**3. Biological nitrogen fixation: A sound alternative to nitrogen fertilizer**

Attempts to reduce the use of nitrogen fertilizers need to incorporate responsible, cost effec‐ tive and environmentally sound options; an improved use of legumes and an increased un‐ derstanding of legume nodulation represent such options. Indeed, this symbiotic relationship between legumes and rhizobia represents the most important nitrogen-fixation association in the world, with an annual production of approximately 200 million tons of ni‐ trogen (Peoples et al., 2009; Jensen *et al.,* 2012). Optimising this symbiosis can increase crop yields and enhance soil fertility, whilst reducing the negative monetary costs and environ‐ mental impacts associated with nitrogen fertilizer use (Hirel et al., 2007; Peoples et al., 2009; Canfield et al., 2010). Hence, increasing our use of legume crops and identifying critical fac‐ tors required to control nodulation are seen as pivotal steps towards reducing our reliance on nitrogen fertilizers and improving agricultural sustainability (*e. g.* , Giller and Cadisch 1995; Vance 2001; Peoples et al., 2009; Rockstrøm *et al.,* 2009; Canfield et al., 2010; Jensen et al., 2012).

Soybean alone is estimated to produce up to 200 kg N ha-1 in aboveground biomass in a sin‐ gle growing season. Of the soybean nitrogen content, 58-68% is estimated to be derived from symbiotic nitrogen fixation (Salvagiotti et al., 2008; Peoples et al., 2009; Jensen et al., 2012). Following harvesting, the remaining portions of the plant, including roots and nodules which represent 30-60% of the nitrogen content, are left to replenish the nitrogen content of the surrounding soil (Mahieu et al., 2007; McNeill and Fillery 2008).
