Microbial responses to elevated atmospheric CO2 and O3 in northern temperate forests

Abstract

Rising atmospheric concentrations of CO2 and O3 have the potential to alter terrestrial ecosystem functioning by modifying rates of photosynthesis, plant growth and the production of plant detritus. Because soil microbial communities are structured by the amount and biochemical composition of plant detritus, rising concentrations of atmospheric CO2 and O3 have the potential to alter the composition and activity of soil microbial communities. The objectives of this study were to gain a basic ecological understanding of microbial metabolic responses to elevated CO2 and O3 and to examine how these responses are modified by different plant communities over the growing season. To test these ideas, microbial activity was studied under Populus tremuloides and a mixture of Betula papyrifera and Populus tremuloides exposed to experimental CO2 (ambient and 560μL L-1) and O3 treatments (ambient and 54.5nL L-1). I employed extracellular enzyme analysis of 1,4 β,D-glucosidase and cellobiohydrolase to assess microbial degradation of cellulose. Averaged among vegetation types, atmospheric CO2 and O3 interacted to significantly influence β-glucosidase activity of subsurface mineral soil, such that O3 counteracted the stimulatory effects of elevated CO2. In the aspen community, exposure to elevated CO2 (-20%) and elevated O3 (-20%) significantly reduced the activity of β-glucosidase, relative to the ambient condition. Additionally, exposure to elevated O3 significantly reduced the activity of cellobiohydrolase (-40%), relative to the ambient condition, beneath aspen. Enzyme activity in the aspen-birch community did not differ by treatment. Regarding seasonality, β-glucosidase activity and cellobiohydrolase activity were greatest in May and steadily declined in July and October for both vegetation types. Different responses between aspen and aspen-birch communities possibly result from changes in plant litter chemistry or a change in microbial community composition of the aspen community. As a result, forest community composition and ontogeny may be important factors in understanding ecosystem responses to elevated CO2 and O3.