The 15N-isotope dilution technique applied to the estimation of biological nitrogen fixation associated with Paspalum notatum cv. batatais in the field

The contribution of associated biological nitrogen fixation to the nitrogen nutrition of Paspaulum notatum cv. batatais was estimated using the ¹⁵N-isotope dilution technique. The plants were grown in the field in concrete cylinders (60 cm dia) filled with soil, to which were added small quantities of ¹⁵N-labelled fertilizer at frequent intervals over 12 months. The pensacola cultivar of P. notatum was used as a non-N₂-fixing control plant. This was justified by the observation that nitrogenase (intact core C₂H₂ reduction) activity associated with the pensacola cultivar was consistently much lower than that of the batatais cultivar.At the first harvest, no evidence for N₂ fixation associated with the batatais cultivar was obtained, probably because of slow establishment of the N₂-fixing association. However, at the subsequent three harvests the batatais cultivar exhibited a lower ¹⁵N-enrichment and yielded more N than the pensacola cultivar. These data together suggested that 8–25% of the N in the batatais cultivar originated from N₂ fixation.The grass Paspalum maritimum was also included in the experiment and exhibited low nitrogenase activity similar to that of the pensacola cultivar of P. notatum. However, the total N and ¹⁵N data of these two grasses were not in good agreement indicating that it is important for the use of the isotope dilution technique that control plants are of very similar physiology and growth habit.