| Characterized with obvious transitional environmental factors and intensive human activities, the soil was seriously degraded in the wind-water erosion crisscross region. Wind and water alternate erosion was the direct reason that caused the regional environmental problems. Through sampling in representative regions, this study compared soil properties between the wind erosion and water erosion regions, analysed the soil quality status, distribution and the factors that affect soil quality in water-wind erosion crisscross region. Using the 137 Cs technology, the coupling effect of soil erosion and soil quality was reflected, and the causes of soil degradation were revealed. Moreover, combined with wind tunner and simulated rainfall experiment, the thesis reflected the changes of soil nutrients, soil particles and soil and water loss characteristics. Finally, the interaction and mechanism of soil degradation under wind and water alternate erosion were analyzed. In order to clarify the contribution of wind and water erosion to soil losses, the thesis using the USLE model, 137 Cs tracer technique and terrain factor analysis to discriminate the erosion rate of wind erosion and water erosion. The main conclusions were as follows:(1) The soil nutrients showed a decreasing trend from southwest to northeast in wind-water erosion crisscross region. The contents of soil organic matter(SOM) and total nitrogen(TN) in the southwest section(Guyuan area) was higher while it was lower in the northeast section(Shenmu area). The distribution of clay and silt contens in different slope aspects was consistent with the variation of soil nutrients. There exsit a significant correlation between soil nutrients and rainfall or wind index, the relationship between soil nutrients and the land use types, altitude, soil mechanical composition were also significantly correlated(P<0.05). In the area with annual rainfall more than 400 mm, the content of SOM and TN were higher while in areas where annual rainfall less than 300 mm, the contents were lower. When the average wind index greater than 100, the content of SOM and TN significantly decreased than that of the area of wind index less than 100(P<0.05). The SOM and TN contents were higher in the area of altitude more than 400 mm while the contents were lower in areas where altitude less than 1700 mm. Moreover, the contents of SOM and TN were significantly lower in areas of sand content more than 80% while the contents were significantly higher in the area of sand content less than 70%.The soil degradation index of sloping cropland in the wind-water erosion crisscross region was-3.27%. Except for Guyuan area, the soil quality in other sampling regions was degraded with the most degraded area in Shenmu sampling area. Soil degradation degree, to a certain extent, reflected the vulnerability of the erosion intensity and erosion environment.(2) In the wind-water erosion crisscross region, wind erosion and water erosion significantly affected the SOM and TN contents through changing the physical properties of topsoils. Differences in soil 137 Cs content and physicochemical properties among different slope positions and slope aspects were mainly due to wind erosion, water erosion and the result of their interaction. Wind-water alternate erosion accelerated soil quality degradation, and caused a serious decline in nutrients of the infertile soils. The average value of 137 Cs was 35.6% higher on the southeast- compared to the northwest-facing slope. Moreover, the 137 Cs were increased along slope from upper to bottom. A significant correlation was found between 137 Cs inventories and the SOM and TN contents(P<0.01 or P<0.05). The impact of slope aspect and slope position on SOM, TN, silt and clay contents were consistent with the variation in 137 Cs inventories. The lowest contents of SOM and TN were observed in upper slope. Overall, there was a linear correlation between 137 Cs content, SOM, TN and soil particles. This relationship can be well decribed by equation y=ax+b.(3) In the wind-water erosion crisscross region, water erosion and wind erosion affected each other and reinforced mutually. By changing soil physical properties, wind erosion promoted the variation of slope surface roughness, runoff rate and erosion rate, and increased the degree of slope erosion. Compared with no wind erosion, wind erosion increased the surface roughness, runoff and erosion rates by 8.12-78.06, 4.5-21.69 and 7.25-38.97 %, respectively, at wind speeds of 11 and 14 m s-1. Linear regression showed that surface roughness, runoff and erosion rates were positively associated with wind speed and rain intensity(P<0.01). The greater rainfall intensity and wind speed, the more obvious slope shape changes.The relationship between runoff and rain duration under different rain intensities after wind erosion were described well by a logarithmic function. Relative to the change of slope runoff, sediment yield process is more complex. The sediment yield first decreased to a minimum and then increased with the duration of the rain. Wind erosion and water erosion exists a significant positive interaction. The interaction effect between wind erosion and water erosion should be considered when predicte total erosion amount in water-wind erosion crisscross region. Moreover, reduce the regional wind erosion can effectively reduce their interaction.(4) The influence of wind-water alternate erosion on soil particles and nutrients was different from single phase erosion, which had some special characteristics. Wind erosion lead to soil particle coarsening, decreased the SOM and TN contents, thereby affecting the changes of soil particles and nutrient in sediment. Wind erosion aggravated the redistribution of soil nutrient, which became the the main reason for the decline in soil quality in wind-water erosion crisscross region. Relative to control, wind erosion decreased the slope surface(0-1 cm) clay(<0.002 mm) and silt(0.002-0.02 mm) content by 7.65%-9.15% and 6.51%-6.74%, respectively, at wind speed of 11 and 14 m s-1. Because of SOM, TN and fine soil mostly absorbed together, wind erosion further reduced the soil nutrient through separating fine soil from slope. Under the condition of wind-water erosion, SOM and TN contents on slope surface decreased by 4.19%-14.16% and 6.67%-13.63% compared with no erosion, respectively; soil clay and silt contents in erosive sediment increased by 0.35%-19.60% and 5.80%-21.10%, respectively; while sand content decreased by 2.40%-8.33%. Moreover, the contents of SOM and TN in erosive sediment decreased by 3.12%-19.66% and 2.69%-12.23%, respectively. The regression equations showed that sediment particles was positive correlation with wind speed and rain intensity(P<0.01).(5) In the research region(Dingbian area) soil erosion rate was ranged from 1513.25 t km-2 a-1 to 8314.29 t km-2 a-1, which means nearly 0.126-0.693 cm topsoil was eroded each year. Wind erosion accounted for 34.94% of total amount of soil losses; while water erosion accounted for 65.06% of soil losses. Water erosion was the main soil erosion types in wind-water erosion crisscross region. Controlling water erosion through some measures could significantly reduce the total erosion amount. The contents of 137 Cs on selected slope ranged from 239.91 Bq m-2 to 1049.23 Bq m-2, and increased along the slope from top to bottom. The change of erosion rate was opposite with 137 Cs content along the slope. The most serious erosion rate was happened on upper position, while it was weaker on the bottom of the slope. Because of wind erosion and water erosion and their interaction, the slope land was dominated by erosion rather than deposition. Wind and water crisscross erosion and their interaction on slope land should be fully considered in prevention and control of soil erosion in the wind and water erosion crisscross region. Comprehensive management for water and wind erosion controlling, especially for water erosion controlling, could reduce soil erosion significantly. |
PDF Full Text Request