Investigating The Soil Erosion Rates On The Cultivated Slopes In The Northeast Black Soil Region Of China Using 137Cs And 210 Pbex Measurements

The fertile black soil in Northeast China is famous for the "granary." However, the long excessive reclamation had led to increasingly serious soil erosion, and cultivated slope had become a major sediment sources. So the serious degradation of quality of cultivated black soil had posed a serious threat to the food security in China. Therefore, an understanding of erosion rules is important for designing conservation programs and deepening the erosion mechanism.In this paper, a straight cultivated for nearly 100 years slope with 500 meters long, 3°slope gradient, southward and a convex cultivated for more than 50 years slope with 350 meters long,northward were selected in Keshan county in Hei Long Jiang Province to study the 50-100 years erosion rates of cultivated black soil, and analysis the soil physical and chemical properties as well as nutrient content, in order to discuss the erosion rules and the degradation process of the cultivated black soil. Major methods contain field sampling and laboratory analysis, ¹³⁷Cs and ²¹⁰Pb_ex composite tracer technology, USLE model, wavelet analysis, SPSS, and so on. The main results as follow:(1) The spatial distribution of soil erosion intensity on the long and straight cultivated slope was studied using ¹³⁷Cs measurement and wavelet analysis. The results showed that there was a fluctuant period of soil erosion intensity with 142m along 4 transects downslope, which suggested that the downslope intensity of soil transporting alternately changed between weak and strong due to the impact of long-term erosive rainfall. Moreover, the best fitting result (R2>0.96) of the soil erosion rates along the four transects was the sum of sine function. It was further convinced the correctness of the fluctuant of the erosion intensity on the study slope; the same study of the convex slope showed the downslope erosion intensity was not periodic fluctuated, so the wavelet analysis was not applicable for the erosion study of other shapes of slope.(2) The ¹³⁷Cs contents in soil samples collected on the slope flat were analyzed and the results showed that ¹³⁷Cs inventory of all samples were less than the ¹³⁷Cs reference inventory, and the primary reason was there existed some wind erosion. Because ¹³⁷Cs measurement could get the total erosion rates and USLE model could measure the water erosion rate, the contributions of water and wind erosion rates were partitioned and the results showed that the annual mean erosion rate on the straight slope and convex slope was 3054 t km² a^-1 and 3548 t km² a^-1 respectively. Meanwhile, the wind erosion rate was 631 t km² a^-1 and 1155 t km² a^-1, 20.7% and 32.6% respectively because of the different slope aspect, which means that nearly 0.5¹mm soil was blew off in the study area every year. The water erosion rate proportion of the two slope was 79.3% and 67.4%, thus water erosion was still the dominant erosion style in the study area. Accordingly, the prevention needs to be considered adequately about the different influence by water and wind and comprehensively designed.(3) By comparing the profile distributing of the SOM content of the two slope, it was found that the SOM content in straight slope cultivated for 100 years and convex slope cultivated for 54 years were 4.3% and 5.6% respectively, soil degradation was more serious in straight longer reclaimed slope obviously. By stepwise regression of the erosion rate and nutrient content on two slopes, it was found that erosion rate Y can be represented by function of nutrient content, for the straight slope, that was: Y=189082.80X-14503.31(R=0.506), X was TP content. And, for the convex slope, Y= 2282.516X-16762.25(R=0.646), X was SOM content. These functions may show the intensive relations between soil nutrient loss and soil erosion. By estimating the nutrient loss of study area, the results indicated that the soil loss amount in straight slope were 241.87,10.94,6.14 t km² a^-1 of SOM, TN and TP respectively, but the nutrient loss on convex slope were more 367.76,14.22,9.14 t km² a^-1 respective of SOM, TN and TP.(4) The soil erosion rate of the two cultivated slope was studied using ¹³⁷Cs and ²¹⁰Pb_ex tracer measurement, the results showed that on the straight cutivated for nearly 100 years slope, due to the little difference of rainfall during the last 100 years in this region, the erosion rate derived from ¹³⁷Cs and ²¹⁰Pb_ex should be similar, and the results showed the erosion rate were 2812 t km² a^-1 and 2575 t km² a^-1 respectively by ¹³⁷Cs and ²¹⁰Pb_ex, though ¹³⁷Cs traced soil erosion rate of more than 50 years while ²¹⁰Pb_ex nearly 100 years, so it can be concluded that the two tracer measurement were both correct in this area. However, on the convex cultivated for 54 years slope, cause the little erosion before reclaimed, the erosion rate in the last 100 years by ²¹⁰Pb_ex was 1890 t km² a^-1, which was significantly less than after reclaimed: 3771 t km² a^-1 derived from ¹³⁷Cs measurement. Using the weighed average method, the erosion rate of the convex slope before reclaimed can be got, it was 319 t km² a^-1 and nearly 12 times less than that of after reclaimed. It could be concluded, because of the excessive reclamation and other unreasonable utilizations by human, the soil erosion had been accelerated seriously. Therefore, the fertile land in black soil region needs to be protected scientifically and quickly.