|White Clover, Trifolium repens.|
White Clover is so common and modest that it’s often ignored. It’s like background noise: always there but barely noticed, at least until it flowers. Beneath its ordinary appearance, though, is an extraordinary ability: It can capture atmospheric nitrogen, N2, and convert it to ammonia, NH3, a first step in making nitrogen usable.
Called biological nitrogen fixation, this process is billions of years old and vital to life as we know it. Although nitrogen gas composes about 78% of the atmosphere by volume, most living things can’t use it. We humans, for example, can’t simply take a deep breath and get the nitrogen we need. We don’t have the molecular machinery to do that.
But some kinds of bacteria do. They possess nitrogenase, a complex enzyme that can break the strong bonds in nitrogen molecules and attach the atoms to hydrogen, making ammonia. Ammonia then goes on to participate in other reactions that make proteins, DNA and other biomolecules. When these compounds decay, or when some of the captured nitrogen leaks into the soil, other plants absorb it. We eat these plants or the animals that graze on them to get our supply of nitrogen. We can’t live without it.
Clover and other legumes house nitrogen-fixing bacteria in nodules on their roots. This symbiosis is of mutual benefit: The plants receive nitrogen from the bacteria, and the bacteria receive energy and carbon compounds from the plants. The nodules also provide a low-oxygen environment for nitrogenase to work. A kind of hemoglobin called leghemoglobin scavenges oxygen that would otherwise disable the enzyme. At the same time, leghemoglobin provides oxygen for cell respiration, the set of reactions that produces the energy to drive nitrogen fixation and other processes.
Nodules on the roots of White Clover hold bacteria that fix
Legumes are the primary biological nitrogen fixers, but a few plants in other families can do the same. Speckled Alder (Alnus incana), Silver Buffaloberry (Shepherdia argentea) and New Jersey Tea (Ceanothus americanus), for example, are non-legumes that also house nitrogen-fixing bacteria in root nodules. Called actinorhizal plants, they are mostly trees and shrubs from temperate regions. They are adapted to nutrient-poor soils, so some of them have been used to restore land degraded by mining, logging, wildfires or other disturbances.
Other fixers live freely in soil, or they live in close association with roots but not inside nodules. The latter includes bacteria that live in the rhizosphere (the near-root environment) of many grasses, including wheat and corn. Some researchers are trying to develop nodulating cereal crops that capture more of the nitrogen they need naturally instead of absorbing it from manufactured, energy-intensive fertilizer, which now supplies most of the nitrogen needed for agriculture. If they succeed, it could be part of the answer to mitigating climate change – and to feeding a hungry world.
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Bernhard, A. (2010) The Nitrogen Cycle: Processes, Players, and Human Impact. Nature Education Knowledge 3(10):25
Diagne, N., Arumugam, K., Ngom, M., Nambiar-Veetil, M., Franche, C., Narayanan, K. K., & Laplaze, L. (2013). Use of Frankia and actinorhizal plants for degraded lands reclamation. BioMed Research International, 2013, 948258. https://doi.org/10.1155/2013/948258
Bakum, J. (2022) Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems. International Maize and Wheat Improvement Center (CIMMYT).