Plant selection and soil effects shaped diazotrophs associated with Miscanthus × giganteus rhizomes and rhizosphere

Background/Question/Methods

Miscanthus × giganteus is a perennial C₄ grass with great potential for producing lignocellulosic biomass. It is widely used in trials in Europe and US as a biofuel feedstock candidate. Previous research indicates that nitrogen fertilization has little or none impact on the dry biomass yield of M. × giganteus, even though the plant removes a considerable amount of nitrogen through annual harvesting. Modeling and nitrogen balance experiments suggested that diazotrophs associated with M. × giganteus may be the source for the extra nitrogen. However, there is no published study investigating the diazotroph communities associated with M. × giganteus.  In this study, we are interested in 1) how the soil chemical factors, especially nitrogen, affect the bacterial communities in the rhizosphere and endosphere; 2) if M. × giganteus may actively select endophytes from the rhizosphere bacteria pool. M. × giganteus rhizome and rhizosphere samples were collected from four field sites, and the plants received three different levels of nitrogen treatments in each site. We examined the diazotroph communities that reside in the rhizomes and rhizosphere of cultivated M. × giganteus using terminal restriction fragment length polymorphism (T-RFLP) targeting the nifH gene. 

Results/Conclusions

Soil chemical factors and textures influenced microbial community composition. Edaphic factors explained 21.95% and 37.72% of variance in diazotroph communities living in endosphere and rhizosphere, respectively. Nitrogen treatments explained little variance in endosphere and rhizosphere diazotroph assemblages. Total soil nitrogen, however, showed a positive linear correlation with endosphere diazotroph richness (r = 0.173, p < 0.001).

Our results also showed that endosphere and rhizosphere harbored discrete diazotroph assemblages. Moreover, although rhizosphere diazotrophs showed site-specific patterns of beta-diversity, the endosphere of M. × giganteus shared a considerable amount of diazotrophs among all sampling sites. Our results suggest that the similar endophytic diazotroph communities observed in different sampling sites may be due to transmission of rhizome-born diazotrophs to progeny rhizomes, or positive selection of endophytic diazotrophs from the rhizosphere pool. Rhizome propagation procedures and plant selection may play important roles in recruiting beneficial microbes for the cultivated M. × giganteus.