This happens due to the possibility  that nitrogen fixing microorganisms need reasonable amount of Phosphorus to fix the nitrogen. once they are supplied with phosphorus rich substrate, their phosphorus requirements are fulfilled and they work more effectively to fix more and more nitrogen which results in enhanced microbial biomass N.

Some relevant informations are in this attachment.

http://www.sciencedirect.com/science/article/pii/S0016706113002851

helpful....?

Rightly said by Dr. Saeed. to fix one molecule the bacteria need atleast 16 moles of ATP, so exogenous supply of P is beneficial