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S.B. Johnson1, R.D. Armstrong2 and K.B. Walsh1

1Department of Biology, Central Queensland University, Rockhampton, Qld 4702
Department of Primary Industries, LMB 6, Emerald, Qld 4720

Declining soil nitrogen fertility is a major problem in central Queensland as it is throughout the northern cereal belt. One potential solution to this problem is the introduction of nitrogen fixing pasture-legumes to the system. The value of a ley legume will be a function of both its dry matter production and fixing capacity. This paper documents nitrogen fixation for a range of potential ley legumes.


Two trials were conducted to evaluate total nitrogen fixation using the 15N natural abundance technique in a range of legumes suitable for warm season planting. The first trial of five legume species involved three consecutive harvests during the 1993/94 growing season. The second trial evaluated twelve species and involved a single harvest at the conclusion of the 1994/95 growing season. The trials were conducted on an open downs cracking clay soil at Emerald which had been previously cropped to wheat for at least 20 years. Dry weight, percentage nitrogen and δ15N values were determined at each harvest. These data were used to determine the percentage of nitrogen derived from the atmosphere and the amount of nitrogen fixed on a per hectare basis.


In the first trial, the total amount of nitrogen fixed by Lablab purpureus peaked after pod fill at 77 kg/ha. This was approximately double that of Macroptilium atropurpureum (38 kg/ha). In comparison both Medicago sativa and Desmanthus virgatus cv. Marc fixed very poorly with rates below 11 kg/ha. These low values may be explained by low dry matter production in M. sativa and an ineffective Rhizobium strain (N. Brandon pers. commun.) with Desmanthus. At Vigna radiata harvest (84 days) 38 kg/ha of nitrogen had been fixed. The total nitrogen removed from the field in grain was also 38 kg/ha giving a net nitrogen balance of zero for V. radiata.

In the second trial the following rates of nitrogen fixed in kg/ha were: L. purpureus 34.9, M. atropurpureum 29.0, M. sativa 14.2 and D. virgatus cv. Marc 4.8. The generally lower rates of fixation probably reflect higher soil mineral nitrogen levels at planting. Data for the following species and cultivars will also be presented: Cajanus cajan, Clitoria ternatea, Desmanthus virgatus cv. Bayamo, Glycine latifolia, Macroptilium bracteatum, Stylosanthes scabra aff. scabra, Vigna trilobata and V. radiata.


This project was jointly funded by the GRDC and an ARC small grant.

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