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 (<2 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.