In this link, they say the heavy use of nitrogen will interupt the Rhizobia symbiotic relationship with the plants.
They can supply the plant with all of its nitrogen needs. In return, the host plant supplies the rhizobium with simple carbohydrates. The plant can give up to 20% of its supply of carbohydrates to the bacteria. However, if there is a sufficient supply of nitrogen in the soil to meet the plantís needs, the plant will not engage in the relationship with the bacteria.