Elevated-CO2-induced changes in the chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter: subsequent mass loss and microbial response in a stream ecosystem

Changes in chemistry of quaking aspen (Populus tremuloides Michaux) leaf litter were examined under ambient (AMB = 360 ppm) and elevated (ELE = 720 ppm) levels of atmospheric CO2. Senesced ELE leaves were significantly higher in phenolic compounds, lignin, and ON than AMB leaves. A 30-d in situ experiment in a northern Michigan stream analyzed changes in leaf mass, the concentration of phenolic compounds as a result of chemical leaching, and the growth responses of fungi and bacteria. ELE leaves lost less mass than AMB leaves after a 30-d incubation. Although ELE leaves were initially higher in total phenolic compounds and condensed tannins, differences between the treatments were no longer observed after 48 h of chemical leaching. Bacterial biomass and community respiration were higher on the AMB leaves for the first 12 d of incubation, whereas fungal biomass and community respiration were higher in the AMB treatment by the end of the 30-d experiment. Fungal biomass was negatively correlated with C:N and positively correlated with bacterial biomass on the ELE leaves, but not on the AMB leaves. These results indicate that a doubling in concentration of atmospheric CO2 could lead to leaf Litter that is more recalcitrant toward microbial breakdown, which may decrease the availability of C and N for higher trophic levels.