Nitrogen Fixation Responses in Sphagnum fuscum to N-Additions to an Alberta Peatland, 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 . Bogs, being ombrotrophic, may be
especially susceptible to increasing N deposition. To examine
responses to N deposition, over five years, we experimentally applied
N (as NH 4 NO 3 ) to a bog
near Mariana Lakes, Alberta, at rates of 0, 5, 10, 15, 20, and 25 kg N
ha -1 yr -1 , plus
controls (no water or N addition). We measured
N 2 -fixation using the acetylene reduction
technique between 2012 and 2015. All measurements were made between
June and August. Mean acetylene reduction rates ranged from 2.2 mg
m -2 da -1 (late
June 2012) to 12.3 mg m -2 da -1 (late July 2014). Averaged across all
measurement dates, rates were highest in the 5 kg N
ha -1 yr -1 treatment, and decreased with increasing N loading. The acetylene
reduction/N2 fixation response to N addition was described by a
segmented linear regression, with rates increasing as N addition
increased to 3.1 ± 1.5 kg N ha -1 yr -1 and decreasing with further increases
in N addition. Water addition alone had no significant effect on N
fixation rates on any of the measurement dates (p >= 0.79). While
increasing N deposition may not substantively change total inputs of
new N to bogs, the form on new N inputs shifts to inorganic N in
deposition, rather than organic N produced by the microorganisms that
are fixing N 2 .