Net N Mineralization Rates in Peat From N-Addition Plots in an Alberta Peatland, 2011-2014

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). From 2011 through 2014, we
quantified net N mineralization in each plot using the in
situ buried polyethylene bag technique. Concentrations of
initial KCl-extractable
NH 4 + -N,
NO 3 - -N, and DIN in
the top 10 cm of peat were unaffected by N inputs. We hypothesized
that as N deposition increases to a level that exceeds the capacity of
the bog vegetation to take up N, net N mineralization in surface peat
would be inhibited by higher
NH 4 + -N availability,
net nitrification would be stimulated by higher
NH 4 + -N availability
(cf. McGill and Cole 1981, Robertson and Groffman 2015), and
concentrations of DIN in porewater at the top of the water table would
increase, as DIN bypasses interception by the ground layer vegetation.
None of these hypotheses was supported. Experimentally added
NH 4 + -N and
NO 3 - -N apparently
appear to be rapidly immobilized. This immobilization prevents
experimentally added DIN from moving downward through the peat to the
bog water table.