Physical And Chemical Properties Of Agricultural Soils During Seasonal Freeze-thaw A Study Of The Response To Winter Irrigation

Freeze-thaw alternation is an abiotic stress acting on the soil,which has a direct or indirect effect on both physical and chemical properties of the soil,and is a more important factor affecting soil water,heat,salt and nutrient transport processes.Winter irrigation during the seasonal freeze-thaw period can calm the soil temperature,accumulate soil moisture in the cultivated layer,compact the soil,and wash out the salt.To investigate the response of soil physical and chemical properties to the effect of winter irrigation during the seasonal freeze-thaw period and to determine the basis for reasonable winter irrigation quotas is a hot topic of current research.This thesis takes the land in the irrigated agricultural area of the desert oasis in the north of China as the research object,Analysis based on indoor freeze-thaw simulations and field trials,Monitoring of physicochemical parameters of cotton field soil by winter irrigation drenching and simulation of soil water and heat migration under different winter irrigation quotas.Soil water and heat transport patterns and spatial and temporal distribution characteristics under different irrigation levels were analyzed,The characteristics of the changes of soil physicochemical parameters during the freeze-thaw process with different initial soil water contents were clarified,And based on the experiment,a numerical calculation study of soil hydrothermal migration process under the action of freeze-thaw cycle was carried out,Numerical calculations were performed using the finite element software HYDRUS-1D.The following main advances were derived:（1）Based on indoor freeze-thaw simulations,the proportion of the water-stable composition of agglomerates>0.1mm decreased by 30.00%and 21.79%after five freeze-thaw cycles for initial soil water contents of W₂ and W₃,respectively,with apparent destruction of large agglomerates and an increase in small agglomerates;As the initial moisture content of the soil increases,the amount of freezing and thawing in the soil gradually increases,with vertical deformation increasing by 35.5%and 48.2%at W₂ and W₃;After five freeze-thaw cycles,the rate of decrease of soil water content with increasing water absorption in different initial water content treatments showed a trend of larger and then smaller;after three freeze-thaw cycles,the soil capacity showed a rapid decrease,with W₁,W₂ and W₃ treatments showing a decrease of 7.31%,5.52%and 7.92%respectively compared to the trend before freeze-thaw,and a slower decrease in capacity than three freeze-thaw cycles.The trend of decreasing soil weight was slowed down by more than 3 freeze-thaw cycles.（2）Based on the indoor freeze-thaw simulation test,under the influence of freeze-thaw cycle,the large pores in the soil column of W₃ caused the Cl^-penetration time to be significantly faster than other initial moisture content soil columns,among which W₂ had the longest penetration time of 4570 min,while W₃ had the shortest penetration time of 1170 min,and the Cl^-penetration curves of different initial moisture content soils were all in the shape of S The penetration curves of W₀,W₁ and W₂ were all"S"shaped,with the W₃penetration curve always above the other initial moisture content soil columns during the penetration phase,and the penetration concentrations of W₁>W₀>W₂ during the initial phase of Cl^-input;after the 1st freeze-thaw cycle,the release of ammonium N in the leachate of the soil column of W₃ was the largest,which was40%higher compared to the control group W₀,and the 3th freeze-thaw cycle,the ammonium nitrogen content in the drench solution increased with the initial water content,and reached a constant value in both the 5th and 10th freeze-thaw cycles.（3）Based on indoor freeze-thaw simulations,the initial water content W₂ and W₃ soils accelerated their cumulative infiltration through the freeze-thaw cycle,while the cumulative infiltration of the initial water content W₁ was lower than that of W₀.At an infiltration time of 1800 s,the cumulative infiltration of W₂ and W₃ was 36.2%and 73.6%more than W₀,respectively,while the cumulative infiltration of W₁ was 11.8%less than W₀.Due to the freeze-thaw cycle,the rate of soil infiltration is significantly higher than W₀ when the initial water content is greater than W₂,with soil infiltration rates showing W₃>W₂>W₀>W₁.（4）Based on the Hydrus-1D model simulation,the simulation evaluation index R²is greater than 0.85,which proves that the model simulation of soil moisture content and temperature in different winter irrigation plots is reliable.Simulated winter irrigation volumes of 80 m³/acre and 70 m³/acre soil temperature and water content were 5.31%and 35.5%higher than 70 m³/acre water content at 25 and 40 cm depth,and 12.22%and12.40%higher than 70 m³/acre soil temperature at 80 m³/acre and 60 m³/acre soil temperature and water content at 80 m³/acre,respectively.The comparison was similar to the former.Overall,the results show that a winter irrigation rate of 80 m³/mu is more conducive to providing good frost protection and moisture retention for overwintering wheat.