Isotopic composition and concentration of total nitrogen and nitrate in xylem sap under near steady‐state hydroponics

Abstract

After root uptake, nitrate is effluxed back to the medium, assimilated locally, or translocated to shoots. Rooted black cottonwood (Populus trichocarpa) scions were supplied with a NO 3 −-based (0.5 mM) nutrient medium of known isotopic composition (δ 15 N), and xylem sap was collected by pressure bombing. To establish a sampling protocol, sap was collected from lower and upper stem sections at 0.1-0.2 MPa above the balancing pressure, and after increasing the pressure by a further 0.5 MPa. Xylem sap from upper stem sections was partially diluted at higher pressure. Further analysis was restricted to sap obtained from intact shoots at low pressure. Total-, NO 3 −-N and, by difference, organic-N concentrations ranged from 6.1-11.0, 1.2-2.4, and 4.6-9.4 mM, while discrimination relative to the nutrient medium was −6.3 to 0.5‰, −23.3 to −11.5‰ and − 1.3 to 4.9‰, respectively. There was diurnal variation in δ 15 N of total-and organic-N, but not NO 3 −. The difference in δ 15 N between xylem NO 3 − and organic-N suggests that discrimination by nitrate reductase is near 25.1 ± 1.6‰. When this value was used in an isotope mass balance model, the predicted xylem sap NO 3 −-N to total-N ratio closely matched direct measurement. K E Y W O R D S black cottonwood, isotope discrimination, nitrate reductase, nitrogen isotopes, nitrogen uptake and assimilation 1 | INTRODUCTION Nitrogen is the primary limiting nutrient for most plants in terrestrial ecosystems (Glass et al., 2002). The various forms and pools of nitrogen in the environment, and in the plant, typically have differences in the relative abundance of 15 N (δ 15 N). These differences in isotopic composition are widely used in the study of nitrogen cycling in ecosystems and individual organisms (Dawson, Mambelli, Plamboeck, Templer, & Tu, 2002). However, interpretation of variation in δ 15 N at natural abundance levels can be problematic. For example, the δ 15 N of an individual plant or one of its parts is not only determined by the external nitrogen source, of which there is often more than one, but also by internal physiological factors, such as different nitrogen uptake mechanisms, different pathways of assimilation and transport, and the possibility of differential recycling of nitrogen within the plant, all of which may discriminate against 15 N (Robinson, 2001). A better understanding of isotope discrimination that may or may not be associated with these various factors is needed before variation in δ 15 N can be routinely used in ecological and/or physiological research. Nitrogen sources in soils include nitrate, ammonium and organic nitrogen. These are distributed unevenly in time and space, in varying proportions that depend on profile position, local vegetation, climate and many other edaphic factors (Amundson et al., 2003). For example, boreal forests generally have less inorganic nitrogen in soils than temperate forests because of colder temperatures and slower rates of mineralization (Näsholm et al., 1998). Also, because they are often acidic, boreal fo...

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