| Mu Us Sandy Land is one of the four maj or sandy lands in China,and also the source of sandstorms in the Beijing-Tianjin-Hebei region,it is listed as one of the key governance areas of "two screens and three belts".For a long time,implementing comprehensive management of sandy land have been the hot spots and problems that need to be solved.On the premise of ecological security,it is a valuable sand management direction to increase the cultivated land area under limited conditions.In recent years,the technology of rapid compounding of soft rock and sand to form soil has been widely used and has achieved results in the land regulation,development and utilization of Mu Us Sandy Land.The engineering method of soft rock and sand compounding only achieves the basic goal of inorganic reconstruction of soil from "sand" to "soil",however,After long-term or certain period of utilization,the quality stability and sustainable use are the testing standards for improving the quality from"sand" to "loam".The quality of the "compound soil" and its evolution are the key factors,Further research is needed to explain the essential problem.In this paper,based on the development process and action mechanism of the soil formed by the combination of soft rock and sand,soil structure,fertility and crop productivity are used as the main standards and criteria for the process from parent material to soil formation,Field positioning test,laboratory simulation and model fitting were used to find the key control factors for the quality stability of compound soil,so as to explore the soil formation mechanism and quality evolution trend of compound soil by combining micro and macro factors.The main research progress and conclusions are as follows:(1)It reveals the types and succession rules of organic-inorganic cementing materials in different proportions of compound soil,and analyzes the internal mechanism of soft rock and sand compound soil from a micro-macro perspective.Based on the analysis of scanning electron microscope and energy spectrometer,with the increase of the cultivation period of the compound soil,the particles of the compound soil began to adhere to each other,and the cementing material between the particles was rich,and a large number of stickies appeared on the surface of the soil particles,which was beneficial to the benign transformation of the compound soil structure and possessed the characteristics of agglomerates,and more iron binding points appeared on the surface of the large aggregate of compound soil.After 9 years of cultivation,the main organic cementitious substances of compound soil agglomeration are total organic carbon,easily oxidized organic carbon and particulate organic carbon.With the increase of the cultivation years of compound soils,the carbon stock management index generally showed an increasing trend,the carbon stock activity,carbon stock activity index and carbon stock management index of 1:1 and 1:2 treatments of each soil layer were significantly increased compared with other treatments,the soil organic carbon stock was in a benign management situation,and the soil improvement effect was better.The inorganic binders in the compound soil are mainly calcium carbonate and free iron oxide,and the 1:2 treatments is more conducive to the accumulation of calcium carbonate and free iron oxide in the 0-30cm soil layer.(2)The key control factors for the stable development of the compound soil structure during the land use process were explored.With the increase of cultivation years,the 1:1,1:2,1:5 compound soil all showed an increase in clay and silt content,a decrease in sand content,and an increase in fractal dimension.The soil texture is generally composed of loam sand-Sandy soil-loamy soil-silty loam type transition,the gradation of the soil particle size of the arable layer tends to be refined and rationalized,and the compound soil texture develops to a good state suitable for crop growth.After 9 years of planting,with the downward movement of the silt and clay,the thickness of the tillage layer increased,and a relatively dense sticky layer was formed in the 30-40cm soil layer.The formation of the sticky layer can effectively prevent the leakage of water and fertilizer,and improve the water and fertilizer retention of the compound soil.After years of cultivation,the total organic carbon has significantly improved the formation of water-stable agglomerates of 0.25-0.5 mm and<0.10 mm particle size,both easily oxidized organic carbon and particulate organic carbon play an important role in the formation of 0.25-0.5 mm water-stable aggregates,the inorganic binder calcium carbonate and free iron oxide promote the formation of 0.5-1 mm water-stable aggregates.(3)The effect of soft rock on the water-holding and water-retaining capacity of the compound soil is studied,and the effects of freeze-thaw alternations on the structure and physical and chemical properties of compound soil were expounded.Using van Genuchten model to fit the water characteristic curve,it is found that the fitting accuracy of water characteristic curve of different particle size soft rock and different proportion of soil is above 0.96.The water holding capacity of compound soil with particle size less than 1mm is stronger than that with particle size less than 2mm.Under the same suction force,the average water content is about 5.2%.The saturated water content of soft rock with different particle size is larger than that of aeolian sand soil,and the saturated water content of particle size<1 mm is higher than 8.24%of that of<2 mm.Due to the small water content of the sand surface,a frozen layer is formed from a distance below the surface,with a maximum freezing depth of 98.0 cm,the surface layer usually forms a dry sand layer,with strong wind erosion.Due to its good water-holding capacity and high surface water content,the compound soil starts to freeze from the surface,which enhances its wind-resistant ability and the maximum freezing depth reaches 116.0cm.The freeze-thaw alternation has a certain dispersion effect on the soil structure of the soil layer,and the higher the clay content,the smaller its destructive effect on the stability of the aggregate structure.The stability of the aggregate structure of the 1:1 compound soil after freeze-thaw alternation is greater than 1:2 and 1:5.The short-term freeze-thaw alternations increased the accumulation of carbon and nitrogen mineralization in the compound soil and improved the structure of the surface soil,but the effect was not dominant.Among them,the increase rate of the 1:2 compound soil was more significant,and the performance of organic matter and nitrogen was better.The results of the study prove that silt,clay and organic-inorganic cement are the most critical driving factors for soil quality improvement in the desertified land in the study area.(4)The succession laws of compound soil quality and crop suitability under different crop planting modes were clarified.With the increase of corn and potato planting years,the content of ?0.25mm water-stable aggregates and WMD value continued to increase,becoming the main component of the agglomerates,of which the small particle size is mainly 0.25?0.5mm,accounting for 36.6%?40.4%;Compared with before planting,the WMD values of 1:1,1:2,and 1:5 compound soils were increased by 1.05 times,1.62 times,and 2.13 times,respectively,and the stability of the aggregates was improved,especially it can effectively promote the formation of 1:2 and 1:5 compound soil water-stable aggregates.Among the 1:2 and 1:5 compound soils,the clay content,fractal dimension value,organic matter content increase the fastest,corn and potato yield is the highest,SYI value is the largest,CV value is the smallest,the yield stability and sustainability are the best.The results showed that the 1:2 and 1:5 compound soils were the most suitable for corn and potato planting,respectively.These two proportions best developed the compound state of inorganic-organic colloid in the compound soil,and improved the stability of soil structure and agricultural adaptability.In summary,after many years of cultivation,the structure of compound soils in different proportions has been increasingly optimized,and compound soils are all developing to the inorganic-organic colloid composite state,realizing the upgrading process from "new soil" to"mature soil".The soil can maintain a good and stable development state after the compounding,and does not need to be compounded after many years,but the nutrient content of the compounded soil is generally low and needs to be improved in the later period.This indicates that the compound soil formation technology reverses the land desertification process into the primary soil formation process and accelerates the soil formation process. |
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