AERIAL TRANSPORT OF ISOTOPIC NITROGEN DURING HYDROPONIC GROWTH OF CORN

Authors: Sharpe, Ronald; Harper, Lowry

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Isotopic nitrogen (15N) has been used extensively to determine the fate of applied N fertilizer and to estimate N use efficiency (NUE). In these studies, NUE of 40 to 60% are common, indicating a large portion of fertilizer N is not being used by the crop. In addition, researchers have been unable to account for 20 to 40% of the applied N and theorized that the "unaccounted-for" N was leached as NO3 or denitrified and lost as N2 or nitrous oxide (N2O). The fate of applied N needs to be accurately determined because both NO3 and N2O are potential environmental pollutants. Corn was grown hydroponically in a greenhouse with and without 15N. The study was designed to quantify isotopic N15 losses from plants, evaluate atmospheric ammonia (NH3) transport between plants, and assess the potential error associated with the use of N isotopes. This study shows that fertilizer N absorbed during the early vegetative growth stage may later be lost from the plants via NH3 volatilization. Previous 15N studies did not consider NH3 volatilization losses and actual N use may be significantly greater than previously indicated because the N lost as 15NH3 can equal 15 to 20% of total plant N (25 to 30 kg ha-1). This study also shows that much of the "unaccounted-for" N losses may be due to NH3 volatilization and not to the environmentally hazardous NO3 or N2O compounds.

Technical Abstract: Isotopic 15N tracer studies have been important tools in the understanding of nitrogen (N) cycling in agricultural and natural soil-plant systems. The objectives of this research were to quantify volatilization losses of 15N under controlled conditions, evaluate 15NH3 transport between plants and assess the potential for error associated with the use of N isotopes. Corn (Zea mays L.) was grown hydroponically in pea gravel in twelve 125-L barrels. All N was applied as NO3 with enrichment periods of full season, planting through anthesis, anthesis to harvest, or no 15N applied. Plant growth and total N accumulation followed normal growth curves and maximum N content was obtained at anthesis. When 15N treatments stopped at anthesis, plants lost 15N (presumably as 15NH3) equivalent to about 37% of total plant N between anthesis and harvest. In treatments in which 15N applications started at anthesis, net N content in the leaves and stems decreased between anthesis and harvest while 15N content increased indicating a substitution of recently absorbed N for N compounds translocated to the fruit. Volatile losses of N indicate that caution must be exercised when conducting and interpreting 15N tests. Significant plant to plant transport of 15NH3 showed that decreases in plant 15N content could not be used to estimate NH3 volatilization for an entire field.