Foliar uptake of atmospheric nitrate by two dominant subalpine plants: insights from in situ triple-isotope analysis.

The significance of foliar uptake of nitrogen (N) compounds in natural conditions is not well understood, despite growing evidence of its importance to plant nutrition. In subalpine meadows, N-limitation fosters the dominance of specific subalpine plant species, which in turn ensures the provision of essential ecosystems services. Understanding how these plants absorb N and from which sources is important in predicting ecological consequences of increasing N deposition. Here, we investigate the sources of N to plants from subalpine meadows with distinct land-use history in the French Alps, using the triple isotopes (Δ17 O, δ18 O, and δ15 N) of plant tissue nitrate (NO3 - ). We use this approach to evaluate the significance of foliar uptake of atmospheric NO3 - (NO3 - atm ). The foliar uptake of NO3 - atm accounted for 4-16% of the leaf NO3 - content, and contributed more to the leaf NO3 - pool after peak biomass. Additionally, the gradual 15 N enrichment of NO3 - from the soil to the leaves reflected the contribution of NO3 - atm assimilation to plants' metabolism. The present study confirms that foliar uptake is a potentially important pathway for NO3 - atm into subalpine plants. This is of major significance as N emissions (and deposition) are predicted to increase globally in the future.