Estimating biological nitrogen fixation in field-grown soybeans: impact of B value

Aim To evaluate the impact of using different B values on the overall estimation of the proportion of nitrogen (N) derived from the atmosphere (%Ndfa) in soybean (Glycine max L.) field trials under two contrasting management systems via the N-15 natural abundance (NA) technique. Methods First, a literature review of the soybean B value was conducted. Secondly, a greenhouse study was performed to determine the shoot B value by plant development stage for four soybean varieties. Lastly, in two field trials, soybean was grown under two contrasting management systems (common practices, low inputs; and intensified practices, high inputs) and the %Ndfa by development stage was estimated by the NA technique using the three sources of B values (literature review, greenhouse: plant development stage and peak biomass). Results The average B value reported in the literature for soybean was - 1.80 parts per thousand, differing on the trial setting (greenhouse - 1.37 parts per thousand and field - 2.62 parts per thousand). The average shoot B value in the greenhouse was - 1.97 parts per thousand (R-7). The variation between shoot B values measured in this study was small and quantitatively not significant across plant development stages and plant organs. Field trials showed that intensification improved yields and increased maximum N-2 fixation rate from 4.9 to 6.2 kg N ha(-1) day(-1) under irrigated conditions relative to the common practices. Conclusions The %Ndfa relative to total N demand ranged from 45 to 56%; at the fixation levels measured in this study, the source of B value had relatively low impact on the estimation of %Ndfa.