Rhizobia within legumes have also been shown to contribute
to the diversity of aboveground plants (Van der Heijden,
2006). Rhizobia fix atmospheric nitrogen for their leguminous
hosts. Nitrogen fixation often results in increased nitrogen
availability for neighbouring non-leguminous species (Hooper
et al., 2005). Grassland mesocosms inoculated with a variety
of rhizobia bacterial species produced greater community productivity
and evenness than mesocosms lacking rhizobia (Van
der Heijden, 2006). This increase in evenness (a component of
diversity) in grasslands is probably a result of facilitative interactions
between leguminous plants and forb species that
reduce competition for the limiting nutrient nitrogen. The
interaction of multiple mutualist partners, or mutualist guilds,
with plant hosts may be the norm (Stanton, 2003), thereby
suggesting that a wide variety of belowground mutualists contributes
to aboveground diversity.