Do Riparian Plants Fix CO2 Lost by Evasion from Surface Waters? An Investigation Using Carbon Isotopes

Abstract

Rivers and streams in many parts of the world contain high concentrations of dissolved carbon dioxide, which is lost to the atmosphere by evasion (outgassing). Recent methodological advances now enable the carbon isotopic composition of this evaded CO2 to be measured directly, with early results from peatland streams showing it to be depleted in 13C relative to the atmosphere. The first direct measurements of the radiocarbon age of evaded CO2 for a stream draining a peatland site in the United Kingdom covered a large range, ranging from modern to >1400 yr BP. We investigated whether a proportion of the carbon fixed by plants growing adjacent to the stream was derived from evasion; this would have implications for the cycling of carbon in such streams, and lead to riparian plants having older, rather than contemporary, 14C ages. 13C analysis of riparian plants at the site suggested that up to 20% of the carbon they fixed was derived from evasion. Although the 14C content of the same samples suggested a similar proportion of plant carbon was derived from evaded CO2, the uncertainty in these estimates was greater. Fixation of evaded CO2 was greatest in plants growing within 1 m of the stream. Although the results for this site suggest a small amount of CO2 lost by evasion from the stream surface is incorporated in plant material, it may be a more important factor at other sites characterized by higher evasion rates, lower wind speeds, and different stream bank morphology.