| Soil aggregates are the basic unit of soil structure and play an important role in regulating soil properties,fertility and ecological functions.Their stability is related to their pore relationships,water movement and storage,soil aeration,biological activity and crop growth.At present,researches on hydraulic stability of aggregates are relatively common,and mainly focus on dissipation,non-uniform expansion,mechanical impact and physicochemical effects.Although the mechanisms of soil aggregate fragmentation caused by the four kinds of actions are different from each other,the four kinds of action mechanisms all have a common feature,that is,soil aggregates are wetted by water before fragmentation,which indicates that the four kinds of action mechanisms may only be surface phenomena that cause soil aggregate fragmentation.However,in the absence of external force,the interaction between particles in the soil-water system may be the internal mechanism that causes soil aggregates to break up.Hydration is gradually considered to be the important mechanism that dominates the interaction between particles.Without this mechanism,dry aggregates cannot disperse again after wetting.However,many scholars have chosen deionized water and alcohol as dispersion media when studying the effect of hydration on the damage degree of soil aggregates.As we all know,alcohol is a polar solvent and water is also a polar solvent.Choosing deionized water and alcohol as dispersion solutions cannot clearly determine the effect of hydration on the stability of soil aggregates.Therefore,this paper takes a non-polar solvent(hexane)as the starting point,through comparative analysis of the fragmentation rate of soil aggregates dispersed in deionized water,alcohol and hexane,discusses the effect of hydration on the stability of soil aggregates,and on this basis,through comparison of the stability of soil aggregates under different rainfall kinetic energy,discusses the stability characteristics of soil aggregates under the combined action of raindrop strike and hydration,and provides theoretical basis for clarifying the hydraulic mechanism of soil aggregate stability and the key factors affecting the stability of soil aggregates.The main conclusions are as follows:(1)Physical and chemical characteristics of soil and micromorphological characteristics of soil aggregatesBy comparing the physical and chemical properties of sandy loam to clay loam series texture soils,it is found that the soil bulk density is mainly concentrated in the range of 1.37 g/cm3~1.60 g/cm3,and the porosity is mainly concentrated in the range of 39.70%~48.33%.Calcium carbonate(CaCO3)is mainly concentrated in the range of 9.89 g/kg to 31.74 g/kg;The clay content is in the range of 18.45%~34.79%,the silt content is in the range of 29.16%~51.12%,and the sand content is in the range of 21.18%~52.39%.At the same time,we analyzed the correlation of physical and chemical properties of a series of textured soils.the results showed that there was a significant positive correlation between soil clay and organic matter(SOM)and cation exchange capacity(CEC).with the change of soil texture,the size of SOM showed as follows: S6(clay loam)> S5(clay loam)> S2(silt loam)> S4(loam)> S3(loam)> S1(sandy loam);CEC size is shown as S6(clay loam)> S5(clay loam)> S2(silt loam)> S4(loam)> S1(sandy loam)> S3(loam).In addition,through comparative analysis of the microscopic characteristics of series of texture soil aggregates,it is found that the basic structural units of clay loam aggregates are mainly flaky particles and flat aggregates,and flaky aggregates mainly formed by surface-to-surface contact.The arrangement type between particles is cementation structure,and the compaction degree is relatively tight.The basic structural unit of loam aggregates is flaky particles,mainly surface-to-surface contact and edge-to-surface contact,and the compaction degree inside the particles is looser than clay loam.The basic structural units of silt loam and sandy loam aggregates are mainly flaky particles,with edge-to-surface contact or surface-to-surface contact.The flaky particles are relatively large,the pores between the particles are relatively large,and the arrangement is relatively loose.(2)Stability characteristics of soil aggregates under hydrationFirstly,the stability difference between remolded soil aggregates and natural soil aggregates was determined by static disintegration method.The results showed that the stability of natural soil aggregates was significantly higher than remolded soil aggregates,which indicated that the stability of soil aggregates could not be accurately reflected by using remolded soil aggregates as test materials.On this basis,we compared and analyzed the stability of natural soil aggregates dispersed in deionized water,alcohol and hexane solutions.the results showed that the strength of the aggregates dispersed in deionized water,alcohol and hexane solutions showed the following order: deionized water > alcohol > hexane.When the aggregates disintegrated within the range of 0.25~2 mm,the strength of soil aggregates dispersed by deionized water was significantly higher than that by alcohol and hexane solution,but the aggregate fragmentation index(ABI)under alcohol and hexane solution dispersion had no significant difference(p < 0.05).When the aggregates disintegrated within < 0.25 mm,the strength of deionized water dispersed soil aggregates was significantly higher than that of alcohol and hexane solution,while the difference of soil ABI under alcohol and hexane solution dispersion was determined by soil texture and soaking time(p<0.05).This shows that the effect of hydration on the stability of soil aggregates cannot be correctly evaluated by comparing alcohol with deionized water,and hexane and deionized water should be selected as dispersion media to clarify the effect of hydration on the stability of aggregates.In addition,comparing the stability of soil aggregates with different textures,it can be found that hydration has the greatest destructive strength on sandy loam aggregates and the smallest destructive effect on clay loam aggregates.At the same time,the soaking time and the distribution of agglomerates after disintegration were analyzed.the results showed that when the agglomerates disintegrated in the range of 0.25~2 mm and < 0.25 mm,the percentage content of agglomerates of 0.05~2 mm gradually decreased while the percentage content of agglomerates of 0.002~0.05 mm and < 0.002 mm gradually increased with the increase of soaking time.(3)Stability Characteristics of Soil Aggregates under Combined Action of Raindrop and HydrationThe stability of soil aggregates is different under different rainfall heights.The research results show that the stability of soil aggregates gradually decreases with the increase of rainfall kinetic energy.Meanwhile,the relationship between rainfall kinetic energy and aggregate stability is fitted.It is found that the exponential function can well describe the relationship between the two,and the correlation coefficient is above 0.95.At the same time,the distribution characteristics of aggregates are also different with the change of rainfall kinetic energy.the results show that when aggregates are broken to the range of 0.25~2 mm,the percentage content of aggregates of 0.05~2 mm decreases with the increase of rainfall height,while the percentage content of aggregates of 0.002~0.05 mm and < 0.002 mm increases.When the aggregates are broken to less than 0.25 mm,the percentage content of aggregates in each grain size fluctuates greatly with the increase of rainfall height.However,as a whole,except for sandy loam(S1),the percentage content of 0.05~2 mm aggregates at rainfall height of 2 m is lower than that of 0.05~2 mm aggregates at rainfall height of 0.5 m,and the percentage content of 0.002~0.05 mm and < 0.002 mm aggregates at rainfall height of 2 m are higher than that of 0.002~0.05 mm and < 0.002 mm aggregates at rainfall height of 2 m.In addition,the stability of different types of soil aggregates also varies with rainfall height.When the rainfall height is 0.5 m,the stability of six types of soil aggregates is as follows: S6(clay loam)> S5(clay loam)> S2(silt loam)> S4(loam)> S3(loam)> S1(sandy loam);When the rainfall height is 1 m,it is S6(clay loam)> S2(silt loam)> S5(clay loam)> S4(loam)> S3(loam)> S1(sandy loam);When the rainfall height is 1.5 m and 2 m,it is S2(silty loam)> S6(clay loam)> S5(clay loam)> S4(loam)> S3(loam)> S1(sandy loam).To sum up,when evaluating the stability of soil aggregates,natural soil aggregates should be selected as the research object.At the same time,we found that comparing deionized water with hexane solution can truly reflect the effect of hydration on the stability of soil aggregates.From the changes of soil aggregate stability under the combined action of water cooperation and raindrop impact,it is found that the stability of soil aggregate is affected by comprehensive factors such as soil clay,CEC,rock fragment contents,soaking time and rainfall kinetic energy.When aggregates are only destroyed by hydration,the stability of aggregates is determined by soil clay,CEC,soaking time and microstructure characteristics of aggregates.When the aggregate is hit by raindrops and hydrated,the stability of the aggregate is determined by comprehensive factors such as rainfall kinetic energy,clay,CEC and rock fragment contents. |
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