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FAQ

Nitrogen (N) is one of the building blocks of life: it is essential for all plants and animals to survive. Nitrogen (N2) makes up almost 80% of our atmosphere, but it is an unreactive form that is not accessible to us.

Humans and most other species on earth require nitrogen in a “fixed,” reactive form. Reactive nitrogen is necessasry for the food production process. Until the discovery of the Haber-Bosch process in the early 1900s, we only had access to naturally occurring sources of reactive nitrogen (such as manure and guano) for food production. The Haber-Bosch process--an industrial process through which we can fix reactive nitrogen--has allowed food production to keep up with the growing human population, but at a cost to the environment.

Today, humans create over 2 times as much reactive nitrogen as nature. In contrast, human activity contributes just 5-10% of CO2  emissions. Much of this reactive nitrogen has accummulated in the environment, where it causes a series of negative impacts to human and ecossytem health. Major sources of this reactive nitrogen include agriculture and the burning of fossil fuels. This nitrogen pollution causes profound environmental impacts, including smog, acid rain, forest dieback, coastal ‘dead zones’, biodiversity loss, stratospheric ozone depletion and increased greenhouse gases. It also affects human health, including respiratory disease and an increased risk for birth defects.

Reactive nitrogen (Nr) includes all forms of nitrogen that are biologically, photochemically, and radiatively active.  Compounds of nitrogen that are reactive include the following: nitrous oxide (N2 O), nitrate (NO3 -), nitrite (NO2 -), ammonia (NH3), and ammonium (NH4 +). Reactive forms of nitrogen are those capable of cascading through the environment and causing an impact through smog, acid rain, biodiversity loss, etc. 

The non-reactive form of nitrogen is N2  and makes up about 80% of our atmosphere.  This form of nitrogen does not contribute to the environmental impacts noted above. 

Unless otherwise noted, the term “nitrogen” or “N” on the N-Print website refers to “reactive nitrogen” – not its non-reactive diatomic form.

What is Virtual Nitrogen?

(Leach et al., 2012)Nitrogen is an important nutrient for food production. Nitrogen can be applied to a field in a variety of forms such as synthetic fertilizer, manure, crop residue (green manure), or nitrogen fixed by legumes. Only some of the applied nitrogen is taken up by the plant--the rest remains in the soil or is lost to the environment through runoff and leaching to the waterways or volatilization to the atmosphere. Some additional nitrogen is lost during processing; only a fraction of the original nitrogen applied actually makes it into the final food product (embodied N). Virtual nitrogen is the rest: virtual nitrogen is any nitrogen that was used in the food production process but is not contained in the final food product that you consume. Stated another way, virtual nitrogen is the nitrogen lost to the environment throughout the food production process.

Virtual nitrogen factors are calculated to represent the entire food production process. These virtual nitrogen factors (VNF) consider N lost during fertilizer application, crop processing, food waste, and more. Once all of these inputs and losses are tallied, the nitrogen that you consume in your food can be subtracted out--the remaining amount is virtual nitrogen. The virtual N factor calculation figure (Leach et al., 2012) conceputalizes the flow of 100 units of new nitrogen through conventional corn production in the United States. The losses on the bottom of the figure circled in red are the total virtual N associated with conventional corn production. The consumed N (circled in red on the right of the figure) is the nitrogen actually consumed by an individual; this is the only part of the applied nitrogen that is not lost to the environment or recycled throughout the process. The virtual N factor is then calculated by dividing the total N losses by the N consumed, yielding a VNF in units (kg N lost) / (kg N consumed). It should be noted that the virtual N factors are calculated over several crop cycles. The dotted arrows represent nitrogen recycled back for future crop cycles, such as crop residue used as fertilizer.

 

The figure to the right (adapted from Galloway et al., 2003 and Galloway et al., 2008) shows how global anthropogenic (or human-caused) reactive nitrogen creation rates have changed over time. The major creation pathways are fossil fuel combustion, legume cultivation, and the Haber Bosch process. Until the 1950s, global reactive N creation rates remained relatively constant. In 1910, the Haber Bosch process was developed by German scientists Fritz Haber and Carl Bosch to convert N2 from the atmosphere into NH3. The Haber Bosch proces was originally used to manufacture munitions. However in the 1950s, we began producing synthetic fertilizer through this process. The Haber Bosch process is responsible for sustaining about 30% of the world's population today (Erisman et al. 2008)--which is apparent in the figure as population grows along with the rate of Haber Bosch N fixation. N creation from the cultivation of legumes and from fossil fuel combustion have only increased slightly since the mid-1800s. Increases in N production over time lead to larger losses of nitrogen pollution to the environment. Different food products demand different amounts of nitrogen fertilizer inputs and have varying nitrogen use-efficiencies, which determine how much of the nitrogen is released into the environment. To learn about how reactive N is created and its adverse effects in the environment, see the background page.

Most municipal sewer systems in the United States have primary or secondary treatment, which filter the human waste and break down its biological content.  However, this level of treatment does not remove most of the reactive nitrogen. Only about 5% of homes in the US are attached to municipal sewer systems with tertiary treatment, which further break down waste to a non-reactive form (N2) that does not harm the environment.

Knowing the level of sewage treatment at your local sewage treatment plant is very important because it can impact your N footprint by about 10%.  To find out if you have tertiary sewage treatment, contact your local municipal sewage treatment plant.

While some elements like electricity production are out of your control, there are many things that you can do to reduce your nitrogen footprint. Here are a few, stratified by food and energy – aspects of the N footprint:

Food

  • Alter your diet: try to choose foods produced on more sustainable farms. These use practices that reduce their amount of nitrogen fertilizer runoff into the water and air. If they raise livestock, they take care to manage their manure so it doesn’t pollute.
  • A protein-heavy diet is nitrogen-intensive. The Dietary Reference Intake (DRI) recommendation is 0.36 grams of protein per pound of body weight per day for adults (or 0.8 g protein per kg body weight).  See page 4 of this document from the Institute of Medicine for more information. If you eat more than your recommended protein intake, try replacing some protein with other types of food like fruits, vegetables and grains.

Energy

  • Limit your household utility usage by using more efficient appliances.
  • Choose more sustainable transportation methods like carpooling, public transit, bicycling, or walking.
  • Reduce your consumption of goods and services by limiting unnecessary purchases, reusing, and recycling

It is important to note that doing many of these things even just one day a week can make a big difference over time.  Most of these lifestyle changes will reduce not only your nitrogen footprint, but also your carbon footprint and your ecological footprint.  These changes will generally lead you to live a more sustainable life.

We already know how to reduce our nitrogen footprint: use less nitrogen fertilizer, eat fewer energy-intensive foods, and try to minimize fuel-heavy forms of transportation, like airplane travel. These suggestions aren’t unique to reducing nitrogen pollution– many people trying to create more healthy communities are calling for these changes. You can go one step beyond your personal lifestyle choices and join others in calling for:

  • More renewable electricity choices from your local utility, such as wind and solar power
  • Increasing government support for sustainable farming practices

For instance, there are current discussions in the US Congress about conservation measures within the national Farm Bill that encourage farmers to grow food more sustainably.  Let your local Representative know you support these measures.