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 in ̄uenced by the nature of the macro-porosity. That macro-porosity can however also be manipulated by choice of tillage operations. In order to investigate the potential impacts of tillage on rates of nitrate leaching from structured soils, a model speci®c to these soils, CRACK-NP was developed. The model, its application and validation for an experimental site on a heavy clay soil (Verti-Eutric Gleysoil) at Brimstone Farm, Oxfordshire, UK, is described. 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 macro-pores, but within the peds they move only by diffusion, internal in®ltration and root uptake (evaporation). The model re ̄ects the in ̄uence of diffusion limitation in the release of solutes to by-passing water. The model was then used to investigate the in ̄uence of variable ped spacings which were created by variations in tillage practices. The results both from the model and from the ®eld data demonstrated that ®ner soil structures, which have larger surface contact areas and shorter diffusion path lengths, present greater opportunities for interaction between peds and the water moving around them, and so release more nitrates through the drainage waters. # 2000 Elsevier Science B.V. All rights reserved.