Given the large contribution of fossil fuel burning to greenhouse gas emission, there is high demand to implement renewable cropping systems for biofuel production. However, the heavy use of nitrogen (N) fertilizers to maximize crop yield leads to additional forms of pollution, such as high levels of nitrate (NO3-) leaching into groundwater. Determining ways to reduce NO3- production and leaching will thus increase the environmental benefit of bioenergy cropping systems. The nitrification of ammonium (NH4+) by bacteria and archaea in the soil is a key step leading to NO3- production. Studies have shown that the candidate bioenergy crop sorghum is capable of exuding secondary metabolites in its rhizosphere that inhibit nitrification. Nevertheless, the relationship between sorghum’s biological nitrification inhibition (BNI) ability and its rhizosphere microbial community has yet to be explored in field conditions. We implemented a field experiment and used 16S rRNA sequencing to compare microbial community composition, nitrifier recruitment, and nitrification rates in bulk and rhizosphere soils to better understand the mechanisms of BNI in sorghum.
Results/Conclusions
As the season progressed the microbial community diverged strongly among bulk and rhizosphere soils (PERMANOVA R2 = 0.040, p-value = 0.001). Nitrification rates were strongly inhibited in mid-season when the plant growth was at its highest rate (F-value = 27.7, p-value <0.010). The relative abundance of archaeal nitrifiers was not significantly different among bulk soil and rhizosphere in mid-season (F-value = 2.93, p-value = 0.10); however, relative abundance of bacterial nitrifier taxa was found to be reduced in the rhizosphere soils compared to bulk soils (F-value = 4.35, p-value = 0.040). Additionally, nitrification rate was significantly associated with the percent of bacterial nitrifiers (R2 = 0.270, p-value < 0.001). Overall, we show that the sorghum rhizosphere environment greatly affected the structure of the microbial community, particularly the recruitment of nitrifiers. The inhibition of nitrification in the rhizosphere appears to be driven by lower recruitment of nitrifying bacteria, rather than archaea, in field conditions. Our knowledge of plant-microbe interactions could expand our ability to leverage natural genetic variation and breed for desirable rhizosphere traits such as reduced nitrification and improved nitrogen retention in agroecosystems.