中文摘要：

农耕地犁耕层土壤中的^210Pbex向犁底层土壤的扩散,相当于无限薄层扩散源向一维半无限介质的扩散。本文对无侵蚀土壤垦殖后土壤中^210Pbex的扩散、直至达到稳定态分布的物理过程给予了数学的描述。垦殖10年后（y=10.37a）,土壤中的^210Pbex可以达到稳定态分布。根据四川内江无侵蚀紫色土农耕地土壤的^210Pbex深度分布,求得紫色土犁底层土壤的^210Pbex入渗系数（D）为0.676cm^-2·a^-1;计算了垦殖后不同时间犁底层中^210Pbex的积累扩散量;给出了犁底层土壤^210Pbex深度分布形态的时间变化。

英文摘要：

When uncultivated land without erosion is cultivated, the ^210Pbex in the depth of tillage, which decreases exponentially with depth, can be mixed in the plough layer. Subsequently, the ^210Pbex of the plough layer would gradually diffuse into the plough pan layer owing to the concentration grads of ^210Pbex between them, and finally a steady state would come forth as a result of balance between diffusion and decay of the ^210Pbex. This process can be considered as one-dimensional diffusion from a thin diffusion source layer with unlimited width to semi-infinite media. This paper describes the process in a quantitative way. The time of the process from initial cultivation to the steady state is about 10 years （y=10.37 a）. As in the case of cultivated purple soil of Neijiang in Sichuan province, the downward diffusion coefficient of soils of the plough pan layer is D=-0.676 cm^2.a^-1, which is achieved according to the ^210Pbex depth distribution of the soils. Subsequently, the ^210Pbex inventory of the plough pan layer of different time after initial cultivation are calculated, and ultimately the process of ^210Pbex distribution in the non-eroded purple soils after cultivation is figured out.