| Called as "ecological ulcer" by local residents eroded red soil region of southern China, the term of "Benggang", originated a Chinese pictograph of geomorphy, has been known to represent a kind of severely eroded ecosystem where source edge of channel collapsed, transported and developed a deep-cut shape, concave-deforming deep-seated failure land form, finally evolved higher stage of hilly and gully erosion, which consisted of representative subsystems, such as upper catchment, collapsing wall, colluvial deposit, channel and alluvial. Benggang located in hilly area of Yangtze River Basin, Pearl River Basin and Southeast Coastal Rivers Basin extensively, the total amount of Benggang that collapsing area exceeded 60 square meters, were 2.39×105, and total erosion area were also 1.2×103 km2 along with the average erosion modulus surpassing 5.90×105 t·km-2·yr-1 at the national scale. The erosion process destroys the land resources, resulting in a large number of sediment deposition of river, farmland, it's a one of important causes for the deterioration of the regional ecological environment.To reveal spatial distribution of surface soil (0-20 cm) and dominant vegetation (Dicranopteris dichotoma) combined system in area of Benggang, three typical adjacent Benggangs representing the vegetation coverage in the range of 2%,20% and 95% were selected within Huang-ni-keng Benggang group of Zhuotian Town, Changting County in southwestern of Fujian Province. Mechanical composition, pH, nutrient contents and Rare Earth Elements (REEs) for 63 soil samples from the system of Benggang including upper catchment, collapsing wall, colluvial deposit and channel were investigated and measured, the contents of REE in Dicranopteris dichotoma body of 30 samples were accomplished to analysis as well by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Results included:(1) Differences of mechanical composition in the soils in the areas of Benggangs under different vegetation coverage were obvious, which was by the order of three Benggangs was sand>silt>clay, and clay content was significant difference with sand and silt between Benggang NO.1 and Benggang NO.2; and the clay was respectively significant difference in three Benggangs.(2) The rang of change surface soil characteristics of within Benggang system including ph, soil organic carbon?total nitrogen?total phosphorus?total K?total iron? available phosphorus and available potassium were 4.82-5.13,0.74-6.24 g/kg, 0.21?0.46 g/kg,0.06?0.07 g/kg,4.06?5.63 g/kg,5.12?5.82 g/kg,0.35?0.66 mg/kg, 21.53?23.78 mg/kg, their average values were 5.01,2.67 g/kg,0.33 g/kg,0.06 g/kg, 4.85 g/kg,5.42 g/kg,0.51 mg/kg,22.59 mg/kg. The results showed that there existed obviously differences of nutrient contents in the soils in the areas of Benggangs under different vegetation coverage by variance analysis. The contents of organic carbon of Benggang NO.3, was significantly more 9 and 6 times respectively than Benggang NO.1 and Benggang NO.2, where their vegetation cover were quite low. Soil organic carbon, pH and total potassium were the significant level (P<0.05); total nitrogen, total phosphorus, available phosphorus, total iron is not a significant difference between Benggang NO.3 and Benggang NO.1, Benggang NO.2.The growth of vegetation, especially Dicranopteris dichotoma had a significant impact on soil nutrient.(3) The feature of total rare earth elements (REEs) in the soil of three different vegetation coverage conditions. The results showed that Benggang NO.3 (374.53 mg/kg)> Benggang NO.2 (211.99 mg/kg)> Benggang NO.1 (194.91 mg/kg), there were some features in the rare earth element content of the soil surface under different conditions of vegetation cover, high degree of vegetation coverage Benggang REE contents; in light rare earth rare earth elements (LREEs) content of the surface soil in the study area between 176.21-278.12 mg/kg, heavy rare earth elements(HREE) in an amount ranging 18.70-96.41 mg/kg, which LREE/HREE ratio was 5.37-10.64; basic change of the rare earths elements within different various locations of Benggang system presented colluvial deposit> channel> collapsing wall> upper catchment; Benggang's, vegetation coverage had a significant impact on the difference of REE distribution.(4) The total rare earth elements of Dicranopteris dichotoma were between 1063.53-1579.55 mg/kg, the content of Dicranopteris dichotoma body withinBenggang NO.1 was 1267.53 mg/kg, the content of leaves was 500.85 mg/kg, the percentage was about 54%, the content of Dicranopteris dichotoma body within Benggang NO.2 was 1579.55 mg/kg, the percentage is 49%; the contents of leaves were 767.90 mg/kg, the content of Dicranopteris dichotoma body within Benggang 3 was 1196.28 mg/kg, the contents of leaves was 798.83 mg/kg (the percentage 31%). REEs contents of Benggang various parts of the body Dicranopteris dichotoma with three different vegetation cover conditions was in the order of colluvial deposit> channel> upper catchment, colluvial deposit due to had long time natural weathering and larger volume could provide favorable conditions for survival and diffusion of Dicranopteris dichotoma; the coverage of Dicranopteris dichotoma was advantageous to the interception of sediment flow from formation of Benggang erosion, and to promoting accumulation of rare earth elements for the surface soil. |
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