| Soil erosion is one of the global environmental disasters restricting the survival and development of the human society, and it seriously restricts the sustained development of the global society and economy. Plentiful countries and regions in the world have started research on preventing and controlling soil erosion, and have obtained breakthrough progresses in terms of the mechanism and process of soil erosion, the evaluation and calculation of soil erosion rate, and the evaluation on the environmental effect of water & soil conservation and ecological environment construction. In terms of soil erosion rate calculation,it's a faster and more convenient method to research soil erosion by means of radioactive nucleus elements.At present, the radioactive nucleus elements widely applied to the tracer research of soil erosion are 137Cs, 210Pbex and 7Be. Wherein, the first two nucleus elements can only provide the medium- and long-term (50 years and 100 years) soil redistribution rate,while 7Be may be applied to the tracer research of soil erosion in short-term or individual rainfall period, can even make up the deficiencies of 137Cs and 210Pbex in the tracer research of soil erosion,vnd can be used together with 137Cs and 210Pbex for the tracer research of soil erosion in different periods of time. However, 7Be tracer technique has started late, its research on some basic problems is not deep-going enough, and especially, there is lack of 7Be soil erosion tracer model which may be applied widely, calculated simply, and applicable to the field slopes in condition of individual rainfall.Based on observing the deposition characteristics and the 7Be vertical distribution on ground surface in Ningzhen, this paper has established an applicable 7Be soil erosion model,and conducted model verification by combining with traditional methods for researching soil erosion; meanwhile, this paper has used this technique to research the soil erosion process of low and gentle slopes in Ningzhen,to analyze the seasonable changes in the soil erosion rate of slopes, to compare the changes with the calculation data of 137Cs and 210Pbex tracer soil erosion rate, and to develop compound tracer research on soil erosion of multiple nucleus elements in Ningzhen. The main results obtained are as shown below:1. Key bases for tracing of soil erosion with 7Be.The deposition rule and surface distribution characteristics of 7Be are the key bases for tracing soil erosion with 7Be. In the researched area, the individual rainfalls were collected by plastic container in the main rainy season of two consecutive years,and monthly atmospheric deposition samples of 7Be were collected from 2010 to 2011.Deposition samples were experimented to obtain residual samples by chemical experiement which was followed measured with a HPGe detector for 7Be. The results showed that the concentration of 7Be in individual rainfalls was observed to be 0.58?4.12 Bq/L, averagely 1.52 Bq/L. In an independent individual rainfall process,the concentration of 7Be at the later stage of rainfall was obviously lower than that at the earlier and middle stages of rainfall; As seen from the amount of 7Be deposition(Amount of deposition = 0.8296 X Rainfall) in individual rainfall process; in the researched area, the amount of 7Be deposition in 2010 and 2011 was 1621.78 Bq/m2 averagely, and the deposition flux was 0.66?14.49 Bq/m2d, averagely 4.44 Bq/m2d.The seasonal variation of 7Be deposition obviously, sequence of average deposition was Spring > Summer > Winter > Autumn; The maximum amount of monthly deposition appeared in April 2010, and the minimum one appeared in November 2010,the change of deposition presented relatively obvious two seasonal peak; In autumn,winter and spring, the average dry deposition was 19.24 Bq/m2, as seen from the ratio of dry deposition among the monthly total deposition, dry deposition in November 2010, January and April 2011 made a high ratio of contributions.In the researched area, the seasonable background value of 7Be in soil that showed Spring and Summer than Autumn. In the soil profile of background value, 7Be was centralized in surface soil, along with the increase of depth downward, the 7Be content presented exponential decrease, and the total geometrical distribution depth reached up to 16?20 mm. The inventory of 7Be in ground flora was 120.78 Bq/kg averagely. Wherein, rank grass had a minimum retention, while kraurotic wormwood had a maximum retention. Ground cover had a 7Be retention and absorption rate of averagely 12.77%, but artificial cultivation of plant had a lower rate.2. The model of 7Be tracing soil erosion was established.The soil erosion rates derived from 7Be measurement model is the key bases with 7Be technic. In this paper, to analysis the existing 7Be tracer models for soil erosion,by combining with the existing tracer models, and based on the assumptions, using a mass balance approach and considering the characteristics of the existing tracer models, and the loss of 7Be due to settlement, erosion and radioactive decay, a soil erosion calculation model and vegetation modified model were established to trace soil erosion for any season of rainfall. The new model verification result shows that,the model can be used to calculate the concrete soil particle redistribution rate according to the distribution of 7Be on ground surface.3. Research on 7Be tracer of soil corrosion in Yellow brown soil slope, Ningzhen Area.In the different periods, the inventory of 7Be at the background sampling points in Ningzhen was 343.33 Bq/m2 averagely. The inventory of 7Be in the soil slopes with a variation scope of 399.00 Bq/m2. The relaxation mass depth h0 of 7Be in soil was 4.57-11.86. The soil erosion amount averagely on slopes in one year by using new 7Be tracer model were 1.52 kg/m2(abandoned land), 5.48 kg/m2(tea gardens) and 4.43 kg/m2(dryland) respectively. In terms of seasonable change in soil erosion, each slope had the largest erosion contribution rate at the time (April 16?July 22,2011). In terms of the annual average erosion rate, the maximum was Tea gardens, and minimum was abandoned land.4. Comparative analysis on soil erosion traced with 7Be, 137Cs, and 210Pbex in Ningzhen AreaThe vertical distributional difference of 137Cs, and 210Pbex was larger. The ratio of specific activity of 7Be/137Cs, and 7Be/210Pbex presented the obvious trend of progressive decrease along with the increase of soil depth. The annual average erosion rate calculated with 137Cs and 210Pbex tracer model were 839.12 t/km2-a and 1441.17 t/km2·a for abandoned land,9175.03 t/km2·a and 8579.37 t/km2·a for tea gardens,5803.48 t/km2-a and 6193.50 t/km2-a for dryland. This result and the result obtained with 7Be model could form mutual corroboration, and showed the new 7Be tracer model was availability. But the results calculated with 137Cs model and 210Pbex model included gully erosion. The average annual soil erosion rate obtained through the compound tracing of the three nucleus elements was 1230.40 t/km2·a (light erosion)for abandoned land, 7743.85 t/km2·a (strong erosion) for tea gardens, and 5475.12 t/km2·a (strong erosion) for dryland. The result of compound tracing shows that,human turbulence has aggravated soil erosion, and the measure of planting on slopes has also affected the soil erosion on slopes. |
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