Growth and nitrogen fixation by Medicago littoralis in pot experiments: Effect of plant density and competition from Lolium multiflorum

The legume Medicago littoralis cv. Harbinger, was grown either alone (1–4 plants per pot) or with Lolium multiflorum (ryegrass) at a total of 4 plants per pot, using two soils of contrasting N status. An ¹⁵N dilution technique was used to distinguish the amounts of plant N due to N₂ fixation and to N uptake from soil. Medic outyielded (dry weight and total plant N) ryegrass in a soil which released low amounts of inorganic N (Roseworthy) but ryegrass outyielded medic in a soil of higher N availability (Avon).For both soils, all combinations of medic and ryegrass plants utilized 70–73% of the inorganic N released on incubation. Competition from ryegrass invariably reduced yields of dry matter, total N, and fixed N of the medic plants, especially in the Avon soil. For both soils, the percentage reduction in the amounts of fixed N resulting from competition from ryegrass was directly proportional to the percentage increase of plant dry matter due to ryegrass. Medic plants grown in Roseworthy soil contained much higher proportions of N due to N₂-fixation than did medic plants grown in Avon soil. The amounts of plant N, fixed N and plant dry weight increased with increasing numbers of medic plants, when grown alone in Roseworthy soil, but not in the Avon soil containing more than two plants per pot. Nevertheless, irrespective of the soil used, medic numbers per pot, or competition from ryegrass, the amounts of fixed N correlated well with total N and with dry matter yields of medic plants. The proportions of fixed N to total N varied consistently in each of the medic plant parts (roots < = leaves < stems < pods).