Plant Responses to Increased Experimental Nitrogen Deposition in a Boreal Peatland

Increased nitrogen (N) deposition onto boreal peatlands and forests is anticipated with further expansion of Alberta\u27s oil sands industry and consequently, an increase in sources of nitrogen oxide emissions. Increased N deposition has the potential to affect peatland flora and alter N cycling patterns in peatlands, therefore it is imperative to investigate at what level of excess N deposition these effects take place. This thesis discusses results from the first two years of a five year N fertilization study being conducted at a peatland complex near the hamlet of Mariana Lake in northeastern Alberta, Canada aimed at quantifying the N critical load for these peatland ecosystems. At the study site there are forty-two experimental plots - half in an ombrotrophic bog, the other half in the poor fen - with varying N fertilization treatments ranging from 0 kg/ha/year to 25 kg/ha/year. To investigate nitrogen uptake by plants at the Mariana Lake study site, I measured nitrogen (N) and carbon (C) concentrations of Sphagnum capitulum tissue and vascular plant foliar tissue. For Sphagnum species, I also analyzed C:N ratios and capitulum N storage. To investigate potential growth response of the target Sphagnum species, measurements were taken for linear growth (the vertical elongation of the Sphagnum shoots), stem mass density (the weight of Sphagnum stems occupying a volume after capitula were removed), and ultimately, net primary production (the product of the prior two measurements). Capitulum mass density (biomass) was measured as well to investigate possible changes in Sphagnum capitulum growth. Also, during the height of the growing season (mid-July, 2011 and 2012), the plant communities in each treatment plot were sampled to provide baseline data necessary for documenting any shifts in plant distribution or community composition that may occur after N additions

Experimental nitrogen addition alters structure and function of a boreal poor fen: Implications for critical loads

Bogs and fens cover 6 and 21%, respectively, of the 140,329 km2 Oil Sands Administrative Area in northern Alberta. Regional background atmospheric N deposition is low (b2 kg N ha−1 yr−1 ), but oil sands development has led to increasing N deposition (as high as 17 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, unaffected by oil sands activities, at rates of 0, 5, 10, 15, 20, and 25 kg N ha−1 yr−1 , plus controls (no water or N addition). At Mariana Lake Poor Fen (MLPF), increasing N addition: 1) progressively inhibited N2-fixation; 2) had no effect on net primary production (NPP) of Sphagnum fuscum or S. angustifolium, while stimulating S. magellanicum NPP; 3) led to decreased abundance of S. fuscum and increased abundance of S. angustifolium, S. magellanicum, Andromeda polifolia, Vaccinium oxycoccos, and of vascular plants in general; 4) led to an increase in stem N concentrations in S. angustifolium and S. magellanicum, and an increase in leaf N concentrations in Chamaedaphne calyculata, Andromeda polifolia, and Vaccinium oxycoccos; 5) stimulated root biomass and production;6) stimulated decomposition of cellulose, but not of Sphagnum or vascular plant litter; and 7) had no or minimal effects on net N mineralization in surface peat, NH4 +-N, NO3 −-N or DON concentrations in surface porewater, or peat microbial composition. Increasing N addition led to a switch from new N inputs being taken up primarily by Sphagnum to being taken up primarily by shrubs. MLPF responses to increasing N addition did not exhibit threshold triggers, but rather began as soon as N additions increased. Considering all responses to N addition, we recommend a critical load for poor fens in Alberta of 3 kg N ha−1 yr−1