Insights into mycorrhizal colonisation at elevated CO2: a simple carbon partitioning model

A simulation model was used to investigate the effect of an increased rate of plant photosynthesis at enhanced atmospheric CO2 concentration on a non- leguminous plant-mycorrhizal fungus association. The model allowed the user to modify carbon allocation patterns at three levels: (1) within the plant (shoot-root), (2) between the plant and the mycorrhizal fungus and (3) wit hin the mycorrhizal fungus (intraradical-extraradical structures). Belowgro und (root and fungus) carbon losses via respiration (and turnover) could al so be manipulated. The specific objectives were to investigate the dynamic nature of the potential effects of elevated CO2 on mycorrhizal colonisation and to elucidate some of the various mechanisms by which these effects may be negated. Many of the simulations showed that time (i.e. plant age) had a more significant effect on the observed stimulation of mycorrhizal coloni sation by elevated CO2 than changes in carbon allocation patterns or belowg round carbon losses. There were two main mechanisms which negated a stimula tory effect of elevated CO2 on internal mycorrhizal colonisation: an increa sed mycorrhizal carbon allocation to the external hyphal network and an inc reased rate of mycorrhizal respiration. The results are discussed in relati on to real experiments. The need for studies consisting of multiple harvest s is emphasised, as is the use of allometric analysis. Implications at the ecosystem level are discussed and key areas for future research are present ed.