Abstract: Biochar amendment may affect abundance and diversity of soil microbial community. However, the effects of biochar amendment on nitrogen-fixing bacterial community in coastal alkaline soils remain unclear. Using realtime quantitative PCR and DGGE profiling, this study investigated the variations of abundance and diversity of nifH genes in a coastal alkaline soil amended with different doses (0, 5%, 10%, 20%, w/w) of biochar during an 84-day incubation. This microcosm experiment showed that the incubation time significantly influenced nifH gene abundance (P<0.05), but no differences were observed among treatments (P>0.05). Following 14-day incubation, the copy number of nifH genes was significantly stimulated. On the 42nd day, the nifH abundance in the 20% biochar treatment was significantly higher than that in the control. For the 10% biochar treatment, however, higher abundance of nifH was observed on the 56th day, compared with the control. NMDS and CLUSTER analyses showed that the samples could be separated into two major groups, the lowdose group (0 and 5% biochar treatments) and the high-dose group (10% and 20% biochar treatments). ANOSIM indicated that the dose of biochar application significantly affected the community structure of nitrogen-fixing microbes in the coastal soil (P=0.001). DGGE analysis and sequencing of the major bands indicated that the relative abundance of Ideonella and Skermanella increased following biochar amendment, whereas the population of Azospirillum, an important nitrogen-fixer, remained relatively stable.

Key words: clay, organic matter, soil compression-rebound, shear strength, triaxial test, oedometer test