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

Abstract

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 15 N 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 −\u20091.80‰, differing on the trial setting (greenhouse −\u20091.37‰ and field −\u20092.62‰). The average shoot B value in the greenhouse was −\u20091.97‰ (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\xa0kg\xa0N\xa0ha −1 \xa0day −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.