Isotopic studies on the uptake of nitrogen by pasture plants. III. The uptake of small additions of 15N-labelled fertilizer by Rhodes grass and Townsville lucerne

Small additions of (15NH4)2SO4 and K15NO3 were used to investigate the nitrogen economy of Townsville lucerne (Stylosanthes humilis H.B.K.) and Rhodes grass (Chloris gayana Kunth.) grown separately and together in pots. Rhodes grass reduced the total yield of nitrogen per plant in associated Townsville lucerne to about half that of Townsville lucerne in pure stand. It appears that the Rhodes grass seedlings competed strongly with Townsville lucerne for available soil nitrogen during the first 5 weeks of the experiment. When the two species were grown together, Rhodes grass took up about 20 times as much 15N as the Townsville lucerne during the first 9 weeks, and about 8 times as much between 9 and 13 weeks after sowing. This ratio was the same for (15NH4)2SO4 as for K15NO3. When the two species were grown separately they took up equal amounts of 15N between 9 and 13 weeks after sowing. Estimates of soil nitrogen uptake at 5 weeks showed that 47% of the nitrogen in the Townsville lucerne growing with Rhodes grass had come from the soil. At 9 weeks the cumulative uptake of soil nitrogen by this Townsville lucerne was only 6%, and at 13 weeks only 3%, of its total nitrogen yield. No significant transfer of unlabelled nitrogen from legume to grass was detected. Attention is drawn to the assumption used in calculating uptake of soil nitrogen by the legume. It is suggested that this tracer technique can be applied to 15N experiments in the field.