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.