Nitrate is an important nitrogen source for Arctic tundra plants

Abstract

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.

Keywords: Arctic tundra plants; nitrogen dynamics; plant nitrate; soil nitrate; stable isotopes.

Fig. 1.

Concentrations of NO₃⁻ in plant leaves (A) and roots (B) across different ecosystems. The box encompasses the 25th to 75th percentiles, and whiskers are the SD values. The line and square in each box mark the median and mean values of studied plants at each site, respectively. Unique letters above the boxes mark significant differences at the level of P < 0.05. Detailed site information, including site abbreviation definitions, and species-specific values are given in SI Appendix, Tables S1 and S2. dw, dry weight.

Fig. 2.

Differences (∆ values) in δ¹⁵N (A) and δ¹⁸O (B) between leaf NO₃⁻ and soil NO₃⁻ across different ecosystems. The box encompasses the 25th to 75th percentiles, whiskers are the SD values, and the red line and red square in each box mark the median and mean values, respectively. Unique letters above the boxes indicate significant differences at the level of P < 0.05. The ∆ values were calculated using replicate values of plant tissues minus mean values of soil in corresponding sites (SI Appendix, Fig. S8 and Table S1).

Fig. 3.

Δ¹⁷O vs. δ¹⁸O plots of NO₃⁻ in soil, leaves, and atmospheric (Atmos, as precipitation or snow) deposition across different ecosystems. The mixing lines of Arctic and tropical sites (y = 2.52x − 4.42 and y = 2.97x + 0.58, respectively) were based on isotopic values of soil NO₃⁻ (n = 18) (54) and snowpack NO₃⁻ (n = 12) (56) at Barrow, and of soil NO₃⁻ (n = 18) and precipitation NO₃⁻ (n = 3) at Jianfengling in tropical China (49), respectively. The mixing line of temperate sites (y = 2.64x + 3.82) was based on isotopic values of soil NO₃⁻ at Japanese temperate sites (n = 22) and precipitation at TML (n = 12) in this study. The mixing line of subtropical sites (y = 2.87x + 0.91) was based on isotopic values of soil NO₃⁻ (n = 29) at subtropical sites and precipitation NO₃⁻ at Guiyang, China (n = 3) in this study. The Δ¹⁷O of soil NO₃⁻ was assumed to be zero.

Fig. 4.

δ¹⁵N values of leaf total N and soil N sources of tundra plants in Alaska. AM, arbuscular mycorrhiza; ECM, ectomycorrhiza; ERM, ericoid mycorrhiza; NM, nonmycorrhiza. The box encompasses the 25th to 75th percentiles, and whiskers are the SD values. The line in each box marks the mean value. Plant δ¹⁵N data were summarized from ref. and those of SI Appendix, Fig. S9B. The empty squares show soil δ¹⁵N data reported at IMT (53) and the blue-filled circle shows data at TFS (21). Soil δ¹⁵N-NO₃⁻ values of other sites are summarized from available data of non-Arctic sites in this study; soil δ¹⁵N-NO₃⁻ values at Barrow are cited from ref. .

Fig. 5.

Proportional contributions (mean ± SD) of soil NO₃⁻, NH₄⁺, and HAA to leaf total N of tundra plants in Alaska. The ¹⁵ε values [0‰ for NM plants, −5.0‰ for AM plants, −6.9‰ for ECM plants, and −7.7‰ for ERM plants (21, 62)] were considered for NO₃⁻, NH₄⁺, and HAA (scenario 1); for NH₄⁺ and HAA only (scenario 2); for HAA only (scenario 3); for none of NO₃⁻, NH₄⁺, and HAA (scenario 4).