Abstract: This study was located at Kezuozhong near Tongliao City, in the Inner Mongolia Autonomous Region of China, and investigated the effects of nitrogen fertilizer application on non-structural carbon and nitrogen metabolite levels in leaves of oats (Avena sativa) grown in a desert environment. Nitrogen fertilizer was applied as urea at 70, 140, 210 and 280 kg·ha^-1 elemental N, and with a no-N treatment as a control (designated N₇₀, N₁₄₀, N₂₁₀, N₂₈₀, and N₀, respectively). Separate fertilizer applications comprising 15%, 40%, 25%, and 20% of the total, respectively, were made at seedling, tillering, jointing, and heading stages of crop development. Soluble sugar, starch, amino acid, soluble protein (SP) content and nitrate reductase activity (NR) were measured for leaf samples collected from various positions at the booting, heading and grain filling growth stages. Sucrose content and activity of sucrose synthase (SS) were also measured at the grain filling stage.It was found that nitrogen fertilizer significantly (P<0.01) reduced the carbohydrate content (including soluble sugars, starches and glucose), with the size of the reduction in proportion to the quantity of N applied. Conversely, free amino acid levels in leaves were increased by N fertilizer and were highest in the N₂₈₀ treatment at booting and heading stages, and were also high in the N₂₁₀ nitrogen treatment at the watery dough stage of grain fill.Similarly, SP content was significantly higher in N-treated plants than in the control (P<0.01) and varied across growth stages, with highest values recorded in the N₂₁₀ treatment at the booting stage, and in the N₂₈₀ treatment at the heading stage. In terms of variation within the plant, SP concentration in the upper leaves was the highest in the N₂₁₀ treatment, while in the middle and lower leaves SP was the highest in N₂₈₀ nitrogen treatment in the watery dough stage. At grain filling stage, in the N₇₀ treatment, SS activities were 3.61, 6.50, and 13.44 μg·g^-1 FW·h^-1, respectively, for upper, middle and lower leaves, and corresponding values for NR activities were 81.6, 84.8, 76.2 μg·g^-1 FW·h^-1. SS and NR activities were significantly decreased (P<0.01 and P<0.05 for SS and NR, respectively) with application of N at higher rates than N₇₀.Topdressing of N fertilizer had no obvious influence on the nitrate-nitrogen content in the oat leaves. The C∶N ratios were largest in the N₀ treatment at the booting stage, being respectively 9.00, 5.70 and 1.48 for upper, middle, and lower leaves. However, the C∶N values of the upper leaves of N₂₁₀and the middle and lower leaves of the N₂₈₀ treatment nitrogen were maximized at the heading and filling stages.This study provides comprehensive data on developmental changes in carbon and nitrogen contents, on activities of key enzymes in carbon and nitrogen metabolism and on yield characteristics of oats grown in a desert soil, and will assist understanding of the coordination of carbon and nitrogen metabolism.

Key words: oat, nitrogen fertilizer, carbon and nitrogen metabolism, carbon to nitrogen ratio