Changes of diazotrophic communities in response to cropping systems in a Mollisol of Northeast China

Nitrogen-fixing microorganisms play important roles in N cycling. However, knowledge related to the changes in the diazotrophic community in response to cropping systems is still rudimentary. In this study, the nifH gene was used to reveal the abundance and community compositions of diazotrophs in the cropping systems of continuous cropping of corn (CC) and soybean (SS) and soybean-corn rotation for growing corn (CSC) and soybean (SCS) in a black soil of Northeast China. The results showed that the abundance of the nifH gene was significantly higher in cropping soybean than in cropping corn under the same cropping system, while remarkably increased in the rotation system under the same crop. The Shannon index in the CC treatment was significantly higher than that in the other treatments, but the OTU number and Chao1 index had no significant change among the four treatments. Bradyrhizobium japonicum was the dominant diazotrophic species, and its relative abundance was at the lowest value in the CC treatment. In contrast, Skermanella sp. had the highest relative abundance in the CC treatment. A PCoA showed that the diazotrophic communities were separated between different cropping systems, and the variation caused by continuous corn cropping was the largest. Among the tested soil properties, the soil available phosphorus was a primary factor in determining diazotrophic community compositions. Overall, the findings of this study highlighted that the diazotrophic communities in black soils are very sensitive to cropping systems.