Effects of solar radiation on litter decomposition in semi-arid grasslands: More to the ultraviolet than meets the eye?

Solar radiation, including ultraviolet-B radiation, plays an important role in the photomineralization of organic matter in aquatic ecosystems, and recently has been shown to be a dominant driver of carbon loss in semiarid ecosystems. At the same time, the mechanistic basis for photodegradative losses of senescent plant material in terrestrial ecosystems is poorly understood. Our objective in this study was to examine the relative importance of different wavelengths of solar radiation on abiotic photodegradation in fire-prone grasslands of central Argentina.  Using plastic filters which attenuated different wavelengths of natural solar radiation, we suspended senescent plant material of native grass species above the soil surface and beneath filters which attenuated radiation in the following manner:  1)’Control’ – transparent to all solar radiation 2)’-UV-B’ – mylar filter that attenuated nearly all solar UV-B (280-315 nm) radiation 3)’-UV’ – lexan filter that attenuated both UV-B and UV-A radiation (280-400 nm) and 4)’ –UV&Blue’ – a combination of filters that attenuated solar radiation less than 500 nm.  We found a highly significant effect of radiation exclusion on mass loss after four months of incubation in the field (P<0.0001), with a reduction of  25%, 50% and 60% in mass loss in treatments of increasingly attenuated solar radiation. These results highlight the importance of UV-B induced photodegradation, but also demonstrate that both UV-A and short-wave visible light can have important impacts on photodegradative carbon losses.  Aspects of litter quality including carbon chemistry or morphology may be critical in determining the susceptibility of different plant litter to photodegradation from solar radiation.