### Contact Info

Tyson Ochsner

Assoc. Professor

Plant and Soil Sciences

Oklahoma State University

368 Ag Hall

Stillwater, OK 74078

Phone: (405)-744-3627

FAX (405) 744-0354

tyson.ochsner@okstate.edu

# Temperature Dependent Degradation Exercises, D.L. Nofziger

I.        Questions about impact of temperature on degradation rate: These can be answered with the aid of the program or by manipulating the equations presented in the help document. To begin, uncheck the box in the lower right corner so the temperature dependent curve is not shown. The remaining 2 lines show the concentration as a function of time at the temperatures in the boxes.

1. If the initial concentration of the chemical is 100 mg L-1, the reference half-life is 50 days at a reference temperature of 25 °C, and the activation energy is 50 kJ/mol,
1. What is the concentration of chemical after 100 days?
2. Complete the following table:
 Temperature (°C) Concentration (mg L-1) at 100 days Half-life (days) 4 5 9 10 14 15 19 20 24 25 26 29 30 31

1. Describe the change in concentration with temperature.
2. Does a change of one degree produce the same change in concentration at 10, 20, and 30 °C? If not, at what temperature is the rate of change most rapid?
3. Describe the change in half-life with temperature.

1. Repeat exercise 1 for chemicals with activation energy of 20 KJ mol-1 and 80 kJ mol-1. Compare the answers to questions c, d, and e for the three activation energies.

II.                 Questions involving impact of temperature changes with time and soil depth on degradation: To answer these questions using the Java software, be sure the upper box in the lower right corner of the window is checked so the temperature dependent degradation curve is shown on the graph and the “Variable” column is shown on the table.

1. Using an initial concentration of the chemical is 100 mg L-1, reference half-life of 50 days, reference temperature of 25 °C, activation energy is 50 kJ/mol, minimum surface temperature of 0 °C, maximum surface temperature of 30 °C, and soil depth of zero, answer the following questions:
1. What differences do you observe between the temperature dependent curve and the constant degradation rate curves?
2. How long does it take for the concentration associated with a single application to be reduced to one half its original value if the chemical is applied on the first day of each month of the year? Explain why these values differ. For which month is this time a minimum? For which month is it a maximum? How do these times compare with the reference half-life?
3. What concentrations are calculated at the end of 365 days for applications on January 1, April1, July 1, and October 1.
4. What temperature produces a constant degradation rate that yields the same concentration after 365 days as observed in part c. above. Is this the average of the minimum and maximum temperatures at the soil surface?

1. Answer question 3 for a reference half-life of 200 days. What differences and similarities do you observe?

1. Repeat exercise 3 for soil depths of 1 m and 2 m. How does the impact of temperature change with soil depth? (It may be useful to review the soil temperature application  as you think of these results.)