It might be due to the kind of root exudates produced under legumes which makes rhizosphere more acidic than cereal. Acidic rhizosphere helps in utilizing P from rock phosphate under legumes. Further, composition of the exudates which attracts more microbes towards rhizhosphere which ultimately helps in P utilization from RP. 

I agree with Dr Kalaivanan. Yes, the root exudates of legumes would aid in the release of P from rock phosphate.

Hi Ghina,

All comments above provide explanations for why legumes seem to be more efficient in utilizing rock phosphate.  Another possibility not mentioned is:

Biological N2 fixation (BNF) itself is a rhizosphere acidifying process, as Hydrogen ions (H+) are a product of BNF (Bolan et al. 1991).  Some publications have reported less root-zone acidification with low rates of BNF (e.g. Betencourt et al. 2012).  This is a special advantage, efficiently N fixing legumes have.

Although cereal crop root system may have microbes capable of associative N fixation, this process is reported to be less efficient than legume N fixation.

This advantage combined with legumes ability to form efficient mycorrhizal associations (As Hani has already commented above) and root exudations that contain organic acids may help legumes to solubilise rock phosphate.

In a side note: some of North American agricultural extension literature suggests that soil acidity resulting from harvesting one tonne of Alfalfa biomass may require about 200 lb of agricultural lime to neutralize (University of Nebraska Extension). We know that Alfalfa is an efficient N2 fixer.   

Cited papers:

(1) Bolan et al. (1991) Plant and Soil. 134: 53-63, (2) Betencourt et al. (2012) Soil Biol. Biochem. 46: 181-190.

-SJ

Possible reason is the type of association with the roots and microbes which help in P utilisation and uptake. Beside this, root exudates mainly organic acid helps in solubilisation of P and increase the available P content.Which may improves the P uptake by the plant.

Yes as a microbiologists i agrees with all of them. Dr. Hani Antoun is absolutely right........................More the microbial network.....more the efficiant one....... for P mobilization and other are right because solubilization of RP will be done by m/os surrounding the roots and free living bacteria will much more where root exudates are more..........................