Abundance of Diazotrophic Community in a Long-Term Crop Rotation System.

Diazotrophic bacteria in soil are key members of microbial communities that convert atmospheric nitrogen to biologically available ammonium. The objective of this study was to monitor the abundance of diazotrophic microorganisms in a long-term crop rotation system by quantification of the nifH gene. The field experiment, Cullars Rotation, was established in 1911; it consists of a three-year rotation of soybean, cotton, winter clover, corn, and wheat. Soil samples were collected from six treatments in June and October 2008 and February 2009 at two depths (0-5 cm and 5-15 cm). The abundance of the nifH gene was measured by quantitative polymerase chain reaction. The nifH gene copy numbers ranged from 6.1 x 10⁵ to 2.3 x 10⁷ copies per gram of dry soil. The lowest gene copy number was found in the treatment without lime application. There was a significant interaction between sampling month and soil depth. In general, the nifH gene abundance was the highest in February and lowest in October. More nifH genes were found in the surface soil than subsurface soil. These results indicate that liming, season, and soil depth were important factors affecting the abundance of diazotrophic microorganisms in this century-old crop rotation system.