The Effect Of Freeze-thaw Action On Environmental Geochemistry Behavior Of Selenium In The Unsaturated Zone Of Rice Planting Area

Selenium is an essential micronutrient for human beings and animals,which plays an important role in human health and biological growth.In ecosystems,the unsaturated zone is closely related to crop growth,which can supply and coordinate the nutrients needed for the growth and development of crops,and the quality of the unsaturated zone directly affects the growth of crops and then affects the yield of crops and the quality of agricultural products.At the same time,the selenium in the unsaturated zone is closely related to human health and can enter the human body through the food chain,water,respiration and other ways.Moreover,the available selenium content and fractions directly affect the migration of selenium,bioavailability and selenium content in crops.As the unsaturated zone is closely related to climate change,irrigation water and other external environments,it is significantly affected by freeze-thaw action and irrigation.At present,there are few reports on the environmental geochemistry behavior of selenium in the unsaturated zone during freeze-thaw and irrigation period.Sanjiang Plain is one of the main grain-producing areas in China and undergoes frequent seasonal freezing and thawing process all the year round.In recent years,with frequent irrigation and freeze-thaw action,selenium content in the unsaturated zone media has decreased year by year.Therefore,in order to clarify the causes of selenium content loss in the unsaturated zone,it is necessary to make an intensive study of migration and transformation of selenium in the unsaturated zone under freeze-thaw and irrigation conditions.Based on this,this paper takes the typical agricultural area in Hamatong River Basin of Sanjiang Plain as the study area,the selenium in the unsaturated zone as the object,and on the basis of investigating and analyzing the geological and hydrogeological conditions of the study area,identify the fraction and distribution characteristics of selenium in the unsaturated zone media,and clear the uptake and enrichment characteristics of selenium in different organs of crops,recognize the source and enrichment cause of selenium in the unsaturated zone,analyze the environmental geochemistry behavior characteristics of selenium in the unsaturated zone under the influence of freeze-thaw and irrigation condition,forecast the migration trend of selenium in the unsaturated zone under long-term irrigation condition,and bring forward some countermeasures and suggestions for the development of selenium-rich agriculture in Sanjiang Plain.This study has great theoretical and practical significances for enriching and developing the theory of environmental geochemistry of selenium,improving and regulating the agricultural ecological environment,and rationally developing and utilizing selenium-rich resources to improve the selenium content of rice crops.Through this study,the main conclusions are as follows:(1)There are significant differences from the vertical distribution of selenium content in the unsaturated zone between the paddy field and upland field.On the whole,the content of water-soluble selenium and the mobility factor of selenium in the unsaturated zone of the paddy field were significantly lower than those in the unsaturated zone of the upland field.Comprehensive analysis shows that the selenium in the unsaturated zone in the study area is a natural source,which is mainly derived from the granite of the Wanda Mountains.Moreover,the enrichment of selenium in the surface layer of the unsaturated zone can be attributed to the effect of human factors such as fossil fuel combustion and fertilization superimposed on the high background value of selenium.(2)Rice accumulates selenium more efficiently than corn and soybean,and the selenium contents in roots,stems,leaves,and grains are relatively high.In addition,compared with soybean,rice and corn can fix most of the absorbed selenium in its roots or stems,resulting in relatively low transfer ability to the grains.Moreover,the selenium content in different organs of crops was closely related to the available selenium content of unsaturated zone media,and other fractions of selenium could be transformed into available selenium for plant use under certain conditions.Besides,the content of selenium varied markedly among different growth periods and different organs of the same crop,and the selenium content in the grains of rice,soybean,and corn decreased with the ripening of crops.(3)Freeze-thaw action has a significant impact on physicochemical properties of unsaturated zone media,and then changs the fractions of selenium in the media of the unsaturated zone.Moreover,the physicochemical properties and selenium fractions of the unsaturated zone media will change with water content and the number of freeze-thaw cycles.Freeze-thaw action also affects the moisture migration in the unsaturated zone and then changes the selenium content in different depth media of the unsaturated zone.Besides,freeze-thaw action will reduce the adsorption capacity of selenium by the unsaturated zone media,enhance the permeability of the media,promote the release of selenium in the unsaturated zone,and accelerate the migration rate of selenium in the unsaturated zone.(4)The flooding process will significantly change the physicochemical properties of unsaturated zone media,and then changs the fractions of selenium in the media of the unsaturated zone.Moreover,the physicochemical properties and selenium fractions of the unsaturated zone media will change with the flooding time.At the same time,in the process of irrigation infiltration,selenium in the unsaturated zone media will continuously dissolve,but mainly Se(VI).Moreover,after leaching,the contents of residual selenium,humic acid-bound selenium,strongly organic-bound selenium and carbonate-bound selenium in the media of the unsaturated zone increased in varying degrees,while the contents of water-soluble selenium,ion-exchangeable selenium,and Fe-Mn oxide-bound selenium decreased obviously.Numerical simulation results show that the selenium of unsaturated zone media enters groundwater will not affect groundwater quality by leaching action under the condition of long-term irrigation.(5)The alternating effects of freeze-thaw and irrigation action can accelerate the loss of selenium in the unsaturated zone media,and further affect the absorption of selenium in the unsaturated zone by crops,which is not conducive to the development of selenium-rich agriculture.Therefore,it is necessary to rationally develop and utilize the selenium-rich land resources to ensure the sustainable development of healthy and green agricultural production activities.For example,in the long-term rice planting activities,it is suggested to introduce the rice water-saving irrigation cultivation technology,appropriate application of selenium fertilizer and other measures to reduce the loss of selenium in the unsaturated zone,and promote the balance of the selenium content in the unsaturated zone.