This present disclosure provides methods and compositions for multiplex amplification, e.g., PCR, of DNA samples compartmentalized on the surface of an array. In certain aspects, the method includes providing a solid substrate having features of immobilized forward and reverse primer pair constructs therein for amplifying a target polynucleotide sequence, contacting the plurality of features with a corresponding nucleic acid sample, and incubating the contacted features under conditions to allow release of the forward and reverse primer constructs and amplification of the desire target polynucleotide. The release of the primer constructs from the substrate can be done prior to or during the incubating step.