Morphology of Soil Iron Oxides and Its Correlation with Soil-Forming Process and Forming Conditions in a Karst Mountain
岩溶山地土壤氧化铁形态及其与成土环境的关系

The quantity and morphology of iron oxides are indicators of soil forming-process and forming conditions. In order to analyze the connection between soil iron oxides and soil forming conditions and degenerative process of karst ecosystem, we have chosen 14 soil profiles on the top and middle section of Jinfo Mountain, a typical karst slope in Chongqing, China. Morphology and contents of soil iron oxides were studied by using chemical selective extraction techniques. We draw conclusions: 1 total iron (Fet) is mainly controlled by parent material and lithology. Significant difference of Fet content exists between soils in Top Mountain (51.49 g·kg-1, mean value from 5 profiles) and soils at the middle sector of North Slope (86.29 g·kg-1, mean value of 9 profiles); 2 the results show low concentration of Fed (29.16 g·kg-1) and low ratio of Fed to Fet (35.40%) in soil clay under conditions of high elevation and low temperature on Top Mountain. In contrast, the results indicate advanced weathering and soil-forming process at middle slope sites due to high temperature; this is supported by high mean values of Fed (43.92 g·kg-1) and ratio of Fed/Fet in clay (60.41%); 3 long humid climatic setting and large numbers of soil organic matter on top of the mountain result in high activation degrees (Feo/Fed) and high complexation degrees (Fep/Fed); mean values of them are 73.51%, 17.21% respectively, which are higher than that of soils at middle slope sites (13.06%, 0.41%); 4 after degradation or deforestation of secondary forestland (pinus massoniana among bushes) at middle section of the hillslope, soil free iron oxides (Fed) and total iron oxides (Fet) decrease as well as soil organic carbon and clay, because of progressively increasing of soil erosion. Average contents of Fet and Fed in clay from 2 shrub profiles are 98.25 g·kg-1, 50.81 g·kg-1 respectively. However, the four tillage soils we have studied reveal lower values of Fet (84.52 g·kg-1) and Fed in clay (47.86 g·kg-1). Soil iron oxides are reliable indicators to estimate degeneration of karst ecosystem and karst rock desertification.