Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330-1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. To evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during 5years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research (CiPEHR) project. Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the 5year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures, and higher levels of soil moisture. Within 5years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools.

Winter air temperature variability is projected to increase in the temperate zone whereas snow cover is projected to decrease, leading to more variable soil temperatures. In a field experiment winter warming pulses were applied and aboveground biomass and root length of four plant species were quantified over two subsequent growing seasons in monocultures and mixtures of two species. The experiment was replicated at two sites, a colder upland site with more snow and a warmer, dryer lowland site. Aboveground biomass of Holcus lanatus declined (-29 %) in the growing season after the warming pulse treatment. Its competitor in the grassland mixture, Plantago lanceolata, profited from this decline by increased biomass production (+18 %). These effects disappeared in the second year. There was a strong decline in biomass for P. lanceolata at the lowland site in the second year. These two species also showed a decline in leaf carbohydrate content during the manipulation. Aboveground productivity and carbohydrate content of the heathland species was not affected by the treatment. The aboveground effects of the treatment did not differ significantly between the two sites, thereby implying some generality for different temperate ecosystems with little and significant amount of snowfall. Root length increased directly after the treatment for H. lanatus and for Calluna vulgaris with a peak at the end of the first growing season. The observed species-specific effects emphasize the ecological importance of winter temperature variability in the temperate zone and appear important for potential shifts in community composition and ecosystem productivity.