| Object:Xinjiang is a typical seasonal frozen soil area,which is affected by the freeze and thaw cycle all year round.Soil structure and water and salt distribution of cotton fields under drip irrigation under mulch are affected to varying degrees.In order to further explore the changes of soil structure and water and salt distribution of cotton fields under freeze and thaw conditions under drip irrigation under mulch for a long time.This paper uses advanced geophysical detection methods to detect and sample six cotton fields with different application years of drip irrigation under mulch to study the above issues,in order to provide theoretical basis and data support for soil improvement and salinization prevention in this area.Methods:The effects of freezing and thawing on soil structure and distribution of water and salt were studied by ground penetrating radar(GPR)and sampling method.Among them,first use the ground penetrating radar 250MHz frequency antenna reflection method to obtain the soil profile structure of the drip irrigation cotton field before and after freezing and thawing,and collect the radar reflection signal,and then use the 1000MHz antenna to detect and collect the relevant radar information,and integrate the collected data at the two frequencies.Radar information is used to inverse the dielectric constant of each soil layer,and finally the Dobson saline soil dielectric model is used to quantitatively invert the water and salt content of each soil layer.Taking into account the sampling work during the radar detection,the sampling depth of each block is divided into 20 cm each layer,and a total of 8 layers are taken.Take the sample number and take it back to the laboratory to determine the soil bulk density,moisture,salinity and other data.Use Excel 2010 and SPSS.24 software for data sorting and analysis,and Origin 2017 for drawing.Results:(1)The soil structure level of the cotton field under the mulch drip irrigation can be detected by the ground penetrating radar 250MHz frequency antenna reflection method,and the thickness between different soil layers can be distinguished according to the two reflection planes in the ground penetrating radar profile spectrum image.Among them,the resolution of GPR profile spectral image is significantly affected by soil water and salt content.(2)Freezing and thawing can improve soil structure.Among them,soil aggregates changed significantly in the surface and middle layer of cotton field under drip irrigation under film,and the proportion of soil aggregates among particles was redistributed during the period.The proportion of large aggregates decreased and that of small and medium aggregates increased.For the soil bulk density,after thawing,the soil bulk density of cotton field with drip irrigation under mulch decreased by 4.43%～5.38%compared with that before freezing.As for soil porosity,the soil total porosity,non-capillary porosity and capillary porosity increased by about 4.96%,0.65%and 4.31%after melting in cotton field with drip irrigation under mulching under 0～140 cm.Based on the quantitative description of the generalized structure index of soil,the generalized structure index of soil from 0 to 140 cm of cotton field under mulching drip irrigation increased after freezing-thawing,and the three-phase structure of each soil layer after thawing was close to the ideal standard type compared with that before freezing.(3)By modifying the parameters A andξin the Dobson dielectric model,the model can be applied to cotton fields under film drip irrigation in Xinjiang.The value range of A is about 0.74,and the value ofξis about 0.14.Among them,the 1000MHz frequency antenna and the saline soil dielectric model can effectively invert the soil volumetric water content of drip irrigation cotton fields under the film.The revised inversion formula R~2 is greater than 0.94,RPD is greater than 1.5,RMSE is less than 0.026cm~3/cm~3,and MRE is less than 12.0%.At the same time,using the combination of ground penetrating radar250 MHz and 1000 MHz frequency antennas,combined with Hu Qingrong’s modified Dobson dielectric model,using the modified parameters,the salt content of drip irrigation cotton fields under the film can be better inverted.The measured value is similar,R~2=0.835,RMSE is 1.43 g/kg.(4)During the whole freezing-thawing stage,the surface soil temperature changes most dramatically.During the freezing period,the soil temperature increases with the increase of soil depth,that is,the deeper the soil depth,the higher the soil relative temperature,which is shown as>at the bottom and>at the middle.In thawing period,the soil temperature showed the>bottom layer and>middle layer.The distribution of soil water and salt in cotton fields under drip irrigation under mulching was significantly changed by soil temperature before and after freezing-thawing.In the early period of freezing,soil moisture in cotton field under drip irrigation under mulching was mostly about 60 cm.During the freezing period,the soil moisture was mostly distributed at about 100 cm,which decreased by about 40 cm compared with that in the early freezing period.In thawing period,the soil moisture was mostly distributed at about 40 cm.The distribution of soil salt is closely related to the distribution of soil water.The soil salt accumulates in the surface layer before and after freezing-thawing.Only in the early thawing stage,the soil salt content in the surface layer decreases,and during this period,the salt in each block migrates downward,and the salt mainly accumulates in the distance of 40-60 cm.At the end of thawing,the salt concentration appears on the surface,that is,the whole freeze-thaw stage presents a state of"salt-accumulation,desalt-reaccumulation".(5)According to the difference of salt concentration between different soil layers,the salt accumulation zone and the aeration zone can be divided in the GPR soil profile spectrum.At the same time,the overall migration of soil salt in cotton field under drip irrigation can be learned from the vertical changes of the GPR soil profile spectrum of the boundary between salt accumulation zone and aeration zone at different times.Conclusion:GPR can detect the soil structure of drip irrigation cotton field,and quantitatively invert the soil water and salt content of drip irrigation cotton field under film through the combination of high and low frequency combined with Dobson saline soil dielectric model.This method provides a new idea for detecting soil structure and water and salt content of drip irrigation cotton field under film.With the intensification of freeze-thaw cycle,soil structure and distribution of water and salt in cotton field under drip irrigation under plastic film were affected.This test in the use of fixed-point sampling and GPR technology of combining the research process,through the research on long-term membrane under drip irrigation cotton fields before and after freezing and thawing soil structure changes of salt and water distribution characteristics,in order to prevention and control of the region’s soil improvement and soil salinization provides critical information and reference,further to ensure sustainable utilization of membrane under drip irrigation in Xinjiang cotton fields. |
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