6.3: Nitrogen Cycling

The Elusive Case of Nitrogen

You have read a little in the previous sections about the cation exchange capacity of soil and how the nutrients K, Ca, and Mg are made available to the plant. Now we explore how the anion nutrients, especially Nitrogen (N), behave in soil and how the plant can access them. Rather than “exchanging” the major anions N, P, and S are considered to “cycle” through the soil in various forms.

Nitrogen (N)

Nitrogen is not very abundant in soil but it is a relatively large component of the plant body. Plants remove nitrogen from the soil and concentrate it in their tissues. Nitrogen needs to be replenished whenever annual crops (except legumes) are grown because they remove nitrogen from the system.

What role does nitrogen play in the plant? The presence of nitrogen is the defining characteristic of all proteins. You’ve probably heard of amino acids, these are simply some basic nitrogen-containing molecules that are the building blocks of even more complex molecules called proteins. This is why nitrogen is in such demand by both plants and animals. It is the foundation of all proteins, and thus all life. It is not synthesized from the air (except by bacteria in the nodules of legume plants) or water. Nitrogen is usually not a limiting factor in normal plant growth; but it is often limited in the high production and yield goals of modern agriculture, even sustainable agriculture.

Besides nitrogen, there are two other anionic macronutrients plants require, phosphorus and sulfur. What is the relationship between the presence (or absence) of these anionic elements in the soil and their availability for the plant as nutrients? The availability is considered in the context of how they cycle from one form to another. Nutrient cycling of anions is complex compared with the availability of the cations which simply “exchange”. We’ll take a look at the nitrogen cycle to see all the forms this nutrient can take and just how complex it is to find out if plants have enough. After that, we’ll review the implications of the cycle for nitrogen fertilizer and farming.

Nitrogen has a lot of options when it comes to forming compounds. It is simply looking for ways to feel stable and balance out that extra electron circling outer orbit. Chemically, relationship-wise, it’s looking to be completed.

Table \(\PageIndex{1}\): Nitrogen Compounds

Shares electron(s) with:	Results in:	We call it:	Comments
Another N	N2	Nitrogen gas	It’s in the air. It makes up 78% of our atmosphere! Super stable triple bond takes energy to break.
Two oxygens	NO2	Nitrous oxide	Potent “greenhouse” gas
Three oxygens	NO3^-	Nitrate – a 'slippery' molecule	If it is not taken up by plant roots it either leaves the soil as NO2 gas or through leaching.
Four H’s	NH4^+	ammonium	Yummy form plants can take up via roots
Three H’s	NH3	ammonia	An intermediate form on the way to NH4^+

The nitrogen cycle is a picture of the different ways and places that nitrogen exists.

Figure \(\PageIndex{1}\): The Nitrogen Cycle. Design: V. Dantoin

The blue arrows above indicate ways that inert atmospheric nitrogen N₂ enters the biological system. The red arrows indicate ways that biologically available nitrogen leaves the plant/soil system.

Definitions in the Nitrogen Cycle:

Fixation (Getting N into the soil)

The process of converting atmospheric nitrogen (N₂) to forms usable by plants. How? Three ways: a) nodules form with micro-organisms and roots in legumes b) lightening or volcanic eruptions provide enough energy to bind molecules of nitrogen together with oxygen or c) industrial manufacturing process which results in fertilizer. To “fix” nitrogen requires energy and that is why it’s an expensive fertilizer. At any one time, 99% of the soil nitrogen is in this fixed, organic form. Fixed nitrogen means it is stuck in, or part of the bodies of soil micro-organisms.

De-nitrification (Losing N from the soil)

Fixed soil nitrogen converts back to \(\ce{N2}\). There are a variety of ways denitrification occurs. Both have to do with water. Rainwater can leach the “slippery” nitrate molecules out of the soil and into the groundwater. Nitrogen can also volatilize - becomes a gas; it leaves the soil and re-enters the atmosphere as inert \(\ce{N2}\) or as greenhouse gas \(\ce{NO2}\). These two processes, fixation and denitrification are always in balance. More sub-cycles occurring in the soil are outlined below:

Mineralization

Fixed, organic nitrogen becomes available nitrogen to the plant as Ammonium (\(\ce{NH4^{+}}\)), this is mineralization. About 2-3% of organic nitrogen becomes mineralized in a year. One complete “turn over” of soil nitrogen occurs then every 30-50 years. Mineralization is also called ammonification because it first becomes ammonia (\(\ce{NH3}\)) which then continues its’ transformation into ammonium which plants use. Soils high in organic matter can contribute a lot of nitrogen to plants through mineralization.

Nitrification

This is the process by which NH₄⁺ produces available nitrogen as the nitrate (NO₃^-) anion. Nitrification results in the production of H⁺ ions and can increase soil acidity (lowers pH). Nitrates are stable in well-aerated soil but leach easily out of wet soil because they combine with H₂O and run away together.

Uptake or Immobilization

Plants take up both NH₄⁺ and NO₃^- through their roots and incorporate them into their bodies. The nitrogen becomes immobile because it is “stuck” in the body of a plant. This is different than mineralization where N from the atmosphere becomes embodied in soil organisms.

Natural Nitrogen Fixation

Most people don’t realize that we are surrounded by an atmosphere full of nitrogen. The air we breathe is 78% nitrogen gas, N_2. This nitrogen gas is very inert because it is triple-bonded to itself. It (N₂) doesn’t want to transform into a biologically available form of nitrogen. If only we could get all that nitrogen into something we can use to grow plants then life would be wonderful, right? How does N₂ from the air become bio-available? Well, in a natural system, nitrogen is fixed in one of two ways. One way it is fixed is by the incredible, tiny, fantastic little bacteria (Rhizobia) associated with root nodules of plants in the legume (and other) families as shown in the image below. There are also some other kinds of free-living bacteria that fix nitrogen. A second (minor) way N enters the biosphere is with a massive injection of lightning energy, or volcanoes. The heat energy in lightning causes the N to combine with oxygen to form compounds that fall to the earth in the rain.

Figure \(\PageIndex{2}\): Roots of soybean plant with root nodules. The interior of the nodule is red showing the nitrogen that is fixed by bacteria. Copyright: Fabian von Beesten, www.gartensoja.de. CC BY 4.0

Synthetic Nitrogen Fixation

The third way nitrogen gets fixed is synthetically, by humans. We use energy from fossil fuels to power the reaction. Here is a short explanation from the website “How Stuff Works”. “In this process, heated nitrogen (from the air) and hydrogen are mixed under very high pressure in a vessel where they combine chemically. The vessel contains a catalyst (usually iron with oxides of aluminum and potassium), which speeds up the chemical reaction. The Haber process is the most widely used process for the commercial production of ammonia. Fritz Haber, a German chemist, developed the process in the first decade of the 20th century. Karl Bosch, another German scientist, adapted the process for industrial use.” This is the Haber-Bosch process. As a side note, Fritz Haber was also a nazi war criminal - his nitrates were used in bomb-making. Is the Haber-Bosch process a blessing (feeds a lot of people) or a curse (significant degradation of the environment) because it allows nitrogen to break free from its natural moorings and be added anywhere fossil fuel manufacturing can make it available?

Demand by plants (to build proteins) is why synthetic nitrogen fertilizers are the most heavily used type of fertilizer in conventional agriculture. Because plants (and resulting animal milk and meat) are removed from a farm by harvest, nitrogen is removed and must be replaced. Synthetic nitrogen is quite expensive and energy-intensive to manufacture. It is an “unnatural” addition to a plant-soil-atmosphere system that was balanced at one time. What we’ve done is take a stable N₂ atmospheric molecule, break it apart, feed some of it to plants as fertilizer, and let the rest escape the soil cycle as either nitrous oxide greenhouse gas or nitrates leached into our waterways.

Industrially fixed nitrogen is something new on the earth. We’ve doubled the amount of fixed nitrogen previously in the natural cycle taking it from a stable N2 form. Unstable soil nitrogen escapes as a reactive greenhouse gas. We now add over 10 million tons of NO₂ to the atmosphere annually. As non-organic corn production rises so will our synthetic N production and its transformation and escape into the atmosphere and into our groundwater. Humans have become an important factor in the nitrogen cycle.

It is estimated that up to half of the nitrogen conventional farmers apply to their corn crops is lost from the system by leaching or denitrification. In high corn yield areas, it is not uncommon for farmers to add 200 lbs of synthetic nitrogen per acre to a corn field. They add it at somewhat lower or higher levels according to the price of the N fertilizer. Nitrogen fertilizer is relatively cheap because of the fracking boom in natural gas and the cheap energy costs. But if 200/lb/A is added, and only half of that is actually used by the crop, the other 100 lbs/A runs out of the system and into the environment. This is a cost externalized to all of us.

Organic Farming and the Nitrogen Cycle

Organic farms are not allowed to use synthetically produced sources of nitrogen like urea or synthetic ammonium. In fact, this restriction is probably what prevents a majority of regular crop farmers from becoming certified organic. They just can’t imagine cropping without synthetic N.

So how do organic farms get nitrogen into the soil? What are the organic options for nitrogen? There are organic fertilizers that can be purchased, but most organic farmers choose to “grow their own” nitrogen.

A “per acre furrow slice” of soil contains about 1000 lbs. of nitrogen for each 1% of organic matter, but as we have learned, most is in the fixed form. There will be about 20-30 pounds of nitrogen mineralized each year for each 1% organic matter in the soil. A 150 bushels of corn/A contains 235 pounds of nitrogen incorporated into plant tissue itself. The natural sources of nitrogen in many conventional soils is small compared to the needs of industrial corn. Where will the organic farmer get the 200 lbs. of nitrogen needed to grow 150 bushels of corn?

High Amounts of Organic Matter

Soils with high amounts of organic matter may supply almost enough nitrogen for a high-yielding corn crop. For example, soil with organic matter of 3.5 percent may contain 3,500 lbs. of Nitrogen per acre. It will mineralize or make available about 105 pounds of nitrogen per acre. Organic farmers seek to increase soil organic matter as a means of increasing the nitrogen available to their plants.

Mineralization Rates

The nitrogen-supplying power of soil is intimately related to the organic matter and mineralization rates of soil.

Organic farmers tend to supply most of their nitrogen needs by using biological processes rather than synthetic processes. Organic farmers “grow their own” nitrogen in a variety of ways; animal manures, green manures, cultivating the mini-livestock in the soil, the nitrogen-fixing bacteria and the worms, and soil biology. The bottom line, organic farmers grow nitrogen by harvesting it from the atmosphere rather than purchasing nitrogen when possible.

Also, organic farms lose very little nitrogen from the system or soil cycle due to denitrification, leaching, and volatilization. Organic nitrogen is slowly released and steady. It is not lost from the system and is therefore available to the plant.