Impact of elevated CO2 on mycorrhizal associations and implications for plant growth

The impact of increasing concentrations of atmospheric CO2 upon plant physiology has been widely investigated. Plant, and in particular root, growth is nearly always enhanced as a direct consequence of CO2 enrichment, with C-3 species generally more responsive than C-4 species. Such alterations in plant productivity will have consequence for below-ground processes and increased carbon allocation to the roots may favour symbiotic relationships. This paper discusses the current information available for the consequences of these changes upon mycorrhizal relationships. Generally mycorrhizal plants grown under CO2 enrichment show enhanced phosphorus uptake but nitrogen uptake is unaffected. This increased nutrient uptake is not correlated with increased mycorrhizal colonization of the roots. Similarly root exudation does not increase under CO2 enrichment but qualitative differences have yet to be assessed. However, it is predicted that total rhizodeposition of materials will increase as will litter inputs, although mineral and biochemical alterations to these plant derived inputs may occur. The consequences of such changes within the rhizosphere are discussed and future research priorities identified.