Data from: Associative nitrogen fixation (ANF) in switchgrass (Panicum virgatum) across a nitrogen input gradient

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 15N2 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.