Pore water concentrations of Nitrogen from N-Addition plots in an Alberta Poor Fen, 2011-2015

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). We
collected surface pore water from all plots several times a year
throughout the 5 year experiment. Over the 5 years of the study,
porewater NH4+-N, NO3--N, and DON concentrations at the top of the
poor fen water table were unaffected by N addition (p = 0.06, 0.30,
0.16, respectively). However, porewater NH4+-N, NO3--N, and DON
concentrations were substantially higher in 2011 than in 2012-2015
(Fig. 11). Water addition alone had no significant effect on porewater
concentrations of NH4+-N, NO3--N, or DON (p > 0.99). We hypothesized that as N deposition increases to a level that exceeds
the capacity of the fen vegetation to take up N, net N mineralization
in surface peat would be inhibited by higher NH4+-N availability, net
nitrification would be stimulated by higher NH4+-N availability, 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 with nitrogen being immediately
taken up by vegetation. It is unclear if longer term study would
reveal similar responses.