Aggregate Stabilization Mechanism And Slope Erosion Characteristics Of Several Typical Zonal Soils

Soil erosion is one of serious environmental problems in China.Soil resistance to erosion is mainly controlled by soil properties.Various types of soils differing greatly in their properties are generally distributed in horizontal zonality under the impact of climate conditions.To date,few attempts have been made to systematically investigate soil structure stabilization and their resistance to erosion for different types of zonal soils.In this study,six types of zonal soils in central south China were selected from north to south,that is,Cinnamon soil,Yellow-cinnamon soil,Brownish red soil,Red soil,Latosolic red soil and Latosol,which derived from common and similar parent materials(loess deposit,quaternary red clay and basalt)with heavy textures.On the basis of the theories of soil physics,chemistry,geography,erosion and mathematical statistics,indoor analysis and field runoff plot rainfall simulation experiments were conducted to investigate the aggregate stabilization mechanisms and the effects of rainfall intensity(45 and 90 mm/h)and erosion degree(no,severe and very severe)on erosion processes and erodibility for different types of soils.Main results were shown as follows:(1)Spatial and profile variations of soil basic properties and aggregate stability for zonal soilsFrom Cinnamon soil to Latosol,the content of exchangeable base cations and 2:1 type clay minerals(vermiculite and illite)decreased while that of free iron and aluminum oxides and kaolinite increased;amorphous iron and aluminum oxides contents increased firstly and then decreased;bulk density and organic matter did not exhibit a distinct trend as free oxides,and increased and decreased with an increase of soil depth,respectively.Aggregate stability displayed significant differences between soil types,profile horizons and land uses.Aggregate dry stability generally increased with an increase of soil depth and decreased from Cinnamon soil to Latosol while aggregate water stability decreased with an increase of soil depth and increased first and then decreased from Cinnamon soil to Latosol.Aggregate dry stability was positively correlated with bulk density(r=0.62,p<0.001),and negatively with organic matter(r=-0.42,p<0.05),while aggregate water stability was influenced by p H,organic matter,bulk density and exchangeable sodium.Aggregate water stability was the least under slaking among the three pre-treatments,followed by mechanical breakdown.The differential swelling mainly disrupted aggregates for the Cinnamon and Yellow-cinnamon soils due to their large contents of vermiculite and exchangeable cations.Free iron and aluminum oxides,kaolinite,1.4 nm intergrade minerals and organic matter played the positive role in aggregate water stability while vermiculite,exchangeable cations and bulk density played the negative one.(2)Distribution of binding agents in various size water-stable aggregatesWater stable aggregates were in bimodal distribution for zonal soils under natural vegetation and they were dominated by the macro-aggregates(>0.5 mm)except for Cinnamon in the eluvial horizon and by micro-aggregates(<0.25 mm)in the subsurface horizons.The distribution of different types of binding agents with water-stable aggregate size varied with soil types and pedogenic horizons.Free iron oxides content decreased with a decrease of aggregate size in the parent material horizons of Yellow-cinnamon soil and Red soil.Similar conditions occurred in free aluminum oxides in the parent material horizons of Red soil,manganese oxides in the parent material horizon of Cinnamon and Yellow-cinnamon soils and in the eluvial horizons of Red soil and Latosol.It indicated that these agents likely contributed to the formation and stabilization of water stable aggregates in soils.Crystalline oxides were dominant in free oxides and were associated with clay particles in the subsurface soil horizons.In the eluvial horizon,soil organic carbon content and C/N decreased with a decrease of aggregate size for Cinnamon and Yellow-cinnamon soils,while the organic carbon content followed an inverse pattern for Latosol and varied in narrow range for other soils.It indicated that organic matter was stabilized by physical occlusion in aggregates and sorption on clay minerals for Cinnamon and Yellow-cinnamon soils,and by sorption on clay minerals for other soils.Compared to clay content,iron and manganese oxides had close linkage with organic matter.(3)Hydrological processes for zonal soils in rainfall-induced erosion.Hydrological processes in rainfall erosion were influenced by the interaction of soil properties and rainfall intensity.Overall,soil infiltration rate decreased initially and reached a steady state during the rainfall while runoff rate and runoff coefficient followed an inverse pattern.Both the infiltration rate and runoff rate were generally enhanced by rainfall intensity.For Cinnamon soil,Red soil and Latosolic red soil,the average infiltration rate and runoff rate separately increased and decreased with an increase of erosion degree,and for Yellow-Cinnamon soil and Latosol,the effects of erosion degree on infiltration rate were influenced by rainfall intensity and runoff rate increased with an increase of erosion degree.The infiltration rate and runoff rate were the largest and least for the Latosol among all soil types,respectively.In addition to rainfall intensity,clay mineralogy(1.4 nm intergrade minerals and illite),soil organic matter,exchangeable base cations,free aluminum oxides and particle density influenced the hydrological processes whose influencing factors also varied with rainfall intensity.(4)Erosion processes for zonal soils in rainfall-induced erosionSediment concentration and soil loss rate were larger at the high than at the low rainfall intensity except for very severely eroded Cinnamon soil and Yellow-cinnamon soil.The temporal variations in sediment concentration and soil loss rate during the rainfall were influenced by the interaction of soil properties and rainfall characteristics(intensity and duration).For Cinnamon soil,and Red soil and Latosolic red soil at the low rainfall intensity,sediment concentration decreased with an increase of erosion degree;for Yellow-cinnamon soil,Red soil,Latosolic red soil and Latosol at the high rainfall intensity,sediment concentration was distinctly lower in the very severe than in the no and severe erosion degrees and no significant differences were observed between the no and severe erosion degree except for Red soil.At the low rainfall intensity,soil loss rate decreased with an increase of erosion degree for Cinnamon soil,Red soil and Latosolic red soil while it followed an inverse pattern for Yellow-cinnamon soil and Latosol.At the high rainfall intensity,soil loss rate was much lower in the very severe than in the no and severe erosion degrees.Rainfall intensity significantly contributed to soil erosion and among soil properties,clay mineralogy(1.4 nm intergrade minerals and illite),cation exchangeable capacity,bulk density,amorphous aluminum oxides and aggregate stability influenced erosion processes.(5)Sediment composition and transport mechanisms for zonal soilsSignificant differences in sediment effective sizes occurred between soil types and erosion degrees,which were influenced by rainfall intensity.The average sediment size was overall larger at the high than at the low rainfall intensity and was much smaller in the very severe than in the no and severe erosion degrees except for the very severely eroded Latosolic red soil.The content of <0.10 mm size fraction gradually decreased while that of >0.10 mm size fraction increased with runoff time at the early stage of erosion,and then both of them reached a steady state as rainfall continued.Effective sediment sizes were bimodally distributed except for the early stage of runoff;the bimodal pattern became more remarkable from Cinnamon soil to Latosol and the mean weight diameter increased first and then decreased.p H,amorphous aluminum oxides,exchangeable sodium and aggregate dry stability,in combination with rainfall intensity influenced sediment sizes.The sediment delivery was dominated by suspension/saltation for Cinnamon soil,Yellow-cinnamon soil,very severely eroded Red soil,very severely Latosolic red soil at the low rainfall intensity and at the early stage of runoff,while by both suspension/saltation and rolling for other treatments.(6)Variations of the erodibility for zonal soilsSignificant differences occurred in soil erodibility between soil types,erosion degrees and rainfall intensities.Soil erodibility varied temporally due to the alteration of surface soil structure during the rainfall and it was least for the Latosol among all soil types.At soil pedon scale,soil erodibility generally decreased with an increase of erosion degree except for Yellow-cinnamon soil and very severely eroded Red soil,and was much lower in the very severe than in the no and severe erosion degrees.Soil erodibility calculated by nonograph was larger than the measured value,indicating that the nomograph cannot be applicable to the heavy textured soils under the short-term rainfall in central-south China.Among soil properties,clay mineralogy(1.4 nm intergrade minerals and illite),particle density,amorphous aluminum oxides and aggregate dry stability influenced soil erodibility and the soil properties selected accounting for soil erodibility varied with rainfall intensity.