A reassessment of the contribution of soybean biological nitrogen fixation to reactive N in the environment

Abstract

The expansion of soybean (Glycine max (L.) Merr) acreage, increasing yields, and recent nitrogen (N) fertilization recommendations could have a major effect on the contribution of biological N fixation (BNF) in soybeans to reactive nitrogen (Nr) in the environment. We used 15N natural abundance to separate fixed N into grain, aboveground vegetative biomass, and roots along a 9-point N-fertilizer gradient to ask: 1) is the belowground BNF contribution sufficiently different from aboveground to affect regional estimates of soybean Nr production based on harvested biomass, and 2) how does N fertilizer affect soybean yield and BNF’s contribution to different tissues? The contribution of root and vegetative biomass to overall plant BNF was five times lower than that for grain. Including this difference in BNF extrapolations translates to 3.5 ± 0.5 Tg Nr yr−1 for total US soybean production, ~37 % lower than earlier estimates that did not differentiate tissue source. Production of Nr ranged between 35 ± 11 and 73 ± 5 g Nr kg−1 grain and was affected by both fertilization and irrigation. In all cases N credits to the next rotational crop were minor. N-fertilization at even very low levels (17–50 kg N ha−1) did not affect yield, but grain N content increased with fertilizer level. The percent BNF contributed to plant N decreased linearly with increasing fertilization, in grain from 49 ± 8 % in unfertilized plots to a plateau of 16 ± 6 % at fertilization ≥85 kg N ha−1; in aboveground vegetative biomass from 77 ± 4 % to a plateau of 11 ± 11 % at 146 kg N ha−1; and in roots from 88 ± 12 % to a plateau of 41 ± 6 % at 146 kg N ha−1. The average whole-plant BNF contribution decreased from ~84 % in unfertilized plots to a plateau of ~34 % at fertilization rates greater than 84 kg ha−1. Results underscore the unnecessary expense and environmental burden of adding N fertilizer to modern soybean varieties, and provide a refined lower estimate for the contribution of soybean N fixation to the US and global Nr budgets of 3.5 and 10.4 Tg Nr yr−1, respectively.