Arctic extreme winter warming events lead to microbial N immobilization and evergreen shrub N limitation

In Arctic soils, wintertime usually means subzero ground temperatures and only little unfrozen water available below a snow cover. While this period has less active nutrient cycling by microbes, some activity means that winter mineralization of e.g., nitrogen (N) can release a pulse of mineral N (ammonium, nitrate) into the soil solution, which can become biological available upon spring thaw. 

In springtime, plants may compete with microbes for the N pulse, but if thaw happens during winter, the N pulse could be immobilized by active microbes, which can decrease the size of the springtime N pulse, and therefore decrease the growing season N addition to the ecosystem.

Many parts of the Arctic have with climate change seen an increase in the frequency of extreme winter warming events (WW events), which are periods of positive temperatures lasting 5-7 days and causing snow to melt and the upper soil layer to thaw. 

In a field scale experiment, we quantified the amount of mineral N released into solution upon soil thaw during a simulated WW event in Disko island, Western Greenland (69.28ᵒN, 53.48ᵒW). We used ¹⁵N tracing to determine which parts of the ecosystem that benefited from this N during the WW event. We further returned the following summer to test whether vegetation was more N limited the summer after a WW event.

Our results show that after 6 days of thaw, 50-60 % of WW-released N was found either in active microbial biomass or stored in the soil, whereas none had been assimilated by the plants. The following summer, we saw that evergreen shrubs subject to the WW event had acquired less N than if they had experienced a stable winter. This indicates that evergreen shrubs are especially sensitive to a smaller spring N pulse, which is in line with studies showing that evergreen shrubs rely more on springtime N uptake. As evergreen shrubs are an important functional plant type in the tundra, increased frequency of WW events could therefore change tundra plant community composition. Our results also indicate that N immobilization during WW events could be a mechanism linking WW events to Arctic browning and decrease in photosynthetic C uptake rates.

Our research sheds light on the little studied impact of climate change-related WW events on the nutrient cycling of Arctic soils and the plant-root competition for N in the future, and we develop methods for studying this phenomenon in the broader Arctic.