Life in the dark: Impact of future winter warming scenarios on carbon and nitrogen cycling in Arctic soils

The high Arctic region is warming faster than other areas globally, with widespread decreases in snow cover and longer growing seasons. This is expected to greatly influence plant-microbial carbon (C) and nitrogen (N) cycling and thus ecosystem functioning, particularly when soils remain unfrozen during the dark winter period and no photosynthesis is possible. To better understand this response, we studied the effects of different winter warming scenarios on soil C and N dynamics. We collected intact soils cores from two contrasting biomes from Svalbard, namely a moss-dominated peat mire and purple saxifrage-dominated dry tundra heath. In the laboratory, we used an Arctic annual light regime and three contrasting winter temperature regimes to simulate a range of climate scenarios: (i) current climate in which the soils were frozen in winter and thawed in the summer, (ii) a year 2050 regime in which the soils were both frozen and thawed in winter, and (iii) a year 2100 regime in which the soils remained unfrozen during winter. We monitored greenhouse gas (GHG) emissions (CO2, CH4 and N2O) and nutrients in soil solution (DOC, NO3−, NH4+, P) over a one-year period. In the peat mire, warmer winters increased CO2 emissions due to increased microbial activity but had little effect on methane or N2O fluxes. Warmer winters also decreased dissolved organic carbon and nutrients in soil solution. In the dry tundra heath, warmer winters increased nitrate availability but did not greatly alter CO2, CH4 or N2O gas fluxes or other nutrients. Warming in the dry tundra health induced earlier spring flowering and shifts in vegetation composition. Overall, we present clear evidence that warmer winter conditions are likely to alter C and N cycling in Arctic soils, with implications for plant productivity and vegetation community composition.