Biological nitrogen fixation (BNF) by crop legumes reduces demand for industrial nitrogen fixation (INF). Nonetheless, rates of BNF in agriculture remain low, with strong negative feedback to BNF from reactive soil nitrogen (N) and drought. We show that breeding for yield has resulted in strong relationships between photosynthesis and leaf N in non-leguminous crops, whereas grain legumes show strong relations between leaf N and water use efficiency (WUE). We contrast these understandings with other studies that draw attention to the water costs of grain legume crops, and their potential for polluting the biosphere with N. We propose that breeding grain legumes for reduced stomatal conductance can increase WUE without compromising production or BNF. Legume crops remain a better bet than relying on INF.

豆类作物的生物固氮（BNF）减少了对工业固氮（INF）的需求。但是，农业中BNF的发生率仍然很低，土壤中的活性氮（N）和干旱对BNF产生了强烈的负反馈。我们表明，增产育种已导致非豆类作物中的光合作用与叶片氮之间有很强的关系，而豆类谷物则显示叶片N与水分利用效率（WUE）之间有很强的关系。我们将这些理解与其他研究相提并论，这些研究引起人们对豆类作物的水费以及它们对氮污染生物圈的潜在关注。我们建议，为降低气孔导度而育种豆类可以增加水分利用效率，而不会损害产量或BNF。与依靠INF相比，豆类作物仍然是更好的选择。