Patterns of retention and utilization of aerially deposited nitrogen in boreal peatlands

Nitrogen dynamics were quantified for two years using N-15 (as NH,CI at 3 g N m(-2) year(-1)) in a bog and a rich fen in continental western Canada. After two years, 81-88% of the applied nitrogen was recovered; 98.4-98.7% of the recovered nitrogen occurred in the top 45 cm of the moss-peat layer and 1.3-1.6% occurred in the above ground shrub component In the bog, 29% was recovered from the top 0-5 cm of Sphagnum fuscum, while in the fen 19% was recovered from this zone in Tomenthypnum nitens. Sphagnum sequestered the applied nitrogen more efficiently than did Tomenthypnum. However, over time Tomenthypnum retained it better in the upper moss layers. Overall, after two years Sphagnum retained 42% and Tomenthypnum 48% in the top 12 cm of the ground layer. The nitrogen of the current year's moss growth contained 26-28% applied nitrogen (% NDFF) after one season and 19-20% after two seasons. Moss production increased from 4-30% annually over the two year period, while production for Betula pumila in the fen and Ledum groenlandicum in the bog did not increase significantly. Decomposition of S. fuscum in the bag was significantly less than T. nitens in the fen after two years, but no differences were evident after nitrogen additions in either peatlands. We conclude that I) nearly all nitrogen is immediately sequestered by the mass layer; 2) much of this new nitrogen is found in the actively growing region and mass production increases; 3) shrubs are little affected by the additional nitrogen during the first two years and direct absorption of nitrogen through leaves is unimportant and 4) newly deposited nitrogen is tightly controlled by the moss layer in both bogs and fens. However retention patterns are different. Subsequent use of nitrogen by vascular plants is determined by nutrient release through decompositional processes deeper in the moss layer.