What is virtual nitrogen?

What is Virtual Nitrogen?

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.