Soil microbial and plant community responses to single large carbon and nitrogen additions in low arctic tundra
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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.
KeywordsBiomass Competition Evergreen Graminoid Nitrogen immobilization Soil depth Tundra
We thank Meghan Laidlaw for harvesting and sorting the plant biomass and Linda Cameron for assistance with sample analyses. We are grateful to Kate Buckeridge, Haiyan Chu, and several reviewers for many very helpful comments on earlier versions of this manuscript. Many thanks to Peter Lafleur and Greg Henry for scientific support, and especially to Steve Matthews and Karin Clark of the Division of Environment and Natural Resources, G.N.W.T. for logistical assistance. Financed by NSERC, CFCAS and DIAND.
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