Nitrogen mineralization, nitrification, and denitrification in a high arctic lowland ecosystem, Devon Island, NWT, Canada

This study examined nitrogen (N) transformations in two sites at Truelove Lowland (Devon Island, NWT, Canada). The two sites (sedge meadow and willow/herb hummocks) were located at the low end and midway along the beach ridge basin topographic gradient typical of Truelove Lowland. Net N-mineralization and nitrification rates were measured using both field (late June through July) and laboratory incubations. Denitrification rates (July) were measured using the acetylene blockage technique. Natural abundance of N-15 was analyzed in plant tissue and soils from both sites. Net N-mineralization rates in the field incubated cores were similar between the two sites, but were higher in the willow-herb hummock soils incubated in the laboratory. Nitrification and denitrification rates were significantly higher in the drier hummock site, Nitrogen stable isotope ratios (delta(15)N) from the hummock site were 1.82 parts per thousand higher in soils, 1.20 parts per thousand higher in moss, and 3.25 parts per thousand higher in Salix arctica leaves compared to ratios from the meadow site, consistent with the long-term enrichment of the N-15 isotope in the hummock site resulting from higher nitrification and denitrification rates. Rates of nitrification along the beach ridge-basin gradient appear to be controlled primarily by differences in moisture, whereas denitrification rates are governed by nitrate concentration. Thus, topographic control of N transformations is both direct and indirect and is exerted on one, rather than multiple, points of the N cycle in this high arctic lowland ecosystem.