Study On The Effect Of Freeze-thaw On Soil Structure And Water-Salt Migration In Long-term Mulched Drip Irrigation Cotton Field

Object: To elucidate the changes of soil structure and its response to seasonal freezing and thawing in longterm mulched drip irrigation cotton fields in typical artificial oases in northern Xinjiang,as well as the temporal and spatial changes of soil water and salt in non-irrigation seasons,and reveal the key processes of salt migration.This study evaluates the long-term applicability of submembrane drip irrigation technology in irrigated agriculture in arid areas,and provides a theoretical basis for preventing and improving soil salinization and freeze-thaw erosion.Methods: Through two consecutive years of field location monitoring,the cotton field with 9-24 years of mulched drip irrigation and a natural wasteland(CK)were selected as the research objects by using the time-space substitution method.The temporal and spatial variation of soil water and salt in the 0-200 cm soil layer was observed,the temporal and spatial distribution dynamics of soil water and salt in long-term mulched drip irrigation cotton field were analyzed,and the characteristics of soil water and salt transport in cotton field with different drip irrigation years during the freeze-thaw process were explored.By comparing different drip irrigation years and the changes of soil physical structure in cotton fields before and after freeze-thaw,the changes of soil structure and seasonal freeze-thaw on soil structure in cotton fields under long-term submembrane drip irrigation were studied.Results:(1)After the wasteland was reclaimed as cotton field,the soil bulk density and solid phase ratio of the tillage layer showed a trend of first decreasing and then increasing with the increase of the application time of mulched drip irrigation,but the change trend of total soil porosity was opposite to the bulk density.Long-term mulched drip irrigation significantly affected the soil generalized structure index and soil triathlon structure distance(P<0.05)in cotton field,and optimized the soil three-phase structure.With the increase of drip irrigation duration,the ratio of both mechanical stability and water-stable aggregates of0.25-8 mm,the stability of aggregates and the organic carbon content in aggregates at each size fraction showed an increasing trend.(2)Long-term mulched drip irrigation significantly affected the spatial and temporal distribution characteristics of soil salt in cotton fields,but had no significant effect on soil moisture.The results of two years’ observation showed that the vertical distribution characteristics of soil salt changed from surface to bottom after the wasteland was reclaimed into cotton fields.With the increase of mulched drip irrigation durations,the salt storage of soil in 0-200 cm soil layer in cotton field decreased significantly(P<0.05).After 24 years of mulched drip irrigation,the desalting rate in 0-40 cm,40-200 cm and the entire 0-200 cm soil layer were 93.40%,87.75% and 90.09%,respectively.(3)In the two-year experiment,the soil freezing duration was 120 and 123 days,respectively.The maximum freezing depth of the soil was 100 cm,and the soil began to melt in both directions from the surface and the deep layer to the thawing period.With the progress of the freeze-thaw process,the soil moisture of 0-40 cm showed an increasing trend,and the increase was the largest in the thawing period,while the water of the 40-200 cm soil layer showed fluctuations.Soil salinity transport may be greatly affected by moisture,and plots vary greatly.An increase in the total salt storage of 0-200 cm soil layers(salt flux was positive)was observed during the two-year freezing period.When the surface soil water storage is large,the salt storage may be reduced by downward leaching of water,or it may accumulate upward during the thawing period as the water is evaporated.(4)Before and after freezing and thawing,the bulk density,solid phase and gas phase ratio of the topsoil in drip irrigation cotton field decreased significantly,and the porosity and liquid phase ratio increased significantly(P<0.05).After thawing,the three-phase structure of soil was significantly improved.Seasonal freezing and thawing significantly increased the proportion of 0.25-8 mm mechanicalstable aggregates and the mechanical stability of aggregates,while significantly decreased the proportion of<0.25 mm aggregates(P<0.05).However,due to the difference of initial water content and salt content,seasonal freezing and thawing may reduce the water stability of aggregates in cotton fields.However,after24 years of drip irrigation,the mechanical stability and water stability of aggregates in cotton fields were significantly enhanced,but not significantly affected by seasonal freezing and thawing.Conclusion: In general,long-term mulched drip irrigation has shown good sustainable application potential in arid and semi-arid areas by reducing soil salinity and improving soil structure,but countermeasures are also needed to address soil salinization that may be induced by seasonal freeze-thaw.