Predicting surface litter decomposition rates in deserts has proven difficult and mass loss rates often exceed estimates. Several recent studies have found solar radiation, particularly UV radiation, to be responsible for significant litter decomposition in arid systems. We assessed the influence of different wavebands of solar radiation on the mass loss of litter in the Sonoran Desert of central Arizona. Three types of litter (leaves or twigs of Larrea tridentata (creosotebush), leaves of Cynodon dactylon (Bermudagrass)) were placed in envelopes that either (1) transmitted all solar radiation wavebands, (2) filtered UV-B radiation, (3) filtered UV-B and UV-A radiation, or (4) filtered UV-B, UV-A and visible radiation. Envelopes were placed on the soil surface in Larrea tridentata-dominated desert and loss was assessed at 3-4 month intervals over fourteen months.

Results/Conclusions

Solar UV-B, UV-A and visible wavebands all appeared capable of accelerating litter mass loss. In general, the solar UV-A waveband was most effective in accelerating mass loss, the UV-B waveband was the least effective, and the visible waveband was intermediate. The greater effectiveness of the UV-A waveband is likely attributable to its relatively high total irradiance (much higher than that of UV-B) along with its relative effectiveness in terms of photodegradation (probably much higher than that of visible). The effectiveness of solar radiation in accelerating litter decomposition was season specific and appeared to depend in part on precipitation and moisture availability. During the wettest season of our study, December through February, solar radiation exposure did not appear to accelerate litter mass loss. Rather, mass loss rates were greatest for litter in envelopes that filtered the greatest amount of solar radiation (all UV and visible radiation). Subsequent experiments found that soil and litter moisture was retained longer after precipitation events under or in envelopes that filtered or shaded all solar UV and visible radiation. This likely improved conditions for microbial activity and biotic litter decomposition. Rates of CO₂ release from soils/litter envelopes were greater from envelopes that filtered UV and visible radiation, supporting our idea that microbial activity was greater in litter and soils shaded from UV and visible radiation. Litter decomposition in our system appears to be strongly influenced by both microbes, which are promoted by moisture and inhibited by solar radiation, and solar radiation, which inhibits the effectiveness of microbes but accelerates decomposition through photodegradation.