Practice Problems for Carbon/Nitrogen Ratios
Agronomy 354
• Carbon content of most natural plant residues is about 40%.
• Nitrogen content of most natural plant residues is quite variable.
• Thus, the C:N ratio of residues can vary considerably depending upon the type of residue involved.
• The rate of breakdown of an organic residue is a function of the environment and the type of organic material being broken down. Some materials are quite readily available (simple sugars), some intermediate (plant residues such as corn stalks and tree leaves containing a mixture of sugars, cellulose, hemicelluloses, protein, and lignin), and some break down slowly (sawdust that contains high lignin levels).
• The rate of release of carbon from a residue (carbon mineralization) and the rate and extent of nitrogen tie-up by microbes (nitrogen immobilization) is a function of the environment (pH, oxygen content, temperature, moisture, etc.).
• The example worked in class (50% decomposition in a 4- to 8-week period) is a reasonable value for residues chopped and buried into a warm, moist soil of neutral pH. With constant conditions, the rate of decomposition is most rapid at the beginning and progressively gets slower with time.
• C:N ratio > 30:1: expect immobilization of N as maximum biological activity is reached.
• C:N ratio < 20:1: expect mineralization of N as maximum biological activity is reached.
• A mixed population of soil microbes will assimilate about 25% of the carbon being decomposed; they have a C:N ratio of about 8:1.

1. Phenylalanine is an amino acid with the formula C6H5CH2CH(NH2)COOH.

a. What is the %C in phenylalanine?

b. What is the %N in phenylalanine?

c. What is the C:N ratio of phenylalanine?

2. Assume 100 lbs of tree leaves (C:N ratio 100:1) are incorporated into the surface 6 inches of a 10 ft by 10 ft garden area. What is a reasonable estimate of the amount of N that needs to be added so that N does not limit plant growth due to immobilization?

3. Six tons of corn-stalk residue are added per acre (40% C and 0.6% N).

a. What is a reasonable estimate of the amount of microbial C present in the soil in microbial tissue 4-8 weeks after application.

b. What is a reasonable estimate of the amount of CO2 liberated during this same period.

c. What is a reasonable estimate of the total amount of microbial N at the end of 8 weeks?

d. What is a reasonable estimate of the amount of immobilization of soil N (inorganic N taken from the soil) at the end of 8 weeks.

4. Two tons of sucrose (table sugar) are added per acre. You should assume that 100% will be readily broken down (rather than the expected 50% of a natural plant residue). Also, the maximum incorporation of C into microbial tissue is expected to be much quicker because of this readily available energy source. Activity likely will be maximized within a week after application in a warm, moist soil.
a. Sucrose is C12H22O11. What is the percentage C and N in sucrose? (C=12, H=1, O=16, N=14)
b. What is a reasonable estimate of the amount of N required to prevent immobilization from the soil (assume 100% breakdown of the sucrose within a week)?