Estimation of N_2 Fixation in Soybean and Cowpea by Using Soil Residual ^<15>N

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Estimation of the amount of N_2 fixed by cowpea and soybean was carried out using concrete plots at the International Rice Research Institute (IRRI). After the application of ^<15>N labeled ammonium sulfate, one crop of rice was grown under upland and flooded conditions. After the harvest of rice, soybean and cowpea in each plot were grown with a non-nodulating isoline of soybean and upland rice as reference plants, respectively. No differences in the growth, N uptake and N_2 fixation of all the plants used was detected in relation to the flooding pretreatment. The percentages of N derived from air (%Ndfa) were 25 and 10 in soybean and cowpea, respectively. There was no significant difference in the estimates of N_2 fixed between the N balance and ^<15>N dilution methods in both soybean and cowpea. Mineral N contents in soil and ^<15>N abundance decreased with time and soil depth. The half life of the ^<15>N abundance in soil mineral N ranged from 63 to 110 days. Data of isotope contents in soil mineral N were used to calculate possible errors in the estimation of %Ndfa. It was inferred that soybean absorbed soil N only from the 0-7 cm layer and only until 46 days after sowing (the highest ^<15>N uptake), the value of %Ndfa would be 33. Whereas, when soybean absorbed soil N uniformly from all the soil layers (the possible lowest ^<15>N uptake), the value of %Ndfa would be 13.
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