Nitrogen Fixation Responses in Sphagnum Mosses to N-Additions to an Alberta Poor Fen, 2012-2015

Development of the oil sands has led to increasing atmospheric N
deposition, with values as high as 17 kg N ha-1 yr-1; regional
background levels <2 kg N ha-1 yr-1. To examine responses to N
deposition, over five years, we experimentally applied N (as NH4NO3)
to a poor fen near Mariana Lake, Alberta, at rates of 0, 5, 10, 15,
20, and 25 kg N ha-1 yr-1, plus controls (no water or N addition).
Between June and August of each year, we measured N2-fixation using
the acetylene reduction technique between 2012 and 2015Averaged across
all measurement dates, whether described by a linear or segmented
regression, N2-fixation rates decreased with increasing N addition.
N2-fixation rates were not different between the water addition only
treatments and the control treatments (p = 0.44). While increasing N
deposition may not substantively change total inputs of new N to bogs,
the form of new N inputs shifts to inorganic N in deposition, rather
than organic N produced by the microorganisms that are fixing N2.