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Effects of a ten-year climate warming experiment on nitrogen cycling in high arctic tundra Rolph, Sandra Gale

Abstract

The effects of a 10-year climate warming experiment on nitrogen (N) cycling in high arctic tundra ecosystems were examined along a soil moisture gradient at Alexandra Fiord, Ellesmere Island, Canada (78°53'N, 75°55'W). Open top chambers were established in 1992 to passively warm five tundra plant communities within the range predicted for a doubling of atmospheric CO₂. Inorganic N availability, measured using ion exchange membranes, was consistently higher in the warmed plots throughout the growing season in three plant communities. Soluble organic N availability increased significantly with warming in a wet sedge meadow. Net N mineralization in buried bag incubations was not significantly affected by the warming treatments; however, net N immobilization was four-times higher in the warmed plots compared to the controls in the sedge meadow. Reciprocal transplantation of buried bags between temperature treatments indicated that the increase in net N immobilization was a result of changes in soil properties during the nine-year experiment, in conjunction with continued temperature enhancement. Significant reductions in litter quality, measured as C:N ratios, were observed for woody and herbaceous growth forms in the warmed treatments at the end of the growing season. Reproductive parts had higher C:N than vegetative parts, and C:N increased with warming. Therefore, previously observed increases in reproductive effort with warming have likely reduced litter quality. Despite this potential for negative litter quality feedbacks to N availability with warming, soil organic matter was not significantly affected by the ninth year of the experiment. We hypothesize that the short-term changes in soil N transformations and increased N availability have contributed to the increases in plant growth observed in the warmed plots, and that shifts in the relative availabilities of NO₃, NH₄, and SON may have contributed to changes in the species composition of the tundra plant communities. However, this negative feedback to greenhouse warming may be strongly constrained by longer-term litter quality feedbacks to soil organic matter quality, and N availability in high arctic tundra ecosystems.

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