Barbara Muesing is our resident City Service Corps member. In this article, she explains the nature of the Nitrogen Cycle, its importance, and how human involvement impacts it both positively and negatively.

You may be surprised to learn that the air we breathe is not mostly made up of oxygen, but of nitrogen gas — about 78%. All living things, from giraffes to humans to decomposers like red wiggler worms and the microorganisms that live in our compost piles under the Queensboro Bridge, need nitrogen to survive. Nitrogen is critically important for our DNA, as well as amino acids, which are the building blocks of proteins.

Unfortunately, our bodies don’t have the energy to take nitrogen directly from the air. Nitrogen gas (N2) has three bonds, which makes separating one nitrogen atom from another very difficult. This process of making nitrogen available for life is called the Nitrogen Cycle. Since the process is so energy intensive, only some abiotic conditions (such as lightning) and certain kinds of microbes are up to the task.

Some of these bacteria, which you may already know from high school biology class, live symbiotically with different types of legumes, including lentils and beans. The bacteria live in the roots of these plants and “fix” the nitrogen, turning N2 into ammonia (NH3), which is a form of nitrogen that plants can readily use. After the plants are able to use the nitrogen for their own growth, it travels up the food chain, eventually ending with humans.

Other microbes, various fungi and prokaryotes (single-celled organisms), replenish the nitrogen in soil by decomposing organic material. In nature, this would go straight into the soil, but in the controlled decomposition of the composting process, we get to choose where those nutrients go, and we try to pick the places that need it most. One important location in NYC is the nutrient-poor soil of street tree beds, where nutrient-rich compost acts as a soil amendment and natural fertilizer.

Finally, the cycle is completed by bacteria that are able to transform nitrogenous compounds back in nitrogen gas, returning it to the atmosphere, and maintaining the balance of nitrogen in the air. This processes called denitrification and anammox (anaerobic ammonia oxidation).

One of the most important inventions of the 20th century was the Haber-Bosch process, which made fixed nitrogen much more widely available. This process allowed people to grow more crops, thus increasing food productivity. As a consequence, it created an enormous boom in population, from just 1.6 Billion in 1900 to now over 7 Billion. Nearly 80% of the nitrogen found in human tissues today originated from this process.

In many regards, the Haber-Bosch process has been a massive success. It has allowed humans to overcome food shortages. Hunger now largely results from inequitable food distribution rather than limitations in growing capacity. However, unabated application of manufactured fertilizers poses serious challenges. One of the issues with this process is that the unused nitrogen in the fertilizer leaches out of the soil into rivers and other bodies of water. This degrades the aquatic habitat and alters the natural ecosystem. One infamous consequence of this nitrogen rich runoff is algal blooms, which sharply decrease the amount of oxygen available in the water that fish and other life need to survive. Some blooms can even be toxic.

The Nitrogen Cycle is one of the most important for life on Earth, and humans have played a significant role in altering it. Fertilizers and fossil fuels have had a huge impact on fixed nitrogen — the type of nitrogen plants are able to utilize. Drastically increasing the amount of available nitrogen, a nutrient that is often the limiting factor of growth in an ecosystem, upsets the nitrogen cycle and can undermine ecosystem health.

Luckily, an alternative solution exists! By working with the natural nitrogen cycle, rather than artificially altering it, judicious application of compost can boost the productivity of soil without pushing the ecosystem out of balance. By dropping off your food scraps with the NYC Compost Project hosted by Big Reuse, so that they can be composted and their nutrients returned to city soils, you are helping to keep the Nitrogen Cycle running smoothly in our urban environment!

For a more in depth look at the Nitrogen Cycle, replete with chemical equations, check out this article from Nature. And for a good overview of human impact on the cycle and ways we can reverse the negative trend, Ensia has a wonderful story.