Development and testing of a model for predicting tillage effects on nitrate leaching from cracked clay soils

Both water movement and nitrate leaching in structured soils are strongly i nfluenced by the nature of the macro-porosity. That macro-porosity can howe ver also be manipulated by choice of tillage operations. In order to invest igate the potential impacts of tillage on rates of nitrate leaching from st ructured soils, a model specific to these soils, CRACK-NP was developed. Th e model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone Farm, Oxfordshire, UK, is d escribed. The model considers the soil as a series of aggregates whose size is also the spacing of the macro-porosity. Water and solutes move in the m acro-pores, but within the peds they move only by diffusion, internal infil tration and root uptake (evaporation). The model reflects the influence of diffusion limitation in the release of solutes to by-passing water. The mod el was then used to investigate the influence of variable ped spacings whic h were created by variations in tillage practices. The results both from th e model and from the field data demonstrated that finer soil structures, wh ich have larger surface contact areas and shorter diffusion path lengths, p resent greater opportunities for interaction between peds and the water mov ing around them, and so release more nitrates through the drainage waters. (C) 2000 Elsevier Science B.V. All rights reserved.