Bio122 F15
HW 7: Decomposition exercise (Ch 22)
Name_________________________________________
Breaking It Down: Decomposition in Dry Environments
(This exercise is based on Austin, A. T. and L. Vivanco. 2006. Plant litter decomposition
in a semi-arid ecosystem controlled by photodegradation. Nature 442: 555–558.)
INTRODUCTION
Photosynthesis gets all the glory. Earlier, we learned how photosynthesis converts CO2
from the atmosphere into the biomass on which all other life depends. What is equally
important is the complex process of decomposition. Without decomposition, we would
quickly be buried in dead organic material. As you learned in Chapter 22, it is the
decomposition of organic matter that allows nutrients to cycle through the system and be
available for plants to uptake and incorporate into new organic material.
Most studies of decomposition in terrestrial ecosystems have examined the role of soil
organisms that decompose plant material and their interactions with climatic and tissue
chemistry. In deserts and in semi-arid regions, however, organic material often turns over
more rapidly than what models of the decomposition would predict, suggesting that some
of the decomposition is abiotic. One possible abiotic cause of degradation is
photodegradation, wherein sunlight degrades organic molecules into smaller molecules.
In addition to occurring naturally, photodegradation is often used for commercial
purposes such as the destruction of pollutants by wastewater treatment facilities.
Amy Austin and Lucia Vivanco at the Universidad de Buenos Aires examined
photodegradation in the semi-arid regions of southern Argentina. Their study site, located
at about 45°S (as far from the equator as northern Maine in the northern United States),
receives an average of 15 cm of precipitation a year, most of that falling in the winter
months. Based on the limited precipitation and the dominant vegetation (perennial
tussock grasses and shrubs), this region is classified as steppe, a type of temperate
shrubland (Figure 1).
Figure 1 The Patagonian steppe in southern Argentina. (Photo by
James Cadwell/CC BY-SA 1.0)
Bio122 F15
HW 7: Decomposition exercise (Ch 22)
To test for the effects of photodegradation on organic material, Austin and Vivanco
exposed leaf litter of the dominant grass species at the site to various treatments. In some,
ultraviolet-B (UVB) radiation was blocked; in others, all light was blocked (blocked
total); and in the control, all light was allowed. Each of the light treatments was
subdivided: in half, a biocide consisting of a bactericide and a fungicide was applied, and
the other half had no biocide.
The researchers continued the light treatments for a little over 18 months, and monitored
the percentage of leaf litter remaining in each of the treatments. Figure 2 (based on
Figure 1A from the paper) shows the mean percentage (and standard errors) of leaf litter
remaining at different times in the various treatments. The greater the amount of organic
mass loss, the greater the amount of decomposition.
Figure 2.
Question 1
Refer to the data at the last sampling date (right-most in Figure 2 above). Notice that the
treatments with the greatest amount of degradation accounted for about 40% loss of
organic matter. How much of this degradation (the 40%) can be ascribed to biotic
degradation (bacteria and fungi), UVB radiation, other wavelengths of radiation, and
other abiotic degradation?
Bio122 F15
HW 7: Decomposition exercise (Ch 22)
Question 2
What implications do these results have for nutrient cycling in other areas around the
globe?