No plant can take up nitrogen from the atmosphere by itself. Legumes and some other plants form sophisticated structures, usually on their roots, that house nitrogen-fixing bacteria. Previous research has shown that less-structured associations between sugar cane and nitrogen-fixing bacteria can meet a good fraction of sugar cane’s modest nitrogen requirements. One study found nitrogen contributions of 100-200 kgN/ha (also roughly equal to lb/acre), a large fraction of what even corn needs, by two different methods.
This week, we learn that some traditional corn (maize) varieties grown in Mexico can also get nitrogen from a loose symbiosis, involving nitrogen-fixing bacteria in mucilage on the surface of above-ground roots. The paper is titled “Nitrogen fixation in a landrace of maize is supported by a mucilage-associated diazotrophic microbiota.” The authors, two of whom I know from my time at UC Davis, present convincing evidence that bacteria in the mucilage are indeed fixing nitrogen and that the plant gets some of it.
But how much nitrogen does that plant get? The key data are in their Table 2. The data are quite variable, so it will be interesting to see whether the highest values can be replicated consistently with the right genotypes and conditions. Looking at median values for corn in the tassel stage, I multiplied total shoot nitrogen (kg/ha) by the % of nitrogen they estimate came from symbiosis, and got 5-20 kgN/ha. For commercial corn production in the US, that would be less than the margin of error, that is, less than the error in our ability to estimate how much a given corn crop needs. So I don’t see much short-term potential for adapting this system to high-yield corn production.
What about long term? Could an improved version of this symbiosis ever supply as much nitrogen as legume root nodules do? Probably not. Legumes monitor the performance of each root nodule and cut off resources to nodules that supply little or no nitrogen. I don’t see any way that corn could preferentially direct resources to the best nitrogen-fixers in its mucilage — or those that are more generous in sharing their nitrogen with their plant host — while cutting off resources to “free-riders” that fix little or no nitrogen. So we could increase nitrogen fixation by breeding for more mucilage-producing roots, but the photosynthetic costs of supporting those roots would soon exceed the nitrogen benefit. Efficiency (nitrogen obtained per carbon invested) is the key to genuinely useful nitrogen fixation.