Isotopic composition and concentration of total nitrogen and nitrate in xylem sap under near steady‐state hydroponics
Yi Hu, Robert D. Guy
Abstract
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.