Competition for 15N labelled ammonium between Trifolium subterraneum L. and Lolium multiflorium L. when grown in a mixture

Grasses and legumes are often grown in association in pastures because total herbage yield and forage quality is often higher than a monoculture grass sward. The competitive ability of the forage species for mineral N influences the stability of the mixed pasture. Mt. Barker subterranean clover (Trifolium subterraneum L.) Gulf ryegrass (Lolium multiflorium L.) were grown in pure stands and in mixtures in 3.8 I pots filled with exploded vermiculite to quantity the competition for applied mineral nitrogen (N). The isotope dilution technique using 15NH4 was used. Also, the effect of increasing rates of fertilizer N on nodulation and fixation by subterranean clover and the rate of N uptake of subterranean clover and ryegrass was measured. The acetylene-ethylene technique and nodulation rating were used to examine the effect of N fertilization on fixation and nodulation. The uptake of N fertilizer by subterranean clover and ryegrass in both pure stands and the mixtures increased as the rate of fertilizer N applied increased. When grown in a pure stand, subterranean clover recovered a comparable amount of fertilizer N, but when grown in mixture with ryegrass, subterranean clover recovered from 44 to 364 mg of N. Ryegrass recovered 2 times the amount of labelled N in 6 hours than subterranean clover. The rate of N uptake was not due to differences in root fresh weight or dry weight. Ryegrass appeared to be a better competitor for NH4 than subterranean clover because of the greater rate of uptake