The effect of winter cover crops on yield of the following spring crops and nitrogen balance in a calcareous loam

The recovery of nitrogen 'retained' through cover crop uptake, delayed plou ghing and immobilization by straw was assessed in a spring cropping rotatio n 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 upo n cover crop management. Late August-sown cover crops which were incorporat ed in February/March tended to reduce spring crop yields and crop N offtake . Adverse effects on soil N supply, seedbed conditions and soil water reser ves were not in evidence and so an allelopathic effect from the decompositi on 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 dril led later and their early destruction was followed by a short fallow period , spring crop yields and N offtake were increased. The soil N balance indic ated 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 imm obilized organic N, in which case it could be made available to subsequent crops over a number of years or lost via other routes. Nitrate concentratio ns in drainage water increased with the number of years under cover croppin g.