Independent, interactive, and species-specific responses of leaf litter decomposition to elevated CO2 and O3 in a northern hardwood forest

The future capacity of forest ecosystems to sequester atmospheric carbon is likely to be influenced by CO(2)-mediated shifts in nutrient cycling through changes in litter chemistry, and by interactions with pollutants like O(3). We evaluated the independent and interactive effects of elevated CO(2) (560 mu l l(-1)) and O(3) (55 nl l l(-1)) on leaf litter decomposition in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) at the Aspen free air CO(2) enrichment (FACE) site (Wisconsin, USA). Fumigation treatments consisted of replicated ambient, +CO(2), +O(3), and +CO(2) + O(3) FACE rings. We followed mass loss and litter chemistry over 23 months, using reciprocally transplanted litterbags to separate substrate quality from environment effects. Aspen decayed more slowly than birch across all treatment conditions, and changes in decomposition dynamics of both species were driven by shifts in substrate quality rather than by fumigation environment. Aspen litter produced under elevated CO(2) decayed more slowly than litter produced under ambient CO(2), and this effect was exacerbated by elevated O(3). Similarly, birch litter produced under elevated CO(2) also decayed more slowly than litter produced under ambient CO(2). In contrast to results for aspen, however, elevated O(3) accelerated birch decay under ambient CO(2), but decelerated decay under enriched CO(2). Changes in decomposition rates (k-values) were due to CO(2)- and O(3)-mediated shifts in litter quality, particularly levels of carbohydrates, nitrogen, and tannins. These results suggest that in early-successional forests of the future, elevated concentrations of CO(2) will likely reduce leaf litter decomposition, although the magnitude of effect will vary among species and in response to interactions with tropospheric O(3).