NITROGEN-FIXATION, ACCUMULATION OF SOIL-NITROGEN AND NITROGEN-BALANCEFOR 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-s own 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 angus tifolius), lentil (Lens culinaris), held pea (Pisum sarivum) and field bean (Vicia faba) were all below 40%. The %Ndfa values were consisten tly higher when barley (Hordeum vulgare) rather than oilseed rape (Bra ssica napus) was used as the non-N-2-fixing reference crop. This was a ttributed to a period of increased N accumulation late in the season f or oilseed rape, which resulted in a lower N-15 content in rape than b arley tissue. The autumn-sown legumes fixed very little N over the aut umn-winter period (less than 10% of their accumulated N). The N harves t index of spring-sown crops was around 70% or greater. As a result, t he 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 t han that for the reference crops. In addition, their residues (both st over and roots), which were returned to plots following harvest, had a higher N content than those of the reference crops. Thus, in comparis on with oilseed rape or barley, there is likely to be a relative benef it to a following crop in terms of N nutrition when it is grown after field peas or lentils.