The most common metric of arbuscular mycorrhizal abundance is percent root length colonized (PRLC) by mycorrhizal structures. Frequently, plants with greater PRLC are assumed to receive more nutrients (such as phosphorus) from their mycorrhizal symbionts, leading to greater plant growth. Nevertheless, the functional significance of this metric remains controversial. I asked whether manipulations of PRLC generally led to higher plant biomass and greater phosphorus content. I conducted a meta-analysis of 91 laboratory- and field-based studies in which mycorrhizal colonization was increased by adding inocula or reduced via fungicides, compared to unmanipulated controls. For each study, I calculated (1) the difference in PRLC between the treatments, and (2) the response ratio of plant biomass (manipulation/control). In 58 of these studies, the response ratio of phosphorus content of host plants could also be calculated.
Results/Conclusions
The response ratio of plant biomass (r2 = 0.262, P < 0.001) and P content (r2 = 0.281, P < 0.001) rose significantly and exponentially as PRLC increased. In both cases, the benefit of higher PRLC became markedly pronounced if PRLC increased 60% or more. Relationships between PRLC and plant biomass or P content did not vary significantly between plant functional groups, or between lab and field studies. It appears that as the extent to which plant roots are colonized by arbuscular mycorrhizal fungi increases, plant growth increases predictably. A likely mechanism for this relationship is increased transfer of phosphorus (and perhaps other nutrients) through the more-prevalent mycorrhizal structures. Because relationships were consistent across plant functional groups, PRLC seems to be a useful metric in a variety of systems.