The diversity and abundance of the nitrite reductase gene nirS were investigated for a free surface flow constructed in both the summer and winter seasons Nitrate was efficiently removed especially by the Typha wetland under anoxic conditions in both seasons Clone library analysis which used the distance based operational taxonomic unit richness program suggested that the Typha wetland sediment samples collected in September provided the highest diversity and richness in terms of corresponding indices (Simpson D Shannon-Weiner (H') and Chao 1) The phylogenetic analysis based on deduced amino acids divided 211 nirS clones into eight clusters Although most of the clones were loosely related to the nirS gene of cultivated denitrifying bacteria some clones were related to the nirS of Xanthomonadales Burkholdenales Rhodocyclales and Hydrogenophdales bacteria over the sampling time respectively The real time polymerase chain reaction results showed that the copy numbers of nirS from both the Acorus and Typha wetlands in the summer (13 2 +/- 2 0 and 1 8 +/- 0 2 x 10(8) copies/g soil) were significantly higher than those of the corresponding nirS genes in the winter (8 6 +/- 1 1 and 0 7 +/- 0 1 x 10(8)copies/g soil) These results suggest that the diversity and abundance of the nirS can affect in regard to nitrogen removal efficiency (C) 2010 Elsevier B V All rights reserved