Reliable quantification of N2 fixation by non-legumes remains problematic

Author(s)
Unkovich, Murray
Herridge, David
James, Euan K
Giller, Ken
Peoples, Mark B
Publication Date
2020-12
Abstract
<p>We write this commentary in response to a recent paper by Barros et al. (2020) which reports rates of N<sub>2</sub> fixation of 0–265 kg ha<sup>-1</sup> (above-ground biomass only with no accounting of fixed N in the roots) by 15 sorghum (<i>Sorghum bicolor</i> (L.) Moench) genotypes at four sites in Brazil. The high values reported require careful consideration because they are up to ten times higher than observed elsewhere in Brazil on sorghum inoculated with diazotrophs specifically selected from parallel glasshouse experiments (dos Santos et al. 2017). Sorghum is a widely-grown poaceous grain crop, but one not known for its N<sub>2</sub>-fixing capability. Based on the data presented, we calculate the rate of N<sub>2</sub> fixation would have needed to exceed 3 kg ha<sup>-1</sup> day<sup>-1</sup> during the first 90 days of growth for the highest fixing sorghum genotype, a rate higher than observed for most N<sub>2</sub>-fixing legumes (e.g. soybean, <i>Glycine max</i> (L.) Merrill; La Menza et al. 2020). Barros et al. (2020) used a methodology that relies on small natural differences in <sup>15</sup> N abundance (usually expressed as δ<sup>15</sup>N; parts per thousand relative to atmospheric N<sub>2</sub>) which generally occur between atmospheric N<sub>2</sub> and plant-available N in the soil, the latter sampled with a known non-N<sub>2</sub>-fixing plant. The N<sub>2</sub>-fixing plants, partly or wholly rely on atmospheric N<sub>2</sub> as an N source for growth, display significantly lower δ<sup>15</sup>N than plants with lesser or no dependence on N<sup>2</sup> fixation. At the two sites at which sorghum δ<sup>15</sup>N were significantly different to the δ<sup>15</sup>N of the non-N<sub>2</sub>-fixing reference plants, the median Ndfa was calculated to be 52% and shoot N fixed 45 kg N ha<sup>-1</sup>.</p>
Citation
Nutrient Cycling in Agroecosystems, 118(3), p. 223-225
ISSN
1573-0867
1385-1314
Link
Publisher
Springer Netherlands
Title
Reliable quantification of N2 fixation by non-legumes remains problematic
Type of document
Journal Article
Entity Type
Publication

Files:

NameSizeformatDescriptionLink