PS 22-16 - Root Zone N Sustainability in Perennial Energy Crops

Background/Question/Methods

Perennial grasses such as switchgrass and miscanthus are ideal bioenergy feedstocks due to their high productivity with low requirements of N input. Previous research has mostly focused on single biogeochemical processes associated with certain grass species, but a comprehensive assessment of sustainability for bioenergy crops is lacking. Here we compared N sustainability among four perennial systems including switchgrass (Panicum virgatum, c.v. Shawnee and Liberty), prairie cordgrass (Spartina pectinata, c.v. Savoy), and miscanthus (‘Miscanthus x giganteus’). We focused on biological nitrification inhibition (BNI), which suppresses ammonia oxidation into nitrate; and biological N fixation (BNF), which converts atmospheric N₂ into NH₄⁺. Thus, our goal for this study is to identify grass species with the highest BNI and BNF.



Soils samples were collected in the field season of 2020 and 2021. We measured BNF by acetylene reduction assay. We determined BNI by comparing nitrification potential of rhizosphere soil with bulk soil. We also sequenced 16S rRNA gene and functional genes including amoA and nifH.

Results/Conclusions

The potential nitrification rates in rhizosphere soils were generally lower under all the grass species than in bulk soil, indicating BNI potential for all bioenergy crops. 16S rRNA sequencing revealed Nitrososphaera as the most abundant nitrifiers. Additionally, the abundance of nitrifiers was lowest in Miscanthus x giganteus but highest in cordgrass. One of the cultivars of switchgrass, Shawnee, displayed significantly higher BNF potentials compared to other grasses. The rhizosphere microbiome of Shawnee was dominated by diazotroph taxa within the genera of Geobacter, Anaeromyxobacter, and Anabaena. Overall, all grasses have exhibited potentials to reduce nitrate loss by nitrification suppression, and Shawnee showed the highest N sustainability potentially by forming stronger associations with diazotrophs in its root zone.