Diversity of nitrogen fixing bacteria in rhizospheric soils of citrus–poplar cropping system of Punjab

Abstract

The agroforestry system based on horticultural crops is economically sound providing food, timber, fuelwood, and many productive and protective benefits to farming families. Diazotrophs are nitrogen-fixing bacteria which possess the nifH gene that codes for the nitrogenase enzyme involved in the reduction of atmospheric dinitrogen to ammonia. The aim of present study was to assess the diversity and functional activities of free-living culturable diazotrophic bacteria in the rhizospheric soil of citrus-poplar cropping system. Morphological, biochemical and molecular techniques were used to study the diversity of diazotrophs in the citrus–poplar cropping system. The physicochemical properties of soil were also studied using standard methods. Soil pH, electrical conductivity, organic carbon, ammoniacal nitrogen and nitrate nitrogen content ranged from 7.5- 8.6, 0.17-0.29 dS m^-1, 0.40-0.85%, 46-67 ppm and 35.5-48.5 ppm, respectively. Total 130 free living diazotrophs were characterized using cultural, morphological, biochemical and functional characteristics. Out of the total, 74 isolates were assessed based on amplification of two nifH (nifH1 and nifH2) primers. The nifH positive isolates were further characterized using restriction fragment length polymorphism of 16S rDNA and the isolates were identified as Stenotrophomonas maltophilia, Stenotrophomonas rhizophila, Rhizobium spp., Pseudomonas fulva, Xanthomonas spp., Agrobacterium spp., Achromobacter spp., Ensifer adhaerens, Bacillus amyloliquefaciens, Xanthomonas theicola and Azotobacter spp. Diverse groups of diazotrophic bacteria were observed in the rhizospheric soils of citrus-poplar cropping system.