| Summary: | Permafrost in high latitude ecosystems currently stores 1330-1580 Pg of carbon (C). As these ecosystems warm, the 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 productivity of warmed tundra plants. The magnitude of the plant response, however, will be mediated by changing nitrogen (N) availability in thawing soil profiles. Nitrogen availability currently limits plant productivity as well as microbial decomposition in tundra ecosystems. We monitored N cycling in response to experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research project (CiPEHR). Within five years of permafrost thaw, soil inorganic N availability increased during both the growing season and winter. Soil warming at CiPEHR prompted a 23% increase in aboveground plant biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum explained 91% of the observed change in aboveground biomass. A soil incubation experiment revealed that there was a large amount of inorganic N stored in permafrost soils and deep soils mineralize more N than their shallow counterparts when thawed. When we compared the flush of inorganic N released during the thaw of permafrost to the potential N mineralized by a warmer and thicker active layer, we saw that these fluxes were similar in magnitude. Depth profiles suggest that the increase in plant available N came predominately from soils below 35 cm. The amount of N in leaf litter significantly increased during the process of permafrost thaw and we believe that plant translocation of N from deep soils to surface soils could accelerate decomposition in shallow, N limited soils. Dry surface soils at CiPEHR were associated with reduced N availability, indicating that decomposition can also be both moisture and temperature limited at this site. Together, these results demonstrate that there is a strong, positive relationship between the depth of permafrost thaw and plant access to N in tundra ecosystems but the future C balance of these ecosystems will be heavily influenced by thaw-induced changes to local hydrology and N mobility through plants and soils.
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