Nitrogen is an essential nutrient required for the production and growth of all plants, vegetation, and living organisms. It makes up 78% of our atmosphere; however, that only accounts for 2% of the Nitrogen on our planet. The remaining 98% can be found within the Earth’s lithosphere; the crust and outer mantel. The Nitrogen found within the nonliving and living fractions of soil represents an unimaginably low fraction of a percentage of all the Nitrogen on our planet. That tiny percent of all total Nitrogen found in our soils is what we can interact with to help or hinder plant production.

To be considered an essential nutrient, an element must satisfy certain criteria:

  1. Plants cannot complete their life cycles without it.
  2. Its role must be specific and defined, with no other element being able to completely substitute for it.
  3. It must be directly involved in the nutrition of the plant, meaning that it is a constituent of a metabolic pathway of an essential enzyme.

In plants, Nitrogen is necessary in the formation of amino acids, nucleic acids (DNA and RNA), proteins, chlorophyll, and coenzymes. Nitrogen gives plants their lush, green color while promoting succulent growth and hastens maturity. When plants do not receive adequate Nitrogen, the leaves and tissues develop chlorosis. However, over-application of Nitrogen can cause even more problems, including delayed maturity, higher disease indigence, lower tolerance to environmental stresses, reduced carbohydrate reserves, and poor root development.

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Chlorotic corn. Image provided by T. Morris, 2018

The Nitrogen Cycle describes the movement of Nitrogen through a landscape. Nitrogen undergoes numerous changes that affect its availability to certain plants and organisms.

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The Nitrogen Cycle. Image provided by T. Morris, 2018

Nitrogen undergoes numerous transformations within a landscape; each transformation represents a distinct chemical reaction or process that acts to further Nitrogen within the cycle. The different transformations are shown in the image provided, but some important ones to keep in mind are Mineralization (organic N -> NH4+), Immobilization (microbial), Denitrification (NO3 to a gaseous form), and Leaching (the loss of dissolved Nitrate into groundwater). There are factors that determine the rates and occurrences of all Nitrogen transformations including pH, temperature, saturation, etc… All of these transformations determine how much Nitrogen is available in your soil for plant uptake. Leaching poses a big problem, when too much Nitrogen is applied via fertilizer, NO3 can be transported in the soil water. Excess leaching can lead to Eutrophication.

Most plants take in Nitrogen as Nitrate, NO3, and Ammonium, NH4+. Generally, Nitrate is absorbed much more than Ammonium, but it is all plant-specific. The combination of both of these forms of Nitrogen can help to improve over-all plant growth when compared to intake of just one. Some plants use symbiotic N2 fixation, where they supply C for fixed Nitrogen from bacteria, actinomycetes, and cyano-bacteria (blue-green algae). This process involves the transformation of N2 to NH3. For instance, Legumes use Rhizobia inside their root nodules to convert N2 to NH4.

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Nitrogen Fixing Nodules. Image from NC State University

Applying the correct amount of Nitrogen is key in reducing leaching, and ensuring your plants are getting the perfect amount for maximum yields. Nitrogen testing proves to be difficult because of the constant transformations it undergoes. Getting your soil tested for other micro and macro nutrients can help provide information on overall soil health, and from there, proper Nitrogen fertilizer recommendations can be made. Talk to anyone from the UConn Soil Nutrient Lab or Home & Garden Education Center for more information on Nitrogen fertilizers and soil testing.

An alarming piece of new research shows decreasing Nitrogen availability with continued global warming. As CO2 levels increase in the atmosphere, essential nutrients are becoming less available to plants. As essential nutrients become less available, forests and ecosystems that usually absorb CO2 would be unable to do so, further increasing the CO2 in the atmosphere. Oligotrophication is the term coined to describe the decreasing productivity of a forest due to the unavailability of Nitrogen. You can read more about this process in the paper “Isotopic evidence for oligotrophication of terrestrial ecosystems” in Nature Ecology & Evolution by Andrew Elmore and David Nelson from the University of Maryland Center for Environmental Science and Joseph Crain of Jonah Ventures.

Joe C.