Nonlinear responses to nitrogen and strong interactions with nitrogenand phosphorus additions drastically alter the structure and functionof a high arctic ecosystem

摘要

Significant changes in ecosystem CO2 exchange and vegetation characteristics wereobserved following multiple additions of nitrogen (N) and factorial additions of Nand phosphorus (P) to prostrate dwarf-shrub, herb tundra in Northwest Greenland.Ecosystem CO2 exchange and vegetation cover and composition were very sensitive tolow rates of N inputs (0.5 g M-2 y-'), indicating that even low rates of atmosphericN deposition may alter high arctic ecosystem structure and function. Increasing Naddition from 1 to 5 g N M-2 y-1 did not alter CO2 exchange or vegetation characteristics,suggesting the ecosystem had become N saturated. Factorial additions of both N andP released the ecosystem from N saturation and dramatically increased grossecosystem photosynthesis (+500%) and ecosystem respiration (+250%), such that theecosystem switched from a small source of CO2 to a small sink for CO2 at middayduring the 2005 growing season. Changes in the component fluxes of CO2 exchangewere largely explained by a doubling of the normalized difference vegetation index, a100% increase in vascular plant cover and dramatic increases in the abundance of severalpreviously rare grass species. Our results clearly demonstrate that high arctic prostratedwarf-shrub, herb tundra is highly sensitive to low levels of N addition and that futureincreases in N deposition or N mineralization will likely lead to change in carboncycling and vegetation characteristics, but the magnitude of the response will beconstrained by P availability.