Effects of warming on plant uptake of post-fire nitrogen in an arctic heath tundra

Postfire nitrogen (N) becomes increasingly important with the rising frequency of fires in arctic tundra, and climate warming is expected to accelerate plant recovery following fire. However, how plants differ in utilizing this postfire N and how their postfire N uptake responds to warming remains unknown. We conducted a fire experiment in combination with a warming treatment using open top chambers (OTCs) in an arctic heath tundra, West Greenland. We investigated the longer-term fate of two postfire N forms by tracing inorganic N (¹⁵NH₄⁺-N and ¹⁵NO₃⁻-N) and pyrogenic N pools (PyOM-¹⁵N) and examined how postfire N was acquired by vegetation at functional group- and species-specific levels. Most postfire inorganic and pyrogenic ¹⁵N (> 67%) was lost over the 4 yr following the fire, indicating limited N fertilization effects on plant recovery. Warming increased moss aboveground biomass and thus enhanced moss uptake of PyOM-¹⁵N. By contrast, warming increased the capacity of graminoids to take up inorganic ¹⁵N (+200%), despite their unchanged aboveground biomass. Our results show that warming alters postfire N cycling by shifting the pathways through which different plant functional groups access fire-derived N, with important implications for vegetation recovery and nutrient feedbacks in a warmer, more fire-prone Arctic.