Associative nitrogen fixation

• Published in: PloS one 2018-06, Vol.13 (6), p.e0197320
• Main Authors: Roley, Sarah S, Duncan, David S, Liang, Di, Garoutte, Aaron, Jackson, Randall D, Tiedje, James M, Robertson, G. Philip
• Format: Article
• Language: English
• Subjects: Grasses; Nitrogen fixation; Physiological aspects; Properties
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0197320

Associative N fixation (ANF), the process by which dinitrogen gas is converted to ammonia by bacteria in casual association with plants, has not been well-studied in temperate ecosystems. We examined the ANF potential of switchgrass (Panicum virgatum L.), a North American prairie grass whose productivity is often unresponsive to N fertilizer addition, via separate short-term .sup.15 N.sub.2 incubations of rhizosphere soils and excised roots four times during the growing season. Measurements occurred along N fertilization gradients at two sites with contrasting soil fertility (Wisconsin, USA Mollisols and Michigan, USA Alfisols). In general, we found that ANF potentials declined with long-term N addition, corresponding with increased soil N availability. Although we hypothesized that ANF potential would track plant N demand through the growing season, the highest root fixation rates occurred after plants senesced, suggesting that root diazotrophs exploit carbon (C) released during senescence, as C is translocated from aboveground tissues to roots for wintertime storage. Measured ANF potentials, coupled with mass balance calculations, suggest that ANF appears to be an important source of N to unfertilized switchgrass, and, by extension, to temperate grasslands in general.