Alterations in the photomineralization of allochthonous DOM related to elevated atmospheric CO2

Abstract

Climate-related shifts in forest composition and chemistry will likely affect the quantity and quality of dissolved organic matter (DOM) entering inland waters, and consequently, carbon dioxide (CO2) evasion. We examined the photodegradation of DOM derived from 2 common riparian plant species (Populus tremuloides and Salix alba) grown at ambient (360 ppm) and elevated (720 ppm) atmospheric CO2 concentrations. Rates and total photolytic CO2 production were determined for sterilized leachates ranging from 5 to 100 mg L−1 dissolved organic carbon (DOC). Based on multiple regression analysis, DOC concentration, followed by plant species, best predicted the rate and total flux of CO2. Photolytic CO2 production increased linearly with DOC concentration; however, the 5 mg L−1 treatment had the greatest rate per unit carbon, suggesting a self-shading effect of increasing DOC. The atmospheric CO2 conditions under which the plants were grown had no statistically significant effect, despite observed differences in CO2 fluxes between ambient and elevated Populus leachates. Fluorescence data suggest differences in photolytic CO2 production among treatments are related to differences in plant chemistry within the humic fraction. Thus, the magnitude of the photolytic CO2 flux from fresh waters in the future will depend primarily on climate-related changes in the quantity of terrestrial DOM inputs and secondarily by DOM source.