© 2007 Heron Publishing—Victoria, Canada
Soil DIC uptake and fixation in Pinus taeda seedlings and its C contribution to plant tissues and ectomycorrhizal fungi
Chelcy R. Ford (1–3), Nina Wurzburger (1), Ronald L. Hendrick (1) and Robert O. Teskey (1)
1. Warnell School of Forestry and Natural Resources, University of Georgia, Athens GA, 30602, USA / 2. Present address: USDA FS SRS Coweeta Hydrologic Lab, Otto NC, 28763, USA / 3. Corresponding author (crford@fs.fed.us) / Received March 1, 2006; accepted May 24, 2006; published online December 1, 2006
Summary
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 –31.7‰). 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+.
Keywords:
anaplerotic fixation, carbon cycling, carbon dioxide, corticular photosynthesis, dissolved inorganic carbon, stable isotope.
