The nitrogen cycle: Explaining where your lost nitrogen is going

With the weather of the spring season, losing nitrogen from our farm fields isn’t just a worry, but rather an unwelcome reality. However, using the word “lost” to describe where your nitrogen (N) has gone isn’t really a good description of what is happening to the N nutrients in your soil. The nitrogen cycle can help provide some perspective to better understand where exactly your N is “lost” to.

While nitrogen is deposited into the soil by a number of different pathways including nitrogen fertilizer, nitrogen fixation by leguminous crops, manure additions, and crop residues, there are fewer ways that nitrogen can be lost from the soil, like leaching, denitrification and volatilization. Regardless, nitrogen can be easily lost from the soil and be unavailable for crop growth. The nitrogen cycle displayed in the included diagram shows how these pathways are connected through a series of biological and chemical reactions.

Of the three loss pathways, leaching and denitrification are the top two concerns this spring. In both cases, N in the form of nitrate – the form most taken up by plants – is most susceptible to loss. Leaching is the loss of nitrate N as water drains through the soil profile, moving out of the range of plant rooting systems. Denitrification, on the other hand, is the conversion of nitrate N to unavailable atmospheric N by soil bacteria in low-aerated, water-logged soils. Both leaching and denitrification result in a decreased concentration of plant available nitrate N within the soil and both of these losses are increased with excessive rainfall.

The characteristics of nitrogen and the nitrogen cycle make it hard to do an accurate in-season estimate of N loss regardless of loss pathway. Scouting fields for chlorosis, or yellowing of older leaves, will give indication if your crop is in need of more nitrogen that can then be applied via sidedress application.

For more information about how N is lost or about the nitrogen cycle, click here.

By Emily Sneller, Michigan State University Extension