PARENT SESSION
Oral Session # 66: Ecological Studies at Biosphere II.
Presiding: ML Martinez Vazquez
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 104.

Toward an isotopic mass-balance for carbon for cottonwood (Populus deltoides) plantations grown under CO₂-enrichment inside Biosphere II, Arizona.

Marshall, John¹, Gafford-Barron, Greg², Grieve, Katie², Murthy, Ramesh², ¹ Dept. of Forest Resources, Moscow, ID, USA² Biosphere II, Tucson, Arizona, USA

ABSTRACT- Rates and stable carbon isotope composition (¹³C) were measured for major fluxes to and from fast-growing cottonwood plantations in three sealed bays at Biosphere II near Tucson, Arizona. Air and tissue measurements were made at a range of CO₂ concentrations 400, 800, and 1200 ppm. The soil flux was measured from PVC rings with a Li-cor 6200 and ¹³C of soil respiration was measured on air equilibrated with the soil atmosphere in inverted 12-L dishes. We measured ¹³C of soluble carbohydrates in leaves to estimate the isotopic composition of recent photosynthate. Leaf-tissue sampled in August had ¹³C averaging -31.4, -40.4, and -44.4 (SE = 0.3) ‰ for the 400, 800, and 1200 ppm treatments, respectively. The pure CO₂ added to the bays, when sampled in October, had an average ¹³C of -10.6 (SE = 0.6) ‰. The atmospheric ¹³C was surprisingly similar. Soil respiration showed a small but consistent difference between the bays. By combining these estimates of fluxes and ¹³C, we hope to construct a closed isotopic mass balance for carbon inside each of the bays.

Key words: stable carbon isotope, carbon-dioxide enrichment, carbon budget, soil respiration