The roles of mycorrhizae, soil microbes, and microbially-mediated soil processes in forest tree responses to rising atmospheric carbon dioxide

Nutrient limitations to forest tree productivity as CO{dollar}sb2{dollar} rises in our atmosphere may be ameliorated by stimulation of root symbiotic fungi (mycorrhizae) that increase plant nutrient uptake and by root-associated (rhizosphere) bacteria that chemically transform essential compounds into forms easily assimilated by plants. Plant photosynthate represents a major source of carbon for soil microbes, thus root symbiotic fungi and rhizosphere bacteria may benefit from increased photosynthesis at elevated CO{dollar}sb2{dollar} and increase plant nutrient uptake. We measured responses of mycorrhizae to CO{dollar}sb2{dollar} enrichment in laboratory and field studies and found differential stimulation of the two major types of plant-fungus symbiosis found in forest ecosystems. Colonization rates and biomass of ectomycorrhizal (ECM) fungi of white oak and shortleaf pine increased in response to CO{dollar}sb2{dollar} enrichment whereas no such increases were seen for vesicular-arbuscular mycorrhizae of yellow-poplar. These results suggest that elevated CO{dollar}sb2{dollar} may lend a competitive edge to ECM plants in forests where the two types of symbiosis coexist. Rhizosphere bacterial populations did not increase when yellow-poplar was grown at elevated CO{dollar}sb2.{dollar} Total plant biomass was greater at elevated CO{dollar}sb2{dollar} than at ambient in all of these studies.; Mycorrhizae, rhizosphere bacteria and plants are all dependent upon stable rates of plant litter decomposition that leads to constantly renewed soil nutrient pools. Rising CO{dollar}sb2{dollar} in the atmosphere may alter the decomposability of plant litter by reducing nitrogen content of litter as well as increasing recalcitrant compounds such as lignin. This would reduce rates of decomposition and could offset the benefits of stimulating root-associated microorganisms. We measured litter chemistry of yellow-poplar and white oak grown in the field at twice-ambient levels of CO{dollar}sb2{dollar} and found no differences in decomposability of their leaf litter. Other investigators, however, using pot-grown seedlings and artificial light have seen changes in litter composition with CO{dollar}sb2{dollar} enrichment. A comparison of potted vs nonpotted seedlings of sugar maple exposed to elevated CO{dollar}sb2{dollar} and elevated (+4{dollar}spcirc{dollar}C) temperature again revealed no effect of CO{dollar}sb2{dollar} enrichment and no interactions between "pot" and CO{dollar}sb2.{dollar} Elevating temperature, however, increased nitrogen concentrations in litter, suggesting the potential for increased decomposition rates with global warming.