NITROGEN-FIXATION IN PEANUT AT VARIOUS CONCENTRATIONS OF N-15-UREA AND SLOW-RELEASE N-15-FERTILIZER

Nitrogen fixation of peanut (Arachis hypogaea L.) was studied using pots (1 kg soil/pot) at various concentrations of N-15-urea and N-15-super-IB (a slow release N fertilizer). The soil used was an Andosol with the following characteristics pH 6.02, CEC 23.4 cmol(+) kg(-1), total-C 20.9 g kg(-1), total-N 2.5 g kg(-1), and a loam texture. The number and weight of the nodules and acetylene reduction activity (ARA) decreased by the addition of N-15-urea but not by the addition of N-15-super-IB even at higher doses. Irrespective of the type of fertilizer or the amount of nitrogen applied, the highest ARA of the crop was observed between 75 and 98 d after sowing (DAS). Nitrogen fixation determined by the A-value method showed that the addition of higher doses of N-15-urea markedly decreased the fixation, unlike that of N-15-super-IB. At 75 DAS the percentage of nitrogen derived fr om fixation (%Ndfa) at 100, 200, and 400 mg N per pot was 61, 54, and 29 (333, 308, and 171 mg N per plant), respectively when N was applied as N-15-urea, and 63, 62, and 61 (333, 343, and 349 mg N per plant), respectively when applied as N-15-super-IB. The %Ndfa at 98 DAS in peanut treated with 100, 200, and 400 mg N per pot was 69, 63, and 52 (719, 688, and 578 mg N per plant), respectively when N was applied as N-15-urea and the %Ndfa at the same doses of N-15-super-IB was 69, 68, and 64 (773, 789, and 787 mg N per plant), respectively. Hence, it can be concluded that the symbiotic nitrogen fixation of peanut was not suppressed by the addition of slow release N-fertilizer even at higher doses under the current experimental conditions.