Abundance and diversity of nitrogen-fixing bacteria in rhizosphere and bulk paddy soil under different duration of organic management

Denaturing gradient gel electrophoresis (DGGE) and quantitative real-time PCR (qPCR) approaches were used to assess respectively the molecular diversity and quantity of the nifH gene sequences in rhizosphere and bulk paddy soil under conventional management and different duration of organic management (2, 3, 5, 9 years). The phylogenetic distribution of clones based on nifH gene sequence showed that taxonomic groups were consisted of Alphaproteobacteria (27.6%), Betaproteobacteria (24.1%) and Gammaproteobacteria (48.3%). Members of the order Rhizobiales and Pseudomonadales were prevalent among the dominant diazotrophs. When the quantity of the nifH gene sequences was determined by qPCR, 2.27 × 10⁵ to 1.14 × 10⁶ copies/g of soil were detected. Except for 2 years organically managed soil, nifH gene copy numbers in organic soil, both rhizosphere and bulk, were significantly higher than in CM soil. Moreover, nifH gene copy numbers in the organic rhizosphere soil (3, 5, 9 years) were significantly higher than in bulk soil. The abundance and diversity of nitrogen-fixing bacteria tended to increase with duration of organic management but the highest number of nifH gene copies was observed in the rhizosphere and bulk soil of 5 years organic management. In addition, analysis of variance and canonical correspondence analysis (CCA) showed that C/N, C and N were important factors influencing the abundance and community structure of nitrogen-fixing bacterial.