Quality, Decomposition and Isopod Consumption of Tree Litter Produced under Elevated CO 2

Rising atmospheric CO 2 is expected to alter plant tissue quality which in turn could affect litter quality, decomposition, and carbon and nutrient turnover. We tested this hypothesis using leaf litter of beech (Fagus sylvatica) and branchlets (wood + bark) of spruce (Picea abies) produced under contrasting CO 2 concentrations in model ecosystems. Both types of litter produced under elevated CO 2 had significantly lower N concentrations, but showed no CO 2 -related differences in carbon and lignin concentrations. Decomposition rates (mass loss) assessed in a natural temperate forest were significantly slower in litter produced at high CO 2 . However, this effect became stronger in beech leaves but gradually disappeared in spruce branchlets over the 331-d exposure period. Irrespective of CO 2 treatment beech leaf litter lost 16% of its initial N content. Spruce branchlets produced at low CO 2 lost 50% of their initial N content, and those produced at high CO 2 lost 26%. Two isopod species representing native macro-decomposers consumed 36% more of the high CO 2 -produced beech litter than they did of low CO 2 -produced beech litter. Only small, and non-significant increases in consumption of high CO 2 -produced spruce branchlets were observed. Isopods feeding on high CO 2 litter also produced more feces than those feeding litter from low CO 2 Our results indicate that CO 2 -induced litter quality changes influence only certain stages of decomposition, and that these stages differ between different litter types. Inhibitory effects of elevated CO 2 , however, may be compensated by the positive feed-back of intensified litter processing of low quality litter by macro-decomposers. Consequently, the entire cycle of litter production and decomposition must be included in the analysis of the potential effects of rising CO 2 on litter decomposition. This includes both micro- and macro-decomposer specific effects.