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Experimental nitrogen addition alters structure and function of a boreal bog: critical load and thresholds revealed
Author(s) -
Wieder R. Kelman,
Vitt Dale H.,
Vile Melanie A.,
Graham Jeremy A.,
Hartsock Jeremy A.,
Fillingim Hope,
House Melissa,
Quinn James C.,
Scott Kimberli D.,
Petix Meaghan,
McMillen Kelly J.
Publication year - 2019
Publication title -
ecological monographs
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.254
H-Index - 156
eISSN - 1557-7015
pISSN - 0012-9615
DOI - 10.1002/ecm.1371
Subject(s) - bog , ombrotrophic , sphagnum , black spruce , shrub , peat , boreal , botany , deposition (geology) , taiga , ecology , biology , agronomy , sediment , paleontology
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·ha −1 ·yr −1 ; regional background deposition is <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 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). Increasing N addition: (1) stimulated N 2 fixation at deposition <3.1 kg N·ha −1 ·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 NH 4 + ‐N, NO 3 − ‐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·ha −1 ·yr −1 for northern Alberta bogs.