OOS 12-1 - Counteracting effects of ultraviolet-B radiation on litter decomposition

Tuesday, August 4, 2009: 8:00 AM
Galisteo, Albuquerque Convention Center
William K. Smith, Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT, Wei Gao, 1USDA UV-B Monitoring and Research Program, Colorado State University, Fort Collins, CO, Heidi Steltzer, Biology, Fort Lewis College, Durango, CO, Matthew D. Wallenstein, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO and Roger Tree, USDA UV-B Monitoring and Research Program, Colorado State University, Fort Collins, CO
Background/Question/Methods

Altered surface ultraviolet-B (UV-B) radiation resulting from a combination of factors that include changes in stratospheric ozone concentrations, cloud cover, and aerosol conditions may affect litter decomposition and, thus, terrestrial nutrient cycling on a global scale.  Although litter decomposition rates vary across biomes, patterns of decomposition suggest that UV-B radiation accelerates litter decay in xeric environments where precipitation is infrequent.  However, much less is known about the effects of UV-B radiation under more frequent precipitation regimes where microbial decomposition is characteristically high.  To evaluate the association between moisture regime and UV-B exposure, a litter decomposition experiment was designed for aspen (Populus tremuloides) litter where both abiotic (photodegradation) and biotic (microbial) factors could be manipulated and quantified.  

Results/Conclusions

We found that experimentally increasing UV-B exposure (0, 7.4, and 11.2 kJ m-2 d-1, respectively) did not consistently increase litter decomposition rates across simulated precipitation frequencies of 4-, 12-, and 24-days.  Instead, in control soil treatments (microbially active), a UV-B exposure of 11.2 kJ m-2 d-1 resulted in a 13% decrease in decomposition rates under the 4-day precipitation frequency, but an increase of 80% under the 24-day frequency.  Furthermore, the same UV-B dose increased litter decomposition rates under the 24-day precipitation frequency by 78% even in conditions where microbial activity was suppressed.  Therefore, under more xeric conditions where microbial decomposition was strongly moisture-limited, greater exposure to UV-B radiation increased decomposition rates, presumably through photodegradation.  In contrast, when microbial decomposition was not moisture-limited, greater UV-B exposure slowed decomposition rates, most likely from the resulting inhibition of microbial activity.  Ultimately, these experimental results highlight UV-B radiation as a potential driver of decomposition, as well as indicate that both the direction and magnitude of the UV-B effect may be dependent on water availability, a factor that may change according to predicted changes in global precipitation patterns.

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