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 & 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 (and shoots at high nutrients), but decreasing C:N ratio in Avena shoots. Growth of plants under elevated CO2 decreased the decomposition rate of Vulpia litter, but increased decomposition of Avena 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 chat 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.