Factors affecting nitrogen fixation by dryland lucerne in central-western New South Wales

Abstract

Eight stands of dryland lucerne (Medicago sativa L.) cv. Trifecta were grown on a red-brown earth at Trangie, New South Wales. The stands were of varying age and plant density. Their biomass production and capacity to fix atmospheric nitrogen were measured on 15 occasions over a 5-year period (1995-99). Biomass production (shoot dry matter) ranged from 0.22 to 4.87 t/ha.year and nitrogen fixed from 1.8 to 78.6 kg/ha.year. Whereas biomass production was highest in summer periods, most nitrogen fixation took place in winter and autumn. Irrespective of stand age, greatest productivity occurred in the early years of the experiment and declined thereafter. Plant density varied from 6 to 21 plants/m² at the commencement of the study and decreased over time. We conclude that the productive life of dryland lucerne stands in this environment is probably limited by frequent periods of moisture stress and high soil temperatures in summer. Stands with a lucerne density of 8 plants/m² or better produced twice as much shoot biomass and fixed nearly double the amount of shoot N as did stands with densities of 7 plants/m² or less. There was no relationship between the age of lucerne stands per se and biomass production or nitrogen fixation. The practical implication of this work for farmers in the dryland cropping zone of central-western New South Wales who wish to maximise nitrogen fixation from the lucerne phase of their cropping rotations is to establish and maintain dryland lucerne at 8 plants/m² or better.