雨滴机械打击和消散作用对土壤团聚体的破坏特征

土壤团聚体是土壤结构的基本单元,其稳定性是描述土壤抵抗侵蚀过程中破坏作用的重要指标。但溅蚀过程中,雨滴对团聚体的消散和机械打击两种破坏作用的相对贡献及其破坏机制尚未明晰。利用酒精和超纯水作为雨滴形成材料,模拟机械打击单独作用及消散和机械打击共同作用,分别在五个高度(0.5、1、1.5、2和2.5 m)对塿土和黄绵土进行溅蚀实验。结果表明:当降雨动能相同时,塿土的溅蚀率均小于黄绵土。同时,超纯水雨滴对土壤的机械打击和消散共同作用所导致的溅蚀率均大于酒精雨滴单一机械打击作用的溅蚀率。随着降雨动能增加,两种雨滴对两种土壤的溅蚀率均呈幂函数增加;团聚体因消散破坏作用和机械打击作用的溅蚀率均亦随之增加。但两种土壤的消散破坏作用和机械打击作用的贡献率分别随着降雨动能增加而减小和增加。在相同降雨动能时,塿土消散破坏作用的贡献率均大于黄绵土,而机械打击作用贡献率均小于黄绵土。研究结果对深入理解溅蚀过程中团聚体破坏机理及评价溅蚀过程中团聚体稳定性具有重要意义。

Characteristics of Mechanical Impact and Slaking Effect of Rain Drops on Soil Aggregates

Objective] Soil aggregate is a basic unit in soil structure and its stability is an important index describing soil's resistance to breakdown in the process of water erosion.However,in splash erosion how raindrops function through mechanical impact and slaking effect on soil aggregates and what are the mechanisms and contribution rates of the two are still unclear.This study is oriented to investigate effects of mechanical impact and slaking effect of rain drops on breakdown of soil aggregates during splash erosion.Method] A series of indoor splash erosion experiments were carried out in the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau,Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources,China.Samples of Loessal soil (Sandy loamy soil) collected from Yan'an and Lou soil (Loamy loam soil) collected from Yangling in Shaanxi province,two typical soils in the Loess Plateau,were tested in the experiments.A home-made needle type rainfall simulator,consisting of three parts:water supply apparatus,needle nozzles and support frame,was used to simulate rain drops of ultra-pure water and alcohol to determine effect of mechanical impact alone and joint effect of mechanical impact and slaking on soil aggregates,separately.The splash erosion experiments were designed to have two type of soils and 5 treatments in height for rain drops to fall,i.e.0.5 m,1 m,1.5 m,2 m and 2.5 m.Result] Results show that in the two soils soil aggregate stability exhibited an order of MWDfw ＜ MWDws ＜ MWDsw.Slaking effect (Fast wetting) was the major mechanism of the breakdown of soil aggregates,and followed by mechanical disturbance (Wetting and Shaking),and then chemical slaking (Slow Wetting) in the end.The soil of loamy clay was higher than the soil of sandy loam in RSI (Relative Slaking Index),suggesting the former is more susceptible to slaking effect than sandy loam soil,while the latter is more to mechanical impact.In splash erosion,when rain drops fell from the same height,splash erosion rate was lower in loamy clay soil than in sandy loam soil,and splash erosion rate caused by rain drops of pure water through the joint effect of mechanical impact and slaking was higher than that caused by drops of alcohol through mechanical impact alone in both soils.Regardless of pathways of the rain drops affecting soil aggregates,splash erosion rate increased with rising kinetic energy of the rain drops,and power function could be used to well describe the relationship between splash erosion rate and rain drop kinetic energy.The splash erosion rates caused by slaking effect and/or mechanical impact of rain drops both increased with rising rain drop kinetic energy or rising height where rain drops fell from.The slaking effect contributed more than 50％ to the splash erosion rate,indicting slaking effect was the main factor causing aggregate breakdown effect,but the slaking effect decreased in contribution to splash erosion rate with rising rain drop kinetic energy,while the mechanical impact acted reversely.In the cases the same in rain drop kinetic energy,the contribution of slaking effect was higher in loamy clay soil than in sandy loam soil,but that of mechanical impact was just the reverse.Conclusion] Contribution rates of slaking effect and mechanical impact vary with rain drop kinetic energy and soil type.All the findings in this study could be of great significance to evaluation of aggregate stability and to in-depth understanding of the mechanism of aggregate breakdown during splash erosion.