Evaluation of the appropriate non-N2-fixing crops to quantify nitrogen fixation by soybean using the 15N isotope dilution method

Experiments for selecting the appropriate non-N2-fixing plants to be used in the accurate estimation of N2 fixation by the 15N isotope dilution method were conducted in different locations to quantify N2 fixation in soybeans under different cultural and agronomic practices in Thailand. The non-N2-fixing control plants, namely Japanese and United States non-nodulating soybeans, maize, sorghum and upland rice, were used in experiments at Kamphaeng Saen (central Thailand) and Khon Kaen (northeastern Thailand), and wheat and barley were used as substitutes for the Japanese non-nodulating isolines at Chiang Mai (northern Thailand). Dinitrogen fixation occurred at Chiang Mai and Khon Kaen, averaging 53% and 68% of the plant N yield respectively. No N2 fixation was observed at Kamphaeng Saen owing to high soil N. Despite the presence of indigenous rhizobia, the two Japanese non-nodulating soybean lines appeared to be the most appropriate non-N2-fixing controls for the 15N isotope dilution method to estimate N2 fixation in Thai soybean. Maize proved to be another appropriate non-N2-fixing crop at some sites. Sorghum, wheat and the US non-nodulating line were not appropriate non-N2-fixing control plants. However, the choice of appropriate non-N2-fixing control plants remains a dilemma, and there is as yet no alternative but to use several possible non-N2-fixing plants in each experiment. (author). 10 refs, 5 figs, 3 tabs