Certified Crop Advisor Practice Exam

What C:N ratio is most likely to result in the immobilization of inorganic nitrogen during decomposition?

1. 15

2. 25

3. 35

4. 45

The correct answer is: 35

The C:N ratio is a critical factor in determining how organic materials decompose and how nutrients, particularly nitrogen, are cycling within the soil ecosystem. When organic matter has a lower C:N ratio, it is more likely to provide nitrogen to soil microorganisms, which use it during the decomposition process. Conversely, a higher C:N ratio indicates that the organic material is carbon-rich and nitrogen-poor. In this context, a C:N ratio of 35 is significant because it indicates a level at which microorganisms will require more nitrogen than is available in the material itself for decomposition. This results in the immobilization of inorganic nitrogen, meaning that the microorganisms will absorb nitrogen from the soil or atmosphere to break down the carbon-rich organic material. When organic materials have a C:N ratio around 25 to 30, it is generally balanced for microbial activity, allowing for an effective exchange of nutrients without significant nitrogen absorption. However, at a C:N ratio of 35 or higher, particularly around 35 to 45, nitrogen begins to be immobilized as the available nitrogen in the soil becomes insufficient to meet the demands of the decomposing microorganisms, leading to a short-term decrease in the nitrogen available in the soil for plant uptake and growth. As a result,