Legume Growth and Nitrogen Fixation as Affected by Plant Competition for Light and for Soil Nitrogen'

395 ABSTRACT Since the legume responses to environmental factors may be different in grass mixtures than in pure stands, a pot experiment was conducted to evaluate the effects of plant top and root competition on the growth and N fixation of Desmodium intortum, cv. ‘Greenleaf’ grown in association with Setaria anceps, cv. 'Nandi' The plants were grown in deep, narrow pots which provided for ± top competition (set close together or far apart) and ± root competition (roots in same or different half sections of the pot). All treatments were grown with and without an initial application of 60 kg N/ha. Greenleaf was a better competitor for light than Nandi in spite of the taller height of Nandi. However in contrast with Nandi, Greenleaf yields and leaf areas were reduced by root competition, especially in the minus-N treatments. There were no significant differences in acetylene reduction activity due to either N levels or competition treatment. Nodule numbers were depressed by both top and root competition at both N levels, but the plants were able to compensate to various degrees by increases in nodule size and to a greater extent by increases in acetylene reduction activity per unit of nodule weight.