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

Abstract

AbstractContributions of plant‐associated biological nitrogen fixation (BNF) may be important to N nutrition of grasses. Eleven ecotypes of Panicum maximum were planted in the field in concrete cylinders containing soil (Typic Hapludult) that had a low but stable 15N enrichment. Brachiaria radicans (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 15N‐enrichment data. At later harvests (February–April 1985), the N concentrations in the plants were lower and all the P. maximum ecotypes accumulated more than the B. radicans. Over this period, the 15N enrichment of all the P. maximum ecotypes was significantly lower than that of the B. radicans, indicating a contribution of plant associated BNF to the P. maximum ecotypes. Contributions of associated BNF to the P. maximum ecotypes were estimated to be between 24 and 38% of total N incorporated, equivalent to between 5 and 10 kg N ha−1 per 30 days. The significant differences observed in ISN enrichment between P. maximum genotypes suggest that further screening and selection of P. maximum ecotypes for high associated BNF is a worthwhile objective.