N2 fixation in irrigated lucerne grown for hay

In Australia, lucerne responds to inoculation with Rhizobium meliloti. There are consequent benefits to N2 fixation during and immediately after establishment but little is known about lucerneʹs capacity to fix N2 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 15N (δ15N) in the total N of shoots of lucerne and non-N2-fixing reference plants (weeds) in the stand and in the extractable mineral N of the soil to a depth of 1.1 m. Although δ15N of shoot N was low in lucerne, it was clear that this did not result from plant uptake of soil N of low δ15N at depth. From the measurements of 15N, Pfix, the proportion of lucerne N derived from N2 fixation, was calculated. In 1988–1989, estimates of Pfix 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 Pfix 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.