Decomposition of Betula papyrifera leaf litter under the independent and interactive effects of elevated CO2 and O3

Litter decay dynamics of paper birch (Betula papyrifera) were assessed at the Aspen free‐air CO₂ enrichment (FACE) facility in northern Wisconsin, USA. Leaf litter was decomposed for 12 months under factorial combinations of 360 vs. 560 μL CO₂ L^?1, crossed with 36 vs. 55 nL O₃ L^?1. To differentiate between substrate quality and environment effects, litterbags were placed in their Native Plots of origin or transplanted into the other treatments. CO₂ enrichment, regardless of O₃ concentration, produced poorer quality litter (high C/N, lignin/N and condensed tannins) than did ambient CO₂ (low C/N, lignin/N and condensed tannins). Substrate quality differences were reflected in the mass loss rates (k‐values), which were high for litter generated under ambient CO₂ (0.887 year^?1) and low for litter generated under elevated CO₂ (0.674 year^?1). The rate‐retarding effects of CO₂ enrichment were neither alleviated nor exacerbated by O₃ exposure. Decay rates varied, however, depending on whether litter was placed back into its plot of origin or transplanted to Common Gardens. The results of this study are species specific, but they have important implications for understanding the processes regulating storage of fixed C and the release of CO₂ from northern forest ecosystems.