Nitrogen Fixation, Denitrification, and Ecosystem Nitrogen Pools in Relation to Vegetation Development in the Subarctic
Pernille L. Sorensen, Sven Jonasson, Anders Michelsen
Nitrogen (N) fixation, denitrification, and ecosystem pools of nitrogen were measured in three subarctic ecosystem types differing in soil frost-heaving activity and vegetation cover. N2-fixation was measured by the acetylene reduction assay and converted to absolute N ecosystem input by estimates of conversion factors between acetylene reduction and 15N incorporation. One aim was to relate nitrogen fluxes and nitrogen pools to the mosaic of ecosystem types of different stability common in areas of soil frost movements. A second aim was to identify abiotic controls on N2-fixation by simultaneous measurements of temperature, light, and soil moisture. Nitrogen fixation rate was high with seasonal input estimated at 1.1 g N m−2 on frost-heaved sorted circles, which was higher than the total plant N content and exceeded estimated annual plant N uptake several-fold but was lower than the microbial N content. Seasonal fixation decreased to 0.88 g N m−2 on frost-heaved moss-covered surfaces and to 0.25 g N m−2 in stable heath vegetation, both lower than the plant and microbial N content. Yet fixation was estimated to correspond to about 2.7 times the annual plant N demand on the moss-covered surfaces but less than the plants' demand on the heath. Surprisingly, we found no denitrification on any surface. Climatic changes in the Arctic will generate a warmer climate and change precipitation patterns. A warmer, drier environment will decrease N2-fixation and thereby N availability to plants and microorganisms, while wetter conditions probably will increase N2-fixation and thereby N supply to the surroundings.