Tolerance of Nitrogen Fixation to Nitrate in Soybean a Progeny (Tolerant x Non-Tolerant) Evaluation

The capacity to maintain high levels of N2 fixation in high N soil, while exploiting mineral N through assimilation, should enable optimal utilisation of both pathways of legume nitrogen nutrition. Although genetic variability for the tolerance of N2 fixation to nitrate has been found in soybean, the agronomic advantage of this character has not been demonstrated. The present study was aimed at (i) characterizing the expression of nitrate tolerance of N2 fixation in different environmental conditions, and (ii) evaluating the agronomic consequences of this character. Tolerant and non-tolerant lines resulting from the same cross (tolerant line Tielingbaime x cultivar Kingsoy) were used. Four lines were observed in four to five environments differing in nitrate availability. Nitrate tolerance was only expressed in high N soils, and was related to a lower inhibition of N2 fixation, rather than to a higher fixation potential on low N soils. Nineteen lines of the same progeny were further evaluated in the field, on a moderately high and a high N soil. Lines maintaining substantial N2 fixation on the two soils (based on 15N determination) had a greater seed yield (by an average of 12 to 15 per cent, than non-tolerant lines). The yield increase was associated with a greater number of seeds per plant, a greater vegetative biomass, and on the high N soil, with no significant decrease in seed N content. The data suggest that this nitrate tolerance of N2 fixation allows for a better agronomic exploitation of N in high N soils.