sequences of nirK gene
The nirK 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 PE317 sequencing mode was used.