In order to amplify the nifH gene,a nested PCR using outer primers FGPH19 (5'-TACGGCAARGGTGGNATHG-3'),PolR(5'-ATSGCCATCATYNTCRCCGGA-3') and inner primers PolF (5'TGC GAYCCSAARGCBHACTC-3') and AQER (5'-GACGATGTAGATYTCCTG3') was performed in a Esco Swift MaxPro Thermal Cycler.