| The pollution,remediation and management of Cd in farmland soil,have been paid more and more attention.Cd input into paddy soil through irrigation is not highly valued because of its low concentration.However,Cd in irrigation water with high biological activity of its occurrence mode and the high irrigation quota leads to the large flux of Cd input in the paddy field.Positioning researches on the balance of Cd in paddy soil are mostly based on qualitative statistics while systematic positioning surveys and analyses are still rare.Therefore,it is of great theoretical significance and practical value to discuss the purification and interception methods of Cd with low-concentration and high-throughput in irrigation water and to clarify the balance of Cd in soil.It also plays an important role in revealing the development trend of Cd pollution,protecting soil quality and formulating sustainable management and prevention measures.A typical paddy field system was designed in central Hunan and a plant pond with a constructed wetland system was also implemented,combining horizontal subsurface flow and surface flow.The irrigation water was purified and treated during the growth phase of rice.Combined with field-grown rice plot experiments,this study investigates the effects of constructed wetland system on the purification efficiency of Cd in irrigation water and the accumulation of Cd in rice.By conducting field experiments,this study located and collected irrigation water,atmospheric deposition,soil and rice samples from January 2016 to December 2018,monitored and analyzed the Cd content in the samples,and studied the balance of Cd input and output.The study results are as follows:(1)The concentrations of Cd in the irrigation water of 2017 and 2018 are 0.001?0.479 ?g·L-1 and 0.011?0.904 ?g·L-1 respectively.The concentration of influent Cd in the constructed wetland system in 2017 ranges from 0.034 to 0.644?·L-1 and that of effluent Cd ranges from 0.002 to 0.189 ?g·L-1.The influent Cd concentration in the system in 2017 ranges from 0.090 to 0.479 ?g·L-1 while that of effluent Cd concentration ranges from 0.001 to 0.111 ?g·L-1.The total purification rate of Cd in the irrigation water by the constructed wetland system is 72.06%.The purification efficiency of each level from high to low is as follows:primary constructed wetland>secondiary constructed wetland>tertiary constructed wetland.The plant pond with the constructed wetland system can effectively intercept the low concentration Cd in irrigation water.The concentration of Cd in the water body is in line with the farmland irrigation water quality standard(GB 5084-2005).In the constructed wetland system,the adsorbent materials including permutite and modified biochar had good adsorption effects on Cd with a purification rate of 7.81%.The content of Cd in the wetland plants Typha orientalis Presl,Pontederia cordata L.and Myriophyllum verticillatum L.at the mature stage after planting increased by an average of 1.55-2.71 times compared to the pre-planting seedling stage.The accumulation amount from high to low is:Typha orientalis Presl>Pontederia cordata L.>Myriophyllum verticillatum L..The average purification rate of Cd absorbed by plants is 32.14%.Field plot experiments show that purified irrigation can reduce the Cd content in brown rice of early and late rice by 10.50%to 24.51%.(2)Field experiments was conducted to study the input of Cd in paddy field system of the study area through irrigation water,atmospheric subsidence and agricultural chemicals.The concentration of Cd in irrigation water ranges from 0.018 to 0.644 ?g·L-1,and the annual input flux of Cd in irrigation water of the 3 years is 1.04,1.94 and 1.42 g·(hm2·a)-1 respectively.Cd concentration in atmospheric deposition ranges from 0.039 to 8.342?g·L-1 and the Cd input fluxes of the 3 years are 6.76,5.67 and 7.68 g·(hm2·a)-1 respectively.Cd content in different agricultural chemicals.ranges from 0.016 to 0.448 mg·kg-1,and the annual input of Cd of the 3 years are 0.44,0.59 and 0.66 g.(hm2·a)-1 respectively.The input fluxes from 2016 to 2018 are 8.240,8.200 and 9.766 g·(hm2·a)-1 respectively.The average annual input is 8.735 g·(hm2·a)-1,of which atmospheric deposition,the main source,accounts for 69.15%?82.04%with an average of 76.61%,followed by irrigation water accounting for 12.62%?23.66%with an average of 16.94%and agricultural chemicals accounting for 5.34%?7.19%with an average of 6.45%.(3)Field experiments was also conducted to study the output of Cd in paddy field system through surface runoff,soil infiltration and the harvest of the aerial part.Cd concentration in surface runoff ranges from 0.033 to 1.109?g·L-1.The annual output of Cd in surface runoff of the 3 years is 0.310,0.596 and 1.167 g·(hm2·a)-1 respectively.The annual output of Cd in soil infiltration of these 3 years is 0.028,0.093 and 0.064 g·(hm2·a)-1 respectively.The Cd content in rice plants ranges from 0.819 to 1.975 mg·kg-1.The annual Cd output of rice plants of the 3 years is 6.339,5.451 and 7.121 g·(hm2.a)-1 respectively.From 2016 to 2018 the output fluxes are 6.778,6.139 and 8.361 g·(hm2·a)-1 respectively.The average annual output is 7.093 g·(hm2·a)-1,and the harvest of the aerial part accounting for 85.27%?95.02%of the total output with an average of 89.69%,followed by surface runoff accounting for 4.57%?13.96%with an average of 9.41%.The soil infiltration output contains the least Cd,ranging from 0.41%to 1.51%with an average of 0.90%.(4)The analysis of Cd balance in paddy field system shows that the total Cd input in the study area is higher than the total output.The net input flux of Cd in paddy soil from 2016 to 2018 are 1.462,2.061 and 1.405 g·(hm2·a)-1 respectively.The average annual net input flux is 1.643 g.(hm2·a)-1,and for 3 years the total net input of Cd in paddy field soil is 4.928 g·hm-2,which exerts cumulative effects on Cd in paddy soil.Studies have shown that Cd in paddy fields presents a net input,and Cd pollution in paddy soils is still under development.In summary,although the Cd content in irrigation water is low(0.018?0.644?g·L-1),the input fluxes ranges from 1.04 to 1.42 g·(hm2·a)-1,accounting for 16.94%of the total input flux.Since the biological activity of the occurence mode of Cd in irrigation water is high,and its migration and transformation are rapid,Cd input into paddy soil through irrigation water still needs attention.The Cd input of the paddy field system in the study area is net input,and the Cd pollution in the paddy soil is still under development.However,straw removal can slow down the development trend of Cd pollution in paddy field. |
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