A 210Pbex mass balance model in cultivated soils in consideration of the radionuclide diffusion

The existing210Pbexmass balance models for the assessment of cultivated soil erosion are based on an assumption that210Pbexis quite evenly mixed within the plough layer.However,the amount of210Pbexdistributed in the soils below the plough depth,like a downward tail in the lower part of the210Pbexprofile,has been largely ignored.In fact,after the initial cultivation of undisturbed soils,210Pbexwill diffuse downward from plough layer to the plough pan layer due to the concentration gradient.Assuming210Pbexinventory is constant,the depth distribution in the two layers of the cultivated soils will achieve a steady state after continuous cultivation for 10.37 years,when210Pbexis evenly distributed in the soils of the plough layer with an exponential concentration decline with depth in the soils of the plough pan layer,and the210Pbexconcentration at any depth will be invariable with time.The work reported in this paper attempts to explain the formation of the210Pbextail in the soil profile below the plough depth by theoretical derivation of the210Pbexdepth distribution process in the two layers of the cultivated soils,propose a210Pbexmass balance model considering210Pbexdiffusion based on the existing model,and discuss the influence of the210Pbextail to the existing model.