Does UV radiation alter short-term leaf litter decomposition in contrasting dryland canopy regimes?

Background/Question/Methods

Decomposition of surface organic matter is an important component of biogeochemical cycling, but drivers in semi-arid ecosystems are not well understood. Some studies suggest that in drylands, where vegetation cover is sparse, UV radiation may be particularly important via its photochemical impacts on litter breakdown and microbial decomposers. We examined this possibility by quantifying mass loss of Prosopis velutina leaf litter in litterbags constructed with fiberglass window screen and perforated UV-manipulating plastic of two types: Aclar (a UV transparent plastic) and Mylar (a UV-B absorbing plastic). Litterbags were deployed at a Sonoran Desert field site in settings with contrasting levels of radiant energy exposure: on bare ground in an intercanopy zone (area between shrubs receiving full sun) and a subcanopy zone (shaded area beneath a P. velutina canopy). Five litterbags from each treatment combination were collected weekly for four weeks. Soil surface temperature, photosynthetically active radiation, and soil moisture were monitored; and phospholipid fatty acid (PLFA) concentrations were used to measure effects on litter lipid profiles and as an index of microbial activity. Daily precipitation was recorded at a nearby monitoring station. UV and canopy treatments were compared using a mixed-effect model controlling for temporal autocorrelation.

Results/Conclusions

Litter mass loss ranged from 10 to 40% over the four-week period. Decomposition rates were highest in the intercanopy zone and lowest in the subcanopy zones (F_{1, 97} = 4.83, p = 0.03), but there were no significant differences between UV treatments. Weekly mass loss dynamics were strongly coupled to the occurrence of precipitation events. Although maximum daily soil surface temperatures in the intercanopy zones (mean + SE = 54.1 ± 0.9 C) were significantly higher than those in the subcanopy (44.2 ± 0.7 C), soil moisture content (0-5 cm) was comparable. Analysis of litter incubated in a controlled environment showed little discernable impact of temperature on PLFA sources (fungal, bacterial, plant). Minor (ca. 10%) reductions in total lipids occurred with continuous incubation at 60 C, but not until 3 weeks of exposure. These results suggest that short-term surface litter decomposition in this hot desert ecosystem is influenced primarily by the amount of total radiant energy exposure and precipitation, but not by UV levels. However, UV radiation may influence longer-term rates via their effects on microbial faunal communities.