Analysis of the Source of Nitrogen During Water Yam (Dioscorea alata L.) Growth Using δ¹⁵N Observations

Abstract

The relationship between nitrogen (N) absorption and growth in water yam (Dioscorea alata L.) is not clearly understood. A previous experiment suggested that water yam is able to grow under poor soil conditions regardless without nitrogen fertilization; however, the source of nutrition was not identified. This study aimed to identify the source of N and the percentage of plant N derived from atmospheric N₂ (%Ndfa) under poor soil conditions. Experiments were carried out on Miyako Island, Okinawa Prefecture, Japan, from April 2014 to December 2015. In experiment 1, water yam growth was compared under urea application (N treatment) and without (control), while in experiment 2 urea (N treatment) or cow manure (CM treatment) application versus without application (control) was compared. N and CM treatment had a positive effect on growth and tuber yield. However, while yellowish leaves were observed in control plants, they grew until harvest, and tuber yield was similar between treatments. The δ¹⁵N values of the N- and CM-treated plants suggested that N was absorbed from the urea and cow manure; however, in the control plants, the N source seems to have been atmospheric. According to the result of δ¹⁵N value, it was suggested that water yam cultivated without N fertilizer absorbed 38.04% N from air.