A review of leguminous fertility-building crops, with particular ...

A review of leguminous fertility-building crops, Defra project OF0316 Executive Summary
similar effect to intercropping, presumably because of a greater demand for soil-N under the
denser crop.
Transfer of fixed N can be substantial in grass/clover mixtures. There is a high turnover of
roots and nodules of living clover and greater opportunity for transfer to occur through
decomposition and mineralisation pathways than in annual crops. In contrast to grass/clover
swards, evidence of N transfer between other legume species and companion crops is often
conflicting.
Knowledge of relative rates of fixation at different seasons and stages of growth is important
for assessing how much of this annual total is fixed by legumes that are grown for less than a
full year, for example, with short-term cover crops or where a ley is cultivated during
summer or early autumn. Relative rates of fixation in grass/clover swards are largely
determined by temperature and moisture supply. The stage of vegetative or reproductive
growth may be more important for annual grain legumes. For perennial legumes, longer-term
fluctuations in factors such as soil mineral-N contents will also influence fixation.
As would be expected from the previously described relationship between the build-up of
soil-N and N-fixation, fixation in grass/clover swards is also likely to decline with increasing
pasture age (independently of changes in the proportion of clover).
Influence of management
Various aspects of management will influence the quantity of N provided by a legume crop.
Effects can be divided between those, such as crop removal, which influence the net
accumulation of N and those that influence the N-fixation process directly. Harvesting of
forage or grain will remove much of the fixed N and reduce the benefit to following crops.
The benefit will be further reduced if straw and other crop residues are also removed from
the field. In contrast, where crops are mulched or grazed, much of the fixed N will be
returned to the soil in plant debris or excreta. Nitrogen-fixation processes will be affected
directly by defoliation of the legume and indirectly through effects on the mineral-N content
of the soil. Other aspects of management affecting N-fixation include position of the crop in
the cropping rotation, duration of cropping, methods of cultivation and applications of
manures and composts.
Cutting and mulching can result in mineral N accumulation, which will reduce fixation. It
may be better to remove the foliage from the field, though this then requires the foliage to be
managed (presumably as forage and, ultimately, as manure). In a mixed sward, decreasing
the cutting frequency could potentially decrease fixation (increased competition from the
grass), but defoliation of forage legumes also causes a dramatic decrease in N-fixation
activity.
Nitrogen dynamics are likely to be very different where the cut herbage is returned to the
soil. However, there is little information about the direct effects of mulching on the Nfixation
process.
Harvesting of forage crops or of grain legumes both remove fixed-N from the field.
Although N accumulated in stubble and roots will remain, the net contribution to the soil N
balance may be small or even negative, particularly with grain legumes which generally
obtain a smaller proportion of their N from the atmosphere. The benefits will be even less if
straw or other crop residues are removed as well as the grain.
Written by S Cuttle, M Shepherd & G Goodlass Page 6