Interspecific variation in leaf–root differences in δ¹⁵N among three tree species grown with either nitrate or ammonium

Key message

Interspecific variation in nitrogen isotope composition of roots and leaves of tree seedlings grown in a steady-state nitrogen environment reflects known variation in sites of assimilation and nitrogen source preference in three tree species.

Abstract

As a time-integrated measure of nitrogen use, discrimination against the heavier stable isotope (¹⁵N) during the uptake and assimilation of inorganic nitrogen has the potential to provide information on interspecific differences in inorganic nitrogen source preference. Here, nitrogen isotope composition (δ¹⁵N) at natural abundance was measured for the roots and shoots from seedlings of three forest tree species: Populus tremuloides (aspen), Pinus contorta var. latifolia (pine) and Picea glauca (spruce). The seedlings were grown hydroponically with low (0.1 mM) or high (1.5 mM) concentrations of NO₃ ⁻ or NH₄ ⁺, or in sand with NO₃ ⁻, NH₄ ⁺ or an equal mix of NO₃ ⁻ and NH₄ ⁺ (0.1 mM). Whole-plant nitrogen isotope discrimination was observed in hydroponically grown seedlings but not in sand culture. Differences in δ¹⁵N between shoots and roots were greater in aspen when grown with NO₃ ⁻ (3.02 ‰) than with NH₄ ⁺ (1.27 ‰). There were no significant differences between the δ¹⁵N of leaves and roots for pine and spruce on either source. Although whole-plant nitrogen isotope discrimination was not observed in sand culture, shoot δ¹⁵N was, again, greater than root δ¹⁵N for NO₃ ⁻-grown aspen. Interspecific variation in nitrogen isotope discrimination was observed in both hydroponics and sand culture. The differences in nitrogen isotope composition under steady-state conditions indicate that interspecific differences in nitrogen source preference are consistent with previous experiments using alternative methods to identify differences in nitrogen uptake and assimilation in the same tree species.