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

Abstract

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. 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.