Famine and Feast: Recent Changes to the Global Nitrogen Cycle

The global human population exploded from 1.5 to over 7 billion during the last century. By the middle of the century, the need for increased of crop production was urgent order to meet the growing demand for food. The invention of synthetic nitrogen fertilizer, coined the ‘Green Revolution’ achieved this goal. Crop yields, or the amount of food produced on a given piece of land, increased by 30% globally, saving many people from starvation. So much nitrogen has been added to the fields since then, however, that much has leaked out into streams, lakes, and estuaries and caused major pollution problems that jeopardize human health and the economic value of fisheries. After drastically altering the global nitrogen cycle to relieve a shortage, society is now grappling with an overabundance. My own research seeks to understand how to remove excess nitrogen from watersheds and remediate the issues related to over-fertilization. 

Nitrogen is the most abundant element in Earth’s atmosphere, yet millions of dollars are spent manufacturing nitrogen-based fertilizer every year. What drives this lucrative industry? The answer is the form of nitrogen in the atmosphere. Nearly 80% of the atmosphere is composed of pure nitrogen gas, which is un-useable to most plants. Harnessing nitrogen from the sky requires a lot of energy. The Haber-Bosh process uses man-made energy (oil, gas, nuclear power) to add hydrogen atoms to nitrogen gas, creating massive amounts of ammonia. Some of this ammonia may end up in household cleaning products, but most is used as fertilizer.

Of course, humans did not invent this process. Several different types of bacteria and algae have special enzymes that allow them to harvest nitrogen from the sky at low temperatures, without the excess energy that the Haber-Bosh process requires. A small number of plants (mostly peanuts and beans, and some common flowers) formed alliances with these bacteria, allowing them to live in tiny holes in their roots. These plants enjoy the advantage of abundant nitrogen, and microbes accept a safe place to live and a small share of the sugary products of photosynthesis in return.

The majority of plants are not so lucky to cohabitate with nitrogen harvesting bacteria. Instead, nitrogen is only available as a recycled organic matter – broken down leaves, wood, or animal scat. For the entire history of plants the small number of organisms controlled the abundance of bio-available nitrogen in the world, and whole ecosystems organized around the principle of recycling. Agricultural fields played by the same rules, and farmers had to carefully recycle crop and animal waste in order conserve nutrients in the soil.  

Haber-Bosh tipped this natural balance. While this invention revolutionized agriculture and increased crop yields around the world, many natural, non-agricultural ecosystems were fertilized in the process. Aquatic ecosystems, especially estuaries, are particularly sensitive to fertilization and respond with a flurry of algae growth. Fertilized algae in an estuary grow like weeds in a garden, choking out other life forms such as fish, mollusks, and even crabs. This phenomenon, known as a Dead Zone, happens regularly in many coastal zones today.

The Haber Bosh process has doubled the rate of nitrogen fixation from atmosphere to land. This excess has shown up in water bodies and caused major ecological problems. 

Dead zones present a major challenge for managing chemical cycles globally. Human activities have doubled the rate of nitrogen fixation since 1950, and synthetic fertilizer is still widely applied to agricultural fields, despite widespread impacts on ecosystems. Unfortunately there is no mechanized process to convert ammonia back to nitrogen gas. Certain microbes can convert ammonium back into nitrogen gas. This process is very slow compared to the Haber-Bosh process, though. Significant progress has been made to mitigate nitrogen pollution in certain critical regions, however. In the past twenty years, efforts to curb agricultural fertilizer applications have reduced pollution. Scientists such as myself have dedicated entire careers to studying the bacterial communities that remove nitrogen from waterways. One way to remove nitrogen is to create or restore wetlands where these bacteria live and provide the conditions necessary for rapid nitrogen removal. Widespread implementation of different ecosystem restoration practices has potentially curbed the load of nitrogen entering the Chesapeake Bay, for instance.