Nitrate is an important nitrogen source for Arctic tundra plants

Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO(3)(−)) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO(3)(−) concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO(3)(−) that is typically below detection limits. Here we reexamine NO(3)(−) use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO(3)(−). Soil-derived NO(3)(−) was detected in tundra plant tissues, and tundra plants took up soil NO(3)(−) at comparable rates to plants from relatively NO(3)(−)-rich ecosystems in other biomes. Nitrate assimilation determined by (15)N enrichments of leaf NO(3)(−) relative to soil NO(3)(−) accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO(3)(−) availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO(3)(−) availability in tundra soils is crucial for predicting C storage in tundra.