Long-term N addition can cause disruption of the legume-rhizobium mutualism, possibly due to the degraded soil diazotroph pool, the weakened plant-microbe feedback pathways, disrupted plant selective mechanisms, and the decreased propensity to form nodules. Since the legume-rhizobium symbioses is the main contributor of natural fixed nitrogen to terrestrial ecosystems, a thorough characterization of diazotroph community structure and legume symbioses efficiency under long-term managements of fertilization is critical to benefit agricultural productivity and sustainability. The Morrow Plots is an agricultural experiment that contains unfertilized plots as well as plots that receive organic and inorganic fertilizer, as well as two rotations that include legumes (soybean and alfalfa). The 147-year duration of this experiment makes it uniquely suited for characterizing the evolutionary consequences of long-term fertilization on legume-rhizobium symbioses in this project led by Angela Kent and graduate student Danyang Duan, in collaboration with Tony Yannarell.