Immobilization in Soils

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Source: George Rehm, University of Minnesota Extension

Many times, discussions of nitrogen reactions in soils favor the nitrification and mineralization processes. The immobilization process may be mentioned in passing; but, is not discussed in-depth.

In the big picture, immobilization is the conversion of nitrate-nitrogen into organic nitrogen. It can be considered as the oposite of mineralization. It is controlled by soil bacteria and, therefore, is a biological process strongly affected by soil moisture and/or soil temperature.

We can look at the processes of immobilization and mineralization as being in equilibrium or, if you wish, working against each other. If nitrate-nitrogen removal from the soil via immobilization is more  than the addition via mineralization, there will be a net loss of nitrate-nitrogen from the soil system. If creation of nitrate-nitrogen via mineralization exceeds removal by immobilization there will be a net addition of nitrate-nitrogen to the soil system.

These competing processes are affected by the type of residue or organic matter that is added to the soil. Frequently, the extent of immobilization is related to the ratio of carbon (C) to nitrogen (N) in the residue or organic matter. This ratio is written or referred to as the C/N ratio.

There is a general rule of thumb that the C/N ratio in soil organic matter is 10:1. This is a relatively narrow ratio in corn stover which might be 200:1 The C/N ratio in alfalfa and clover tissue is closer to 10:1.

If a measure of nitrate-nitrogen is used as a measure of immobilization over time, there will be a reduction shortly after the addition of residue. Then, there there will be an increase over time. Initially, soil nitrate-nitrogen is used by soil microorganisms for growth and development. Then, nitrate-nitrogen concentration increases as decomposition progresses. The amount of increase or the speed of increase varies with the C/N ratio of the residue. The wide C/N ratios are equated to a slow release of nitrate-nitrogen via mineralization.

In a perfect world, it would be nice to be able to predict the amount of N immobilized or mineralized. This prediction would aid considerably in formulation of N fertilizer guidelines. However, it is not possible to make this prediction based on some measure of N in the soil. Several researchers have attempted to develop such a test and none have been completely successful.

Will the addition of fertilizer N decrease the amount of nitrate-N immobilized and subsequently increase nitrate-N produced by mineralization? This practice has worked in laboratory studies; but, has not proven to be effective in field research. Thus, addition of fertilizer N to enhance residue decomposition should not be a suggested management practice in Minnesota.

In crop production today, the immobilization process becomes a more important consideration as yields increase. Higher grain yields produce more corn residue that is returned to the soil. This is especially important when corn follows corn in rotation. The use of banded fertilizer placed close to the seed can help to overcome the temporary loss of N via immobilization.

Since immobilization is a biological process, it is directly affected by soil temperature. Therefore, any management practice that increases soil temperature in early spring and/or/early summer should reduce the amount of N that is immobilized. As a consequence, more nitrate-N should be available. Removal of any residue over the row is one of those management practices. The clean band over the row should probably be 10 to 14 inches wide. There are several pieces of equipment on the market that can accomplish this task.

The previous discussion has focused on immobilization of N in soils. The immobilization process also applies to sulfur. Although measurements of sulfur immobilization are very limited, this could be an inportant process as corn yields increase each year. Increased immobilization of sulfate-sulfur could be part of an explanation for the greater frequency of reports of response of corn to fertilizer sulfur observed during the early part of the growing season. But, we don't know for sure the importance of this process in the world of sulfur fertilization.

Everything considered, it appears that as yields increase, more consideration should be focused on the process of immobilization in soils.

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