Epsilon Open Archive

Abstract

This thesis deals with perennial ryegrass (Lolium perenne L.) catch crops and their short- and long-term effects on nitrogen leaching and nitrogen turnover in soils. Results are presented from three field experiments on a sandy soil in south-west Sweden, where undersown catch crops were used in cropping systems with and without applications of liquid manure. The effects of different tillage practices on soil mineral nitrogen and leaching were also studied. Two coupled simulation models, which describe water flow and nitrogen transformations and transport in soil, were used for calculations of nitrogen mineralization and soil nitrogen balances. A more detailed study of the residual effects of ryegrass on the nitrogen supyly to the subsequent crops and nitrogen leaching was performed in lysimeters, using 'N-labelled ryegrass. Undersown catch crops efficiently reduced nitrogen losses when mineral fertilizer or manure was applied at normal rates (90-1 10 kg Nha). Over five years, undersown catch crops reduced nitrogen leaching by 60%, on average, compared with soil which was conventionally tilled in August-September. Incorporation of catch crops affected nitrogen mineralization mainly during the first growing season following incorporation, when approximately 20-30% of catch crop nitrogen was released. The results emphasize the importance of an early onset of nitrogen mineralization in spring after incorporation of catch crops. This is necessary in order to overcome the soil-depletion effect of nitrogen uptake induced by the catch crop. Simulations showed that incorporation of catch crop material in late autumn instead of spring can result in a time distribution of nitrogen mineralization more suitable for a subsequent cereal crop, but this was not verified by the results of the lysimeter experiment. It seems important to obtain further knowledge of how to improve the degree of synchronization between nitrogen mineralization after incorporation of catch crops and nitrogen demand of the subsequent crops. According to simulations, the main part of the catch crop nitrogen contributed to a long-term accumulation of soil organic nitrogen (+I0 kg N per hectare and year), while it slowly declined in autumn-tilled soil given mineral fertilizer (-30 kg N per hectare and year). However, the accumulation of soil organic nitrogen due to the catch crops was very modest compared with the total amount of organic nitrogen in the soil.