What is reactive N and why should I care?

Nitrogen (N) is an important component for all plant and animal growth, essential for the development of proteins, and important in other living functions.

In our environment, N exists in many forms. In its inert gaseous state, N2, it is very stable and makes up over 70 percent of the earth’s atmosphere but is unavailable for plant and animal growth. The many other forms of N in the environment, including ammonia (NH3), ammonium (NH4), nitrite (NO2), nitrate (NO3), nitric oxide (NO), and nitrous oxide (N2O) are collectively referred to as reactive N or Nr.

These forms of N are all interrelated and are constantly in flux in the environment. Through the conversion of N2 to NH3 and NH4 (ammonia and ammonium), N becomes available for life uses. This conversion of N2 to its ammonia forms takes place under two natural processes:

• N2 is converted to NH4 by bacteria living in the nodules of legume plants (clovers, alfalfa, beans, peas); and

• A burst of energy from a bolt of lightning converts N2 to NH4.

In the early 20th century, the advent of the Harber (or Harber-Bosch) process for converting N2 to NH3 was commercially adopted and the manufacture of commercial N fertilizers began. Each process requires significant inputs, either bacterial or energy, to complete the conversion.

Increased human activity, including burning fossil fuels, clearing land, and manufacturing commercial N fertilizers, has made significant contributions to the Nr in the environment. The net effect has been a significant increase in the reactive forms of N in our environment. Since the 1960s, N use has increased over 300 percent.

What’s the concern?

This increased release and conversion of N has positively impacted all of our daily lives. The nitrogen used in agriculture has made it possible for farmers to meet the worldwide demands of a growing population’s daily needs for food, fiber and shelter for instance. But the increasing levels of reactive N are leading to environmental concerns.

Specific examples of problems with excess Nr in the environment include:

• Larger and more frequent algae blooms in lakes and coastal areas that receive waters from N- enriched rivers;

• High levels of nitrates (NO3) in ground water have been directly linked to increased cases of “blue baby syndrome,” a health concern in infants under six months old and issues for the elderly;

• Nitric oxide (NO) and nitrogen dioxide (NO2), collectively referred to as NOx, contribute to smog and haze and have been linked with asthma in children and adults;

• Nitrous oxide (N2O) is a greenhouse gas that also contributes to “acid rain”; and

• Ammonia (NH3) deposition has been associated with the undesirable changes in forest growth.

What is ag’s role?

Reactive nitrogen in the environment is an emerging issue. Agriculture is considered a major source contributor of excess Nr and therefore the industry will be looked to for solutions. When one considers the increasing cost of N inputs, contributing to the solution may actually be a win-win situation for the producer, agriculture and the environment.

Practices that may reduce Nr associated with agricultural production include:

• Apply the N source close to the time of crop uptake.

• Avoid surface-applied N fertilizers, especially on high pH soils and hot/dry conditions.

• Avoid nitrogen fertilizer applications in the fall for next season’s crops.

• Take all reasonable N credits from past cropping practices, reducing purchased N accordingly.

• Increase use of cover crops to maintain nutrients in the root zone during the non-cropping season.

• Reduce ammonia losses from manure storage.

• Explore new technologies that hold and retain N in the soil.

• Increase use of conservation measures to reduce topsoil losses and retain nutrients in the root zone.

Because of agriculture’s dependence on nitrogen for continued yield improvements, it is an issue the industry will want to monitor as the impact of Nr in our environment is explored. — Drs. Gerald May and Natalie Rector, Michigan State University Extension

[Editor’s note: This is a shortened version of an Extension article published in 2011. Edits, including the removal of inline citations, were made due to space restrictions. Despite its age, it was edited and included as an explanatory companion piece to “Sustainability: Not a fad”]

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