Nitrogen (N) availability strongly inﬂuences the structure and function of ecosystems (e.g. Vitousek & Howarth, 1991), but only a relatively small number of microbial groups have the ability to convert the N2 in our atmosphere into biologically available forms.This process, N2 ﬁxation, is the dominant source of new N to the biosphere outside of anthropogenic inputs (Vitousek et al., 2013).Some N2-ﬁxing microorganisms live independently on plant leaves, on decomposing organic material, and in soil (Reed et al.,2011), while others have co-evolved with a few higher plant taxa to form symbioses that ﬁx N2 in root nodules (e.g. Sprent & Raven,1985). The relationship between these legumes and their root nodule symbionts (rhizobia) is one of the most well studied plant –microbe symbioses. Yet, many important questions about the controls, interactions, and implications of legume N2 ﬁxation remain unanswered. In this issue of New Phytologist (pp. 690–699),Wolf, Funk, & Menge elegantly address a fundamental set of questions about N2 ﬁxation in their examination of how herbaceous legumes, their symbionts, and external N availability interact to govern legume access and storage of N.