| Summary: | 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). In September 2013-2015, we measured annual leader extension of Picea mariana on four trees (0.5-2.5 m tall) per plot. Using a visually estimated P. mariana density at Mariana Lakes Bog of 1 tree m -2 , and N concentrations in P. mariana needles, we calculated annual N assimilation attributable to new needle production. Annual leader extension and tree growth increased with increasing N input, with the response becoming more pronounced from 2013 through 2015. Picea mariana needle N concentrations also increased with increasing N input, at a modest, but significant rate that was consistent across all years while water addition alone had no significant effect on P. mariana leader extension, growth, needle N concentration, or N assimilation in any of the years of measurement (p >= 0.54). At Mariana Lakes Bog, the effect of N addition on P. mariana NPP became more pronounced over time. We cannot determine the extent to which the progressively steeper P. mariana growth response to N deposition represents a cumulative effect of added N or is related to interannual differences in temporal climatic variables.
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