@ Sanjoy, in lake sediments we have enough labile-P, which can be extracted by NH4Cl as well as NH4F is used to extract Al bound P beside this in sequential extraction we are using NaOH to extract Fe-bound P and HCl for Ca-bound P.

Thank you sir, for your answer. But I would like to know more about the reaction mechanisms that are happening these processes. Why Lo-P is extracted only with NH4Cl ?

I believe that it has to do with the nature of the phosphates present in the sediments as Professor Tarafdar notes above. Iron and aluminum phosphates are often sequentially removed via these extractions and the mechanism, though I don't know it, is likely adding an anion that has a higher affinity for sediment binding sites, or that it brings P into solution by forming a phosphate salt.