N-2 FIXATION IN IRRIGATED LUCERNE GROWN FOR HAY

In Australia, lucerne responds to inoculation with Rhizobium meliloti. There are consequent benefits to N-2 fixation during and immediately after establishment but little is known about lucerne's capacity to fix N-2 in mature stands. Root nodules are difficult to find and it is commonly held that mature plants rely on combined N deep in the soil profile. A study of the N economy of an established stand of irrigated lucerne at Ginninderra, ACT is reported. Populations of R. meliloti were counted in soil profiles to a depth of 1.1 m. Large numbers were found near the surface (0-10 cm). Smaller numbers(ca. 100 g(-1) soil) occurred uniformly down the profile. Populations of this size are adequate for nodulation. Measurements were made of the natural abundance of N-15 (delta(15)N) in the total N of shoots of lucerne and non-N-2-fixing reference plants (weeds) in the stand and in the extractable mineral N of the soil to a depth of 1.1 m. Although delta(15)N Of shoot N was low in lucerne, it was clear that this did not result from plant uptake of soil N of low delta(15)N at depth. From the measurements of N-15, P-fix, the proportion of lucerne N derived from N-2 fixation, was calculated. In 1988-1989, estimates of P-fix ranged from 79 to 92%. During 1989-1990, the sixth growing season, the lucerne in the most productive treatment cut 7.41 t dry matter ha(-1), containing 198 kg N ha(-1); the mean P-fix was 45.5%, giving 90.0 kg N fixed ha(-1). These data indicate that this established stand of lucerne fixed nearly half of its total N and was not reliant upon mineral N deep in the soil profile.