Abstract

This work assesses the possibilities of reducing N-losses from green-manure crops in cropping systems with iceberg lettuce and leek, by matching the availabilty of N mineralised from green.manure crops with N-demand of field vegetables. The influence of the choice of green-manure species (A), combination of species (B) and time of incorporation (C) was examined by analysing the temporal pattern of N-mineralisation from green-manures, and the N-uptake and root growth of the vegetable crops. In one of two field experiments, the effect of autumn and spring incorporated Persian clover, white clover and yellow-sweet clover was examined (A, C). In the other field experiment, the effect of inter-row red clover strips incorporated at different times during the cropping period was investigated (B). Net N-mineralisation from a mixture of perennial ryegrass and red clover was evaluated in an incubation experiment (C). Uptake of N by perennial green-manure species kept N protected in the green-manure tissues until incorporation into soil and thus prepared the way for a match between N- availability and N-demand. The amount of N incorporated differed with green-manure species. However, because the chemical composition influenced the percentage of N mineralised, no effect of species could be observed in the temporal pattern of N- mineralisation or crop yields. Different combinations of species in the incubation experiment and time of incorporation in the field did, however, influence the temporal pattern of N-mineralisation. Combining red clover with rye grass resulted in net N- immobilisation followed by a net N-mineralisation rate that temporarily exceeded that of clover alone. However, a similar pattern was not observed when inter-row red clover strips of different ages were incorporated into the soil. The change in temporal pattern of N-mineralisation from red clover was instead proportional to the intervals with which the red clover strips were incorporated. This synchronisation effect was counteracted by uptake and transfer of N in crop rows by the clover strips between rows. In treatments with the latest time of incorporation the competition between the vegetable crop and clover strips together with low intensity and lateral spread of vegetable roots contributed to low N-upta