• 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.

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)?