Purpose
Nitrogen fixation by free-living diazotrophs from the atmosphere is an important pathway for nitrogen input into the soil. However, there is little information regarding soil diazotrophic community composition and diversity under long-term fertilization in rice paddy ecosystems.
Materials and methods
Using the ¹⁵N₂-tracing method and nifH gene as a molecular marker, we investigated the abundance, structure, and activity of soil diazotrophic community in soil at a 30-year-old filed experimental site treated with four different fertilizer management practices: control (non-fertilization), chemical NPK fertilizers, NPK plus rice straw (NPK+RS), or NPK plus chicken manure (NPK+OM).
Results and discussion
Among all the treatments, the NPK+OM treatment significantly improved the soil nutritional status. Fertilization increased both bacteria and nifH gene abundances, with the highest values (p < 0.05) found in the NPK+OM treatment. The potential nitrogen fixation rate ranged from 14.6 to 118 μg kg^-1 day^-1, and the highest rates (p < 0.05) were also observed in the NPK+OM treatment. Long-term chemical NPK fertilization decreased the diversity of diazotrophic community, whereas NPK+RS and NPK+OM treatments maintained the diversity of diazotrophic community. Long-term fertilization changed diazotrophic community as compared to non-fertilization, but there were no significant differences among fertilized treatments. Most nifH sequences were closely linked to Alphaproteobacteria, which was dominated by the genera Bradyrhizobium. Relatively higher Cyanobacteria abundances were observed in the unfertilized soil as compared with fertilized soil.
Conclusions
Our results suggest that long-term fertilization increased the abundance of diazotrophs and changed their community structure, and combined use of chicken manure and chemical NPK fertilizers can significantly improve the activity of diazotrophic community.