rotation is a farming tradition that wildly adopted by Asian countries and it also used as a main culture manner in south China. The area of paddy-upland rotation farmland covered above 1400 millions hm² in China and it consists of 30～40% of the rice-land resources. Paddy-upland rotation system has an intrinsic peculiar characteristic that the conditions of thermal energy and oxidation- deoxidization change heavily along the periodic varying of season, and this may lead to an evident influence in the course of mercury transformation, transport and accumulation in soil. Because of lack of systematic informations measured about this circumstance, the purpose of this paper is to study the characteristics of mercury release fluxes and its chemical forms transformation, in the paddy-upland (dry-wet) rotation system. This research would exhibit an important significance for improvement on farther understanding the regional or global mercury resource and its release intensity.Here, the study was conducted by both in simulating experiment, laboratory analysis and field investigation in situ. The field investigation site was located at National Purple Soil Fertility and Fertilizer Efficiency Monitored Base (E:106°24'33.3'' N:29°48'36.2"), which situated at Southwest University, Beibei District in Chongqing, China. The research area belongs to the purple hilly land with an elevation of 239 m. The subtropical earth monsoon climate is presented in there. According to statistics, the mean annual temperature is 18.4°C, annual precipitation is 1105.5 mm, and annual duration of sunshine is 1276.7 h. The tested soil was the gray brown purple soil (Purpli-Udic Cambosols), which is a neutral purple soil subtype and developed from the purple mud stone and shale of Saximiao Association of Jurassic Period. It is the most purple soil types, and it occupies about 40% of the area of purple soils and most of the grain base in Chongqing distribute in it. So it is the typical representative of purple soils. The research area covered 1200 m² and was ploted out to 13 divisions.Different chemical form of mercury has different transference activeness in soil, and it can dominate the release flux of theirs, ultimately. Wherefore, this study had analyzed the mercury existence forms and its variety in different farming seasons while in the flooding period (rice growth term), the intermission period and the upland period (wheat/rape growth term), respectively. Besides, the study had studied the characteristics of mercury exchange and distribution between the soil and water interface in rice growth period by means of simulating flooding experiment. The results revealed that, the mercury content and species distribution of paddy-upland rotation soil varying periodically along with the alternating change of farming period, namely dry-wet change by turn. Total mercury content in the upland soil is higher than that of flooding soil. Residual Hg is the predomination species in the upland soil, and it has a lower activity. Contrarily, soluble Hg is the predomination species in the flooding soil and it has a higher activity. The essential reason of this situation is caused by the periodic change of oxidation-deoxidization conditions in farmland. The results of simulating flooding experiment demonstrated that the mercury concentration of water column increased with the increasing content of soil Hg. Mercury concentration in water have two peak values after flooding, the first peak occurred earlier (3^rd d), the latter appeared relatively later (about 12^th d). After the wet-dry process, partly soil mercury evasion into the air presented as Hg⁰, and which is mainly the production of acid-soluble and H₂O-soluble forms Hg deoxidized through biological or abiological transforming course. During flooding period, the concentration fluctuation trend between DGM and Hg²⁺ in water column is reverse, and it implies that they could transform to each other at suitable environmental condition. The data of mercury fluxes measured in the simulating flooding experiment course showed that, the divalent Hg is easier exchange, transformation and finally evaporation to atmosphere than other forms of mercury in the soil-water-air interface.Mercury fluxes measurement were performed by dynamic flux chamber (DFC) cope with the Lumex^? Multifunctional mercury analyzer RA-915⁺during different farming season while in the flooding period (rice growth term), the intermission period and the upland period (wheat/rape growth term) respectively, at all plots in the investigating field. The characteristics of mercury flux in different farming seasons and covered crops were discussed. Simultaneously, mercury flux influencing factors such as meteorological, soil properties, fertilizer management and microbial communities were also discussed. Finally, an annual emission of mercury to the atmosphere from paddy-upland rotation farmland in Chongqing was estimated. The results indicated that, the average value of mercury fluxes in investigated area was higher than that of global background by 1 magnitude. Therefore, the results imply that the paddy-upland rotation system may be an importance resource which is a potentially contributor to the atmospheric mercury content. During whole dry-wet rotation farming seasons, the mean mercury exchange flux at water-air interface in rice land is the highest with the value of 82.9±73.3 ng m^-2 h^-1, next to the soil-air interface in intermission period with the value of 24.0±17.1 ng m^-2 h^-1, the mean flux of soil-air interface in wheat/rape growth period was the lowest with the value of 11.0±3.5 ng m^-2 h^-1. The annual average mercury flux in whole fanning seasons is 39.3±53.2 ng m^-2 h^-1.Mercury fluxes of water/soil-air interfaces from purple soil rotation farmland exhibit a significantly diurnal and seasonal variation. There was evidently difference of fluxes measured from different season and interface, the decreasing order is as follows warm season rice land (water/air), warm season intermission leisure land (soil/air), warm reason rice/wheat land (soil/air) and cold season rice/wheat land (soil/air). Mercury evasion exhibits a strong diurnal cycle variation, with the highest during midday, while lower in the morning and early evening. Mercury fluxes over air/surfaces had bi-directional exchange in the morning and early evening in cold season. Results of statistical analysis showed, there was positive correlation between mercury flux and total Hg content in paddy-upland rotation farmland (water: r=0.463^*, soil: r=0.322^*), on the contrary, there was significantly negative correlation between mercury flux and soil organic matter content (r=-0.688**). Climatic factors had effected evidently to mercury flux in the water/soil-air interfaces. Regression analysis of data measured over water/soil surface indicated that mercury flux appear to be most significantly positive relation to solar radiation (r=0.730^**) and soil/water temperature (soil: r= 0.514^**; water: r = 0.171^*), and significantly negatively correlated with relative humidity over soil surface (r=-0.748^**), and weaker correlated with relative humidity over water surface and soil moistness. Analysis of data measured also suggested that exponential correlation of mercury flux with soil/water temperature in some plots (soil: R²= 0.3047^**; water: R²=0.7044^**). Arrhenius equation could be applied to explain the link between the kinetics rate constant of this process and the soil/water temperature, so the activation energy (Ea) was then calculated to be 21.5 / 27.7 kcal mol^-1 from the slope of best fit line. The values were higher than the mercury vaporization heat of 14.5 kcal mol^-1, implying that the mercury emission from soil /water surface is not the vaporization of elemental mercury alone.Likewise, results of this research suggested that, farming manner, include crop category, coverage density and fertilizer management, would be an important influencing factor that can effect evidently to mercury flux from the soil/water surface. Especially, the fertilizer management can cause the changes of soil property, microbeial communities and mercury existence form and its transformation. Sequentially, it dominates the course of mercury accumulation and emission in soil.Moreover, data obtained from microorganism cultivation revealed that, a negatively correlation (r= -0.580^*) between the amount of anaerobic bacterial communities (cfu) in the flooding soil and mercury fluxes over water surface during the rice/rape growth term. Two categories of bacterial populations, sulfate-reducing bacteria (SRB) and anaerobic cellulose-decomposing bacteria (ACDB) , were the dominating populations for influencing mercury release flux during the flooding terms, the correlation coefficients were -0.630^*(SRB) and -0.558^* (ACDB), respectively. Nevertheless, the correlation between bacteria communities and mercury fluxes in upland was insignificant. This means that the mercury flux in upland was dominated by other environmental factors.Associating with climate characteristics of Chongqing, a preliminary estimation of regional mercury budget was carried out based on the fluxes data of the experimental plots which measured by dynamic flux chamber. The total mercury emission to the atmosphere from dry-wet rotation farm fields was annually about 565 kg in Chongqing.