| X-ray CT scanning technology is an emerging method for non-destructive detection of 3D structure of soil pores.The introduction of this technology provides an important method for quantitatively studying the relationship between soil pore structure and water-salt movement.Based on the long-term improvement of saline-alkali land in the Inner Mongolia Hetao Irrigation District,this study aimed at solving the problem of unclear water-salt regulation mechanism of agronomic improvement measures in saline-alkali land,such as straw barrier and subsurface construction,and attempted to use quantitative monitoring and analysis of pore structure parameters as a breakthrough to reveal the mechanism of salt control.Through the combination of indoor soil column simulation and field positioning test,CT is used to quantitatively analyze the pore structure parameters of different media under different conditions,and the relationship between soil pore characteristic parameters and water-salt movement is clarified.The main progress of the paper is as follows:?1?Explain the pore structure characteristics of different physical forms of corn straw compartment and its influence mechanism on the infiltration process.A soil column simulation experiment was carried,including no straw interlayer?CK?,segmented straw with 5 cm length?SL?,straw pellet?SK?,and straw powder?SF?.The results showed that the total porosity of the 40-45 cm layer under SF was significantly higher than that under CK,SL and SK by 29.25%,12.09%and 12.61%.The porosity with pore diameter<1 mm under SF significantly increased at each stage of irrigation water infiltration,which thereby increased the volume of the enclosed gas and resulted in a slower infiltration rate.Correlation analysis showed that there was a significant positive correlation between the time of wetting front through the straw layer and the total porosity,the porosity of pore diameter<1 mm before and after the infiltration,but the effect of this macropores weakened after the end of infiltration,the effect of<1 mm porosity in straw interlayer was still significant.Compared with CK,the average water content of SL,SK and SF in 0-40 cm soil layer increased by 18.04%,5.15%and 15.13%respectively after infiltration,and the desalination rate in 0-40 cm soil layer increased by 6.31%,7.16%and 6.44%,respectively.The cumulative phreatic evaporation of SL and SF were 41.20%and 49.00%lower than CK,and the salinity return rate in 0-40 cm soil layer were 36.57%and 47.08%lower than CK,respectively.?2?It is clear that the effective time of straw interlayer is 3 years,and 0.1-0.5 mm equivalent diameter pore has great influence on salt distribution.Two treatments,CK?without straw compartment?and SL800(straw deep burial with 800 kg·666.67m-2),were set up through three-year field experiment.The results showed that there was no significant difference in salt content,bulk density,saturated hydraulic conductivity between SL800 and CK after three years,but compared with CK,the aggregate content of 0.25-2 mm in upper and middle soil layer increased by 1.98-1.37%.From the perspective of pore structure,there was no significant difference between SL800 and CK in 0.1-0.5 mm equivalent diameter porosity in each soil layer.This part of the pores accounts for 56.17%-78.17%of the total porosity,which plays a leading role in salt distribution.However,SL has significantly increased the pore density of>1 mm in middle soil layer,which can promote the formation of preferential flow,and to some extent is conducive to soil and water conservation.?3?The effect and mechanism of salt inhibition and fertilizer conservation by organic fertilization in 10-30 cm soil layer were clarified.Using the field micro-area test method,four field management practices were studied:ploughing without plastic mulching?CK?,ploughing plus plastic mulching?PM?,subsurface?10-30 cm?treatment with organic manure?OM?,and subsurface treatment with OM plus plastic mulching?PM+OM?.The results showed that there was no significant difference in the salt content of OM and PM+OM treated with CK and PM after spring irrigation in 2015,but it promoted the salt to deep leaching.In 2016-2017,the average salt content in 1 m soil of PM+OM was reduced by30.26%and 7.77%than CK,respectively.The PM+OM showed a strong inhibition of salt return after harvesting.Compared with CK,in 10-30 cm soil layer during 2015-2017,the pH value of PM+OM decreased significantly,the organic matter,available nitrogen,available phosphorus and available potassium increased by 8.11-126.64%,42.86-104.87%,79.41-827.36%and 185.05-400.37%than CK,and the soil microbial flora structure and abundance were improved,the content of water-stable aggregates was increased.Compared with CK,the yield of OM and PM+OM treated sunflower in2015-2017 increased by 4.78-49.16%and 11.15-45.08%,respectively.CT showed that compared with CK,the subsurface organic fertilization treatment significantly increased the number of pores in subsurface by 42.57%,the total porosity increased significantly by 11.94%,and the connectivity was significantly improved by 78.49 times?P<0.05?.Among them,the equivalent diameter<1.5 mm porosity has a significant effect on improving the water retention effect of the tillage layer after spring irrigation.The equivalent diameter<0.5 mm porosity has the most significant effect on promoting the salt leaching effect after spring irrigation.In conclusion,CT scanning technology can be used as a means to quantitatively study the characteristics of pore structure and the regulation mechanism of water and salt in straw barrier and subsurface construction.Determining the role,from the results of water and salt regulation and the actual application of the field,the original 5 cm straw compartment can be used as the best measure in the field.However,the straw interlayer has no significant effect on the regulation of water and salt after three years of burial.This weakening effect is mainly related to the pore size of 0.1-0.5 mm.At the same time,the construction of fertile subsurface measures can significantly increase the content of available nutrients and rationally regulate the distribution of water and salt in the tillage layer,thus achieving significant yield increase.After three years of implementation of the measures,the content of water-stable aggregates,the soil porosity and connectivity increased,forming a complex porous soil structure.This study provides a scientific basis for using CT scanning technology to reveal the improvement mechanism and optimization measures of saline-alkali land. |
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