Sanctions and mutualism stability: why do rhizobia fix nitrogen?

Abstract

Why do rhizobia expend resources on fixing N2 for the benefit of their host plant, when they could use those resources for their own reproduction? We present a series of theoretical models which counter the hypotheses that N2 fixation is favoured because it (i) increases the exudation of useful resources to related rhizobia in the nearby soil, or (ii) increases plant growth and therefore the resources available for rhizobia growth. Instead, we suggest that appreciable levels of N2 fixation are only favoured when plants preferentially supply more resources to (or are less likely to senesce) nodules that are fixing more N2 (termed plant sanctions). The implications for different agricultural practices and mutualism stability in general are discussed.