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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