Background/Question/Methods: Rates of plant litter decomposition in arid ecosystems cannot be accurately explained by current biogeochemical models. Models that include temperature, moisture, and litter chemistry as primary predictors tend to under-estimate litter decomposition rates in arid ecosystems. Further, litter mass loss in arid ecosystems tends to follow a linear decay model, in contrast to asymptotic and exponential decay models that generally explain mass loss patterns in mesic environments. Recent evidence suggests that photodegradation, the breakdown of molecules by solar radiation particularly in the ultraviolet (UV) range, may explain observed patterns in litter mass loss in arid ecosystems. We tested the hypothesis that photodegradation would play a larger role in decomposition in arid ecosystems than in more mesic ecosystems using a long-term cross-site manipulative field study. Litter in arid ecosystems tends to have greater exposure to solar radiation because of low cloud, canopy, and litter cover. We also hypothesized that the role of photodegradation would be greater in litter with a higher lignin to nitrogen ratio, as lignin absorbs UV radiation and, unlike biological decomposition, photodegradation does not require nitrogen inputs. We manipulated the amount of UV radiation reaching the litter layer at three grassland sites in Minnesota, Colorado, and New Mexico, USA, that vary widely in aridity and ambient solar radiation. Litterbags filled with litter of two contrasting lignin to nitrogen ratios were placed at each site under screens that either passed all solar radiation wavelengths or passed all but UV wavelengths, but were equally permeable to precipitation. We collected litterbags at four times over a two-year period and measured mass loss, carbon fraction loss, changes in litter nitrogen content, and extracellular enzyme activity at each date.  

 

Results/Conclusions: Mass loss and decay rates at all three sites and for both litter types were higher when litter was exposed to UV radiation. However, the role of photodegradation in the decomposition process was not consistently greater in the more arid sites or for litter with a higher lignin to nitrogen ratio. In addition, while exposure to UV radiation increased rates of decay, litter under both UV treatments followed the same general decay model. These results suggest that photodegradation plays an important role in the decomposition process that is relatively consistent across sites. Further, our results suggest that factors besides photodegradation by UV radiation may explain observed patterns in litter mass loss in arid ecosystems.