| The Three Gorgers Reservoir is the largest scale of reservoir in the world,which is an annual adjustment type river reservoir.The reservoir area is 600 km long from Sandouping in Yichang to Jiangjin in Chongqing,with the highest water level 175 m and the lowest water level 145 m.The reservoir forms a large area of hydro-recession belt between month 4 and month 9 every year,with an area up to 349 km2.With the comfortable condition for crops,many land on the hydro-recession belt became agricultural land where the farmers plant rice,corns and vegetables,etc..Consequently,it became a considerable question how the agricultural activity impacted ecosystem of the hydro-recession belt.As we all known,reservoir was considered as a ?sensitive ecosystem to mercury?,and it was the key of reservoir mercury risk that the behavior of mercury in the hydro-fluctuation zone.The agricultural activity maybe influence the behavior of the mercury in the hadro-fluctuation belt,and with the different agricultural pattern the effects may be different.Therefore,this study selected the different utilized soil for planting rice,corns,vegetables and the unutilized soil covered grass in the hydro-fluctuation belt of the Three Gorgers Reservoir in Qukou town,Kai County,Chongqing province,where is located in the middle part of the reservoir,and continuously sample in rigid site for a year.The distribution,migration and transformation characteristics of mercury in the soil were studied,and it is expected to provide some base data for analysing mercury pollution risk in the Three Gorgers Reservoir.The research results as follows:?1?The soil utilization types in the Three Gorgers Reservoir's hydro-fluctuation belt include: paddy,cornfield,vegetable field and meadom.The three tillage types mainly distrubuted on the top of the hydro-recession belt where the elevation is between 165 – 175 m and the hydro-recession period maintain up to 200 d every year.The agricultural activity obviously decreased the THg and MeHg concentration in the hydro-recession belt soil,which may increase the reservoir mercury load with the Hg escaping from soil go into the reservoir by runoff.THg concentration in the top layer soil range between 25.80 – 68.74 ng·g-1,with meadom > vegetable field > paddy and cornfield?P < 0.01?.The MeHg concentration range between 0.075–0.852 ng·g-1 with meadom > paddy,cornfield and vegetable field?P < 0.01?.The average THg level in meadom soil is?56.35 ± 6.84?ng·g-1 and the average MeHg level is?0.572 ± 0.198?ng·g-1;Comparely,the THg level in vegetable field is?37.35 ± 2.45?ng·g-1 and the MeHg is?0.259 ± 0.147?ng·g-1.Consequently,it is very obvious that agricultural activity decrease the THg and MeHg level in hydro-recession belt soil.?2?The MeHg concentration in the hydro-recession belt soil depend on the available Hg concentration.Thera is a significant positive correlation between the MeHg concentration and the available Hg concentration?P < 0.01?,with the correlation coefficient 0.796;There is a same correlation between MeHg and THg concentration?P < 0.01?,but with the lower correlation coefficient 0.571;The MeHg concentration has no significant correlation with the water soluble Hg.The MeHg proportion to THg in the soil mainly depend on the MeHg concentration?P < 0.01?,with a significant positive correlation coefficient 0.855;There is a significant positive correlation between the MeHg proportion and the available Hg?P < 0.01?,with a correlation coefficient 0.550;There is no significant correlation between MeHg proportion and THg concentration or water soluble Hg.?3?The MeHg concentration variation characteristics of paddy soil,corn soil and meadom soil are similar with a trend that firstly increasing and then decreasing.The peak of MeHg concentration appears between 1 and 2 month after the water recession,and then the MeHg concentration decrease gradually to a stable level.For paddy,the MeHg concentration peak of soil at hydro-recession period is about 3 times than the MeHg concentration before the hydro-recession period,but there is no significant difference in the MeHg concentration between the MeHg concentration stable period and the waterflooding period.Similarly,the MeHg proportion has a same change characteristic,but with a peak at 1 month after water recession and 4 times MeHg concentration in the water recession period than before.The result indicated that the mercury methylation in hydro-fluctuation belt soil is stimulated mightily by water recession which may change some environmental conditions of the soil.Therefore,water recession may increase the MeHg pollution risk of the crops like rice on the hydro-fluctuation belt.?4?In hydro-recession period,there is no obvious relation between the mercury speciation and temperature,but a negative correlation between the MeHg proportion and air temperature?P < 0.05?.It can stimulate the loss of TH,MeHg and available Hg in soil that the soil p H decrease,due to the THg,MeHg and available Hg concentration in soil have significant positive correlations with the soil pH in hydro-recession period?P < 0.01?.Furthermore,agricultural activity is a main factor to decrease the soil pH;and the soil pH may decrease as hydro-recession time goes on,due to the pH has a negative correlation with the hydro-recession time?P < 0.05?.The THg,available Hg and water soluble Hg concentration in soil may decrease with the decrease of soil humidity?P < 0.01 or 0.05?,but no significant effect with the MeHg concentration or mercury methylation.So the flooding,rain and runoff may be the main way where the Hg in the hydro-fluctuation belt soil go into the reservoir. |
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