Photodegradation in species from temperate South America: Direct effects of solar radiation on litter decomposition in terrestrial ecosystems

Background/Question/Methods The importance of litter quality as a major control on litter decomposition has been explored extensively in mesic and humid terrestrial ecosystems; lignin and lignin:nutrient ratios have been identified as key characteristics determining the rate of organic matter turnover. Recent experimental evidence has shown that, in semiarid ecosystems, photodegradation can be important in controlling litter decomposition; however, correlations between litter characteristics and decomposition are much less obvious. Our objective in this study was to evaluate the susceptibility to photochemical mineralization in a range of temperate species in order to quantify the importance of abiotic photodegradation on mass loss and correlate these losses with litter quality characteristics. We harvested litter from 25 native and introduced species growing in temperate ecosystems in Argentina ranging from mesic forests to semiarid steppe of Patagonia and natural grasslands and agroecosystems of the Pampa region. In a factorial experiment, we subjected litter of the twenty-five species in transparent plastic cups to three attenuation treatments: 1) Control (>95% solar radiation transmission): 2) ultraviolet block (attenuation of radiation <400 nm [UV-B and UV-A]) and 3) UV and shortwave visible block (attenuation of radiation <600 nm). Litter was collected and weighed at two time points over a growing season and evaluated for mass loss. Initial samples of all species were analyzed for water and ash content, lignin, cellulose and nutrient content, specific leaf area and tensile strength.

Results/Conclusions There was a highly significant effect of both litter type (P<0.001) and light treatment (P<0.0001) on abiotic mass loss. While direct photodegradative losses were relatively small overall, the magnitude of the effect with attenuation of UV and visible light was 12% and 65% respectively. Species varied widely in their susceptibility to photodegradation, but surprisingly, some of the species considered the most biotically recalcitrant (e.g. Araucaria araucana, Nothofagus dombeyi) had the largest photodegradative losses. Ín addition, low lignin litter types (particularly grasses, Zea mays and Poa ligularis) were most affected by the UV-attenuation treatment. Both lignin content and specific leaf area were positively correlated to abiotic mass loss, while nutrient content and C:N ratios demonstrated no relationship with photodegradation. These results suggest that a wide variety of plant litter types could be susceptible to photodegradation, and that the predictions of litter quality based on parameters developed in mesic ecosystems where biotic degradation dominates mass loss may not be adequate in ecosystems where photodegradation plays a dominant role in litter decomposition.