Microbial nitrogen uptake in winter warming manipulation experiments in Arctic tundra (Disko Island; Greenland)

Due to the above global average warming, the winter climate in West Greenland is increasingly characterized by warm spells causing snow melt and soil exposure. These events might activate soil microbes and associated nutrient cycles, with consequences for the tundra ecosystem, even in the following summer.

Here we studied effects of winter warming in a dry heath tundra ecosystem in Blæsedalen on Disko Island, West Greenland near Arctic Station, characterized by low shrub vegetation (Betula nana, Salix glauca, Vaccinium uliginosum, Empetrum nigrum, Cassiope tetragona). We established replicate 0.5 m² plots equipped with custom-made heating probes that were pre-installed in the growing season aiming to warm up the surrounding soil in the following winter campaign to a depth of 15 cm during a week. Nitrogen (N) transformation pathways from organic N (proteins and amino acids) to ammonium (inorganic N) and microbial N uptake were quantified in the tundra soil using ¹⁵N labelling techniques, accompanied by greenhouse gas flux measurements.

In situ warming resulted in soil CO₂ loss, and activated microbial CH₄ uptake. Under laboratory conditions, we could also detect several freeze-thaw induced emission peaks of N₂O and N₂. For the first time nanoSIMS analysis revealed rapid soil microbial uptake of amino acid alanine into microbial cells in winter soils. Furthermore, optical microscopy and Raman spectroscopy analysis of microbial communities provided insights into both their immediate response to the warming and into memory effects in the following growing season, with increased microbial activity in the samples taken from plots that have been warmed.

In sum, we highlight a fast activation of microbial N turnover due to Arctic winter warming events, which results in changes in nutrient cycling that persist in the following growing season.