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1 Department of Soil Science, Swedish University of Agricultural Sciences, P.O. Box 7014, SE-750 07, Uppsala, Sweden (e-mail: lars.bergstrom{at}mv.slu.se)
2 Swedish Environmental Research Institute (IVL), PO Box 21060, SE-100 31, Stockholm, Sweden
Agricultural non-point source pollution by plant nutrients and pesticides can cause severe environmental disturbances, such as deterioration in the quality of surface water and groundwater. In order to prevent this, the development and implementation of appropriate countermeasures are necessary, which requires knowledge of critical soil functions in the vadose zone. The nutrient source, the transport pathway and the availability of solutes in soil are some important conditions that affect leaching. Our results show that organic nitrogen sources are often more susceptible to leaching than inorganic N fertilizers, due to poor synchronization between the N demand of the crop and the release of inorganic N from the organic N source. Large amounts of leachable N are left in the soil after the growing season. Preferential flow, in combination with where soil solutes occur, is critical for establishing safe loading rates. In some cases, the solute is located in smaller pores of the soil matrix, and is thereby protected against preferential flow and leaching. In other cases, especially soon after application of a fertilizer or pesticide, transient flow-peaks rapidly displace the solutes through macropores in the vadose zone, which can cause large leaching loads and associated water-quality problems.
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