(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.)
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In terrestrial ecosystems, carbon enters via photosynthesis by plants and other primary producers, and the decomposition of organic material returns carbon to the atmosphere. Without decomposition, dead organic material such as leaf litter would accumulate.
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.
Table 1
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 (UV-B) 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. Table 1 shows the densities of bacteria and fungi after different treatments.
Question 1
What effect does the biocide have on the fungi and the bacteria?
Question 2
Which treatment has the highest amount of bacteria?
Figure 1
The researchers continued the light treatment for a little over 18 months, and monitored the percentage of leaf litter remaining in each of the treatments. Figure 1 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.
Question 3
Which two treatments had the most leaf litter remaining?
Question 4
Which two treatments had the least leaf litter remaining?
Question 5
Based on the above, what do you conclude about the effect of photodegradation by light in decomposing leaf litter? Justify your answer.
Question 6
What can you conclude about the effect of UV-B light on photodegradation? Justify your answer.
Question 7
What can you conclude about the effect of bacteria and fungi on decomposition of the plant material? Justify your answer.
Figure 2
Based on the results in Figure 1, and plotting the natural log of the remaining leaf litter against time, the researchers were able to estimate the rates of decomposition (k) in each of the treatments. Figure 2 shows the rates of decomposition in each of the treatments.
Question 8
How much lower (as a percent) is decomposition in the blocked treatment relative to the full sun treatment?
Question 9
Assuming that all of the light is blocked in the “blocked total” treatment, what percentage of decomposition is via photodegradation?
