As a kid I remember being impressed by a story of several people adrift in a lifeboat in the open ocean. After a few days all their fresh water ran out and, in what is surely an horrific irony, they found themselves thirsty while surrounded by water.
In some ways this lifeboat predicament is akin to the situation of plants. The atmosphere is nearly 80% molecular nitrogen, and yet nitrogen is considered one of the most limiting nutrients in terrestrial ecosystems. How so?
An old oak in a savanna beneath a blue winter sky. The grey-green on branch tips are lichens and the emerald green of interior branches are mostly mosses. Many lichens and some mosses associate with bacteria that fix nitrogen from the air.
It comes down to a basic chemistry problem. Nitrogen as a free gas is actually a strongly bonded pair of nitrogen molecules that doesn’t react readily with anything. It is perfectly happy to be itself, thank you. But life needs nitrogen to make amino acids, which are the building block of proteins. So some very nifty biochemistry has evolved to pull nitrogen out of the air and turn it into a form that living beings can use. This process is called “nitrogen fixation” and certain groups of bacteria are very good at it.
A well known nitrogen fixation pathway is the symbiosis between legume plants (Fabaceae) and species of Rhizobia bacteria. Legumes provide the bacteria with a home and some food within root “nodules” and the bacteria make water soluble forms of nitrogen that the plant can use to grow. A rough rule of thumb in farming is that one good stand of a legume cover crop (meaning it is not allowed to go to seed) is good for a couple good stands of a grain crop such as wheat.
A clover is a kind of legume that associates with bacteria to fix nitrogen. This is how farmers get nitrogen naturally.
Although the legumes and Rhizobia have the most well known symbiosis of this kind, many plants have similar relationships. I was reminded of this recently while walking through a woodland. The understory of a forest is not the kind of place you find dense populations of legumes, so if you want to see where the nitrogen is coming from you need to first look up in the trees.
The leaves have fallen, but the branches are covered with lichens and mosses, many of which make homes for bacteria that fix nitrogen. The winter is their season, as the light is not being captured by tree leaves and the humidity is high. Littering the ground are lichen-laden fallen twigs and scraps from this canopy growth. As these decompose the nitrogen enters the mat of soil humus and eventually makes its way into the massive hulk of the old oaks.
A leafy lichen found on the forest floor, perhaps in the genus Lobaria, brings nitrogen into the ecosystem.
If your curiosity about the nitrogen cycle and agriculture is piqued, I recommend following this series of investigative articles. My position is that nitrogen is abundant and practically free when farms are managed using sound agroecological principals. Know your plants, know your microbes, know your fungal networks, and know how to work with these relationships and the ecosystem will provide.