PARENT SESSION
Oral Session # 12: Biogeochemistry I: Carbon Dioxide Cycling.
Presiding: D Markewitz
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 204.

Consequences of elevated carbon dioxide and ozone atmospheres for leaf litter decomposition at Aspen-FACE.

Lindroth, Richard ¹, Parsons, William², Bockheim, James³, ¹ University of Wisconsin, Madison, WI, USA² Universite Laval, Ste. Foy, Quebec, Canada³ University of Wisconsin, Madison, WI, USA

ABSTRACT- The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO₂-mediated shifts in nutrient cycling (via changes in litter chemistry) as well as by interactions with other environmental pollutants, such as ozone (O₃). We evaluated the independent and interactive effects of enriched CO₂ (560 ppm) and O₃ (1.5 x ambient) environments on decomposition of trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) leaf litter at Aspen FACE (Rhinelander, Wisconsin, USA). Litter mass loss and chemical composition were monitored over two years. Reciprocal transplants of litterbags were employed to separate the effects of substrate quality (chemistry) from those of environment (fumigation treatment). Calculation of mass loss rates (k-values) revealed significant treatment effects that varied between tree species. For aspen, both CO₂ and O₃ retarded mass loss, and their effects were additive. Changes in loss rates were due primarily to shifts in litter chemical composition rather than to changes in the environment in which decomposition occurred. For birch, enriched CO₂ and O₃ independently tended to accelerate decomposition, but in combination did not do so. Accelerated decomposition was due to changes in both chemical composition and treatment environment. Differences in initial leaf litter quality were maintained between the species and sustained for at least one year among the fumigation treatments. For both aspen and birch, starch and condensed tannins were initially higher under elevated compared to ambient CO₂, regardless of O₃ level. Elevated C/N and lignin/N ratios were sustained under CO₂ enrichment, and likely contributed to slower litter decay rates for aspen (+CO₂ and +CO₂+O₃ treatments) and birch (+CO₂+O₃ treatment). These results suggest that nutrient cycling dynamics of north temperate forest ecosystems will change under atmospheric conditions predicted for the future.

Key words: decomposition, ozone, CO₂, FACE