SELECTION OF ECOTYPES OF PANICUM-MAXIMUM FOR ASSOCIATED BIOLOGICAL NITROGEN-FIXATION USING THE N-15 ISOTOPE-DILUTION TECHNIQUE

Ecotypcs of the tropical pasture grass, Panicum maximum, were screened by means of the 15N isotope dilution technique for their ability to support associated biological nitrogen fixation (BNF). The ecotypes were planted in pots containing 3kg of a purple latosol (allic rhodalf) mixed with 1.5g pot-1 of 15N-labelled organic matter (compost). The experiment consisted of 26 treatments (25 ecotypes and the non-N2-fixing control plant, Brachiaria arrecta, IRI 442) with four replicates and was laid out in a randomized complete-block design. B. arrecta and P. maximum ecotype KK. 16 accumulated more N than most other ecotypes at the first two harvests (high soil N availability), but recovered the least N at the final two harvests (low soil N availability). At the last two harvests these two grasses had the highest 15N enrichment. The uptake pattern of labelled N of ecotype KK 16 was similar to that of the other P. maximum ecotypes, but distinctly different from that of B. arrecta, and for this reason KK16 was considered to be a better control plant and capable of obtaining little or no N from associated BNF. The other P. maximum ecotypes derived between 16 and 39% of their total N from BNF at the final two harvests. © 1990.