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Why Hasn’t Biological Nitrogen Fixation Fixed Nitrogen Scarcity in the World?

Posted by Andrew McGuire | May 5, 2022

“N limitation to primary production and other ecosystem processes is widespread.” Vitousek et al. 2002.

Most environments are limited by the lack of nitrogen. This explains why the world stays green and why it pays to follow the nitrogen. But why is this so? With this widespread limitation, organisms that can fix nitrogen (biological nitrogen fixation) would seem to have an advantage. Why don’t they proliferate and eliminate the nitrogen deficit? Why can’t we just let biological nitrogen fixation fix our nitrogen fertilizer problem?

There is no one answer, but researchers (Vitousek et al., 2002, 2013) believe that these disadvantages explain it:

Close view of plant roots.
Root nodules on a bean plant. While nodules are required for nitrogen fixation, they are costly for legumes to make and maintain. Licensed photo from Adobe Stock.

  • Energy. Fixing nitrogen takes a lot of energy. If organisms do not have to fix nitrogen to get what they need, they don’t. When they do, the energy costs mean they cannot grow as fast as non-N-fixing organisms. We see this in the high yields of corn—given sufficient N—compared to soybeans.
  • Shade tolerance. Because of the high energy cost of N-fixation, some legumes (trees, but not tropical) are less shade tolerant, making them less competitive in mixtures with non-N-fixing plants.
  • Oxygen levels. Biological N fixation requires low levels of oxygen. N-fixers must form structures, like the nodules on legume roots, to keep out oxygen. This too is costly and not required by non-N-fixers.
  • Phosphorus and other nutrient levels. Nitrogen fixers require some nutrients in higher amounts than non-N-fixers. P, K, Fe, and molybdenum. Again, this limits N-fixation in environments with low levels of these nutrients. Alfalfa growers may not have to apply N fertilizer, but they will not make much hay without adequate soil-P levels. “Thus, N limitation to primary production on a whole-system level may be limitation by P (or another non-N nutrient) in disguise.” (Vitousek et al. 2013)
  • Grazing. Because they need the high N content, herbivores (cattle, beetles, and such) often eat N-fixers over non-N-fixers, giving the latter an advantage. Graziers know this as the problem of keeping the legume-grass ratio constant over years.

There are no solutions. There are only trade-offs. — Thomas Sowell

So biological nitrogen fixation is not the free high-protein midday meal that we often think it is. Benefits? Yes, but don’t forget the costs; win-wins often mean you just haven’t looked closely enough.

References

  • Vitousek, P.M., K. Cassman, C. Cleveland, T. Crews, C.B. Field, et al. 2002. Towards an ecological understanding of biological nitrogen fixation. The Nitrogen Cycle at Regional to Global Scales. Springer, Dordrecht. p. 1–45.
  • Vitousek, P.M., D.N.L. Menge, S.C. Reed, and C.C. Cleveland. 2013. Biological nitrogen fixation: rates, patterns and ecological controls in terrestrial ecosystems. Philosophical Transactions of the Royal Society B: Biological Sciences 368 (1621): 20130119. doi: 10.1098/rstb.2013.0119.

7 thoughts on "Why Hasn’t Biological Nitrogen Fixation Fixed Nitrogen Scarcity in the World?"

  1. Ann Hirsch says:

    Nevertheless, BNF is environmentally more stable than other approaches. The question is how many people can BNF feed, or is the question something else. For example, how much can the planet’s environment take with regard to the current strategies being used. See books by Vaclav Smil.

    1. Andrew McGuire says:

      There is a hierarchy of such questions, but the first must be “how do we feed people?” and then “how do we minimize the environmental impacts?”
      And yes, Smil’s books are great.

  2. Great piece. I have linked to this from our new knowledge exchange platform at FarmPEP.net – hope that is OK?

    https://farmpep.net/node/258

    1. Andrew McGuire says:

      Yes, that is fine.
      That is a very interesting site/service.

  3. Kevin Foster says:

    Andrew
    Hi
    Nice summary!
    We often use spray / graze technique here (at a 3 or 5 true stage) in Western Australia to “sweeten” up the broad leaves weeds in our annual subclover pastures, so sheep selectively target the weeds and gives us a clover dominant pasture

    Legumes can be out competed by grasses early in the season too as they dont use energy to nodulate as you pointed out and you can’t graze to early, or you can disadvantage the legume

    The other problem is that the “croppers” often want more N up front or want a large amount of N available over a short time frame rather, than a slow release of biological N

    Kind regards
    Kevin

    1. Andrew McGuire says:

      Your management shows that it is not a trivial thing to be able to manage legumes in a mixture!
      Thanks for the comments.

  4. Tony says:

    You got me with Thomas Sowell:). Segway to bioengineering! Thanks for the article.

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