Nitrogen fixation by Ulex europaeus (gorse) and Cytisus scoparius (broom)

Abstract

A series of glasshouse and laboratory experiments was carried out to enable comparison of two woody perennial legumes, gorse and broom, with other legumes, nodulated non-legumes and other biological nitrogen fixing systems.

Both species had distinct juvenile phases in which broom closely resembled herbaceous species in appearance, but adult plants of both species bore little resemblance to each other or to other legume species. Nodule development was similar to that of other legumes, but mature nodules exhibited structural adaptations to longevity - meristematic activity, a well developed vascular system and numerous cytoplasmic granules in cortical cells. Acetylene reduction and ¹⁵N₂ fixation continued for much longer following excision than has been observed in other legumes. In all experiments, broom nodules exhibited higher rates of acetylene reduction and nitrogen fixation than did gorse nodules.

The first detectable product of nitrogen fixation in excised nodules - ammonia - was rapidly incorporated into amide and ∝ amino groups and another unidentified fraction. The principle free amino acid in nodules and sap was asparagine. Its preponderance increased as plants aged.

Whole nodulated plants and excised nodules of both species exhibited a relatively low temperature optimum for growth and nitrogen fixation (22°C). They were very sensitive to elevated temperatures. Results indicated that gorse and broom have relatively low light requirements.

When aeration was sufficient, combined nitrogen had little effect on growth of nodulated plants. Nodulation in both species was reduced by increasing amounts of combined nitrogen. High levels (100 mg/1) of nitrate and ammonia caused considerable inhibition of nitrogen function.

Both species showed large responses to phosphate, but were able to grow and fix nitrogen when supplied with low amounts of phosphate.

Boron deficiency reduced nitrogen fixation. Nodulation was increased to compensate for this.

Considerable amounts of nitrogen can be contributed to the ecosystem in gorse end broom litter. Direct transfer between gorse or broom and Pinus radiate is likely to be small and may be masked by competition for other nutrients.

These findings are discussed with respect to the use of gorse and broom to overcome nitrogen deficiency in reafforestation on the Moutere Gravels, in Nelson, N.Z.