sequences of nirS gene
The nirS genes were amplified. To distinguish the amplicons that originated from different soil samples, barcode oligonucleotides were added at the 5 end of each primer. After amplification, amplicons for each sample were pooled in equal quantities and to be sequenced on the Illumina MiSeq platform (Illumina, San Diego, CA). the 600 cycle sequencing kit and PE315 sequencing mode was used.