NITROGEN-FIXATION IN PEANUT DETERMINED BY ACETYLENE-REDUCTION METHOD AND N-15-ISOTOPE DILUTION TECHNIQUE

Seasonal pattern of nitrogenase activity of peanut (Arachis hypogaea L.) was studied under concrete plot conditions using samples of six different soils collected from Chiba Prefecture. Nitrogen fixation was determined using the N-15-isotope dilution, N-difference, and acetylene reduction methods. Peanut derived 41-63% of its total nitrogen from fixation depending on the soil types, based on measurements using the N-15-isotope dilution technique. The largest amount of nitrogen was fixed in Humic Andosols followed by Light-coloured Andosols and Sand-dune Regosols and the smallest in Dark Red soils. Amount of nitrogen fixed that was estimated by the N-difference method was 20-30% larger compared to the use of the isotope dilution technique. Calculation of nitrogen fixation based on the determination of acetylene reduction activity using a theoretical conversion factor of one-third N2 reduced per C2H2 reduced markedly underestimated the amount compared to the isotope dilution method. The highest acetylene reduction activity was observed during the period from 95 to 120 d after sowing irrespective of soils.