Background
The authors write that "ectomycorrhizal (ECM) fungi, forming the dominant type of symbiotic association with trees in boreal forests, receive as much as 25% of the total carbon assimilated by plants," and that, in return, "the extraradical fungal mycelium is directly involved in mobilization and uptake of nutrients which are, in part, passed on to the host plant." This important function is accomplished via the fungal exudation of a variety of low molecular weight organic compounds, polymer degrading enzymes, siderophores, polymeric carbohydrates and fatty acids, the dominant components of which -- low molecular weight organic acids, saccharides, amino acids and peptides -- "play important roles in enhancing mineral weathering, nutrient mobilization and uptake by plants."

What was done
Seedlings of Scots pine (Pinus sylvestris) trees were grown in the laboratory in liquid culture for a period of six weeks with either no ECM fungi or one of eight different such species associated with their roots, during which period they were exposed to air of either 350 or 700 ppm CO2, after which period a number of analyses were performed to identify and quantify the variety of exudates produced by the fungi.

What was learned
Johansson et al. report they observed "a clear impact of elevated CO2 on exudation of soluble low molecular weight organic compounds," and that these exudates "increased by 120-270%" due to "the increased carbon availability to the plant-fungus system," which was driven by the increased atmospheric CO2 concentration employed in their study that increased net CO2 assimilation rates (i.e., photosynthesis) by approximately 40% for both ECM and non-mycorrhizal seedlings, and which led to a mean increase of 27% in the growth (i.e., total biomass production) of the seedlings infected with the eight different species of ECM fungi, but which led to only a 14% increase in the biomass of the non-infected seedlings.

What it means
The four researchers concluded that the phenomena they observed "may contribute to nutritional feedback mechanisms to sustain tree growth when nutrients become limiting," such as some have hypothesized might occur over time in trees growing on low-fertility soils in CO2-enriched air (see Nitrogen (Progressive Limitation Hypothesis) in our Subject Index). The findings of this study, however, as well as those of the study of de Graaff et al. (2009) -- which was published in the same issue of Soil Biology & Biochemistry -- clearly indicate that earth's plants are well equipped to deal with this hypothetical roadblock to higher plant productivity in a CO2-enriched world of the future.

Reference
de Graaff, M.-A., Van Kessel, C. and Six, J. 2009. Rhizodeposition-induced decomposition increases N availability to wild and cultivated wheat genotypes under elevated CO2. Soil Biology & Biochemistry 41: 1094-1103.