Characterization Of Aggregate Stability And Its Effects On Erosion Processes At Slope Land In Purple Soil Region

Purple soil widely distributed in Southwest China and southern hills and mountains.For the rainfall is abundant,concentrated and frequent,the ground vegetation coverage is poor,soil erosion is serious in purple soil area.Purple soil erosion seriously threatened the local ecological environment and stability,therefore the research of purple soil erosion mechanism and process is particularly important.Using combination of methods of wild artificial simulated rainfall and laboratory analysis,we studied the characterization of aggregate stability and its effects on erosion processes at slope land in purple soil region.Through the analysis of purple soil stability features, clarified failure process,result,and mechanism of purple soil in artificial rainfall simulation conditions,the inner relation between soil stability features index and sediment yield,we determined the characterization of aggregate stability and its effects on erosion processes at slope land in purple soil region.Main conclusions are as follows:(1) Using combination of traditional method of aggregate stability,wet sieving,decentralized method,and LB-method that could differentiate aggregate breakage in rainfall condition,we analyzed the aggregate stability characterization of slope purple soil systemly and discovered that stability of the soil samples was not consistent because of the difference of aggregate initial size, pretreatment and measure.(2) Results of aggregate stability by wet sieving:content of＞0.25mm water stable aggregate (WSA) showed mulberry land(72.50%)＞alfalfa land(37.72%)＞badlands(36.63%)＞vegetable land(35.20%);PAD＞0.25mm showed vegetable land(60.88%)＞badlands(59.73%)＞alfalfa land (57.89%)＞mulberry land(25.26%);MWD and GMD showed mulberry land＞alfalfa land＞badlands＞vegetable land,which is also the overall trends of aggregate stability.Micro-aggregate was mainly composed with sand(1～0.05mm) and powder(0.05～0.005mm).DR showed mulberry land(88.87%)＞badlands(85.84%)＞alfalfa land(84.81%)＞vegetable land(80.99%);CDI variation was mulberry land(56.08%)＞alfalfa land(50.28%)＞badlands(48.88%)＞vegetable land(42.27%). Micro-aggregate stability and soil anti-erodibility is poor as DR and CDI increasing.(3) Broken aggregate particle size distribution mainly concentrates on 0.15～0.05mm level. Using MWD and GMD measure aggregate stability,mulberry land has the highest stability after two kinds of processing,Secondly is alfalfa land and badlands,vegetable land aggregate stability is the worst.(4) Results of aggregate stability by LB-method:5～2mm size broken aggregate content was rarely and 0.15～0.05mm size had most content after fast wetting treatment;broken aggregate had similar distribution characteristics after slow wetting treatment and wet-stirring treatment,which reflected Purple soil stability had poor stability and anti-erodibility in the storm,irrigation conditions.By the three treatments MWD and GMD order was MWD_SW＞MWD_WS＞MWD_FW,GMD_SW＞GMD_WS＞GMDFw.RSI showed vegetable land(0.67)＞badlands(0.64)＞alfalfa land(0.55)＞mulberry land(0.48);RMI variation was badlands(0.40)＞vegetable land (0.32)＞alfalfa land(0.22)＞mulberry land(0.11),which showed slaking funiction had bigger destructiveness to vegetable land and badlands,and mulberry land soil kept a high stability after mechanical shaking treatment.Aggregate was more sensitive to slaking funiction by fast wetting treatment and mechanical shaking treatment caused by external stress,these two kinds of effects were the main mechanisms of purple soil.(5) In the first hour of rainfall duration,flow production curve of the soil samples was at a relatively low level in the before 36 minutes,fluctuation curves was at small scale.36-60 minutes,for the complete saturated soil moisture and super seepage flow producing,flow production rate began to increase and flow curve in fluctuation at bigger scale.Sediment curves is basically the same rule. In the second hour of rainfall duration,due to the soil moisture content has reached high level, sediment yield quantity are improved to varying degrees.Sediment yield changes reflected in the process of continuous rain,to the purple soil that had poor stability and wassensitive for mechanical broken,the influence of raindrops hit on slope surface runoff was mainly caused by surface soil structure damage;to the purple soil that had high stability and was not sensitive for mechanical broken,the influence of raindrops hit on slope surface runoff was compaction effect on the surface soil.(6) Related between aggregate stability parameter and sediment yield was more apparent in the first hour of rainfall duration.Related between aggregate stability parameter after LB-method and sediment yield was poor,which explained purple soil aggregate damage was not caused by a single breakage mechanism.Aggregate stability parameter by wet sieving as the index that measured purple soil erodibility is more reasonable.