The current invention provides for a microbial consortium comprising one or more ammonium-oxidizing strains which are generally recognized as safe (GRAS) and which are selected from the group of Nitrosomonas, Nitrosospira, Nitrosopumilus, Cenarchaeum, Nitrosoarchaeum, Nitrosocaldus, Caldiarchaeum; and one or more GRAS strains which are commensal to the one or more ammonium-oxidizing strains, and which are selected from the group of Acinetobacter, Alcaligenes, Arthrobacter, Azospirillum, Azotobacter, Bacillus, Beijerinckia, Enterobacter, Erwinia, Flavobacterium, Rhizobium, Serratia and Deinococcus.