Experimental nitrogen addition alters structure and function of a boreal bog: critical load and thresholds revealed

Bogs and fens cover 6% and 21%, respectively, of the 140,329 km(2) Oil Sands Administrative Area in northern Alberta. Development of the oil sands has led to increasing atmospheric N deposition, with values as high as 17 kg N center dot ha(-1)center dot yr(-1); regional background deposition is <2 kg N center dot ha(-1)center dot 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 NH4NO3) to a bog near Mariana Lake, Alberta, unaffected by oil sands activities, at rates of 0, 5, 10, 15, 20, and 25 kg N center dot ha(-1)center dot yr(-1), plus controls (no water or N addition). Increasing N addition: (1) stimulated N-2 fixation at deposition <3.1 kg N center dot ha(-1)center dot yr(-1), and progressively inhibited N-2 fixation as N deposition increased above this level; (2) had no effect on Sphagnum fuscum net primary production (NPP) in years 1, 2, and 4, but inhibited S. fuscum NPP in years 3 and 5; (3) stimulated dominant shrub and Picea mariana NPP; (4) led to increased root biomass and production; (5) changed Sphagnum species relative abundance (decrease in S. fuscum, increase in S. magellanicum, no effect on S. angustifolium); (6) led to increasing abundance of Rhododendron groenlandicum and Andromeda polifolia, and to vascular plants in general; (7) led to increasing shrub leaf N concentrations in Andromeda polifolia, Chamaedaphne calyculata, Vaccinium oxycoccos, V. vitis-idaea, and Picea mariana; (8) stimulated cellulose decomposition, with no effect on S. fuscum peat or mixed vascular plant litter decomposition; (9) had no effect on net N mineralization rates or on porewater NH4+-N, NO3--N, or DON concentrations; and (10) had minimal effects on peat microbial community composition. Increasing experimental N addition led to a switch from new N being taken up primarily by Sphagnum to being taken up primarily by shrubs. As shrub growth and cover increase, Sphagnum abundance and NPP decrease. Because inhibition of N-2 fixation by increasing N deposition plays a key role in bog structural and functional responses, we recommend a N deposition critical load of 3 kg N center dot ha(-1)center dot yr(-1) for northern Alberta bogs.