Standing dead plant litter of emergent macrophytes constitutes a significant fraction of wetland detritus, yet the processes controlling the decomposition of this litter are not fully understood. Recent studies have shown that solar UV radiation promotes litter decomposition in dryland systems, but the role that UV plays in more humid systems is less clear. We conducted a field study to examine the effects of solar UV-B radiation (280-320 nm) on the decomposition of leaf litter of Spartina patens and Sagittaria lancifolia, two common salt tolerant marsh species of wetlands in southern Louisiana, USA. Senescing foliage was collected, air-dried, weighed and placed in UV-B-transparent (Aclar; +UV-B treatment) or UV-B-absorbing (clear polyester; cutoff near 290 nm; -UV-B treatment) litterbags.  Litterbags were then placed back into the marsh at two heights: canopy (~70cm above soil; S. patens, S. lancifolia) or ~10cm above soil (S. lancifolia only). Samples were collected, dried and weighed at monthly intervals (n=15/month/treatment/height) from March to August 2008.
Results/Conclusions

When averaged over time, leaf dry mass loss in S. lancifolia was significantly greater in the –UV-B treatment (p<0.001), but treatment differences were greatest at the lowest height level. Among sampling periods, leaf dry mass loss was greatest when solar UV-B levels were below midseason maxima and temperatures where relatively modest (i.e. daily max <30C), suggesting conditions more favorable for microbial activity. No UV-B treatment effects were detected in S. patens (p=0.74), however >60% of dry mass still remained at the end of the study. These results indicate that, in contrast to dryland ecosystems, solar UV-B inhibits rather than promotes decomposition of litter in this subtropical marsh and further suggest that UV-B effects are mediated by microbial rather than abiotic (photodegradation) processes.