Estimation of Biological Nitrogen Fixation Associated with 11 Ecotypes of Panicum maximum Grown in Nitrogen‐15‐labeled Soil1

Contributions of plant-associated biological nitrogen fixation (BNF) may be important to N nutrition of grasses. Eleven ecotypes of were planted in the field in concrete cylinders containing soil (Typic Hapludult) that had a low but stable N enrichment. (cv. IRI 442) was used as the non-fixing control, and the plant aerial tissue was harvested eight times over a 390-day period. At the first harvest of the experiment (17 May 1984), the concentration of N in all the grasses was high and, subsequently, declined during the first 270 days of growth, reflecting the decreasing availability of soil N. Until soil N availability had become the main factor limiting plant growth, it was thought unlikely that any significant contribution of BNF would occur, and this was confirmed by the N-enrichment data. At later harvests (February–April 1985), the N concentrations in the plants were lower and all the ecotypes accumulated more than the . Over this period, the N enrichment of all the ecotypes was significantly lower than that of the , indicating a contribution of plant associated BNF to the ecotypes. Contributions of associated BNF to the ecotypes were estimated to be between 24 and 38% of total N incorporated, equivalent to between 5 and 10 kg N ha per 30 days. The significant differences observed in ISN enrichment between genotypes suggest that further screening and selection of ecotypes for high associated BNF is a worthwhile objective.