Soil microbial and plant community responses to single large carbon and nitrogen additions in low arctic tundra

Plant production and community composition in many mid- and high latitude ecosystems is strongly controlled by nitrogen (N) availability. We investigated the effects of large factorial additions of labile carbon (C) (sucrose) and N (NH4NO3) in a single year on soil microbial and plant biomass pools over subsequent years in a widespread low arctic mesic tundra ecosystem. Soil microbes took up large amounts of N within weeks of its addition, and this accumulation was maintained over at least 2 years. Microbial biomass C was unaffected, strongly suggesting that the addition had rapidly elevated microbial N concentrations (by ∼50%). Microbial biomass N and root N (per unit soil volume) decreased with depth down through the organic and mineral layers in all treatments, indicating that most of the added N was retained within the top 5 cm of the organic layer 2 years after the additions. In contrast to N, the C additions had no significant effects. Finally, plant shoot N concentrations 3 years after the additions were significantly enhanced primarily in the evergreen species which dominate this ecosystem-type, resulting in a ∼50% increase in evergreen shoot N accumulation but no corresponding change in biomass. Our study demonstrates a very rapid and substantial microbial N sink capacity that is likely to be particularly important in determining N availability to plants over weekly to annual timescales in this tundra ecosystem. Furthermore, the results suggest that the moderate increases in tundra soil N supply expected due to climate warming could be largely immobilized by microbes, resulting in slower and more evergreen-dominated plant community responses than are predicted from long-term, annually repeated, high-level fertilisation studies.