Data from: Rainfall events stimulate episodic associative nitrogen fixation in switchgrass

Associative N2 fixation (ANF) is widespread but poorly characterized,
limiting our ability to estimate global inputs from N2 fixation. In some
places, ANF rates are at or below detection most of the time, but
occasionally and unpredictably spiking to very high rates. Here we test
the hypothesis that plant phenology and rainfall events stimulate ANF
episodes. We measured ANF in intact soil cores in switchgrass (Panicum
virgatum L.) in Michigan, USA. We used rain exclusion shelters to impose
three rainfall treatments with each receiving 60 mm of water over a 20-day
period but at different frequencies. We concurrently established a
treatment that received ambient rainfall, and all four treatments were
replicated four times. To assess the effects of plant phenology, we
measured ANF at key phenological stages in the ambient treatment. To
assess the effects of rainfall, we measured ANF immediately before and
immediately after each wetting event in each treatment involving rainfall
manipulation. We found that the previous day’s rainfall could explain 29%
of the variation in ANF rates within the ambient treatment alone, and that
bulk soil C:N ratio was also positively correlated with ANF, explaining
18% of the variation alone. Wetting events increased ANF and the magnitude
of response to wetting increased with the amount of water added and
decreased with the amount of inorganic N added in water. ANF episodes thus
appear to be driven primarily by wetting events. Wetting events likely
increase C availability, promote microbial growth, and make rhizosphere
conditions conducive to ANF.