Disclosed herein are methods for forming ammonium salts of C4 diacids in a fermentation process with simultaneous removal of divalent metal carbonate salts. The pH of fermentation broths obtained during the production of fumaric, maleic, malic, and/or succinic acid by a microorganism is controlled by using alkaline oxygen containing calcium or magnesium compounds in the hydroxide, oxide, carbonate or bicarbonate forms—forming divalent metal salts of the diacids that are partially or wholly insoluble in the broth. The calcium or magnesium salts of the diacids are substituted with ammonium by introduction of ammonium salts at elevated temperature and pressure dissolving precipitated divalent metal cation salts of the diacids and forming soluble ammonium salts thereof. Carbonate in the form of CO2 or bicarbonate is simultaneously added to the fermentation media at the elevated temperature and pressure. The temperature and pressure are then reduced forming insoluble divalent metal carbonate salts that are separated from the solubilized ammonium diacid salts. The recovered metal carbonate salts can be recycled as pH control materials in subsequent fermentation reactions. Also disclosed is use of the solubilized ammonium diacid salts directly as a reagent for hydrogenation to form the derivatives N-methyl-2-pyrrolidone (NMP) gamma-butyrolactone (GBL) and 1,4-butane-diol (BDO) in single pot reactions.