Variation in N2 fixation in subarctic tundra in relation to landscape position and nitrogen pools and fluxes

Biological N-2 fixation in high-latitude ecosystems usually exhibits low rates but can significantly contribute to the local N budget. We studied N-2 fixation in three habitats of East European subarctic tundra differing in soil N stocks and fluxes: N-limited vegetated peat plateau (PP), frost formations of bare peat called "peat circles" (PC) with high availability of soil N, and vegetated upland tundra (UT) with low to intermediate N-availability. Nitrogen fixation was measured at field conditions twice during summer 2011 by acetylene reduction assay, and N-2 fixation rates were verified by 15N(2) fixation assay. Response to variation in nutrients, carbon, and temperature was studied in complementary laboratory experiments. Further, we aimed to link N-2 fixation rates to N deposition and major N transformation rates (gross and net mineralization, plant N uptake) including high N2O emissions recently found from PC. We hypothesized that N2O emissions in PC were fueled partly by biologically fixed N. Contrary to that hypothesis, N-2 fixation was found solely in PP (0.01-0.76 mg N m(-2) d(-1)), where N-2 was fixed by moss-associated cyanobacteria and heterotrophic soil bacteria. The low N and high P availability corresponded with the occurrence of N-2 fixation in these soils. Nitrogen fixation represented only a small portion of plant N uptake in PP. Conversely, bare PC (as well as vegetated UT) lacked N-2 fixation and thus N2O efflux is most likely fueled by release of mineral N to the soil through internal nutrient cycling.