Tracer studies of carbon source utilization in a wetland on the Canadian shield

It is generally agreed that the uptake of C by plants via their root systems is <2% of total C assimilated. However, the potential for very high specific activity 14C in the root zone near low to intermediate level radioactive waste management areas, as the result of upwelling contaminated groundwater or soil gas, makes it imperative to establish more precise upper limits for this assimilation pathway. Studies have been undertaken at field sites at the Chalk River Laboratories to address this concern. In order to check the hypothesis that the transpirational stream may be an important pathway for C assimilation in plants growing under anoxic or wetland conditions, several direct comparisons of small plants, potted and planted, have been carried out in a discharge zone at which both atmospheric and soil pore water C contain heightened levels of 14C. Although the measurements reported here must be considered preliminary, they suggest that no more than 0.1% of the C assimilated by a plant growing under these conditions is delivered via the roots. Prior to performing these experiments the vertical gradient in 14CO2 specific activity in the air at this site was measured both by analyses of lichens growing at different heights on one tree, and by direct collection of CO2 with passive alkali traps. This gradient has been used to provide an estimate of the % of primary biomass production subject to this heightened signal, and on that basis a revised inventory of the discharged 14C that is tied up in vegetation and deposited locally as litter is recommended.