Dynamic Variation Of Mercury Species In Soil In Wet-Dry Rotation Environment

As a global pollutant, biogeochemical cycle course of mercury is always the research hotspot in field of environment science. It is considered that global total amount of mercury imported to atmosphere is about 7500 t annually, and amount of mercury from native source nearly occupy 1/2 of the total. In native release sources of mercury, soil is the most important native source, and mercuric granary of sediment of reservoir and lake were bleeding of mercury from soil. According to research, majority of artificial release mercury entered atmosphere will settle down in alout one year. Therefore, soil isn't only the source of mercury, but also the sunk of it. For the moment, there have been many researches aiming at mechanistic of transportation and transformation of mercury, and we have known to a certainty mercury exchange flux of the environmental surface and influence factors of it, but there is few report about research of release characteristic of mercury in soil in wet-dry rotation environment. Based on above causes, the main cultivate soil of Chongqing including purple soil and yellow soils as well as the alluvia in three gorge reservoir were regarded as objects of investigation in this study by field investigation combined with simulated test. Spot monitoring were studied by RA-915⁺ mercury analyzer and dynamic fluxes chamber technique, and researches about dynamic variation of mercury species and release characteristics in wet-dry rotation . environment were carried out in laboratory simulation, too. The results showed:In the process of alternation of wetting and drying in simulated test, soil mercuric content would fall off, and descending amplitude of yellow soil was lower than it of purple soil and alluvia. Acid soluble species was main species of soil mercury before and after flooding, and aquaregia soluble species and alkali soluble species followed, and content of aqua soluble species and auricome soluble species were undermost. Variation of total mercury mostly incarnated variation of acid soluble species, and aquaregia soluble species which had opposite trend to alkali soluble species was indicative of uptrend, and as to aqua soluble species and auricome soluble species, there were no distinct variation rule. Variance amplitude of treatment A (mercurization) was greater than it of treatment B (unmercurization). There were no correlation between any species and total mercury of treatment A, but there were excessively significant positive correlation between acid soluble species and total mercury of treatment B. Acid soluble species and alkali soluble species of treatment A had positive correlation with content of organic matter, but only total mercury of treatment B had positive correlation with it. Alkali soluble species of treatment A had negative correlation with pH, but in treatment B, it is the total mercury which had negative correlation with pH.During the drying period, mercuric content of each plot kept relatively stable, content of aquaregia soluble species was the highest, contents of acid soluble species and alkali soluble species followed and came close to each other, contents of aqua soluble species and auricome soluble species were the lowest. The order of Coefficient of variation is aqua soluble species > auricome soluble species > alkali soluble species > acid soluble species > aquaregia soluble species. Acid soluble species had excessively significant positive correlation with total mercury. All of species and total mercury had significant or excessively significant correlation with content of organic matter, there was negative correlation between alkali soluble species and organic matter among the rest, and other species and total mercury had positive correlation with organic matter. There were negative correlation between auricome soluble species and pH of soil, and between aquaregia soluble species and pH. Alkali soluble species had positive correlation with pH.During flooding period, total mercury in the water varied along with total mercury in soil. For the first flooding, content of total mercury in water of treatment A fall off at first, then rose up, and was rising up constantly for the second flooding. Content of total mercury in water of treatment B had uptrend. Active mercury was main species in water, and variance of total mercury mostly incarnated variance of active mercury. Content of dissolved gaseous mercury was very low and kept relatively stable. Only yellow soil of the three types, there was excessively significant negative correlation between active mercury and pH of water, correlation coefficients of treatment A and treatment B were respectively -0.776^** and -0.793^**. There were significant positive correlation between active mercury and E_h of the water and the soil. Active mercury of treatment A of yellow soil had significant correlation with water temperature, and correlation coefficient was 0.659^*, and correlation coefficients of treatment B of purple soil and alluvia were respectively 0.822* and 0.778*. There were no significant correlation between total mercury and water temperature in treatment A, which had opposite situation to treatment B.During the flooding, the higher the content of mercury in soil, the higher the mercury exchange flux in water/air surface. For the first flooding, mercury exchange flux of treatment A came close to each other, as well as treatment B. Flux of treatment A continuously reduced, and in treatment B, flux of yellow soil had downtrend which was opposite to purple soil and alluvia. For the second flooding, flux of all treatments fall ? off a lot, and flux of alluvia descended the most. In this period, flux of treatment A and B had nearly the same variance trend. Flux of yellow soil and purple soil fluctuated, but flux of alluvia slowly rose up at first, and then slowly descended. Variance of flux in water/air surface mainly contraried with that of active mercury. There were differences between three types of soil in the influences which caused by meteorologic factor and hydrochemistry.Comparison showed that the higher the content of mercury in soil, the higher the flux in soil/air surface. In treatment A, except yellow soil, flux of mercury in soil/air surface was much higher than it in water/air surface, and alluvia had highest flux. Flux of mercury in soil/air surface of treatment B was lower than that in water/air surface, and there were no distinct differences between three types of soil. During the drying period, in treatment A, flux of yellow soil continuously descended, and flux of purple soil and alluvia rose up at the beginning, then fall off. Flux of treatment B descended slowly. In treatment A, only flux of yellow soil had correlation with air temperature, and correlation coefficient was 0.479*. Flux of yellow soil and alluvia had significant correlation with relative humidity, and correlation coefficients were respectively -0.574^* and -0.498^*. Flux of purple soil and alluvia also had significant correlation with intensity of illumination, and correlation coefficients were respectively 0.459* and 0.504*. There were no correlation between flux and meteorological factors in treatment B.In 13 plots, it is the rape-wheat rotation plot which had the highest flux, and it is the weeds plots which had the lowest flux. There was a little difference between flux of each plot, as well as variance of it, and the scope was mainly -50 ng m^-2 h^-1100 ng m^-2 h^-1 within which flux of all plots fluctuated. As a whole, flux of all plots had excessively significant correlation with meteorological factors, and correlation coefficients with atmosphere pressure (x1), relative humidity (x2), air temperature (x3), soil temperature (x4) and intensity of illumination (x5) were respectively -0.238^**, -0.234^**, 0.333^**, 0.278^** and 0.287^**,and the optimum regression equation is y=-5.302+2.692x₃-2.611x₄+0.035x₅ (R²=0.146).