Saturday, 15 July 2006

Catenary Variations of Soil Morphology in Relation to Conversion of Subtroplical Soils to Agriculture.

Zhao Qiguo1, Bin Zhang1, Ming-zhu Wang1, Yuan-shu Jing1, A. Thimm2, and H. Zepp3. (1) Chinese Academy of Sciences, Inst of Soil Science, Nanjing, China, (2) Ruhr- Univ Bochum, Geographical Inst, Bochum, Germany, (3) Ruhr- Univ Bochum, Geographical Inst, Bochum, Germany

Land use conversion may modify soil morphological properties along the slope due to soil and water erosion. We examined one indigenous forest soil catena covered with Mason pine (Pinus Masoniana) and three agricultural soil catenas with known history of land use conversion from the forest to peanut (Arachis hypogaea) cropping in subtropical China. The objectives of the study were to characterize soil morphological properties along the slope and to elucidate landscape processes involved in relation to the land use conversion. Compared to the soil profiles along the forest catena, the soil profiles in the agricultural catenas revealed soil erosion occurred with an estimated depositional rate from 12 to 16 cm year-1 at the footslopes. The soil erosion process was confirmed with the spatial distributions of soil organic carbon (SOC) content and particle size fraction (PSF). The SOC content in the A horizon was greater on the backslope or upper footslope (0.60 to 0.76 g kg-1 cm-1) than on the shoulder or lower footslope (0.26 to 0.46 g kg-1 cm-1) in the non-eroded catena. The SOC contents in the Ap horizon of the agricultural catenas were lower on the backslopes (0.16-0.22 g kg-1 cm-1) than on the footslopes (0.31-0.34 g kg-1 cm-1), indicating lateral translocation of soil particles. The SOC content in the depositional A1 horizons at the lower footslope were higher than in the Ap horizon in the clayey soil catena of agriculture, indicating soil particle deposition. Clay enrichment ratio was defined by the increase in clay content in the subsurface soil compared to than in the surface soil horizon. The PSF within the soil pedons was similar in the non-eroded catena, with low clay enrichment ratio on the flatter slope positions (7 to 8% on the shoulder and lower footslope) and slightly higher clay enrichment ratio on the slope positions (22-32% on the backslope and upper footslope). The clay enrichment ratio was, much greater in the agricultural catenas. It was 25 to 183% on the shoulder and lower footslope and 3-53% on the backslope and upper footslope in the non-terraced agricultural catenas. The clay enrichment ratio ranged from 218% to 274% at all slope positions in the terraced agricultural catena. These results suggested that the stratification of soil texture probably may result from clay movement with infiltration which was more dominant on the flat position than on the sloping position. In addition, the silt content was greatest at the upper footslope and the clay content tended to increase down the slopes especially on the sandy agricultural catenas. The yearly around monitoring data on soil water potential in 2001, consistent with soil color, indicated that interflow occurred over the clay enrichment horizons. These results allowed us to propose a conceptual model which suggests soil erosion on the soil surface and clay movement through the soil may response for the soil textural stratification along the slope, which probably intensify nutrient and pollutant transports with the interflow over the stratified horizons.


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