NITROGEN-FIXATION, ACCUMULATION OF SOIL-NITROGEN AND NITROGEN-BALANCE FOR SOME FIELD-GROWN LEGUME CROPS

The N-15 dilution technique was used to measure N-2 fixation by field-grown spring-sown leguminous grain crops over two seasons and autumn-sown leguminous forage crops over one season. Nitrogen balances for the various crops were also calculated. The percentages of accumulated N derived from N-2 fixation (%Ndfa) for spring-sown lupin (Lupinus angustifolius), lentil (Lens culinaris), held pea (Pisum sarivum) and field bean (Vicia faba) were all below 40%. The %Ndfa values were consistently higher when barley (Hordeum vulgare) rather than oilseed rape (Brassica napus) was used as the non-N-2-fixing reference crop. This was attributed to a period of increased N accumulation late in the season for oilseed rape, which resulted in a lower N-15 content in rape than barley tissue. The autumn-sown legumes fixed very little N over the autumn-winter period (less than 10% of their accumulated N). The N harvest index of spring-sown crops was around 70% or greater. As a result, the overall N balance (quantity of N fixed minus quantity of N removed in harvested grain) was negative for all legume crops. In the case of field peas and lentils, the N balance was considerably less negative than that for the reference crops. In addition, their residues (both stover and roots), which were returned to plots following harvest, had a higher N content than those of the reference crops. Thus, in comparison with oilseed rape or barley, there is likely to be a relative benefit to a following crop in terms of N nutrition when it is grown after field peas or lentils.