Effects Of Water Erosion On Organic Carbon Of Hilly Purple Soils And SOC Forecast Model

As one special kind of soils,purple soil covers an area of 0.16 million square kilometers in China.In the Sichuan Basin,it is widely distributed and the rate of purple soil area to the totalfarmland amounts to 68%.The features of purple soil-distributed region such as abundant and centered rainfall,frequent rainstorms and poor vegetation cover result in severe soil erosion of these regions.The research about effects of soil erosion on soil productive forces is one focus in the field of water and soil conservation.The soil productive forces are decreased mainly resulting from theeffects of soil erosion on the content of soil nutrients,especially organic carbon.On the premise of analyzing the last 17 years' observation data about rainfall and vegetation coverage in Xiejiawan,water and soil conservation station of Suining,Sichuan,the relationships between the reduction ofsoil organic carbon and impact effects of soil erosion such as slope,farming measures,vegetation coverage and rainfall were investigated and analyzed,respectively.Besides,the losses of organic carbon caused by water erosion in purple soil-ed area with grey model were also forecast and evaluated.(1) Under the condition of water erosion,the losses of organic carbon in purple soil ascended with the increase of the slope,and the annual loss yield amounted to 3.8357 t per square kilometer.(2) Under the same slope and rainfall,the ridge in a field contributed less to the losses of organic carbon of purple soil than slope farming,and even less than traditional farming.However,the bared land deeply dig was easy to bring about the losses of organic carbon of soil.For example,under the condition of the rainfall of 55.2mm,the organic carbon losses of soil resulted from verticaland oriental ridges were 45.0576% and 90.1310% lower than that of bared land,and the loss resulted from vertical ridge were 82.0375% lower compared to that of oriental ridge.(3) There was a negative effect of vegetation coverage on the losses of organic carbon inpurple soil under the condition of water erosion,that is to say,higher vegetation coverage,relived losses of organic carbon and loss degree.These suggested that there was a gradual effect of vegetation coverage on the losses of organic carbon of soil.The sensitivity analysis of the fit Logistic model revealed that there was a turning point of effects of vegetation coverage on the losses of organic carbon of soil when the avegetation coverage was 66%.It meant that losses of organic carbon of soil suddenly decent when the vegetation coverage amounted to 66% or so.Thus,the vegetation coverage should account for 66% or more ifthe loss of organic carbon of soil was wanted to be controlled from the vegetation coverage aspect.(4)The effects of rain on the losses of organic carbon in purple soil were embodied mainly byrainfall parameter and rain intensity affected less.The reason might result in the fact that the soiltype belonged to the sloping farming,rain brought about sloping water erosion with strong runoffand thus severe loss of organic carbon of soil resulted.There was a significant seasonal effect of rain on the loss of organic carbon in purple soil and avariation trend characterized by flood season as the peak in one year existed.(5)The grey model based on the data about the annual loss of organic carbon of purple soil in Xiejiawan was finally built and listed as follows:SOC⁽⁰⁾(t+1)=-23．8855(e^-0.4158t-e^-0.4158(t-1)),(t=0,1,2,…,7)The testing accuracy of grey model amounted to first class and the average stimulated deviationwas less than 20%.The special evolution dynamics of integrated management of organic carbon ofpurple soil could be well described and explained by the grey model.