Experimental And Numerical Simulation Study On Drainage And Irrigation Measures Based On Soil Environmental Regulation Effect For Salinized Cotton Field In Southern Xinjiang

Drip irrigation under plastic film has created significant water-saving benefits in the arid areas of South Xinjiang.However,the localized humidity formed by drip irrigation under the film can only leach salts to the bottom of the tillage layer.Salinification induced obvious under the soil evaporation and rising groundwater during plant growth stage.The problem of secondary soil salinization has become more prominent due to poor irrigation and drainage measures and the shortage of freshwater resources.This thesis studied the effects of brackish water in underground piping on soil salt dislodgement through indoor physical and field experiments.Analysis of soil microbial community structure and cotton growth response to water-salt regulation under brackish water drenching in non-reproductive period and freshwater drip irrigation in reproductive period.A coupled model of soil water-salt transport and cotton growth under brackish water drenching and freshwater drip irrigation under underground pipe conditions was constructed,and drainage and irrigation measures suitable for saline land improvement in South Xinjiang were proposed.The results of the study can not only achieve the simultaneous improvement of soil salinity drenching efficiency and cotton yield,but also provide new ideas for the efficient utilization of saline land resources in the arid areas of South Xinjiang.(1)The effect factors of soil-salt dislodgement of brackish water with underground piping mainly include the depth of underground pipe,the amount of irrigation and the mineralization of brackish water.Under a certain amount of irrigation water,the salinity discharge and infiltration rate of the culvert pipe drainage decrease with the both increase of the pipe depth and its water storage capacity.And under the same conditions of underground pipe depth,the salt discharge and infiltration rate of underground pipe drainage both increase with the increase of irrigation water.The salinity concentration of underground pipe drainage is significantly higher than the mineralization of brackish water,indicating that brackish water has a replacement effect on soil salinity.The salt replacement amount is positively proportional to the irrigation volume and infiltration rate,and inversely proportional to the depth of burial of the culvert pipe and the mineralization of brackish water.Based on HYDRUS-1D software,a soil water-salt transport model was established in the underground pipe drainage area under brackish water drenching conditions.The simulation results showed that the irrigation volume was greater than 3180 m~3/ha and 3990 m~3/ha when the depth of the drain was 140 cm and 160 cm,the amount of salt discharge from the drain was more significant than the amount of salt brought into the salt replacement zone of the drain by the brackish water(2–5 g/L).The soil salinity(0–60 cm)reached the critical value of salt tolerance of cotton,which could meet the requirements of cotton growth.(2)During the non-growth period,brackish water drenching significantly increased the water content of 0-40 cm soil layer in each treatment,and the increase of water content of 0-40 cm soil layer in the dark pipe drainage treatment was significantly higher than that in the undrained treatment(CK).The soil desalination rate was positively proportional to the initial soil salinity.Using DRAINMOD-S software,a model of water-salt transport of soil by brackish water on underground pipe drainage during the non-growth period was constructed.During the growth period,the water content of the surface soil(0–30cm)increased under the buried piping drainage condition which facilitated water uptake by the cotton plant roots.While the water content of the deep soil(30–100 cm)decreased under the buried piping drainage condition,which avoided water-logging in the cotton.The salt accumulation was obvious in the surface soil of CK.The salt content in the 0–30 cm soil decreased significantly under buried piping drainage condition.The increased spacing between buried pipes was not conducive to the desalination of the surface soil,but there was no obvious influence.The underground piping drainage treatment significantly increased cotton seedling growth,plant height,leaf surface area,dry matter quality and yield by improving the water and salt environment but decreased cotton growth as the depth of underground piping and the spacing between the plants increased.Using the software DRAINMOD-S and Aqua Crop,a coupled water and salt transport and cotton growth model with brackish water drenching and freshwater drip irrigation was established;the model also considered the underground piping draining system.The model simulated better groundwater level,soil profile salinity,soil ECe and cotton growth.Using the simulation results of salinity leaching of underground pipe drainage soil by brackish water during the non-growth period as the initial value(four brackish water mineralization levels,five irrigation levels,four depths of the underground piping and five spacings between piping),the scenario simulation of cotton yield under 400 conditions of the soil salinity was conducted.The simulation results showed that when the underground pipe spacing was 30 m and the depth was 0.8 m,soil salinity drenching with brackish water with irrigation volume of 1500 m~3/ha and mineralization degree of 2 g/L could achieve the simultaneous improvement of soil salinity drenching and cotton yield during the non-growth period.(3)The underground piping drainage treatment had a significant effect on the Alpha diversity of the soil bacterial community but not affect the soil fungi.During the cotton seedling stage,the effect of underground piping drainage treatment on the soil microbial community was significant.The depth of the underground pipe significantly changed the soil microbial community structure at the cotton bolling stage.The microbial biomass and abundance were significantly higher in the seedling stage than the bolling stage.During the cotton seedling and bolling stages,the most abundant soil bacteria were Proteobacteria,accounting for 74.96%and 77.05%of the bacterial community.The most abundant soil fungi were Ascomycota,accounting for 85.06%and 83.87%of the fungal community.Soil salinity,ammonium nitrogen,and nitrate nitrogen were the main factors explaining the differences in soil bacterial flora,while ammonium nitrogen was the main factor explaining the differences in soil fungal flora at the seedling stage of cotton.Nitrate nitrogen and ammonium nitrogen explain the differences in soil fungal flora at the bolling stage.A null model was used to reveal that the main driving forces for the structuring of the soil bacterial and fungal communities are stochastic and that bacteria are more susceptible to the selective action of environmental factors than fungi.The Mantel test confirmed that the bacterial community was closely related to the soil environment and cotton growth indicators.The cotton seedling was positively correlated with the bacteria Pseudarthrobacter and Thiobacillus,negatively associated with the fungi Cephalotrichum,Chaetomium,and Fusarium.Furthermore,there was a significantly negative correlation with soil salinity and a significant positive correlation with nitrate nitrogen.In contrast,only the bacterium Fictibacillus showed a significant positive correlation with the cotton leaf area and yield at the bolling stage.