Decomposition of litter produced under elevated CO₂: Dependence on plant species and nutrient supply

We investigated the effect of CO₂ concentration and soil nutrient availability during growth on the subsequent decomposition and nitrogen (N) release from litter of four annual grasses that differ in resource requirements and native habitat. Vulpia microstachys is a native grass found on California serpentine soils, whereas Avena fatua, Bromus hordaceus, and Lolium multiflorum are introduced grasses restricted to more fertile sandstone soils (Hobbs and Mooney 1991). Growth in elevated CO₂ altered litter C:N ratio, decomposition, and N release, but the direction and magnitude of the changes differed among plant species and nutrient treatments. Elevated CO₂ had relatively modest effects on C:N ratio of litter, increasing this ratio in Lolium roots (rind shoots at high nutrients), but decreasing C:N ratio in Arena shoots. Growth of plants under elevated CO₂ decreased the decomposition rate of Vulpia litter, but increased decomposition of Arena litter from the high-nutrient treatment. The impact of elevated CO₂ on N loss from litter also differed among species, with Vulpia litter from high-CO₂ plants releasing N more slowly than ambient-CO₂ litter, whereas growth under elevated CO₂ caused increased N loss from Avena litter. CO₂ effects on N release in Lolium and Bromus depended on the nutrient regime in which plants were grown. There was no overall relationship between litter C:N ratio and decomposition rate or N release across species and treatments. Based on our study and the literature, we conclude that the effects of elevated CO₂ on decomposition and N release from litter are highly species-specific. These results do not support the hypothesis that CO₂ effects on litter quality consistently lead to decreased nutrient availability in nutrient-limited ecosystems exposed to elevated CO₂.