Solar radiation exposure accelerates decomposition and biotic activity in surface litter but not soil in a semiarid woodland ecosystem in Patagonia, Argentina

Photodegradation of senescent plant material has been identified as an important vector of aboveground carbon (C) loss in aridland ecosystems, but the consequences for biotic activity and soil C in the field are not well understood. We established an experiment in a semiarid woodland in Patagonia, Argentina with attenuation of solar radiation and additions of leaf litter to evaluate impacts of photodegradation on changes in labile C and biotic activity in aboveground litter and surface soils. Litter decomposition was significantly accelerated by exposure to solar radiation. Moreover, labile sugars (hexoses and pentoses), microbial enzymatic activity (β-glucosidase activity) and available carbohydrates for cellulase degradation (saccharification) all significantly increased in sunlight-exposed litter. None of these stimulatory effects were observed in the surface soils exposed to sunlight. On the contrary, soil microbial biomass and β-glucosidase activity in surface soils were significantly greater only with litter addition and attenuated sunlight. Our results suggest that photodegradation of plant litter (production of volatile compounds through photochemical mineralization) and photofacilitation (stimulation of biotic activity due to change in litter chemistry with exposure to sunlight) generate rapid turnover of C in aboveground litter. The consequences of this accelerated C turnover may be that a fraction of leaf litter decomposes and is directly released back to the atmosphere as CO2 and never enters soil organic matter pool. Taken together, these results highlight the functional importance of solar radiation in determining the C balance in semiarid ecosystems.