Estimation of N2fixation in soybean and cowpea by using soil residual15N

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Abstract
Estimation of the amount of N2 fixed by cowpea and soybean was carried out using concrete plots at the International Rice Research Institute (IRRI). After the application of 15N 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 N2 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 N2 fixed between the N balance and 15N dilution methods in both soybean and cowpea. Mineral N contents in soil and 15N abundance decreased with time and soil depth. The half life of the 15N 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 15N uptake), the value of %Ndfa would be 33. Whereas, when soybean absorbed soil N uniformly from all the soil layers (the possible lowest 15N uptake), the value of %Ndfa would be 13.