The extent of mycorrhizal colonization of roots and its influence on plant growth and phosphorus content

The most common metric of arbuscular mycorrhizal fungal (AMF) abundance is percent root length colonized (PRLC) by mycorrhizal structures. Frequently, plants with greater PRLC are assumed to receive more nutrients (such as phosphorus, P) from their mycorrhizal symbionts, leading to greater plant growth. Nevertheless, the functional significance of this metric remains controversial. In this review, I discuss whether manipulations of PRLC generally led to changes in plant biomass and P content, and whether AMF taxa and plant functional groups influence these relationships. I conducted a meta-analysis of laboratory- and field-based trials in which mycorrhizal colonization was directly altered compared to unmanipulated controls. For each trial, I calculated (1) the difference in PRLC (Delta PRLC) between the treatments, and (2) the response ratio of plant biomass. In a subset of these studies, the response ratio of P content of host plants could also be calculated. The response ratio of plant biomass and P content rose significantly and exponentially as Delta PRLC increased. Nevertheless, Delta PRLC explained only a fraction of the variation in response ratios in each case. Moreover, AMF taxa varied in their effects on biomass per unit Delta PRLC. In addition, plant functional groups differed in effects on plant P content per unit Delta PRLC, with C4 grasses responding most strongly. It appears that as the extent to which plant roots are colonized by AMF increases, plant growth and P content often increase, although substantial variability exists among trials. As others have found, a likely mechanism for this relationship is increased transfer of P (and perhaps other nutrients) through the more-prevalent mycorrhizal structures.