Solar UV inhibits and promotes decomposition of standing dead litter in a southern marsh ecosystem

Background/Question/Methods 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, however first year results from our field study showed that solar UV radiation (280-320nm) inhibited decomposition in certain species in a more hydric marsh ecosystem, at least over the short-term (6-months). As a follow up to this initial study, we conducted a longer duration second year of field studies to further examine length of exposure and the partition of solar UV (UV-B (280-320 nm) and UV-A (320-390nm)) radiation on the decomposition of leaf litter of Sagittaria lancifolia and Spartina patens, two common salt tolerant marsh species of wetlands in southern Louisiana, USA. Senescing foliage was collected, air-dried, weighed and placed in perforated UV-B-transparent (Aclar; +UV treatment), UV-B-absorbing (clear polyester; -UVB treatment, cutoff near 290 nm) or UV-B and UV-A-absorbing (Llumar; -UVBA treatment, cutoff near 390nm) litterbags.  Litterbags were then placed back into the marsh suspended 20cm above the soil. Samples were collected, dried and weighed over a 7-month period for S. lancifolia and an 11-month period for S. patens.

Results/Conclusions Overall, there was a significant UV effect for both S. lancifolia and S. patens on leaf dry mass loss (p=0.009, p=<0.004 respectively), as well as a significant interactive effect of UV and time (p=0.001, p=<0.001 respectively). S. lancifolia litter exposed to the +UV treatment had significantly less mass loss over the first 5-months (p=0.004) of the experiment, however there was no significant difference between -UVB and -UVBA treatments. After 5-months the +UV treatment switched and had significantly greater mass loss (p=<0.001) for the remaining 2-months. Similar results were found in S.patens with the +UV treatment switch occurring at 4-months with significantly greater mass loss (p=<0.001) for the remaining 7-months. Again there was no significant difference between -UVB and -UVBA treatments. These results support our previous study, indicating that, in contrast to dryland ecosystems, solar UV inhibits decomposition of litter in this subtropical marsh. However, this longer study also demonstrates that, similar to dryland ecosystems, solar UV promotes decomposition, albeit later in the decomposition process. This temporal switch suggests that the absence of UV favors the biotic (microbial) processes working on the more labile compounds in the litter, and once depleted the presence of UV thus favors the abiotic (photodegradation) processes.