| As a necessary trace element for human beings,selenium has biological effects such as anti-oxidation,anti-aging,promoting immunity,preventing cancer,inhibiting cancer and improving reproductive ability.With the function and function of selenium being emphasized by the public,rice and other crops which are naturally rich in selenium are paid special attention to.Selenium is enriched in the unsaturated zone by primary or man-made factors.Only water-soluble selenium,ion exchangeable selenium and carbonate bonded selenium can be absorbed through the root of crops and then consumed by human beings to supplement selenium.Therefore,the selenium content in crops is not only related to the total selenium content in soil,but also depends on the bioavailability of soil selenium.Sanjiang plain,as an important commodity grain base in China,has the resource advantage of developing green selenium-rich agricultural products.In recent years,the phenomenon of "changing from dry land to paddy land" is very common in Sanjiang plain,alternating wet and dry irrigation was also promoted by the government.The unique irrigation method of paddy field will inevitably lead to the change of selenium distribution and bioavailability in the paddy field vadose zone,and then affect the soil's selenium supply capacity to crops.This paper took the typical agricultural area of the 852 farm in Hamatong River Basin of Sanjiang Plain as the study area,with dry field transform paddy field vadose zone as the object.It used the field investigation and laboratory experiment to study distribution and bioavailability of Selenium in the unsaturated zone under the influence of changing dryland into paddy field.It is helpful for developing natural selenium-rich rice in the study area,and it provide a scientific basis for the local agricultural production and rational development and utilization of land resources.The main achievements of this paper are as follows:(1)Drought-to-water modification will change the characteristics of vadose zone.With the increase of years of drought to water,the clay content,organic matter,iron and manganese oxide content were lower in the vadose zone,and the silt content,moisture content and humic acid content were higher.(2)The vertical distribution characteristics of total selenium content in the profile of aero-vadose zone in paddy fields are consistent with that in dry lands.Irrigation in paddy fields promotes selenium migration from vadose zone to groundwater,so the content of water-soluble selenium and selenium bioavailability in paddy field vadose zone are generally lower than those in dry land vadose zone,and the longer the period of drought to water change,the lower the selenium bioavailability in vadose zone.(3)Laboratory experiments show that the increase of moisture content of the media in the vadose zone,the pH and Eh values in the vadose zone tend to increase,and the content of free iron oxide increases.When the water content of vadose zone is between 30% and 40%,the increase of water content is beneficial to the accumulation of effective selenium.The alternation of dry and wet in the inclusion zone stimulates the activity of microorganisms,enhances the humus of organic matters,and makes the content of strong organic bound selenium,iron and manganese oxide bound selenium lower on the whole,which promotes the conversion of exchangeable selenium into more stable residual selenium.(4)The results of soil column leaching showed that the long time eluviation caused the loss of selenium in the vallation zone and reduced the bioavailability of selenium in the vallation zone.Compared with the traditional flooded irrigation,the content of carbonate bound selenium in the vadose zone increased after alternating wet and dry irrigation,which promoted the humus of organic matter and increased the content of humus bound selenium,and the effective selenium content in vadose zone decreased less. |
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