Field measurement of symbiotic nitrogen fixation in an established lucerne ley using super(15)N and an acetylene reduction method

Lucerne is an important forage legume in the south and south-east of Sweden on well-drained soils. However, data is lacking on the apparent amount of nitrogen derived through N sub(2) fixation by field-grown lucerne. The experiment was performed in a lucerne ley grown 40 km north of Uppsala. The input of nitrogen through fixation to the above-ground plant material of an established lucerne (Medicago sativa) L.) ley was estimated by super(15)N methodology during two successive years. The amount of fixed N was 242 kg N ha super(-1) in 1982 and 319 kg N ha super(-1) in 1983. The proportion of N derived from the atmosphere (% Ndfa) was 70% and 80% for the two years respectively. In addition an in situ acetylene reduction assay was used in 1982 to measure the seasonal distribution of the N sub(2) fixation and in 1983 to study the effect of soil moisture on the N sub(2) fixation process. The seasonal pattern showed great dependence on physiological development and harvest pattern of the lucerne ley. The maximum rate of N sub(2) fixation occurred at the bud or early flower stage of growth and was followed by a rapid decline as flowering proceeded.