Effect of different fertilization treatments on indole-3-acetic acid producing bacteria in soil

Purpose Soil microorganisms directly affect the growth of plants. Especially, plant growth-promoting rhizobacteria (PGPR) play an important role in plant growth. There are many studies about the effects of different fertilization treatments on soil microbial community structure; however, the effects on PGPR, including indole-3-acetic acid (IAA)-producing bacteria have not been previously reported. The objective of this study is to determine the effects of different types of fertilizers on IAA-producing bacteria. Materials and methods The field trial was completed in the North China with a winter wheat and summer corn rotation system. IAA-producing bacteria were screened from soil treated with different fertilizer (non-nitrogen fertilizer (CK), controlled-release fertilizer (CR), chemical fertilizer (CF), and organic fertilizer (OF)) which was established in September 2005. Quantity of IAA produced by bacteria was determined by spectrophotometer. IAA-producing bacteria were identified based on 16S rDNA sequence. Community structures and phylogenetic relationships of IAA-producing bacteria were analyzed by online Basic Local Alignment Search Tool search engine, biosoftware of DNAMAN and Molecular Evolutionary Genetics Analysis. Results and discussion Compared with CK treatment, CF and CR treatment increased soil pH values, while OF treatment decreased pH. The three types of fertilizers all increased soil organic carbon and total nitrogen, with OF treatment causing the significant increase. Soils treated with OF or CR fertilizer could significantly increase the number of culturable bacteria compared with CF or CK treatment. Fifty-three IAA-producing bacteria (14 strains from CK, nine from CF, eight from CR, and 22 from OF) were identified based on 16S rDNA sequence. The Shannon-Weiner index of IAA-producing bacteria isolated from CK and OF (2.06 and 2.45, respectively) was significantly higher than those from CF and CR (0.50 and 0.95, respectively). Arthrobacter sp. was the most prevalent group of IAA-producing bacteria. Conclusions The fertilizers increased soil organic carbon and total nitrogen, particularly the organic fertilizers. Controlled-release fertilizers and organic fertilizers can promote growth of soil-culturable bacteria and IAA-producing bacteria. These may be reasons why organic fertilizers and controlled-release fertilizers can promote crop growth. Different fertilization treatments affected IAA yield mainly through modifying the quantities of microorganisms, rather than changing the IAA-producing ability of the same microorganisms. Pedobacter sp. which can produce IAA has not been described previously.