A pot experiment was carried out on a Typic Haplustert to study the time-course of N2 fixation in soybean (cv. Punjab-I) by 15N dilution technique using rice (cv. Kranti) as a nonfixing reference crop. Di-nitrogen derived from atmosphere (%Ndfa) in soybean on the 45th day after sowing was 37.6 per cent and thereafter %Ndfa value increased steadily and attained maximum (63.3) at the 75th day after sowing. From biological N2 fixation point of view, the period between the 55th and 75th day after sowing was found very important as during this period 61.2 per cent of the total biologically fixed N was accumulated. Between the period 45th and 55th day and 65th to 75th day, the rate of growth of root biomass (excluding nodules) was very high while in contrast, the crop had maximum nodule dry weight (1.88 g/pot) during the period from 55th to 65th day after sowing. Amount of fertilizer N translocated into the root was maximum (24.3 mg N pot−1) on the 55th day and declined gradually till maturity. The %Ndff and fertilizer N content value of nodule indicated that a measurable amount of fertilizer N entered into the plant system through root nodules. Fertilizer N uptake by the whole plant on the 55th and 95th (final harvest) day after sowing suggests that the crop was in net loss of 23.7 mg fertilizer N per pot at maturity.
Time-course, 15N N2-fixation, soybean, root and nodule growth, fertilizer N uptake and distribution in plant
