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Abstract

Leaching of nitrogen (N) is an unwanted effect of agriculture practices which contributes to eutrophication of surface waters. This thesis examined how soil tillage practices affect leaching losses of N from arable fields. This was done directly by measuring N in drainage water and indirectly by studying the dynamics of soil mineral N (SMN) within the soil profile and involved long-term and short-term studies of tillage of different timing and intensity. The impact of time of tillage in autumn on soil structure and crop yield was also examined.

It was found that adapting soil management and tillage to crop rotation in order to synchronise SMN with crop demand could substantially decrease leaching losses of N. Tillage effects on SMN were mainly attributable to interruption of N uptake in crops and weeds, rather than stimulation of N mineralisation. Timing of tillage proved to be important in this regard. In clay soils, however, delayed tillage in autumn (previously shown to reduce N leaching losses in coarse-textured soils) resulted in operations being performed when soil water content was higher than optimal, causing a poor tillage outcome and decreased yield. From the results it can be concluded that a reduction in N leaching by delaying autumn tillage may be achieved on lighter clay soils in warmer and moister areas of Sweden whereas the effects may be small on heavier clay soils.

The results indicated potential for further reductions in N leaching, especially in autumn-sown crops, by early sowing, a short time interval between tillage and sowing and undersowing of a catch crop in spring. SMN accumulation in winter wheat during autumn and winter was not influenced by tillage method. Therefore changing from conventional to reduced tillage or direct drilling of winter crops is unlikely to reduce overall N leaching losses. The importance of high biomass production during autumn and winter was identified and should be the main focus in strategies for mitigating N leaching losses from arable fields.