Effects of UV Exposure and Litter Position on Decomposition in a California Grassland
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The importance of photodegradation in surface litter decomposition has recently been recognized in arid and semi-arid terrestrial ecosystems, yet its importance in decomposing dense litter and the mechanisms through which it acts remain unclear. We investigated how ultraviolet (UV) radiation exposure and litter position affected decomposition processes in a California annual grassland. In a split-plot design, we exposed Bromus diandrus litter to two levels of UV radiation (UV pass and UV block) at two aboveground locations (at the top, suspended above the litter layer, and at the bottom of the litter layer) for 1 year. We found that UV radiation increased the litter decay constant by 23% at the top location over 1 year, consistent with the occurrence of photodegradation. Surprisingly, UV radiation also increased the litter decay constant by 30% at the bottom location over 1 year. We speculate that photodegradation indirectly increased microbial decomposition through priming effects. Overall, litter in the top location had a 29% higher decay constant than litter in the bottom location. In terms of litter chemistry, exposure to UV radiation increased loss of hemicellulose by 26%, but not loss of lignin. Litter in the bottom location exhibited greater loss of the cell solubles fraction and greater nitrogen immobilization, but lower loss of hemicellulose than litter in the top location. Our results demonstrate that litter position significantly regulates the contribution of photodegradation to overall decomposition, both through direct (top location) and indirect (bottom location) effects. Therefore, better quantification of both direct and indirect effects of photodegradation can greatly improve understanding of biogeochemical cycling in grasslands.
Keywordsphotodegradation decomposition UV-A UV-B invasive species hemicellulose cellulose lignin litterbag drylands
We thank Dad Roux-Michollet, Keri Opalk, Kenneth Marchus, Matt Mass, Viviane Vincent, Molly Zimmerman, and Heather Christian for their assistance in the field and lab. We thank Oliver Chadwick and Carla D’Antonio for their valuable comments on the experimental design and on this manuscript. We thank Kate McCurdy, Eric Massey, and the University of California’s Sedgwick Reserve for providing the study site, field facilities, and technical support. This work was supported by the National Science Foundation under DEB-0935984.
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