Long-term effects of host-specific soil microbiota on plant interactions

It is increasingly recognised that resource competition and plant-soil feedback (PSF) effects can jointly determine outcomes of plant interactions. However, it is less clear whether PSF modulates intraspecific or interspecific competition, or intrinsic growth rate. Thus, it remains to be answered whether PSF alters coexistence predictions by changing competitive ability of interacting species (fitness differences) or rather by altering self-limitation (niche differences). Here, I examined the effects of host-specific soil inoculum, including target and competitor species inoculum, and non-specific inoculum on pairwise interactions of four pairs of grassland perennials. To explore whether the effects of PSF were persistent and dependent on resource availability, interactions were studied over a two-year period under control and fertilized conditions. These data were then used to estimate fitness differences and niche differences and to predict species coexistence. I found that a host-specific inoculum can promote the coexistence of competing plants in two ways. First, it increased niche differences due to less intense interspecific competition in the inoculum of a competitor. Second, a competitively inferior species was more likely to be found in the conspecific inoculum due to its negative effect on the intrinsic growth rate. These effects persisted throughout the experimental period but were absent after nutrient addition. In conclusion, PSF can promote plant coexistence by increasing niche differences and reversing competitive dominance. Although PSF effects can have long-term effects on competitive plant interactions, they depend on abiotic (nutrient) contexts. Therefore, they are less likely to mitigate competitive inequality and prevent competitive exclusion after anthropogenic fertilization.