Isotopic studies on the uptake of nitrogen by pasture plants. IV. Uptake of nitrogen from labelled plant material by Rhodes grass and Siratro

Rhodes grass (Chloris gayana Kunth.) and Siratro (Phaseolus atropurpureus D.C. var. Siratro) plants were grown separately and together in pots of soil containing 15N-labelled ground Rhodes grass plant material (carbon/nitrogen ratio 44). The added plant material immobilized nitrogen during the first 6 weeks of the experiment, and nitrogen uptake by Rhodes grass was still less than that of the control at 15 weeks. Both the grass and the nodulated legume took up 15N throughout the experiment, despite the reduction in availability of unlabelled soil nitrogen. Siratro grown alone took up as much 15N as Rhodes grass grown alone. When the plants were grown together about one-third of the 15N uptake went into the Siratro and two-thirds into the Rhodes grass. The Siratro roots had a higher 15N enrichment than the tops, indicating that relatively more of the unlabelled nitrogen fixed from the atmosphere went into the Siratro tops, compared with the labelled nitrogen taken up from the soil. With Rhodes grass the tops had a higher enrichment than the roots. The difference between the extractable mineral nitrogen in the pots of soil under grass and in the bare pots accounted for uptake by the grass at 3 weeks but underestimated it at 15 weeks. In addition, there was a significant difference in enrichment between the total extractable mineral nitrogen in the soil and the nitrogen taken up by the Rhodes grass. From the uptake and partition of 15N it was calculated that 43–50% of the nitrogen in the Siratro plants at 3 weeks was taken up from the soil (the rest was attributed to symbiotic fixation); at 15 weeks only 2–4% of the nitrogen in the Siratro was from the soil. Very little nitrogen was transferred from the Siratro to the associated Rhodes grass.