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Isotopic evidence for episodic nitrogen fixation in switchgrass (Panicum virgatum L.)

Roley, Sarah S., Xue, Chao, Hamilton, Stephen K., Tiedje, James M., Robertson, G. Philip
Soil biology & biochemistry 2019 v.129 pp. 90-98
Panicum virgatum, annuals, fertilizer application, field experimentation, grasses, greenhouses, growing season, isotope labeling, leaves, metagenomics, microbial communities, nitrogen, nitrogen fertilizers, nitrogen fixation, nitrogen-fixing bacteria, nutrition, perennials, plant tissues, rhizosphere, roots, soil, stems
Perennial grasses can assimilate nitrogen (N) fixed by non-nodulating bacteria living in the rhizosphere and the plant's own tissues, but many details of associative N fixation (ANF) remain unknown, including ANF's contribution to grass N nutrition, the exact location of fixation, and composition of the associated microbial community. We examined ANF in switchgrass (Panicum virgatum L.), a North American perennial grass, using ¹⁵N-enriched N₂ isotopic tracer additions in a combination of in vitro, greenhouse, and field experiments to estimate how much N is assimilated, where fixation takes place, and the likely N-fixing taxa present. Using in vitro incubations, we documented fixation in root-free rhizosphere soil and on root surfaces, with average rates of 3.8 μg N g root⁻¹ d⁻¹ on roots and 0.81 μg N g soil⁻¹ d⁻¹ in soil. In greenhouse transplants, N fixation occurred only in the early growing season, but in the field, fixation was irregularly detectable throughout the 3-month growing season. Soil, leaves, stems, and roots all contained diazotrophs and incorporated fixed N₂. Metagenomic analysis suggested that microbial communities were distinct among tissue types and influenced by N fertilizer application. A diverse array of microbes inhabiting the rhizosphere, and possibly aboveground tissues, appear to be episodically contributing fixed N to switchgrass.