Using~7Be To Document The Response Between Soil Erosion Intensity And Vegetation Coverage On The Slope

In recent years, due to the national policy of returning farmland to forest or to grassland, vegetation coverage gradually improved and the erosion patterns have changed.Therefore new research methods are needed to find and exploit to deal with the emergingissue. The~7Be tracer technique has been successfully applied to study the soil erosion, but~7Be tracer technique can only be applied to bare slope, so~7Be tracer technique isintroduced for soil erosion study of vegetation coverage slope has an important scientificsignificance. It could broad the application fields for the~7Be tracing technology. Thisresearch to analysis the concentration of~7Be in undisturbed soil and the deposition flux of~7Be in Ansai. At the same time, quantify of~7Be mass activity in plants and the effects~7Bein soil through~7Be tracer model building and verification with vegetation according to theobservation samples in runoff plots, and using the modified model to quantitative analysisof the response relationship of soil erosion intensity and vegetation cover in wastegrassland slope. The research conclusions are as follows:(1) The deposition flux of~7Be was obvious of annual and interannual changes in studyarea. The annual deposition flux was1759±416Bq m-2. The deposition flux was mainlyaffected by precipitation, a high of50%deposition of a year in summer and a low of5%only in winter. Compared with the other researched areas, the study area had a highdeposition flux of~7Be, which was possibly to be involved in the great contribution of dust,frost and dew to the deposition.(2) The change of the activity of~7Be within day and month mean value in undisturbedsoil appeared a distribution curve with single-peak. Within a day, the activity of~7Be rangedfrom89.2to941.8Bq m-2, and the mean value is392±210Bq m-2. The highest activity of~7Be appeared in August or in September, the lowest often in winter or the early spring because of the effect of the precipitation and time distribution.(3) The activity of~7Be in the mixing withered plants in study phases ranged from212±46Bq kg-1to964±122Bq kg-1. However, the mixing live plants are ranged from173±86Bq kg-1to703±18Bq kg-1. The activity of~7Be in the mixing withered plants washigher than it in the mixing live plants. The activity of~7Be in the mixing withered plantshad the same tendency of changes with it in the mixing live plants within a year. Thedifference in the activity of~7Be between the mixing withered plants and the mixing liveplants was large in the end of spring and the beginning of the summer. Then, it was smallerin summer and autumnal. However, it was highest in rainy season of September, then, ithas decreased. The activity of~7Be in the same ground cover had a same tendency ofchanges in different years, and the values were close to each other in spring but theincreasing rates were of a significant difference in rainy season. The results showed thatthere was a significant positive correlation between the activity of~7Be in mixing plants andthe decayed cumulative precipitation. And the80%of the variability could be explained bydecayed cumulative precipitation.(4) The activity of~7Be in the same plant increased in rainy season. However, therations of the increase in different plants were different, which was mainly affected by thestem ratio and leaf area index. The activity of~7Be in the same plant was different betweenleaves and stems, and the activity of~7Be in leaves were higher than in stems, and the ratioof activity of~7Be in leaves to it in stems was significantly different in different plants.(5)~7Be tracing technology was applied to document soil erosion on the bare plots、weed plots and millet plots in2011and2012, and compared soil erosion rate with thecalculated rate. The relative errors exceed100%, and results indicated that vegetationcover had obvious effect on the estimate of soil erosion rate on the weed plot using~7Bemeasurement. Therefore, a factor of vegetation had been introduced into the Walling’smodel of converting~7Be activity to soil erosion rate for estimating soil erosion rate on theweed slope surface. It was found that the soil erosion rates calculated by modified modelwere well close to the measured values on weed plot, the relative errors decreased by10%-26%, which illustrated that the modified model could be well used to estimate therates of soil loss on the weed slope surface. These findings provide effective means forfurther study on the relationship between vegetation cover and soil erosion. (6) The analysis shows that some relationship between vegetation coverage and soilerosion intensity. The global aspect was that soil erosion rate is high in vegetation scarcityand low in the lush vegetation. However, there was no significant correlation in bothvegetation coverage and soil erosion intensity. The scale effects and threshold of vegetationcoverage to soil erosion are main reasons for results.