Effects of elevated CO2 and shade on the decomposition of senesced tree foliage: impacts on microbial activity

We examined microbial respiration and carbon/nitrogen content of decomposing leaf material in microcosms used for growth studies of the treehole mosquito, Aedes triseriatus. Leaf material originated from birch and oak trees exposed to conditions of shade/sun and elevated/ambient levels of CO2. Microbial respiration as measured by CO2 production was generally greatest on birch leaves grown under shaded conditions, however, ANOVA indicated possible light X CO2 interactions. There were also strong interactions between species of leaf and CO2 levels, but oak leaves grown under elevated CO2 supported significantly higher microbial respiration rates than oak leaves grown in an ambient CO2 atmosphere. Birch leaves grown under elevated CO2 also generally supported higher rates of microbial respiration. However, light effects were much more pronounced and birch leaves grown under full sun and elevated CO2 conditions supported relatively low microbial respiration. Microbial respiration varied inversely with leaf carbon:nitrogen ratio and directly with nitrogen content across treatments, however, initial carbon and nitrogen content of leaf material was not a consistent predictor of microbial respiration. In general, mosquito production paralleled microbial respiration, suggesting a tight link between the two trophic levels. These data indicate that interactions between available light and CO2 on parent plant material could have variable, species-dependent effects on microorganisms and secondary consumers in aquatic, detritus-based systems.