| 1.Karst area which covers more than one of three equal parts of our country is a kind of vulnerable eco-system. As well as the loess plateau, the karst area in Southwest China is the most serious region of poverty and degradation of environment in China. In recent years, part of the vulnerable ecosystem has been destroyed because of the increase of the population, the tension between people and nature, and the increasing breakage of woodland and grassland later. Soil quality and its change are close connected with the evolvement of karst ecosyetem. Usually, increase of soil erosion rate happens and decline of soil quality appeares after the damage of karst vegetation. So it is very important and necessary for us to study the soil physical and chemical properties and its influencing factors, to study the soil quality and its evolvement under different land use patterns, to reveal the mechanism of soil degradation and its correlation with rock desertification. It is advantageous for us to understand the result of evolvement of karst environment and the importance of keepping good vegetation conditions. It's helpful to adjust the land use patterns and restore the ecosystem in rock desertification area.Research area of this paper locates in Nanchuan county, Chongqing municipality. It's a typical karst region. It consists of two parts, the Mountaintop of Jinfo Mountain, and the middle and lower section of north slope of the Mountain. Five profiles on the top and nine at the middle and lower section were chosen according to vegetation types or land use types. Physical and chemical properties of thirty eight soil layers have been measured. At the same time, 35 soil samples were chosen based on different soil erosion degrees in research area, for studying the soil erosion rate and the spatial distribution of soil erosion and sedimentation on typical karst slope by using cesium-137technique.2.Soil quality on top of the Mountain is very high because of having high rates of coverage and little breakage from people. But there has obvious difference on soil properties and its quality because of the different vegetation types and the variety of landform.On top of the Mountain, soil bulk densities are very small, the values of broadleaf, bamboo and meadow profiles are all less than 1.20g/cm3 except for Cryptomeria fortunei Hooibrenk soil (1.23g/cm3, mean value from three soil layers), the least value appears in broadleaf profiles. Soil texture is of a higher nature, mean values of clay in soil profiles ranges from 11.25％to 19.32％, the values of meadow and Cryptomeria fortunei Hooibrenk profiles are a little higher than that of broadleaf and bamboo, because of its gentle slope and accumulating of clay. Structure coefficients are all larger than 85％, aggregate degrees are all larger than 70％in soil profiles, the results exhibit high quality on soil anti-erodibility. All of the profiles have high content of soil water stable aggregates,＞0.25mm water stable aggregates range from 83.61％to 96.55％, the highest values of＞5mm,＞3mm and＞2mm water stable aggregates exist in broadleaf profiles, bamboo profile is in the next place.On top of the Mountain, thin soil layers associated with steep slope where mainly grows broadleaf is obviously affected by limestone rocks, resulting in high pH and calcium carbonate(1.7g/kg-10.5g/kg) and low exchangeable acidity (1.60 mmol/100g), thicker soil layers exists in the area with gentle slope, in where exchangeable acidity is high (5.76mmol/100g, mean value from 3 profiles) and pH is low. Soil with gentle slope is well developed and cation leaching is strong, soil calcification is weak.On top of the Mountain, fulvic acid hold the large number of humic acids, obviously, it is affectted by low temperature. Broadleaf and bamboo profiles have high content of humic acid and HA/FA, reveals good properties on humic acids.Soil organic matter is abundant, the bigest value appears in broadleaf profiles, bamboo is in the next place. The highest value of total nitrogen exists in broadleaf profiles. The highest value of available nitrogen appears in bamboo profile, broadleaf is the second one. Bamboo and broadleaf profiles have higher Available phosphorus than that of Cryptomeria fortunei Hooibrenk and meadow profile.The integrated soil quality index of four main vegetation types on the top of the Mountain is broadleaf (67.53％), bamboo (57.86％), grassland (42.32％), Cryptomeria fortunei Hooibrenk soil(23.38％).3.At the middle and lower section of the Mountain, the land use types include secondary forestland of Pinus massoniana Lamb interspersed with shrub, grassland, farmland and abandoned farmland. In general, secondary forestland evolves into grassland or farmland after it's deforestation, in recent years, a little percentage of farmland was abandoned.Soil physical properties including soil bulk density, soil texture, soil structure and anti-erodibility are degenerative after damage of secondary forestland of Pinus massoniana Lamb and shrub. After degradation of Pinus massoniana Lamb ecosystem, soil bulk density increases, the content of clay and all sorts of water stable aggregates decreases, at the same time, soil structure coefficient and aggregate degree come down. Contents of＞0.25mm water stable aggregates among defferent land use types is farmland (72.91％), grassland (77.02％), abandoned farmland (82.38％), Pinus massoniana Lamb soil(86.83％).Soil pH value exhibit a tendency of augmentation after the destroy of Pinus massoniana Lamb eco-system, and the content of calcium carbonate in profiles increases at one time.Active iron oxide in soil profiles and free iron oxide in clay decrease with the decline of soil organic matter and clay. Organic matter, clay and ferric oxide are cementations of soil structure, it can partially explain the reason why the content of water stable aggregates and its stability decline after reclamation.The ratio between humic acid and fulvic acid declines after the breakage of secondary forestland, it indicates that Pinus massoniana Lamb soils have higher quality on soil humus than that of other karst eco-systems in this research area.Contrast to secondary Pinus massoniana Lamb soils, nutritive material including total nitrogen, total potassium and available nitrogen are reduced, the decrease of clay is responsible for the reduction of total potasium.The result of soil degradation index and the integrated soil quality index indicate that the soil qualities of dry land, paddy field and grassland are lower than that of secondary forestland of Pinus massoniana Lamb interspersed with shrub. Soil quality of one of the two abandoned profiles is high, it reveales that soil quality comes back after stopping of farming. At the same time, there maybe has plentiful organic fertilizer during the period of farming.4. Based on 137Cs technique, we have measured and calculated soil erosion rates of different erosion types, studied the spatial distribution status of erosion and sedimentation on karst slopes according to analytical data. It shows that the soil erosion rates of woodland and grassland is small, mean value of the three profiles we have chosen is 112.5 t/(km2.a). The erosion rates of cultivated profiles with relatively gentle slopes range from 190.3 to 1138.4 t/(km2.a), its mean value is 565.5 t/(km2.a). Nevertheless, the erosion rates of cultivated land with steep slopes range from 452.0 to 3759.4 t/ (km2.a), its average is 2264.8 t/(km2.a). Contrast to purple land and loess plateau, the soil erosion rate in karst region is small, but it's very dangerous because of the slow speed of weathering and pedogenesis of carbonate rock.5.Our research shows that the destroy of forest and reclaiming in steep slope are responsible for the degradation of limestone soil, the increase of soil erosion rates and the deterioration of karst eco-system mostly.So it is very important and necessary to keep good vegetation conditions in karst area.
|
PDF Full Text Request