Mechanism Research And Optimal Regulation Of Water And Salt Transport And Nutrient Loss Under Controlled Drainage In Salinized Irrigation District

Under the background of water shortage,unreasonable application of nitrogen fertilizer,soil salinization,nitrogen pollution and many other problems becoming increasingly serious in Hetao Irrigation District.Utilize controlled drainage technology to improve salinized soil and realize efficient utilization of water and fertilizer.Maintain the balance between soil water,salt control and nutrient loss.It is of great practical significance for the sustainable development of agricultural ecosystem in Hetao Irrigation District to seek a suitable drainage mode under the control of food security and environmental risk.This study focuses on the distribution characteristics of soil water content,salt content,nitrogen content and their loss laws.The mechanism of"water saving,salt control,fertilizer conservation and emission reduction"under the condition of controlled drainage of salinized oil sunflower farmland was revealed in Hetao Irrigation District.Through field experiments,the water and salt transport law,nitrogen loss,environmental pollution and the response of crop growth and yield to the drainage mode of moderately salinized oil sunflower farmland soil were studied,and a reasonable drainage mode was proposed based on the HYDRUS-2D model.The main findings are as follows:1.The response law of soil moisture content to the controlled drainage mode of salinized soil is clarified.After irrigation in the growth period,the controlled drainage significantly increased the soil moisture content of 0-100 cm,and the most obvious was 0-40 cm.The controlled drainage is 1.67%～5.86%higher than the free drainage treatment.The controlled drainage ensures that the crops have sufficient water absorption and utilization,and improves the soil water availability.Hydrogen and oxygen isotope technology revealed the water source and utilization contribution rate of oil sunflower.The water absorbed by oil sunflower in soil is mainly 0-40cm in 0-10d after irrigation,and the contribution rate of 0-40cm soil water in 2021-2022 is 55.10%～78.66%.Irrigation during the growth period of oil sunflower is the largest source of soil water,with a contribution rate of 84.21%～90.19%,followed by groundwater contribution rate of 7.37%～14.44%,and precipitation of 1.39%～1.63%.At the same time,the contribution rate of different drainage methods to soil water is significantly different,and the contribution rate of controlled drainage irrigation water is higher than that of free drainage.Controlled drainage can improve the recharge of irrigation water to soi l water after irrigation.On the other hand,controlled drainage plays a positive role in ensuring water supply in the later stage of crop growth,thus providing a suitable environment for crop growth and laying a foundation for increasing production.2.The influence of controlled drainage on soil salt leaching was discussed.The change of soil salt content from surface to uniform after spring irrigation.The desalinization rate of soil treated by underground drainage is 48.98%～50.08%,and that of open drainage is 38.37%.There is a great difference in the space of the drainage treatment of the subsurface pipe.The distance from the concealed pipe is 7.09%～7.38%different from the leaching effect of the soil salt content on the subsurface pipe.For open ditch drainage treatment,the average soil desalting rate in two years at the middle point of the open ditch is 37.54%,and the average soil desalting rate in two years at0.4m away from the open ditch is 40.44%,with a difference of 2.90%.Free drainage treatment after irrigation in the growth period has the highest desalting rate in the growth period,with an average desalting rate of 8.15%.Most of the salt in the controlled drainage treatment migrated to the deep soil,which also led to serious salt return in the later stage of the controlled drainage growth,with the salt return rate of30.40%～32.29%.The open ditch drainage does not fully leach the soil salt during spring irrigation,resulting in a high salt content in the soil during the growth period of oil sunflower.Therefore,under strong evaporation,the maximum salt return rate is51.41%.3.The law of soil nitrogen transport and transformation and loss by controlled drainage was clarified.After irrigation in the growth period,the content of NH₄⁺-N,NO₃^--N in soil with controlled drainage of 40cm（CWT1）in the growth period is the highest.The NH₄⁺-N content of 0-40cm soil layer treated with CWT1 was significantly higher than that of controlled drainage 70cm（CWT2）,free drainage（FD）,open ditch dr ainage（OD）by23.04%～28.75%,33.62%～44.17%,10.44%～15.18%（P<0.05）.The controlled drainage during the growth period has a certain effect on preventing NO₃^--N infiltration from polluting groundwater,and can better keep the nitrogen in the soil in the cr op root layer（0-40cm）.The difference between NH₄⁺-N,NO₃^--N in the soil decreased obviously in the middle and late stages of oil sunflower growth,indicating that the controlled drainage of 40cm（CWT1）during the growth period promoted the soil nitrogen absorption in the later stage of the crop.The loss of NO₃^--N in each treatment is 60.46%～107.54%higher than that of NH₄⁺-N.CWT1 treatment stabilized the change of groundwater buried depth,thus reducing the hydraulic gradient of runoff,and the drainage flow rate decreased,the ability of water to carry nitrogen was relatively weakened,and the controlled drainage extended the retention time of soil water in farmland,so that the loss of NH₄⁺-N and NO₃^--N was reduced.4.The impact of controlled drainage on farmland water and soil environment is clarified.Different drainage methods have a significant impact on the loss of nitrogen and phosphorus.Increasing the height of drainage outlet can significantly reduce the loss of nitrogen and phosphorus.The amount of loss of ammonium nitrogen,nitrate nitrogen,total nitrogen and total phosphorus in free drainage treatment was the highest,0.28,0.51,1.07 and 0.41 kg/hm² respectively.The amount of loss of nitrogen and phosphorus in controlled drainage of 40 cm（CWT1）during the growth period was the lowest,and the amount of loss of ammonium nitrogen,nitrate nitrogen,total nitrogen and total phosphorus was 0.16,0.29,0.88 and 0.29 kg/hm² respectively.Compared with the reduction rate of ammonium nitrogen,nitrate nitrogen,total nitrogen and total phosphorus loss in free drainage（FD treatment）,the control of drainage 70cm（CWT2treatment）can reduce the loss of ammonium nitrogen,nitrate nitrogen,total nitrogen and total phosphorus by 31.24%,25.82%,9.57%and 16.94%,respectively.The reduction effect of controlled drainage 40cm（CWT1 treatment）is outstanding,and the reduction degree is much higher than that of CWT2 treatment,which can reduce the loss of ammonium nitrogen,nitrate nitrogen,total nitrogen and total phosphorus by42.00%,40.32%,17.09%and 28.11%respectively.Controlling drainage can reduce the loss of nitrogen and phosphorus from farmland after irrigation during the growth period,which not only provides a favorable environment for the growth of oil sunflowers,but also plays a positive role in environmental protection.5.The response of oil sunflower growth,yield and quality to drainage control was clarified.For the medium salinized soil and the subsurface drainage engineering cond itions commonly used in Hetao irrigation area,the subsurface drainage method can increase the emergence rate of oil sunflower by 2.50～3.59 percentage points.Controlled drainage during the growth period to increase the ratio of effective plants by3.44%～6.46%.After 50 days of emergence,oil sunflower can significantly increase plant height by 4.50%～13.35%and leaf area index by 3.34%～14.71%.Increase yield by 3.07%～10.47%and improve water use efficiency by 3.04%～8.75%.The controlled drainage treatment has a positive effect on improving the quality of sunflower.The controlled drainage treatment can increase the crude fat content of sunflower by3.53%～10.19%and the oleic acid of sunflower by 0.05%～7.28%,but has no significant effect on saturated fatty acid,stearic acid and palmitic acid.Controllled drainage treatment can also increase the accumulation of tocopherol in sunflower.6.The effects of different drainage methods on soil water content,salt content and nitrogen content were simulated by using the HYDRAS-2D model,and the optimal drainage mode was determined.In the moderately saline soil of Hetao irrigation area,considering the project cost and the risk of collapse of the deep open ditch during use,the drainage effect of the concealed pipe is poor or the concealed pipe is submerged.At the same time,the water content,salt content,desalting rate,salt return rate,nitrate nitrogen content and loss of the main root layer soil（0-40cm）in the growth period of oil sunflower are considered.Reduce soil salt content during spring irrigation,increase soil water content and nutrient content after irrigation in growth period,reduce water and nutrient loss,and minimize soil salt return rate in later growth period.That is to provide suitable water and fertilizer conditions for crop growth and minimize salt stress.Therefore,the best drainage method recommended in this thesis is that the buried depth of concealed pipes used in the non-growth period of crops is 130cm,the spacing is 10m,and the cont rolled drainage depth in the growth period is 50cm.