Particulate and dissolved organic matter were generated from foliage of the emergent macrophyte Typha latifolia and the riparian tree Populus tremuloides grown both under ambient (AMB, 360 ul/L) and elevated (ELEV; 720 ul/L) atmospheric CO2 concentrations. Percentage lignin, C : N ratios, and percentage total phenolic compounds were significantly higher (15-100%) in live and senescent plant tissues grown in ELEV than in AMB CO2 conditions. Fungal biomass and rates of growth were 25% and 50% lower on the more recalcitrant particulate organic matter of ELEV Populus and Typha, respectively, both of which had appreciably lower N content than tissues of AMB grown plants. Bacterial development on the surfaces of ELEV tissues was nearly an order of magnitude lower than on AMB grown tissues. Leaching of dissolved organic carbon (DOC) was greater from ELEV CO2-grown leaves than those grown on AMB CO2. Bacterial production rates on leachates from Populus were much higher than those from Typha, and rates were significantly lower on those released from tissues from both species grown on elevated CO2. Leaves of the different plant species were allowed to decompose for up to 13 weeks under natural conditions. At specific intervals, supernatant leachate DOC was removed, sterilized by filtration, and exposed to natural light (full sunlight or PAR without UV). Simple organic substrates or CO2 generated photolytically were determined and the effects of the products on bacterial productivity were assayed at intervals for several days. Complete photolytic mineralization to CO2 of DOC freshly released (24 hr) from both plant species was higher (ca. 35%) from plants grown on ELEV than from AMB CO2. After longer periods of decomposition of leaves and leachates, photolytic mineralization of DOC to CO2 was reduced but that from ELEV-grown tissues was still greater than from AMB. Citric, acetic, malic, formic and other acids were cleaved from humic macromolecules by short (<6 hr) exposures to sunlight. Although PAR was not as effective as full sunlight with UV components, photolytic generation of fatty acids by PAR was clearly a major photodegradation process. Bacterial production was always signficantly enhanced by brief (<6 hr) exposure of leachates to either full sunlight or to PAR only.