Ecological Archives A017-044-A1

Jacqueline E. Mohan, James S. Clark, and William H. Schlesinger. 2007. Long-term CO2 enrichment of a forest ecosystem: implications for forest regeneration and succession. Ecological Applications 17:1198–1212.

Appendix A. Mass balance equation calculating the CO2 concentration in the understory of elevated plots at the Duke Forest FACE experiment.

Photosynthesis typically discriminates against 13CO2, such that the tissues of C3 plants are depleted in 13CO2  (-20 o/oo δ13C) relative to the CO2 of their atmosphere.   For plants in ambient plots, leaf tissue δ13C of -28 o/oo is expected, based on the global value of atmospheric CO213C = -8 o/oo).  Understory plants may show further depletion since the atmosphere in the forest understory is often influenced by soil respiration, which carries δ13C less than -8 o/oo (Andrews et al. 1999).  CO2 in the fumigated plots should have δ13C of -21, based on a mixing model of 365 µl/L of ambient CO2 at -8 o/oo and 200 µl/L of supplemental CO2 with δ13C of -43 o/oo, and leaf tissue in fumigated plots should average δ13C of -41 o/oo.  We used the current-year leaves of  Lonicera japonica (Japanese honeysuckle) collected at ≤15 cm from the soil surface in each subplot in July 1997 to ascertain the δ13C of the atmosphere and thus the adequacy and accuracy of the CO2 fumigation (Fig. 1 in text).  The mass-balance equation, where (mean ambient CO2 concentration × mean δ13C of ambient air) plus (200 µl/L elevated CO2 differential × δ13C of the supplemental CO2 source) equals (understory CO2 concentration in elevated plots × mean δ13C of elevated air) shows the actual CO2 concentration available to plants growing in the understory of the elevated plots.  We assumed an ambient CO2 concentration of 365 µl/L, and calculated the δ13C of ambient and fumigated understory air by subtracting the C3 photosynthetic fraction of -20 o/oo from the mean δ13C of the respective L. japonica foliage samples (Fig. 1 in text).  Thus, the equation [(365)(-13) + (200)(-43)] = -23 × [CO2]elevated understory.  The solution to this equation indicates a CO2 concentration of 580 µl/L in the understory of fumigated plots.  Given the low coefficients of variation of the ambient and elevated foliar δ13C signatures (0.85% and 2.09%, respectively), the CO2 levels were reasonably well controlled.  The CO2 concentration in the 0–15 cm of the ambient understory was likely slightly higher than the global average due to soil respiration (Andrews et al. 1999). 

LITERATURE CITED

Andrews, J. A., K. G. Harrison, R. Matamala, and W. H. Schlesinger. 1999. Separation of root respiration from total soil respiration using carbon-13 labeling during Free-Air Carbon Dioxide Enrichment (FACE). Soil Science Society of America 63:1429–1435.



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