137Cs depth distribution in Haplic-Udic ferrosols of Southern China and its implication for soil erosion

The depth distribution of Cs-137 (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 Cs-137 inventories associated with uncultivated soils on or close to the crests indicate the relatively low Cs-137 in situ retention ability of the soil. Therefore, the use of Cs-137 techniques in erosion studies on uncultivated Haplic-Udic Ferrosols may create some significant difficulties and limitations, including identifying the suitable Cs-137 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 Cs-137 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 Cs-137 because no significant relationship was found between the depth distribution of Cs-137 and that of organic matter. It was also demonstrated that the Cs-137 fallout is associated mainly with the clay fraction, whereas the organic matter is associated mainly with the silt fraction. For Haplic-Udic Ferrosols, the water-induced erosion removes preferentially the clay associated with the relatively high concentration of Cs-137 and low content of organic matter.