After dropping out of the corporate world, I started taking science classes at Arizona State University, hoping to figure out what to do next. At the time I had some vague idea of studying conservation biology in order to work with endangered species. One of the first classes I took was undergraduate ecology. The professor was old school; his lecture notes were in notebooks that were yellowed with age and he wrote on the chalkboard. Definitely no PowerPoint. Nonetheless, the professor's lecture about the nitrogen cycle significantly influenced my new career path.
So what is the big deal about nitrogen?
First, all life needs nitrogen because it is a key component of proteins and DNA. Our bodies have 1 atom of nitrogen for every 13 atoms of carbon. Second, the most common form of nitrogen on Earth is N2 gas, which is about 80% of the atmosphere. We breathe it in and out, but cannot use it in that form.
How do we get nitrogen?
From our food. This leads to the question, how does nitrogen get into our food. And the answer is interesting.
During that ecology class, I first realized that microorganisms play a huge role in the environment; they go about their business whether or not humans are around. A select group of microorganisms take atmospheric N2 gas and “fix” it into usable forms. These microbes include some types of cyanobacteria (also known for their toxic blooms in lakes and coastal areas) and other free-living bacteria. Nitrogen-fixing bacteria have also teamed up with leguminous plants (e.g. soy, alfalfa, clover, peanuts). These plants house bacteria in specialized root nodules and give them carbon for energy. In return, the bacteria provide nitrogen that enables the plants to grow.
When plants and other living organisms die, the process of decomposition releases nitrogen back into the environment where it becomes available for plants and algae that cannot fix their own nitrogen. Nitrogen is a critical component of fertilizers used in crop production.
The part of the nitrogen cycle that really caught my attention was denitrification where, in the absence of oxygen, bacteria convert nitrate (NO3-) to nitrite (NO2-) to nitric oxide (NO) to nitrous oxide (N2O) to N2 gas. Denitrification interested me because it completes the nitrogen cycle by returning fixed nitrogen to the atmosphere. But it is also interested me because not all nitrate ends up as N2 during denitrification. Some escapes as nitrous oxide, a gas that is 300 times more potent than carbon dioxide as a greenhouse gas. Nitrous oxide emitted today stays in the atmosphere for about 100 years. (This is the same nitrous oxide you may remember from the dentist.)
Denitrification occurs in soils and sediments. It has even been observed in the digestive tracks of insects. It is a process that frustrates farmers, who apply fertilizers to crops only to have some of it denitrified. But denitrification can also remove excess human-related nitrogen from the environment, mitigating potential negative consequences.
My first experience with scientific research was in the laboratory of Dr. Nitrogen. I did field experiments around Phoenix, Arizona to understand how atmospherically deposited N affected microbial communities in desert soils. It was fascinating to me that nitrogen from agriculture and fossil fuel combustion could volatilize into the atmosphere, be transported long distances (even to remote areas), and affect downwind ecosystems. For my doctoral studies, I joined the laboratory of Dr. Limnology. Keeping the nitrogen theme, I studied denitrification rates and nitrous oxide production in sediments of lakes receiving either high or low inputs of atmospheric nitrogen deposition.
Simplified Nitrogen Cycle
Simplified diagram of the nitrogen cycle
The ability to "fix" nitrogen changed agriculture
Humans developed the ability to fix nitrogen through an industrial process, named “Haber-Bosch” after Fritz Haber and Carl Bosch. Haber developed a method to synthesize small amounts of ammonia in the laboratory. Bosch scaled up Haber’s approach in order to produce ammonia industrially. It is an understatement to say that Haber-Bosch has revolutionized the fertilizer industries and agricultural systems globally. We all depend on Haber-Bosch for the food we eat. But industrial nitrogen fixation has dramatically changed the amount of nitrogen that circulates between land and the atmosphere. Nitrogen leaks from the agricultural systems into the environment, where it has unintended consequences. Excess nitrogen alters the composition of plant and animal species, is a component of acid rain, and contributes to harmful or nuisance algal blooms in aquatic ecosystems. There are negative human health effects as well. Nitrate can accumulate in groundwater drinking wells and be harmful at high concentrations (50 mg/L). Ammonia released by intense animal production contributes to breathing problems.
I still work with nitrogen today, but my work has moved from the field to indoors. A big “aha” moment was when I realized that it was possible to make budgets for nitrogen in the same way that I made financial budgets in my previous career. I use computer models and large datasets to quantify the amount of nitrogen moving from land to surface waters and understand its effects.
Maybe I am a little obsessed
I have a necklace, coffee mug, and tote bag that bear the periodic table symbol for nitrogen. I use nitrogen in my Twitter handle. I collect science books about nitrogen. I even asked a poet to write about nitrogen. My only frustration is that I haven't found any jokes about nitrogen.