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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 15 N (δ 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 δ 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 δ 15 N at depth. From the measurements of 15 N, 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. |
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