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.