Johnson 9527907 Peatlands, dominated by ericaceous shrubs and Sphagnum moss, cover 400 million ha in the circumboreal region. These ecosystems are characterized by slow decomposition and nutrient cycling, and accumulation of peat (primarily Sphagnum moss litter). Ombrotrophic (rain-fed) peat bogs represent one of the most nutrient-deficient of all ecosystems because peat accumulates above the surrounding landscapes and, therefore, these systems receive nutrient inputs form only atmospheric sources. Consequently, ombrotrophic peat bogs are thought to be strongly limited by nitrogen (N). Our field and laboratory net N mineralization rates from ombrotrophic peat bogs are much lower than minertrophic peatlands and even lower than arctic tundra. Furthermore, it appears that only about 1/4 of the annual N requirement of ericaceous shrubs and Sphagnum moss can be met by inorganic sources from atmospheric inputs (mostly deposition, N fixation is negligible) and N mineralization. Although inorganic N is in short supply in peat bogs, organic N is present in large quantities as partially decomposed peat and, in lesser amounts, as dissolved organic nitrogen (DON). In organic soils, some DON occurs as 'free' amino acids, previously considered as unavailable for plant uptake. Recent laboratory investigations and the few field studies that have be conducted indicate that ericaceous mycorrhizal plants (and even several arctic non-mycorrhizal sedges) can take up amino acids. The goal of this project is to determine if uptake of amino acids (and perhaps more complex organic N forms) by mycorrhizal ericaceous bog shrubs can account for the apparent N deficit in the plants' annual uptake requirements. If the uptake of organic N is a primary form of N nutrition for ericaceuous plants in organic-rich peat bogs, it would fundamentally alter ideas about N limitation and N cyclinging, these systems and perhaps in other Ericace-dominated ecosystems, e.g. heathlands and nutrient-poor oak forests. This proje ct is focused on the possible short-circuiting of the decomposition-mineralization cycle by uptake of amino acid N and nitrogenous sugars by ericaceous plants via ericoid mycorrhizas in an organic-rich bog ecosystem. The objectives of this project are to 1) determine if ericaceous bog plants take up amino acids and amino sugars under field conditions, either directly or via mycorrhizas, 2) characterize the levels of 'free' amino acids and amino sugars in the soil, and determine the release of dissolved organic N, especially from relatively liable roots and leaves, and 3) determine if ericacous bog plants require mycorrhizas for their amino acids uptake. The project plan includes: a field test of uptake of isotopically-labelled glycine (amino acid) and glucose amine (nitrogenous sugar) by mycorrhizal ericaceous plants, field and laboratory incubations of litter to measure release of DON, field and laboratory studies of mycorrhizal dependency, and N turn-over studies using 15N-labelled decomposing roots and leaves.
