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 (NO₃^-) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO₃^- concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO₃^- that is typically below detection limits. Here we reexamine NO₃^- use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO₃^- Soil-derived NO₃^- was detected in tundra plant tissues, and tundra plants took up soil NO₃^- at comparable rates to plants from relatively NO₃^--rich ecosystems in other biomes. Nitrate assimilation determined by ¹⁵N enrichments of leaf NO₃^- relative to soil NO₃^- 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 NO₃^- 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 NO₃^- availability in tundra soils is crucial for predicting C storage in tundra.