Estimation of N_2 Fixation of Soybean by Comparison of Different ^<15>N Labeling Methods

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The effect of different ^<15>N labeled sources on the estimation of N_2 fixation by soybean was investigated in the Philippines, using a non-nodulating isoline as a reference plant. the soil was placed in 1.5×1.5 m concrete plots. The labeled urea or rice straw was applied 1 month before soybean was sown in the soil which had received a labeled ammonium salt prior to the cultivation of two crops. Plots without these additions were also used. The N_2 fixation during 0-36 days after sowing was negligible. The estimation of N_2 fixed at 67 days of plant growth in the plots which received labeled urea or straw was not statistically different from the estimation in the plots with residual ^<15>N. The estimations based on both the N balance and ^<15>N dilution methods were not statistically different. the percentage of N derived from N_2 was 41 and the quantity of N_2 fixed was estimated at 33 kg N/ha. The least variation was found for the estimation of N_2 fixation in the plots with residual ^<15>N. this fact is attributed to the stability of ^<15>N in the soil, as the ^<15>N abundance in soybeans in the plots with residual ^<15>N only showed the least variation among the replicates and did not change with the sampling time.
社団法人日本土壌肥料学会の論文