Nitrogen immobilization is the temporary loss of available soil nitrogen resulting from the decomposition of cover crop residue. Nitrogen immobilization is also referred to as nitrogen tie-up or nitrogen penalty and can result in additional fertilizer costs for farmers planting a nitrogen-hungry crop like corn.

When discussing the challenge of nitrogen immobilization, it is important to understand an alternative outcome called nitrogen mineralization, which can also result from residue decomposition. With mineralization, plant-available nitrogen is released by microbes as they breakdown the terminated cover crop.

C:N Ratio

The C:N ratio or carbon-to-nitrogen ratio is how many carbon units there are to every nitrogen unit in organic material (plant tissue and residue). For example, a C:N ratio of 40:1 means there are 40 units of carbon to every 1 unit of nitrogen in that plant material. This ratio will influence whether nitrogen is immobilized or mineralized and to what extent.

Mechanisms of Nitrogen Immobilization

Plant residue is broken down and decomposed by soil microbes that feed on this organic material. These microbes’ ideal food has a C:N ratio between 20:1 and 30:1. When their food source (plant residue) has a higher C:N ratio, insufficient nitrogen is available for their biological functions. This means the microbes must utilize nitrogen available in the soil to meet their needs.

The amount of nitrogen microbes must consume from the soil is the nitrogen penalty since it is temporarily unavailable for plants like cash crops. Eventually, this nitrogen will be released by the microbes and made plant available, but it is often too late for that year’s cash crop.

Mechanisms of Nitrogen Mineralization

Nitrogen mineralization is essentially the opposite of immobilization. In this scenario, the plant residue feeding the microbes has a lower C:N ratio and contains more nitrogen than they need. In return, the microbes deposit this excess nitrogen into the soil, which is available for plants. This results in a nitrogen credit for farmers and creates a cost saving by lowering the nitrogen fertilizer required for the following cash crop.

C:N Ratio Variability

It is important for management to account for differences in C:N ratios for cover crops.

Cover Crop Species

Cover crop species have different C:N ratios that will determine if they result in nitrogen immobilization or mineralization.

In general legumes and many brassicas have lower C:N ratios that will result in nitrogen mineralization. Cover crops or cover crop mixes with a C:N ratio between 20:1 and 30:1 will have minimal to no effect on available nitrogen. Finally, grass covers that have higher C:N ratios creating immobilization.

Maturity

The C:N ratio of any cover crop species will generally increase as the season progresses and the plant matures. C:N ratios tend to stay lower through the vegetative stage of development, before they develop more “woody” structures.

Varieties

C:N ratios for different varieties of a cover crop species may vary. Ask your cover crop seed retailer if they have any information to help you make an informed decision.

Management Decisions

How a farm handles nitrogen immobilization will depend on its rotation, goals, and other practices.

A legume cash crop like soybeans can fixate nitrogen, meaning nitrogen immobilization is not a concern and will not negatively affect that crop. Before a field goes into soybeans, a grower may utilize a cover crop like cereal rye to help with weed suppression without fear of additional fertilizer costs.

Nitrogen immobilization is more of a concern before a nitrogen-hungry cash crop like corn. Growers can elect to avoid high C:N ratio cover crops, terminate earlier during the vegetative stage to lessen the amount of nitrogen immobilized or decide to take on the added cost of additional nitrogen inputs. That extra cost may be worth it for growers trying to enhance certain benefits of high C:N cover crops.

Nitrogen Calculator Decision Support Tool

An online tool developed by Precision Sustainable Agriculture can help farmers better understand how much nitrogen is immobilized and when it will be released.

The Nitrogen Calculator will chart nitrogen tie-up and release for 18 weeks past cover crop termination and compare this to the nitrogen demands of the following cash crop.

References

• Cates, A. (2021, May 18). Cover crop residue and C:N ratios. Minnesota Crop News. https://blog-crop-news.extension.umn.edu/2021/05/cover-crop-residue-and-cn-ratios.html

• Kladivko, E. & Agronomy Department, Purdue University. (2016). Cover crops for soil nitrogen cycling. https://ag.purdue.edu/department/agry/agry-extension/_docs/cover-crops/covercropsnitrogen.pdf

• Vieira Branco Ozorio, D., Basche, A., & Koehler-Cole, K. (2022, September 9). How Much Nitrogen Does My Cover Crop Take Up and When Do I Get It Back? CropWatch. https://cropwatch.unl.edu/2020/how-much-nitrogen-does-my-cover-crop-take-and-when-do-i-get-it-back