137Cs DEPTH DISTRIBUTION IN HAPLIC-UDIC FERROSOLS OF SOUTHERN CHINA AND ITS IMPLICATION FOR SOIL EROSION

The depth distribution of 137Cs (cesium-137) and its relationship with organic carbon and different particle size fractions in Haplic-Udic Ferrosols in Yujiang, Southern China, are described in this paper. Compared with other sites, the lower 137Cs inventories associated with uncultivated soils on or close to the crests indicate the relatively low 137Cs in situ retention ability of the soil. Therefore, the use of 137Cs techniques in erosion studies on uncultivated Haplic-Udic Ferrosols may create some significant difficulties and limitations, including identifying the suitable 137Cs reference site and its validity on and near the crests. The impact of the soil erosion process to a depth of more than 4 to 6 cm on the sloping dry farmland and dry terrace farmland indicates that the widely accepted assumption of uniform 137Cs depth distribution within the plough layer may not apply in these soils, although it does basically hold true for the paddy terrace field, in the area dominated by Haplic-Udic Ferrosols. For uncultivated Haplic-Udic Ferrosols, the organic matter may not control the downward migration of 137Cs because no significant relationship was found between the depth distribution of 137Cs and that of organic matter. It was also demonstrated that the 137Cs fallout is associated mainly with the clay fraction, whereas the organic matter is associated mainly with the silt fraction. For Haplic-Udic Ferrosols, the waterinduced erosion removes preferentially the clay associated with the relatively high concentration of 137Cs and low content of organic matter.