The Ecology of Nitrogen Fixing Plants

Nitrogen is a key nutrient in life; it is one of the elements in amino acids, and thus occurs in proteins and is an essential part of all living organisms. Although nitrogen is abundant in the Earth's atmosphere (making up about 78% of the atmosphere), atmospheric nitrogen is extremely stable, owing to the triple bond between its atoms. In order to be used by living organisms, nitrogen must be processed into forms usable in biological processes, through a process called fixation.

The process of nitrogen fixation

Nitrogen fixers are plants that have evolved the ability to fix nitrogen, through a symbiotic relationship with anaerobic bacteria that live in special nodules on the plant's roots. The atmospheric nitrogen is processed into ammonia, which is then able to be used as a building block in larger molecules like amino acids or nucleic acids like DNA and RNA, which in turn build proteins and make life possible.

The bacteria that fix nitrogen can only carry out their process in the absence of oxygen. Because nitrogen and oxygen occur together in the atmosphere, these organisms have evolved mechanisms to create an oxygen-poor environment, such as using up the remaining oxygen via respiration or using special proteins to bind to the oxygen. Nitrogen fixing plants create a space in their root system in which these bacteria can thrive.

The competitive advantage of nitrogen-fixing plants

In environments where nitrogen is a limiting factor in plant growth, nitrogen-fixing plants have a huge advantage, as they have access to an essentially unlimited supply of nitrogen. This allows them to benefit from rapid growth, quickly and thoroughly out-competing neighboring plants that lack the ability to tap into the boundless reserves of nitrogen in the air.

Nitrogen fixing plants thus have an advantage in these environments, which include disturbed ecosystems, as well as arid desert ecosystems, barren rocky environments, or environments with sandy soil that does not hold nutrients well.

In nutrient-rich environments, where factors like light are more a limiting resource than nitrogen, nitrogen fixers often have a distinct disadvantage, because they have invested resources and growth into their root nodules and are not receiving many benefits from them.

Applicability for human uses

Humans can benefit from nitrogen-fixing plants in gardening and agriculture, especially when the continual harvest of material takes nitrogen out of the ecosystem. Crop rotation, the intermingling of crops, or using cover crops or nitrogen-fixing trees spaced in between rows of crops, are all ways in which gardening, farming, or agriculture can benefit from these plants. These plants can also be used in ecological restoration when dealing with degraded lands that have been stripped of soil.