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Abstract: A deterministic model for assessing the risk of groundwater contamination by nitrate from land-based sludge disposal was evaluated. A controlled large-lysimeter experiment was set up to monitor movement of nitrate through soil. Four large lysimeters of 900mm length were packed with Manawatu fine sandy loam (a Dystric Fluventric Eutrochrept), on top of which 200mm of municipal sewage sludge was applied. One of the lysimeters was planted with pasture (Lolium perenne and Festuca arundacea), one with a willow tree (Salix sp. ‘Tongoio’), another with a poplar tree (Populus sp. ‘Kawa’), and one was left bare. Bromide was used as a conservative tracer. Movement of bromide and nitrate was analysed in the effluent from the base of the lysimeters. The processes of water and nutrient transport were modelled using a mechanistic scheme based on Richards’ equation for water transport and the convection–dispersion equation (CDE) for nutrient transport. These equations were both linked to a sink term for plant uptake. The model simulated well the transport of water and movement of bromide in the four different lysimeters. The agreement between measured and simulated nitrate leaching was also reasonable considering the simplified model. Uptake of nitrogen by trees reduced the quantity of nitrogen available for leaching. The model could aid development of sustainable management of land-based sewage sludge disposal in terms of nitrate leaching. The next step will be to further develop the model for heavy metal movement, as heavy metals are common co-contaminants of sewage sludge. [Copyright &y& Elsevier]

Abstract: To study the effect of different soil tillage practices and the consequences of soil deformation on the functioning of the pore system, we performed unsaturated leaching experiments (by applying a suction of −10kPa) on undisturbed soil columns from the Ap-horizon of a luvisol derived from glacial till (agricultural site at Hohenschulen, North Germany). We compared two different tillage practices (conventionally tilled to 30cm depth, and conservational chiselled to a depth of 8cm-Horsch system) with respect to soil strength, pore connectivity and their effect on the fate of surface-applied fertilisers. The soil strength was measured by determining the precompression stress value (PCV). The conventionally tilled topsoil had a PCV of 21kPa at a pore water potential of −6kPa, while the conservation treatment resulted in a slightly higher PCV of 28kPa, suggesting a slowly increasing soil strength induced by the formation of aggregates under reduced tillage practice. The leaching experiments were modelled using the convection dispersion equation (CDE) and a modified version of the mobile–immobile approach (MIM), which included three water fractions: mobile, immobile and totally immobile water. From the CDE mobile water fractions (θ m) ranging from 47 to 67% were found, and θ m was slightly higher in the ploughed seedbed compared to the conservation-tilled one. This could be due to higher aggregation in the latter one. Dispersivities were relatively large, ranging from 44 to 360mm, but no difference was found for the treatments. The MIM could simulate the drop in concentration when leaching was interrupted, but overall did not improve the simulation, despite the larger number of fitting parameters. Compacting the soil with loads of 70kPa prior to the leaching experiment did not affect solute transport in the conservational tilled soil. In the conventional-tilled soil, however, the dispersivity decreased and the mobile water content increased compared to the non-compacted soil, suggesting that the former one is less prone to deformation by mechanical loads. [Copyright &y& Elsevier]