Degradation Rates in the Carbon Cycle

Design Approach: The design approach for organic materials is based on their half-life in the soil system.

For compounds with a half-life of up to 1 year, the amount allowed to accumulate in the soil should not exceed twice the amount applied annually. This is achieved by application schedule that is equal to one half-life of the material that is being used.

Biological reactions in the soil related to soil texture and structure, moisture content, temperature, oxygen level, nutrient status, pH, and type and number of microbes.

Because of need for mixing, surface vegetation is not typically a treatment component in systems designed for short half-life organic materials.

The degradation rate and half-life of organic compounds is determined in the laboratory by a series of soil respirometer tests.

Representative samples of soil and material to be evaluated are mixed in a proportional range and placed in sealed flasks, which in turn are placed in an incubation chamber. Humidified, carbon dioxide-free air is passed through each flask.

The carbon dioxide evolved from microbial activity in the flask is picked up in the air and then collected in columns containing 0.1N sodium hydroxide.

The sodium hydroxide solutions are changed about three times a week and then titrated with hydrochloric acid. For anaerobic incubations, gas samples are withdrawn from the incubation flasks and methane is measured on a gas chromatograph.

Typical incubation period is up to 6 months. Control tests are run at 20C, but if field temperatures are vary by more than 10C, the half-life at these other temperatures should also be determined.

Soil respirometer tests should be supported by measuring the organic fractions of the original sample and that of the final soil-material mixture.

Degradation rates are determined with Eqs. 1 and 2 For total and organic carbon degradation, respectively:

Equation 1
Dt = (0.27) ([CO₂]w mg - [CO₂]s mg) / C mg

• Dt = fraction of total carbon degraded over time t.
• 0.27 = proportion of C in CO₂ by molecular weight.
• [CO₂]w = cumulative CO₂ evolved by soil-material mixture.
• [CO₂]s = cumulative CO₂ evolved in unamended soil
• C = carbon applied with the added material.

• Dto = fraction of organic carbon (o) degraded over time t.
• Cro = amount of residual carbon in the organic fraction of the final soil-material mixture.
• Cs = amount of organic carbon extracted from the unamended soil.
• Cao = amount of carbon in the organic fraction of the applied material.

The half-life for a specific material is determined by Equation 3.
Equation 3
t1/2 = 0.5 t (days) / Dt

• t1/2 = half-life of the added material, days.
• t = time period used for data from Eqs. 1 and 2, days.
• Dt = fraction of carbon degraded over time t (or Dto).

Based on aerobic degradations, rapidly degraded materials will have a Dt value of >0.20, moderately degraded materials will have a Dt of 0.10 to 0.19, while slowly degraded materials will have a Dt of less than 0.10. For example, wastes from the fermentation industry have a Dt of 0.22 to 0.25 (rapid), most leaf litters have a Dt of 0.12 to 0.15 (moderate), while most aromatic organics will have a Dt of 0.03 to 0.08 (slow).

The most usual type of experiment is to hold all variables constant (soil moisture, incubation temperature, soil pH, time of incubation) and vary the concentration of the added material. However, the concentration of the added material could be held constant and any of the other variables changed as part of the evaluation. In the bioremediation industry, these type of tests are often conducted where the soil-mixture has been sterilized and microbes in various combinations are added to find the best "consortia" of microbes that will degrade the material in question.

Sample Problem
Find the Dt and half-life values for a poultry compost litter where the material will be applied to a strip-mined soil for reclamation. The following data were obtained with respirometer tests:

Applied carbon (C) = 1350 mg
CO₂ produced (90 days) = 2,300 mg (waste + soil) and 200 mg (soil only).

Solution
1. Use Equation 1 to determine evolved carbon dioxide on a total carbon basis:
Dt = 0.27 ([CO₂]w - [CO₂]s)/C
D90 = (0.27)(2300 - 200)/1350 = 0.42
This number is the degradation rate (fraction degraded) from respirometer data, and indicates that this material is rapidly degradable.

2. Determine the half-life for the material using Equation 3.
t1/2 = 0.5 t/Dt
= (0.5)(90)/0.42 = 107 days = 0.29 year

With a high Dt and a short half-life, this material is very suitable for the reclamation site. One large or several small applications could be considered.