Laboratory experiments involving ultraviolet (UV) irradiation of dicyanoacetylene (C4N2) trapped in water ice at 10 K have been conducted and monitored by infrared spectroscopy (FTIR). By the support of isotopic experiments and theoretical calculations, the irradiation of a DCA/H2O ice mixture at lambda > 230 nm has been found to be a possible source of NH4+HCO3- (ammonium bicarbonate) and NH4+HCOO-(ammonium formate). These latter compounds can arise from a proton-transfer reaction between H2O and the CN radical, which is issued from photolyzed C4N2.