Studies of biodiversity and ecosystem function (BEF) have long focused on the role of nitrogen (N)-fixing legumes as a functional group that occupies a distinct and important niche relative to other plants. Because of their relationship with N-fixing rhizobial bacteria, these legumes access a different pool of N than other plants and therefore directly contribute to increases in productivity and N-cycling. Despite their recognized importance in the BEF literature, the field has not moved far beyond investigating the presence/absence of the legume functional group in species mixtures. Here, we synthesize existing information on how the diversity (species richness and functional diversity) of both legumes and the rhizobia that they host impact ecosystem functions, such as nitrogen fixation and primary productivity. We also discuss the often-overlooked reciprocal direction of the BEF relationship, whereby ecosystem function can influence legume and rhizobial diversity. We focus on BEF mechanisms of selection, complementarity, facilitation, competitive interference, and dilution effects to explain how diversity in the legume–rhizobia mutualism can have either positive or negative effects on ecosystem function—mechanisms that can operate at scales from rhizobial communities affecting individual legume functions to legume communities affecting landscape-scale ecosystem functions. To fully understand the relationship between biodiversity and ecosystem function, we must incorporate the full diversity of this mutualism and its reciprocal relationship with ecosystem function into our evolving BEF framework.
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