Study Of Soil Erosion And N And P Loss Of Purple Soil Under Different Land Uses

Soil erosion has become one of the most important environment problems all over the world,which leads to serious soil loss and makes limited soil resources subjected to serious damage and lowers fertility of soil. As a kind of rocky soil which is formed in short term, purple soil is easy to be eroded. The area of purple soil is 78.7% of that of cultivated land in Three Gorge Reservoir area, among which most is sloping field. Lots of literature demonstrated that in one hand slope cropland is the major sources of silt in rivers, in another hand the serious soil and water loss in slope cropland makes the soil layer in hilly country thinner and causes the nutrient to lose which in turn decreases the productivity and impedes the sustained development of local agriculture. Therefore it is necessary to study the soil erosion and nutrient loss of purple sloping land in Three Gorge Reservoir. This study contrasted the runoff and silt loss of four runoff plots under different purple land use patterns in Houhe Small Watershed, Yubei District, then explored the soil nutrient loss under different land use patterns in runoff plots. Combined with indoor by rainfall simulators and aimed at the effect of different rainfall intensity and gradient on purple soil nutrient loss and N and P loss in Yubei District, the research studied further the relationship between different factors and soil and water loss and N and P loss. It also contrasted the indoor bare purple soil and open field in different cropping patterns to study the effect of vegetation on soil and nutrient loss, which will in turn provide foundation for the evaluation, prediction, prevention and control of soil and nutrient loss of purple sloping land. The results of this research are as follows:1. Rainfall, vegetation cover and surface gradient are the main factors that influence the surface runoff and soil loss, among which the surface runoff relates closely to the rainfall. The relationship between rainfall and soil loss in economic plants land, wasteland, slope cropland is linear and the significance is obvious, but the significance of relationship between soil loss of terraced land is not obvious. The surface runoff and soil loss relate negatively to vegetation cover.2. Under different land use patterns, total N loss has a large percent of NO₃-N.The law of N and P loss per area is about the same:the loss in slope cropland is the biggest, then is bare land and economic plants land, and the lowest is the terraced land. Under four land use patterns, N loss relates positively to rainfall and runoff. And the relationship between total N loss and rainfall is logarithmic function under the four land use patterns except economic plants which it's better to fit in quadratic function. The relationship between ammonia and nitrogen loss and rainfall in terraced land and economic plants is fitting better, and is logarithmic function. The relationship between NO₃-N loss and rainfall is fitting better in terraced land and slope cropland than in the other two cropping patterns and is logarithmic function.3. The relationship between total N loss and runoff is linear in the four cropping patterns. The relationship between ammonia and nitrogen loss and runoff is linear in economic plants, but is logarithmic in terraced land. The relationship between NO₃-N and runoff is logarithmic in the four cropping patterns except slope cropland which is linear. The relationship between total P loss and runoff is logarithmic in economic plants, terraced land and slope cropland, among which the significance in terraced land is the most obvious. P loss also relates to vegetation cover, and increasing vegetation cover can decrease P loss. During the same rainfall DP/TP is the biggest in slope cropland among the four land use patterns and this shows that the movment of P in surface runoff of slope cropland in this watershed is mainly water soluble.This result is inconsistent with most research and the reason remains further research.4. In man-made rainfall simulation, runoff is positively relevant to rainfall intensity and gradient. Under same gradient, the relationship between rainfall intensity and runoff is significantly linear while under same rainfall intensity, gradient is an important factor that influences runoff and the relationship between runoff and gradient 5°～20°is significantly linear. When concerned with the beginning time, gradient has little effect on runoff and rainfall intensity is the major influencing factor that large rainfall intensity makes runoff appear in advance more easily. In rainfall simulation, silt is relevant to runoff linearly, and the significance is obvious, which proves outdoor tests. Because the runoff is linearly relevant to rainfall intensity significantly, the silt is also positively relevant to rainfall intensity, that is the larger the rainfall intensity, the more the silt loss.5. Comparing outdoor tests with indoor simulation tests, it is concluded that silt and total N and P loss per volume of runoff in bare land is largely more than those in wasteland and slope cropland under the same condition.6. Among four different land use patterns, soil loss, runoff, N and P loss are the least in terraced land, followed by economic plants and the most are in slope cropland. So we may conclude that more vegetation cover, good water and soil conservation measures, rational cultivation and land use patterns can lighten water and soil loss, protect water and soil resources and prevent soil fertility deteriorationefficiently.