EFFECTS OF INCREASED SOLAR ULTRAVIOLET-RADIATION ON BIOGEOCHEMICAL CYCLES

Increases in solar UV radiation could affect terrestrial and aquatic b iogeochemical cycles thus altering both sources and sinks of greenhous e and chemically-important trace gases (e.g., carbon dioxide (CO2), ca rbon monoxide (CO), carbonyl sulfide (COS)). In terrestrial ecosystems , increased UV-B could modify both the production and decomposition of plant matter with concomitant changes in the uptake and release of at mospherically-important trace gases. Decomposition processes can be ac celerated when UV-B photodegrades surface litter, or retarded when the dominant effect involves changes in the chemical composition of livin g tissues that reduce the biodegradability of buried litter. These cha nges in decomposition can affect microbia[ production of carbon dioxid e and other trace gases, and also may affect the availability of nutri ents essential for plant growth. Primary production can be reduced by enhanced UV-B, but the effect is variable between species and even cul tivars of some crops. Likewise, the effects of enhanced UV-B on photop roduction of CO from plant matter is species dependent and occurs more efficiently from dead than living matter. Aquatic ecosystem studies i n several different locations have shown that reductions in current le vels of solar UV-B result in enhanced primary production, and Antarcti c experiments under the ozone hole demonstrated that primary productio n is inhibited by enhanced UV-B. In addition to its effects on primary production, solar UV radiation can reduce bacterioplankton growth in the upper ocean with potentially important effects on marine biogeoche mical cycles. Decomposition processes can be retarded when bacterial a ctivity is suppressed by enhanced UV-B radiation or stimulated when so lar UV radiation photodegrades aquatic dissolved organic matter (DOM). Photodegradation of DOM results in loss of UV absorption and formatio n of dissolved inorganic carbon (DIC), CO, and organic substrates that are readily mineralized or taken up by aquatic microorganisms. The ma rine sulfur cycle may be affected by UV-B radiation resulting in possi ble changes in the sea-to-air emissions of COS and dimethylsulfide (DM S), two gases that are degraded to sulfate aerosols in the stratospher e and troposphere, respectively.