Home > Nutrient Management > Essential Nutrients > Nitrogen |
NitrogenOf all the essential nutrients, nitrogen is required by plants in the largest quantity and is most frequently the limiting factor in crop productivity.
Nitrogen Forms and FunctionForms of nitrogen available for plant uptake
|
Form of Nitrogen | Formula | Availability for plant uptake |
Nitrogen gas | N2 | Although 78% of our atmosphere is nitrogen gas, this form of nitrogen must be transformed to usable forms before it is available for plant uptake. |
Ammonia | NH3 | Ammonia is a gas. Ammonium can escape from the surface of the soil under certain conditions and is harmful to plants in high quantities. Ammonium is the basic building block of commercial nitrogen fertilizers. |
Ammonium | NH4+ | Soil particles attract and retain ammonium on cation exchange complexes. This form may be directly taken up by plants. |
Nitrate | NO3- | Nitrate is the second form of nitrogen which is available for plant uptake. In most soils, nitrate is highly mobile. However, in the highly weathered soils of Hawaii, nitrate is stored in soils with ‘anion exchange capacity’ and becomes less mobile. |
Nitrite | NO2- | Nitrite is an intermediate product in the conversion of ammonium to nitrate (nitrification). It is usually present in low quantities, but is toxic to plants. |
Organic Nitrogen | Various compounds | Organic nitrogen must be converted to ammonium before it is used by plants. This conversion occurs with time and is known as mineralization. |
Though complex, the nitrogen cycle:
In this section, we will discuss eight major transformations of nitrogen in the soil: nitrogen fixation, mineralization, immobilization, nitrification, denitrification, volatilization, and leaching.
Although atmospheric nitrogen gas(N2) makes up approximately 78% of the air, it cannot be directly used by plants. Instead, atmospheric N2 only becomes available to plants through three unique processes. The final product of each of these processes is ammonium, which is then available for plant uptake.
The three processes which convert atmospheric nitrogen to ammonium
Certain soil organisms have the special ability to convert atmospheric nitrogen to ammonium. These organisms include several species of bacteria, actinomycetes, and cyanobacteria.
In the soil, nitrogen fixating organisms can form special relationships with plants, called “symbiotic” associations. Symbiotic is a term that means “living together.” Although a symbiotic relationship can be antagonistic, the symbiosis that occurs during biological nitrogen fixation is generally mutual and beneficial.
Legume-Rhizobium SymbiosisThe most abundant symbiotic relationship in nitrogen fixation forms between legumes (i.e. alfalfa, soybeans, etc.) and the Rhizobia bacteria species.
The image below shows cross sections of soybean (Glycine max) nodules. Nodules in the first row are highly effective at converting N2 from the atmosphere into ammonium. The nodules in the second row are moderately effective at biological nitrogen fixation, while the bottom row of nodules do not fix any nitrogen at all. Notice the lack of color in the interior of these nodules, which indicates that they do not have an active ‘nitrogenase system’. A ‘nitrogenase system’ is the bacterial enzyme that is necessary to convert N2 gas into ammonium through this biological process.
Figure 6. Relationship between color and effectivity of nodulation in soybean.
The next image shows how a well nodulated soybean root system looks when it contains many highly effective rhizobia in the soil or is applied as an inoculant.
Figure 7. Depiction of effective nodulation of soybean by rhizobia.
Specificity
Some Rhizobium species are only capable of nodulating a particular legume species and cannot successfully nodulate other legumes.
This phenomenon is known as Rhizobium specificity. However, not all Rhizobium are legume-specific. Thus, some may nodulate a number of different legumes.
The figure below gives examples of some common cross-inoculation groups that may assist you in selecting the proper rhizobial inoculant for a particular legume host. The proper combination of rhizobia and legume will result in the optimal nodulation and most nitrogen fixation. From this figure, we see that using soybean rhizobia with a soybean plant forms an effective symbiosis, while using a soybean rhizobia with a leucaena plant does not. However, cowpea rhizobia is capable of nodulating both mungbean and peanut.
Figure 8. Specificity of Rhizobia for successful nodulation of certain legumes.
Biological Nitrogen Fixation Management Program
The occurrence of the symbiotic relationship is heavily dependent upon a variety of soil conditions. If your program incorporates nitrogen fixation, the following considerations can determine your success.
Table 6. A summary of biological nitrogen fixation measurements by different legumes.
When does inoculation of legumes with rhizobia increase yield and biological nitrogen fixation?
This question is commonly posed by farmers who are deciding when to apply an inoculant to their crops. Since inoculation is relatively inexpensive (less than $5.00/acre), growers should error on the side of caution and inoculate their legume crops unless they have good evidence that inoculation is not needed.
The figure below is a conceptual model which integrates all the factors controlling biological nitrogen fixation. Additionally, it explains when the inoculation of legumes with rhizobia will result in an increase in plant growth and biological nitrogen fixation activity. The model is based on the fact that if the plant’s need for nitrogen is greater than the nitrogen that is supplied by both the existing soil nitrogen and the rhizobia already present in the soil, the inoculation of supplementary effective rhizobia will result in increased yield and biological nitrogen fixation.
Figure 9. The various Factors that control nitrogen fixation.
Amount of Nitrogen Fixed by Legume
When nitrogen is converted to ammonium during biological nitrogen fixation, ammonium becomes available to the legume and the microorganism that fixes it. Typically, the bacteria can fix anywhere between 20 and 80% of the total legume N.
Small amounts of ammonium can also be released by roots of the legume into the rhizosphere, or the surrounding soil.
Availability of Nitrogen in Subsequent Cropping Systems
What about subsequent cropping systems? Can a legume rotation benefit later plantings of a nonlegume crop?
Research shows that yields of nonlegume crops can increase when following a legume rotation. It is believed that the legume rotation increases the N content of soil, thus making it an effective nutrient management strategy.
However, when the legume is incorporated into the soil, the major benefit of the legume rotation lasts only during the first year following the legume rotation.
Other symbiotic relationshipsLegumes and Rhizobia are not the only species that can establish a mutual symbiotic relationship needed for nitrogen fixation to oocur.
“Free-living” nitrogen fixating organisms are also capable of nitrogen fixation, but are not associated with any plant species.
Since the 1950s, ammonium-based fertilizers have been manufactured using the Haber-Bosch technique. In this catalytic process, N2 reacts with hydrogen under 1,200 degrees Celsius and 500 atm.
Since the production of chemical fertilizers requires large inputs of fossil fuel, chemical fertilizers can be relatively expensive.
The impact of the Haber-Bosch technology on agriculture has been very dramatic. The Haber-Bosch technology enables high-analysis ammonium fertilizers to be produced quickly. As a result, the reliance on biological N fixation and manures as N sources has declined.
Atmospheric nitrogen additionsNitrogen is deposited onto the earth’s surface by:
Table 7. This table presents estimates of the different sources of atmospherically fixed nitrogen that was deposited onto the earth in the latter half of the twentieth century. Biological sources account for around 20% of total nitrogen deposition.
When absorbed by plants, ammonium and nitrate are incorporated into plant cells as organic, or living, forms of nitrogen. When plants die, microorganisms break down, or decompose, dead plant cells. During the decomposition about plant matter, organic nitrogen is once again converted to inorganic ammonium and released into the soil.
The process that converts organic N to ammonium is called mineralization and plays a significant role in the management of nitrogen.
To calculate the amount of nitrogen which can potentially be mineralized from your organic fertilizer source, click on the link below:
http://www.qpais.co.uk/nable/minrate.htm
The amount of ammonium that is released to the soil through mineralization depends on several factors:
Since the microorganisms living in the soil need both carbon and nitrogen, net mineralization occurs when C:N ratio is less than 20:1. This means for every two parts of carbon, there should be 1 part nitrogen for net mineralization. If you are applying organic amendments to your soil, it is important to become familiar with the C:N ratio to ensure N availability.
After mineralization of N occurs, ammonium can be:
Immobilization is process that converts inorganic nitrogen to organic nitrogen. It is the reverse reaction of mineralization.
Immobilization occurs when decomposing organic matter contains low amounts of nitrogen. Thus, immobilization occurs if the source of organic matter has a high C:N ratio. Microorganisms, who also need nitrogen to live, scavenge the soil for nitrogen when plant residues contain inadequate amounts of nitrogen. As inorganic ammonium and nitrate are incorporated into the cells of living microorganisms, the total N levels in the soil are reduced. Immobilization can ultimately result in nitrogen deficiencies.
When nitrogen is immobilized in the soil, there may be little nitrogen available for crop growth. As a result, plants can suffer from nitrogen deficiency and develop a yellow coloration. This is the reason why organic materials with a high C:N ratios, such as grass clippings and grain stover, are usually composted before they are incorporated into the soil or planting. This allows time for soil microorganisms to decompose the materials and begin to release nitrogen and other nutrients back into the soil
The processes of mineralization and immobilization are constantly occurring simultaneously. As organic matter decomposes, inorganic nitrogen will be released into the soil. As both plants and microorganisms grow, they utilize the nitrogen in the soil. Once plants and microorganisms die, they decompose and release inorganic nitrogen to the soil through mineralization.
Even though mineralization and immobilization are both occurring, we can determine which process, mineralization or immobilization, predominates.
The answer is the C:N ratio of the decomposable organic matter.
The C:N ratio is a characteristic of all organic matter, which includes:
If your program involves the addition of organic residues, it is important to know its C:N ratio. This knowledge allows you to predict whether net mineralization or net immobilization will occur. If the residue has a wide C:N ratio range, it may be necessary to apply additional amounts of nitrogen to your soil or choose a residue with a narrower range.
In most aerobic soils under optimal soil conditions, ammonium is rapidly converted to nitrate by soil bacteria through a process known as nitrification.
As you can see from the outline of steps above, the intermediate product of nitrification is nitrite. If conditions are unfavorable to undergo the second step of nitrification, nitrite can leach into the ground water and pose as a health risk.
The process of nitrification produces hydrogen ions. When large quantities of ammonium-containing fertilizers are applied to soil over time, this process can acidify the soil. See figure below for a simplified presentation of the nitrification process.
Figure 10. Basic process that causes soil acidity by ammonium fertilizers.
There are many factors that affect nitrification. Since nitrification is mediated by microorganisms, environmental factors that affect biological life will also influence nitrification. In general, the optimal conditions for most plant growth are also the optimal conditions for nitrification:
Nitrate is generally a very mobile in most soils. Excessive amounts of nitrate that are not taken up by plants is subject to leaching. Nitrate leaching can have an adverse effect on the environment.
Denitrification is the biological process in which nitrate is converted to atmospheric N2.
A second loss of nitrogen to the atmosphere is due to volatilization. By definition, volatilization is the loss of gaseous ammonia to the atmosphere.
Note the distinction between ammonia and ammonium. Although similar in form, ammonia is a gas that can escape from the soil into the atmosphere. You may be familiar with ammonia, which is characterized by its pungent smell.
Factors affecting volatilizationThere are several factors that affect volatilization:
In our discussion on cation and anion exchange, we mentioned that cations, such as ammonium, are attracted to soil particles that have a cation exchange capacity. Since most surface soils have a cation exchange capacity, ammonium is retained by soil particles.
In contrast, nitrate is not retained by cation exchange capacity.
Not only is nitrate leaching an economic loss to the farmer, it is also an environmental concern. The conditions that lead to nitrate leaching follow:
In Hawaii, there are soils that can have a high anion exchange capacity under acidic conditions, which reduces nitrate leaching. This phenomenon primarily occurs in the acidic subsurface soil layers that have the greatest anion exchange capacity. As a result, researchers have found that subsurface soil layers in Hawaii can retain nitrate and may prevent it from leaching into the ground water.
The fertilizer formulation can alter the pH of soil. Generally, fertilizers with high proportions of total nitrogen and are derived from ammonium sources (such as urea, ammonium sulfate, ammonium phosphate or ammonium nitrate) can acidify soils with repeated applications. Most fertilizers provide the “Lime Equivalent” on the bag’s label. The lime equivalent is the amount of limestone (calcium carbonate) it takes to neutralize the acidifying effects of using one ton of a particular fertilizer.
In contrast, other fertilizers can increase soil pH. These fertilizers are usually low in ammonium, but high in nitrate. Additionally, these fertilizers sometimes contain calcium from calcium nitrate. The lime equivalent is also given for these fertilizers, but it indicates the equivalent liming effect rather than the lime needed to offset acidity.
The lime equivalent is only an estimate. The actual acidifying effect of the fertilizer is influenced, to some degree, by soil conditions that affect the transformation of the ammonium to nitrate and also by how much ammonium the plant assimilates before this transformation occurs. If growers know the history of their fertilizer applications over time, they can use the lime equivalent to predict when lime additions are likely needed.
See the table below for some fertilizer formulations and note the relationship between the percentage of nitrogen in the fertilizer that is derived from ammonium and the lime equivalent needed to counter this acidifying effect.
Table 8. Fertilizer composition can change the pH of soil
Fertilizer |
Ammonium nitrogen |
Fertilizer’s Potential: |
|
Acidity |
Basicity |
||
|
% |
lbs lime/ton fertilizer |
|
2-7-7 |
90 |
1700 |
|
24-9-9 |
50 |
822 |
|
20-20-20 |
69 |
583 |
|
20–10-20 |
38 |
393 |
|
20–0-20 |
25 |
40 |
|
15-5-15 |
28 |
|
135 |
--data derived from various commercial fertilizer labels