Immobilization and Mineralization of Nitrogen in Agricultural Soils

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- Let's talk about nitrogen.

Nitrogen makes up almost 80% of Earth's atmosphere.

It's found in all organisms, and it's one of many nutrients essential to plant growth and development.

I'm Casey Guindon, a Penn State field and forage crops educator, and I would like to discuss the importance of nitrogen management in agricultural soils.

Plants can only absorb nitrogen from the soil in the form of nitrate or ammonium.

Soil microbes play a vital role in the amount of nitrogen that becomes available to plants.

As they break down organic residues and convert it into their own biomass, they can either immobilize nitrogen, making it unavailable to plants, or mineralize nitrogen so that it is available to plants.

Whether or not microbes mineralize or immobilize nitrogen depends on how much nitrogen is present in the residues that they are decomposing.

We often refer to the amount of nitrogen in residues as the carbon to nitrogen ratio, where a high carbon to nitrogen ratio indicates low nitrogen content and a low carbon to nitrogen ratio indicates a high nitrogen content.

If the carbon to nitrogen ratio of a residue material is higher than approximately 25 to one, the amount of nitrogen in the ratio is less than what the microbes need to build their biomass as they decompose in the residues.

As a result, they pull mineral nitrogen from the surrounding soil, making it unavailable for plant uptake.

This process is called the immobilization of nitrogen.

On the other hand, if the carbon to nitrogen ratio in the residue is less than 25 to one, the amount of nitrogen available in that residue is more than what the microbes can incorporate into their own bodies.

Through the decomposition of the residue, the excess nitrogen is released as ammonium, one of the forms that is available for plant uptake.

This process is called mineralization of nitrogen because ammonium is an inorganic or mineral form of nitrogen.

Weather also plays an important role in the breakdown of soil organic matter and residues that can supply nitrogen to growing plants.

Microbes operate best at soil temperatures above 50 degrees Fahrenheit, so these processes occur more rapidly as the temperature increases.

Additionally, an ideal soil moisture for plant growth also optimizes microbial activity.

Soils that are poorly drained or too dry may experience relatively slow rates of microbial decomposition.

Let's see the process of mineralization and immobilization in action.

These three pots represent soils with different additives.

The pot on the right without a label contains only soil.

No nitrogen containing amendments were added.

The middle, labeled S, contains soil with ground wheat straw added.

Wheat straw had a carbon to nitrogen ratio of approximately 80 to one.

The left pot, labeled L, contains soil with legume hay added.

The carbon to nitrogen ratio of legume hay is approximately 17 to one.

Corn was planted in each pot to observe the changes in plant available nitrogen early in the growing season.

Early in the season, the differences between the corn in the pot with no additives and those with organic additives are difficult to see.

However, the changes in nitrogen supply become more visible as the growth and development of the plants continue.

A few weeks later, you can see a distinct difference between the pot with legume hay labeled L and the pot with wheat straw labeled S.

The corn in the pot that contains the legume hay amendment is much greener than the pot with the wheat straw.

As the growing season progressed, the nitrogen in the legume hay become available through the microbial process of mineralization.

The extra nitrogen in the legume was converted to a plant available form and contributed to the growth and development of the corn plants.

Conversely, the pot with wheat straw had less nitrogen available.

The microbes consumed some of the nitrogen in the surrounding soil that would have otherwise been utilized by the growing corn plant.

As a result, the corn plants exhibit nitrogen deficiency, and their growth and yield potential are limited.

There are many management techniques we can use in our soils that can promote the mineralization of nitrogen throughout the growing season of corn and other commodities.

First, it's important to consider the carbon to nitrogen ratio of cover crops when they're terminated.

Certain cover crops like cereal rye have the potential to immobilize nitrogen if they're terminated at a late and highly mature stage.

Applications of materials like manure can contain a significant amount of organic nitrogen.

The residual organic nitrogen content of manure from previous years of applications can be a significant source of additional mineralized nitrogen and should be accounted for in cropping plants.

Additionally, improving the drainage of agricultural fields can help to increase the amount of nitrogen that is made available to growing plants by helping the soil to remain at the optimal moisture level for microbial activity.

It also reduces the risk of nitrogen loss to denitrification, a microbial process that occurs in oxygen-depleted soils.

Nitrogen is one of the most significant nutrients we can add to fields to promote crop growth.

By being aware of the cycle that nitrogen must go through to become available to crops, we can be better stewards of those nutrients and get the most out of our fertilizer dollar.