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CHAPTER 5

EFFECTS OF ENHANCED SOLAR ULTRAVIOLET RADIATION ON BIOGEOCHEMICAL CYCLES


R. G. Zepp (USA), T. V. Callaghan (UK), and D. J. Erickson (USA)


Summary

Effects of increased UV-B on emissions of carbon dioxide and carbon monoxide (CO) and on mineral nutrient cycling in the terrestrial biosphere have been confirmed by recent studies of a range of species and ecosystems. The effects, both in magnitude and direction, of UV-B on trace gas emissions and mineral nutrient cycling are species-specific and operate on a number of processes. These processes include changes in the chemical composition in living plant tissue, photodegradation (breakdown by light) of dead plant matter, including litter, release of carbon monoxide from vegetation previously charred by fire, changes in the communities of microbial decomposers and effects on nitrogen-fixing microorganisms and plants. Long-term experiments are in place to examine UV-B effects on carbon capture and storage in biomass within natural terrestrial ecosystems. Studies in natural aquatic ecosystems have indicated that organic matter is the primary regulator of UV-B penetration. Changes in the organic matter, caused by enhanced UV-B reinforced by changes in climate and acidification, result in clarification of the water and changes in light quality that have broad impacts on the effects of enhanced UV-B on aquatic biogeochemical cycles. Increased UV-B has positive and negative impacts on microbial activity in aquatic ecosystems that can affect carbon and mineral nutrient cycling as well as the uptake and release of greenhouse and chemically-reactive gases. Photoinhibition of surface aquatic microorganisms by UV-B can be partially offset by photodegradation of dissolved organic matter to produce substrates, such as organic acids and ammonium, that stimulate microbial activity. Modeling and experimental approaches are being developed to predict and measure the interactions and feedbacks between climate change and UV-B induced changes in marine and terrestrial biogeochemical cycles. These interactions include alterations in the oxidative environment in the upper ocean and in the marine boundary layer and oceanic production and release of CO, volatile organic compounds (VOC), and reactive oxygen species (such as hydrogen peroxide and hydroxyl radicals). Climate related changes in temperature and water supply in terrestrial ecosystems interact with UV-B radiation through biogeochemical processes operating on a wide range of time scales.

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