[Agronomy article] Why Crops Cannot Directly Utilize Atmospheric Nitrogen (feat. Nitrogen Cycling in Soil)?
Crops utilize carbon dioxide (CO2) from the atmosphere to undergo photosynthesis. In other words, photosynthesis is the process in which crops, within their chloroplasts, utilize light energy to synthesize glucose (C6H12O6) from carbon dioxide (CO2) and water (H2O).
Photosynthesis occurs in the chloroplasts located in the leaves of crops, where the Calvin cycle takes place, resulting in the production of glucose. The substance generated in this process is commonly referred to as photo assimilates or assimilates. These assimilates are temporarily stored either as sucrose in the mesophyll vacuole or as starch in the chloroplast stroma. The process of converting these assimilates into sucrose or starch is defined as carbon allocation.
The photosynthetic end-products, starch and sucrose, will be transported to different organs of the crops to be used as the primary energy source. This process is known as carbon partitioning.
I have a curiosity here. Crops utilize carbon dioxide from the air efficiently, but why can’t crops utilize nitrogen directly from the air? If crops could use nitrogen just like they use carbon dioxide, there would be no need for the hassle of applying nitrogen fertilizers. It makes me wonder why crops, despite nitrogen making up 78% of the atmosphere, cannot utilize it and instead only absorb the minute amount of carbon dioxide from the air.
The form of nitrogen present in the atmosphere is N2, where two nitrogen atoms are combined. So, why can’t crops directly utilize this N2 just like they do with carbon dioxide?
Today, let’s explore some basic information about nitrogen.
Story about Nitrogen
Nitrogen is a non-metal chemical element with an atomic number of 7.
The fact that the atomic number of nitrogen is 7 means that it also has 7 protons.
Protons are positively charged particles that make up the atomic nucleus. The number of protons in an atom is equal to its atomic number.
Atoms are electrically neutral, meaning that typically the number of protons and electrons in an atom is the same. Therefore, nitrogen would have seven electrons orbiting its atomic nucleus.
The electron configuration of nitrogen is represented as 1s2 2s2 2p3. This means that in the first electron shell of nitrogen, there are two electrons, and in the second electron shell, there are five electrons. The electrons in the second electron shell would occupy both the s orbital (spherical shape) and the p orbitals (dumbbell shape).
Thus, a simplified representation of nitrogen’s atom and electron configuration would be as follows:
In the atmosphere, nitrogen exists in the form of N2, where two nitrogen atoms are combined. It was mentioned that nitrogen has 5 electrons in its outermost electron shell. When two nitrogen atoms bond, three electrons from each atom’s outermost shell form pairs and are shared with each other.
This is called a covalent bond. And because there are 3 electron pairs being shared, it is referred to as a triple bond. This bond is quite stable and strong.
Crops are unable to break the covalent bond of nitrogen themselves in order to directly absorb it. Furthermore, no aboveground organ of the crop can directly utilize atmospheric nitrogen. The root is the only organ capable of absorbing nitrogen, and it does so in the form of ions, transporting them throughout the crop.
So, the question arises:
How do crops convert nitrogen in the form of N2, which is bound by covalent bonds, into ions that can be utilized by the roots?
I mentioned that nitrogen is not absorbed directly by crops from the air, unlike CO2. Instead, it is absorbed through the roots in the form of NO3– (nitrate) or NH4+ (ammonium) from the soil. In other words, nitrogen molecules (N2) in the air cannot be utilized by crops because they are not broken down.
However, nitrogen is an essential requirement for crop growth. In the 19th century, German scientists Fritz Haber and Carl Bosch made a significant contribution to food production by using the Haber-Bosch process to synthesize ammonia (NH3) from nitrogen in the air and hydrogen, thereby making a significant contribution to food production.
Haber-Bosch process N2 + 3H2 → 2NH3
In other words, the synthesis of ammonia from nitrogen and hydrogen has made it possible to produce nitrogen fertilizers. Now we can directly apply nitrogen fertilizers to the soil and supply nitrogen to crops.
Here, a fundamental question arises:
“How do crops absorb nitrogen?”
If ammonia (NH3) is supplied to the soil, do crops simply absorb this ammonia? Or do they prefer a specific form of nitrogen?
From now on, let’s discuss how nitrogen fertilizers are transformed in the soil and utilized by crops.
1) Nitrogen cycle in soil
As I mentioned, nitrogen is a highly stable molecule composed of a triple bond. As a result, crops cannot directly utilize the nitrogen present in the atmosphere.
I’ll explain four main pathways how nitrogen get into the soil. The image below is a sketch I personally drew to depict the nitrogen cycle. I will use this diagram to explain the nitrogen cycle.
1) Lighting (plus precipitation)
When lightning strikes occur in the atmosphere, it generates heat and causes nitrogen to combine with oxygen, creating nitrates (NO3–). These nitrates, along with rainfall, permeate into the soil, allowing crops to access and utilize nitrogen.
2) Nitrogen fixation by free-living heterotrophs
Certain bacteria that form nodules on the roots of legumes and crops help the crops by symbiotically fixing atmospheric nitrogen. This process, known as nitrogen fixation, allows crops to utilize atmospheric nitrogen.
3) Organic matter (decomposition)
In addition to nitrogen fixation, crops can utilize nitrogen through a chemical process in which organic forms containing nitrogen, such as amino acid groups (NH2), are converted by soil microorganisms into ammonia (NH3) or ammonium (NH4+). This decomposition process of nitrogen-containing compounds into ammonia or ammonium by soil microorganisms is known as ammonification.
When ammonia (NH3) reacts with water, it separates into ammonium ions (NH4+) and hydroxide ions (OH–). The ammonium ions (NH4+) serve as a nitrogen source for crops, supplying nitrogen atoms to the crops. In the soil, ammonia (NH3) can be utilized by crops in its ionized form as ammonium (NH4+). However, it can also be converted into nitrite (NO2–) by nitrifying bacteria called Nitrosomonas. Subsequently, nitrite (NO2–) is further converted into nitrate (NO3–) by Nitrobacter. This process of the conversion of ammonia (NH3) to nitrate (NO3–) is known as nitrification.
Nitrification The process in which ammonia (NH3) is converted into NO2 or NO3 by nitrifying bacteria is called nitrification.
2 NH4+ + 3 O2 → 2 NO2- + 2 H2O + 4 H+ (Nitrosomonas) 2 NO2- + O2 → 2 NO3- (Nitrobacter) or NH3 + O2 → NO2− + 3H+ + 2e− NO2− + H2O → NO3− + 2H+ + 2e−
4) Fertilizer
The forms of nitrogen that crops can absorb are ammonium (NH₄⁺) and nitrate (NO₃⁻). In paddy fields, rice can directly absorb ammonium (NH₄⁺), while in upland fields, crops absorb nitrate (NO₃⁻). In other words, in paddy fields, rice can absorb nitrogen immediately after ammonification, while in upland fields, crops need to go through both ammonification and nitrification processes to be able to absorb nitrogen.
Additionally, the anion nitrate (NO₃⁻) carries a negative charge, allowing it to be readily available for crops uptake without being adsorbed by the soil. This enables rapid nutrient supply. On the other hand, ammonium (NH₄⁺) carries a different charge from the soil, causing it to be adsorbed by the soil, resulting in delayed absorption by crops.
Thanks to the aforementioned Haber-Bosch process, we are now able to synthesize ammonia (NH3) from atmospheric nitrogen. This allows us to supply nitrogen directly to the soil using nitrogen fertilizers.
If we apply a fertilizer with the composition of 46-0-0, such as urea (CO(NH₂)₂), to the soil, it will undergo ammonification and nitrification processes, eventually supplying nitrogen to field crops in the form of nitrate (NO₃⁻).
However, I suddenly have the following question:
Can nitrogen fertilizer be supplied directly as nitrate (NO₃⁻) from the beginning?
Yes, it is possible. Nitrogen fertilizers that are supplied in the form of nitrate (NO₃⁻) are referred to as nitrate-based fertilizers.
Nitrate (NO₃⁻) fertilizers are quickly absorbed by crops because they provide nitrogen in a readily available form. However, it is generally known that paddy soils are more suitable for ammonium (NH₄⁺) nitrogen, while field crops prefer nitrate (NO₃⁻) nitrogen. Nitrate nitrogen does not readily adsorb to the soil and is prone to leaching with water (because soil particles are negatively charged), making it unsuitable for paddy fields. In paddy fields, when water percolates down, nitrate nitrogen does not adsorb to the soil but instead accompanies the water, eventually undergoing denitrification in the reduction layer.
What if we use ammonium nitrate?
What if we use a nitrogen fertilizer that contains both ammonium (NH₄⁺) and nitrate (NO₃⁻)? Since crops can uptake both ammonium and nitrate depending on their needs, wouldn’t it be more efficient for crop growth? Is there such a fertilizer available? There is a fertilizer called ammonium nitrate (NH₄NO₃) that contains both ammonium and nitrate.
Ammonium nitrate NH3 + HNO3 → NH4NO3
Do you remember the explosion incident that occurred in Lebanon a few years ago? There was an accident in which stored ammonium nitrate in a warehouse at the Beirut port exploded. Ammonium nitrate is a hazardous substance. However, it is also used as a nitrogen fertilizer in crop cultivation.
Let’s take a look at the nitrogen cycle when ammonium nitrate is applied as a fertilizer:
1) Application:
Ammonium nitrate is applied to the soil as a fertilizer. Ammonium nitrate contains both nitrate ions (NO3–) and ammonium ions (NH4+).
2) Ammonium uptake:
Ammonium ions (NH4+) are taken up by the roots of crops. Ammonium ions serve as a nitrogen source for the crops.
Crops tend to absorb nitrate (NO3–) more rapidly due to its high particle mobility. On the other hand, the uptake of ammonium (NH4+) by crops is relatively slower compared to nitrate. Ammonium ions have a higher tendency to be adsorbed by soil particles (because soil particles are negatively charged). As a result, most of the ammonium (NH4+) will undergo nitrification (the process of converting ammonium to nitrate) before being absorbed by crops.
3) Nitrification:
Ammonium ions (NH4+) in the soil are converted into nitrate ions (NO3–) by soil microorganisms. This process is called nitrification. Nitrate ions serve as an additional nitrogen source for the crops.
Ammonium (NH₄⁺) will be converted to nitrate (NO₃⁻) by soil bacteria. During this process, nitrous oxide (N₂O) and nitric oxide (NO) will be lost to the atmosphere.
4) Nitrate uptake:
Nitrate ions (NO3–) are absorbed by the roots of crops. Nitrate ions provide nitrogen to the crops, aiding in their growth and development.
5) Denitrification:
When there is excessive moisture in the soil, creating anaerobic conditions, soil bacteria will convert nitrate (NO₃⁻) into nitrite (NO₂⁻), ultimately leading to the conversion of nitrous oxide (N₂O) and nitric oxide (NO). These gases will be lost to the atmosphere.”
6) Immobilization and Mineralization:
Mineral nitrogen, such as ammonium (NH₄⁺) and nitrate (NO₃⁻), in the soil will move into the organic matter present in the soil. The activity of soil microorganisms is primarily stimulated by ammonium (NH₄⁺). Immobilized nitrogen cannot be directly utilized by crops and needs to undergo a process called mineralization. In other words, the mineralization of soil organic matter is the process of releasing ammonium (NH₄⁺) into the soil.
7) Ammonia volatilization:
The volatilization of ammonia (NH₃) occurs when ammonium (NH₄⁺) is converted to ammonia (NH₃), resulting in its loss to the atmosphere. The conversion of ammonium (NH₄⁺) to ammonia (NH₃) is easier at high soil pH and high temperatures. If this conversion takes place at the soil surface, the loss can reach its maximum level.
8) Leaching:
The leaching of nitrate (NO₃⁻) into the soil occurs during periods of winter fallow when percolating rainfall washes away mineralized nitrate (NO₃⁻) below crop residues and roots. Proper nitrogen fertilizer application can increase nitrogen use efficiency during crop cultivation and reduce the risk of nitrate leaching.
Reference
Strock, J. S. “Ammonification.” Encyclopedia of ecology, five-volume set. Elsevier Inc., 2008. 162-165.
