Life in the Dark: Impact of Future Winter Warming Scenarios on C and N Cycling in Arctic Soils

Climate change is likely to have a profound influence on C and nitrogen (N) biogeochemical cycling. Increasing temperatures may greatly shorten the period of winter freezing. In extreme scenarios, Arctic soils may remain unfrozen during the winter, when there is permanent darkness. However, there is little information about how these different winter warming scenarios will affect C and N cycling in high Arctic tundra ecosystems. In this study, we assessed the effects of simulated warmer winters 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 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 2100 regime in which the soils remained unfrozen during winter. We monitored greenhouse gas (GHG) emissions and nutrients in soil solution throughout a year. Our results revealed that a simulated warmer winter increased CO2 efflux from the peat soil, however, total GHG emissions were not significantly different from the current climate regime. This suggests that the peat soil continues to be a C sink even if microbial decomposition of C was greater than photosynthetic C fixation in winter. We also found that the 2100 climate scenario shifted in tundra ecosystems from purple saxifrage to other plant species by late summer. This was probably due to increased decomposition and the availability of N. We conclude that warmer winters on the future may have minor effects on total soil GHG emissions but that tundra vegetation is more sensitive to change.