The recovery of nitrogen ‘retained’ through cover crop uptake, delayed ploughing and immobilization by straw was assessed in a spring cropping rotation on a chalk loam in Eastern England (1989–96). The effect of annual cover cropping on yield of the subsequent spring crops and on the soil N balance was also investigated. The recovery of retained N was in part dependent upon cover crop management. Late August-sown cover crops which were incorporated in February/March tended to reduce spring crop yields and crop N offtake. Adverse effects on soil N supply, seedbed conditions and soil water reserves were not in evidence and so an allelopathic effect from the decomposition of the rye cover crop, previously reported by others, may be responsible for the reduction in yield of spring crops. When the cover crops were drilled later and their early destruction was followed by a short fallow period, spring crop yields and N offtake were increased. The soil N balance indicated that over the course of the experiment there was a positive N input to the system due to continuous cover cropping. This input may be held as immobilized organic N, in which case it could be made available to subsequent crops over a number of years or lost via other routes. Nitrate concentrations in drainage water increased with the number of years under cover cropping.
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Garwood et al. (1999) studied this question.