| Purple soil region are abundant in hydrothermal resources and soil fertility, playing an important role in agriculture development of China. Soil erosion has long been recognized as a major environmental problem in the purple soil region where slope farming is commonly practiced. Human activities can accelerate or reduce soil erosion and exerts a first-order control on soil erosion. Therefore, the aims of this study were to investigate the impact of human agricultural cultivation and protection on soil erosion. Field simulated rainfall experiments were used to exam the effect of vegetation removal and hoeing tillage on soil erosion processes. The effects of soil and water conservation measurements on runoff, erosion and plant production were analyzed through long term plots monitoring. The changes of soil properties in citrus orchards with different conservation measurements also were analyzed. The main content of this dissertation are as following:1. The surface and subsurface runoff were simultaneously collected during rainfall experiment with an original adaptation of common experimental frame. Results showed that the average runoff coefficient of subsurface runoff generated from wasteland was 4.7 times larger than that from cultivated land. The subsurface runoff in cultivated land was increased after rainfall started and then decreased, however, subsurface runoff in wasteland kept steady after increased. The runoff coefficient of subsurface runoff on cultivated land was increased with rainfall intensity. However, the subsurface runoff coefficient was not significantly affected by rainfall intensity on the wasteland. The average surface runoff coefficient in cultivated land were 1.0 times large than that in wasteland. Soil erosion rate in cultivated land were significant higher than in wasteland. The average sediment concentration in cultivated land and wasteland were 2.3 g/L and 42.2g/L, respectively2. The runoff generation was changed by vegetation removal and hoeing tillage. The ratio of subsurface runoff to total runoff was reduced from 30.3% to 6.2% caused by vegetation removal practice. For hoeing tillage practice, the subsurface runoff occupied 29.1% of total runoff during low rainfall intensity, and that ratio reduced quickly with rainfall intensity increased. Vegetation removal and hoeing tillage also significantly increased soil erosion. The average soil erosion rates at vegetation removal and hoeing tillage plots were 3.0 and 10.2 times larger than that at vegetation cover plots, respectively. Results of this study reflected both runoff generation mechanism and soil erosion were changed as a consequent of altering land use on steeplands. Thus, conservation practices with maximum vegetation cover and minimum tillage should be useful for reducing surface runoff and soil erosion on steeplands.3. The effects of cropping system on soil loss (C values) were analyzed through calculating crop stage, rainfall erosivity, and SLR in the USLE model. The results reflected the C values of 22 cropping system changed from 0.16 to 0.65 in the sloping crop land of purple soil. The order of C values of different cropping system were intercrop/rotation<rotation<monoculture. In monoculture, the C values of rape and potato were 0.65 and 0.60, respectively, however, the C values of cotton and sweat potato only were 0.29 and 0.33, respectively. In the rotation cropping system the C values were similar among wheat-cotton, maize-rape, wheat-sweat potato, potato-soybean systems, that values approximately 0.2. The C value of wheat-soybean intercrop system was the lowest value among 22 cropping system, that value was 0.2. The relationship between crop stage and rainfall erosivity in that stage were significant influenced annual soil erosion.4. Five years field monitoring results reflected that the effectiveness of terracing influenced by land use. After terracing, the average annual runoff and soil erosion rate were reduced by 23.0 and 52.0% in arable plots; in orchard plots, the average annual runoff and soil erosion rate were reduced by 60 and 68%, respectively. Moreover, the terracing measurement reduced the frequencies of events of high runoff amount and high soil erosion rate in orchard plots, but only reduced the frequencies of events of high soil erosion rates in arable plots. The terracing measurement also increased crop production. After terracing, crop production increased by 13.0% and 15.0% in arable and orchard plots, respectively. For contour strip-cropping measurements, the average annual runoff and soil erosion rate in plots with contour hedgerow were 36 and 67% lower than in plots without conservation measurements, respectively. These results showed the contour hedgerows measurement could be regarded as the best measurement in sloping arable land, and the terracing measurement in citrus orchard could be acceptable based on the ecological and economic benefits of that measurement.5. Based on ten years of field observation and K-mean clusters, erosive rainfall events were grouped into three rainfall regimes. Rainfall regimeâ… was the group of events with strong rainfall intensity, high frequency, and short duration. Rainfall regimeâ…¢consisted of events with low intensity, long duration, and high rainfall amount. Rainfall regimeâ…¡was the aggregation of events of high intensity and amount, and less frequent occurrence. The results showed that event runoff coefficients were not significantly different among rainfall regimes. However, the average soil erosion rates in rainfall regimeâ… andâ…¡were significantly higher than that in regimeâ…¢. The average erosion rates under rainfall regimeâ… ,â…¡, andâ…¢were 1.1,2.0 and 0.5 kg, respectively. The effect of rainfall regime on soil erosion also was changed by terracing and land use. Terracing significantly reduced runoff and soil erosion, and compensated the effects of rainfall regime on soil erosion, which indicated that runoff and erosion in terraced system may little influenced by climate change. On unterraced cropland, soil erosion rate in rainfall regimeâ… is significantly higher than that in regimeâ…¢. However, but the situation did not exist in unterraced orchard. Based on these results, it was suggested more attention should paid to the timing of rainfall events in relation to crop development and the high erosion on unterraced citrus orchard to control soil erosion in this area.6. Soil conservation measurements significantly increased saturated hydraulic conductivity, aggregate stability, soil organic matter, and available N, P, K, but decreased bulk density. The terracing with grass cover measurement had the best effectiveness in improving soil fertility among the three measurements. The average soil organic matter, available P, N, K in TCG were 0.3,0.5,1.8 and 1.7 rates larger than in SC, respectively. Compared to control plot, silt content was increased in SCH, while clay content was increased in TC and TCG. There was a downslope increase in clay content and total N but a decrease in gravels in SC plot. However, that trend did not exist in the other plots. In the field scale, the total nitrogen was significantly higher near the hedgerow in SCH.
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