Aggregate Mechanical Stability and Relationship with Aggregate Breakdown Under Simulated Rainfall

Li Zhaoxia1,Yang Wei2,Cai Chongfa1,Wang Junguang1

(1.Soil and Water Conservation Research Centre of Huazhong Agricultural University,Wuhan,China;2.Hubei Water Resources Research lnstitute,Wuhan,China)

Abstract:Soil erosion caused by water is a major form of soil degradation.Scientists suggest that this phenomenon can be predicted based on aggregate stability.However,most of these scientists have focused on the water stability of aggregates;only a few have devoted efforts to study the mechanical stability of aggregates.To investigate the relationship of aggregate breakdown with mechanical stability and the possibility of predicting soil erosion based on mechanical stability,we collected eight kinds of Ultisol aggregates(3～5mm)from subtropical China and exposed these aggregates to simulated rainfall(60mm h-1).Mean weight diameter(MWD)and splash loss after rainfall were analyzed and compared with aggregate water stability and mechanical stability.Aggregate water stability was determined by wet sieving methods;aggregate mechanical stability was tested by aggregate penetration resistance and tensile strength.Wet sieving method,tensile strength,and penetration resistance of dry aggregates exhibited a good relationship with aggregate MWD(R2≥0.838,0.775,and 0.593,respectively)underrainfall and splash(R2=0.758,0.865,and 0.563,respectively).In contrast,no relationship was found between penetration resistance of wet aggregates and MWD or splash.Thus,MWD of wet sieving method and tensile strength were the better indexes that could be used to estimate fragment size distribution and splash detachment.Tensile strength also correlated with clay contents(r=0.793*),Fed(r=0.735*),and Feo(r=0.911**).With further studies,a simpler and faster method could be developed to estimate soil resistance to erosion based on tensile strength in this region.

Key Words:Tensile strength,aggregate penetration resistance,simulated rainfall,aggregate breakdown

Financial Disclosures/Conflicts of Interest:This work was supported by the National Natural Science Foundation of China(grants 40930529,41171223,and 41001164).

As a major form of soil degradation,soil erosion caused by water is one of the most important environmental problems worldwide and has received considerable attention.This phenomeno nis influenced by aggregate breakdown,which affects water infiltration,soil crustability,and soil erodibility.Further more,aggregate breakdown induced by simulated rainfall is a method that provides the most relevant information about water-induced soil erosion(Legout et al.,2005a).Aggregate stability is the ability to retain a specific arrangement or to resist breakdown when soil is exposed to different stress conditions.Aggregate stability can be classified as mechanical stability and water stability.Several scientists have focused on the relationship between aggregate water stability and soil erosion.For instance,Legout et al.(2005a,2005b),Yan et al.(2008),and Wang et al.(2012)developed physical equations for aggregate water stability and soil erosion parameters.

Numerous methods,such as the wet sieving and the Le Bissonnais(1996)methods,have been developed to assess aggregate water stability.These methods often involve a combination of wetting procedures and subsequent mechanical agitation,which are consistent with the mechanisms of aggregate breakdown caused by rainfall.Aggregate water stability has been widely used to predict soil resistance to water erosion.However,these methods are limited by applying unquantified arbitrary mechanical energy,thus the resulting stability index can only be related to a particular context(Letey,1991;Le Bissonnais,1996;Amézketa,1999;Zhu et al.,2009).These methods are also complicated and time consuming because at least four main processes are involved,including weight measurement,wet sieving,oven drying,and calculation.In these processes,special devices are needed and restrict the applicability of such methods.

Aggregate mechanical stability is widely used to study soil tillage,soil erosion by wind,and so on.However,studies on soil erosion by water are insufficient.Aggregate mechanical stability mainly includes tensile strength(TS)(Dexter and Kroesbergen,1985;Watts and Dexter,1998;Dexter and Watts,2001)and aggregate penetration resistance(APR)(Misra et al.,1986,1988a,1988b;Shein et al.,2009).

The tensile strength of aggregates is defined as the stress,or force per unit,required to cause soil to fail in tension(Dexter and Watts,2001).Tensile strength is an important valuable parameter used to investigate individual soil aggregates because this parameter can be determined by simple tests and is sensitive to soil structure(Dexter and Kroesbergen,1985;Watts and Dexter,1998;Mosaddeghi et al.,2006);tensile strength can also be used for aggregates with different sizes.Barzegar et al.(1994)further indicated that tensile strength is related to aggregate water stability.Abu-Hamdeh et al.(2006)and Yang et al.(2012)investigated the relationship of tensile strength with soil erosion characteristics of aggregates that vary in size,revealing that the detachment rate is negatively correlated with tensile strength.However,the relationship of tensile strength with soil erosion characteristics of different soils remains unclear.

Penetration resistance is the force required to push a cone deeper into the soil divided by the cross-sectional area of its base(i.e.,pressure).The relationship between bulk soil penetration resistance and soil erosion has been considered;bulk soil penetration resistance is considered to be a good index of soil erosion(Al-Durrah and Bradford,1982;Callebaut et al.,1985;Bradford et al.,1992;Maestre et al.,2002;Stavi et al.,2008).Misra et al.(1986)developed a technique to determine the penetration resistance of aggregates.However,the relation between APR and soil erosion is unclear.

Ultisol is the dominant soils type in southeastern United States,Southeast Asia,and other subtropical and tropical areas.Ultisol covers approximately 1.14 million km2 in southeastern China.Uneven topography,improper soil management and land use,and poor soil properties result in Ultisol degradation,which has become one of the most serious environmental problems in China(Zhao et al.,2000;Zhang et al.,2004).

In this study,the tensile strength and penetration resistance of Ultisol aggregates and their relationships with aggregate breakdown under simulated rainfall were investigated.The proposed method is based on aggregate breakdown induced by simulated rainfall,thereby providing relevant information on soil erosion caused by water(Legout et al.,2005a).Hence,other methods are compared with this standard.The objectives of this study are as follows:(i)to understand the relationships of tensile strength and penetration resistance with aggregate breakdown under simulated rainfall and(ii)to compare the efficiency of the method used to assess aggregate breakdown and soil detachment rate by using aggregate mechanical stability and water stability.