Organic litter follows an exponential decay curve. The formula for this decay is Mt = M0 e-kt where:
- M is the mass of litter at a certain time;
- M0 is the initial mass of litter;
- e is the base of the natural logarithm;
- k is the decomposition constant; and
- t is the amount of time passed since the initial measurement.
k, in equations determining the amount of litter remaining after a certain amount of time, is affected primarily by climate and litter quality.
The two primary climatic factors contributing to the rate of decomposition are temperature and moisture levels. However, because the two influence each other so much, k can be measured as a function of temperature and moisture. Climates that are warm and wet (but not too wet) increase the k-value, while areas that are cold and dry cause lower k-values. For example, litter on a rain forest floor decays much more quickly than litter in a forest with temperate summers and cold winters.
The fact that the rate of decomposition follows an exponential decay curve is related to the CENTURY model, explained further on the Lecture 5 notes page. The cell walls of plants are made primarily of three types of molecules: cellulose, lignin, and hemicellulose.1
- Cellulose, primarily glucose, is extremely labile. It is also a major component of plant material - up to 45% of dry mass.2 Cellulose is comprised of long strings of repeated and identical bonds which require only a few enzymes to decompose3. Because a large percentage of plant material is extremely easy to degrade, there is a steep decay curve in the beginning of the process. In the CENTURY model, decomposition of cellulose is equivalent to the metabolic pool.
- Hemicellulose is found in the microfibrils of cell walls. Hemicellulose is comprised of a variety of monomer sugars, including xylose, galactose, rhamnose, and arabinose.4 Because of this variety, it requires several more enzymes to break down bonds. It is also comprised of shorter chains than cellulose, has a more random and branched structure, and can bind with the tough lignin in cell walls. Hemicellulose, therefore, decomposes at a much slower rate than cellulose. In the CENTURY model, decomposition of cellulose is equivalent to the active pool.
- Lignin is the most resistant to decomposition. This is because lignin is composed of complex molecules in a randomized structure5. Lignin also forms a variety of chemical bonds with other polymers, providing it with the complexity to strengthen woody cells and the cells of plan stems. Because of the complexity of the polymers and the variety of bonds, lignin is especially difficult for organisms to decompose. This decomposes at a significantly slower rate than either cellulose or hemicellulose, which causes the decay curve to flatten out (though still decrease gradually) over time. In the CENTURY model, decomposition of lignin is equivalent to the passive pool.
The ratio of lignin to nitrogen may provide a good indication as to how labile the litter is, and the percentage of lignin has a strong effect on k-values.
Determination of k-values
As stated above, the decomposition constant k is primarily controlled by climate and litter quality. Higher k-values correspond to faster rates of decomposition, while lower values correspond to slower decay. Scientists have used empirical results to determine the decomposition constant k in different areas, by placing a known amount of litter in mesh bags (litter bags) in areas with different soils, climates, and environments, and measuring the mass remaining after certain amounts of time.