| Severe soil erosion in the Loess Plateau brings about low and unstable land productivity and continuous decrease in soil nutrients.Soil erosion manily casued by nature factor and human activity.It is important to study the response of soil erosion to nature and human factors for evaluating soil and water conservation.Luoyugou catchment of Tiansui city,Hujiawan catchment of Yanan city,Nianyangou catchment of Yulin city,and Laoyeman catchment of Shenmu were choose as study area.Soil samples were colected from all four catchments.Analysed stoichiometric character of C,N,and P of soil from different land use types.Combined with rainfall erosivity and sediment yield,we investigated the historical erosion environment of each catchemnt.The main results were as follows:1.Compared with the results of the second soil survey in Shaanxi Province in 1992,the content of soil carbon and nitrogen in the surface soil which have vegecation increased,and the content of soil phosphorus decreased slightly in the typical watershed of the Loess Plateau.The result shows that the soil organic matter input in the study area is higher than that in 1992?C:N in different land-use types soil was lower in Hujiawan and Xianyangou small watershed,indicating that the accumulation rate of organic carbon was lower than its decomposition rate in the two catchments.However,The average value of soil C:N in Laoyeman small watershed is 32.60,which indicated that the accumulation rate of organic carbon was higher than the decomposition rate.The average of N:P of Hujiawan,Laoyeman and Nianyangou was all lower than 14,which indicated that the soils of the three catchments were all in the state of high mineralization rate and that soil in the three catchment lack for N.2.Among all stoichiometry characteristic index,N of Hujiawan catchment were significant different in different land use types.Both in Nianyangou and Laoyeman small watershed was C:P which in slope land was siginificant different in diferent land use types.It indicated that N and C:P were sensitive index to the change of land use types to be important index to response of land use change to human activity.3.According to the obvious inflection point of the cumulative anomaly percentage curve of sediment quantity,the sediment yield of the dam controlled watershed can be divided into four stages.In Hujiawan small watershed,it is divided into:1975—1977,1978—1981,1982—1991 and 1992—2003;In Nianyangou small watershed,it is divided into:1960—1964,1965—1983,1984—1986 and 1987—1990;In Laoyeman small watershed,it is divided into:1978—1986,1987—1989,1990—1997 and 2001—2013.The sediment amount was standardized with rainfall erosivity and nitrogen content or C:P,and the contribution rate was calculated by Horot formula.The soil erosion modulus of the Hujiawan small watershed during the 1st and 4th stages was 1103 t/(km~2·a)?6973 t/(km~2·a)?2142 t/(km~2·a)and 1498 t/(km~2·a);In the 1~2 stage,the contribution rate of rainfall is 95.60%and 78.68%respectively.The contribution rate of artificial factors to the small watershed is 4.4%and 15.9%respectively.In the 3~4 stage,the contribution rate of rainfall is 97.42%and 57.12%while the contribution rate of human activity is 2.31%and 0.08%respectively.The rainfall is the main factor to promote the erosion and sediment yield in the third stage.The soil erosion modulus of the Nianyangou small watershed during the 1st and 4th stages was 26224 t/(km~2·a),5899 t/(km~2·a),25838 t/(km~2·a),and 14095t/(km~2·a);During the 1,3,4 stage,the contribution rate of human activity factors were 0.3%,52.63%and 40.08%respectively,and the contribution rates of rainfall were 98.86%,39.41%and 25%respectively.Natural factors and anthropogenic factors are the main factors of erosion and sediment yield in stage 1 and 3 respectively.;The soil erosion modulus of the Laoyeman small watershed during the 1st and 4th stages was 10091 t/(km~2·a)?14746 t/(km~2·a)?7257t/(km~2·a)?6057 t/(km~2·a);In the first and second stage,the comprehensive effects of anthropogenic factors on erosion and sediment yield of small watershed were increased by 0.11%and 2.83%,and the contribution rate of rainfall in the first stage was 82.08,which was the main factor of erosion and sediment yield in this stage.In the third stage,the anthropogenic factor was the effect of sediment reduction.4.C,N and P decreased from south to north on the Loess Plateau.The spatial variation of C:N was mor sinificant than P and N:P.During the period from 1978 to 1983,among the three catchment,Niayangou catchment had a relatively smooth variation of C,N and P.During the period from 1984 to 1985,beside C:P and P the other Stoichiometric character had a significant variation of the all three catchments.During the period from 1988 to 1990,C,N and P had a significant variation in Laoyeman catchment. |
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