Soil DIC uptake and fixation in Pinus taeda seedlings and its C contribution to plant tissues and ectomycorrhizal fungi

Plants can acquire carbon from sources other than atmospheric carbon dioxide (CO2), including soil-dissolved inorganic carbon (DIC). Although the net flux of CO2 is out of the root, soil DIC can be taken up by the root, transported within the plant, and fixed either photosynthetically or anaplerotically by plant tissues. We tested the ability of Pinus taeda L. seedlings exposed to 13C-labeled soil DIC and two NH4+ availability regimes to take up and fix soil DIC. We also measured the concentration and distribution of the fixed soil DIC within the plant and mycorrhizal tissues, and quantified the contribution of soil DIC to whole-plant carbon (C) gain. Seedlings exposed to labeled DIC were significantly enriched in 13C compared with seedlings exposed to unlabeled DIC (6.7 versus -31.7per thousand). Fixed soil DIC was almost evenly distributed between above- and belowground biomass (55 and 45%, respectively), but was unevenly distributed among tissues. Aboveground, stem tissue contained 65% of the fixed soil DIC but represented only 27% of the aboveground biomass, suggesting either corticular photosynthesis or preferential stem allocation. Belowground, soil DIC had the greatest effect (measured as 13C enrichment) on the C pool of rapidly growing nonmycorrhizal roots. Soil DIC contributed ~0.8% to whole- plant C gain, and ~1.6% to belowground C gain. We observed a slight but nonsignificant increase in both relative C gain and the contribution of soil DIC to C gain in NH4+-fertilized seedlings. Increased NH4+ availability significantly altered the distribution of fixed soil DIC among tissue types and increased the amount of fixed soil DIC in ectomycorrhizal roots by 130% compared with unfertilized seedlings. Increased NH4+ availability did not increase fixation of soil DIC in nonmycorrhizal roots, suggesting that NH4+ assimilation may be concentrated in ectomycorrhizal fungal tissues, reflecting greater anaplerotic demands. Soil DIC is likely to contribute only a small amount of C to forest trees, but it may be important in C fixation processes of specific tissues, such as newly formed stems and fine roots, and ectomycorrhizal roots assimilating NH4+.