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.