Mycorrhizal benefit to plants is most frequently evaluated through growth differences between mycorrhizal (M) and non-mycorrhizal (NM) plants. These growth differences are often considered to be due to differences in belowground C expenditure, or in cost efficiency, i.e. amount of nutrients acquired per C expended.

We searched published reports for relations between plant growth and belowground C allocation, C use efficiency, or nutrient uptake, in ectomycorrhizal (ECM) versus non-mycorrhizal plants. We found a similar number of cases of negative, null or positive effects of ECM on plant growth. These effects were not correlated with differences on belowground C allocation or C use efficiency between M and NM plants. In contrast, they were very strongly correlated with mycorrhizal effects on plant N gain.

A comprehensive analysis of the published data therefore provided evidence that C is an excess, rather than a costly, resource, and that the outcome of the symbiosis depends only on whether mycorrhizae result in increased or decreased nutrient acquisition compared with NM plants, and not on cost efficiency differences between M and NM plants. Consequences of this finding for the regulation of resource exchange between symbionts and the nature of the symbiosis are discussed.