Field studies of nitrogen application on Greek oregano (Origanum vulgare ssp. hirtum (Link) Ietswaart) essential oil during two cultivation seasons

The effects of four levels of fertilizer nitrogen application (0, 40, 80 and 120 kg N ha−1) on oil yield, concentration, and composition of Greek oregano (Origanum vulgare ssp. hirtum (Link) Ietswaart) during the second and third seasons from its field establishment were examined. Oil concentration per plant increased significantly from about 1.5% to 2.0% in the second to about 5.5% (v/w) in the third season and it was higher in the inflorescences when compared with leaves. No significant effects of fertilizer application on oil concentration were detected. Oil yield showed significant peaks at nitrogen rates of 80 kg ha−1 in both seasons and was significantly higher in the third (between 57 and 83 l ha−1), when compared with the second season (between 17 and 27 l ha−1). Such a response was ascribed to the positive nitrogen effects on herbage yield. As regards oil composition, nitrogen fertilization exhibited some significant effects only in the third, more humid, season. Thus, it positively affected linalool content in inflorescences at the rate of 80 kg N ha−1and carvacrol content in leaves at the rates of 40, 80 and 120 kg N ha−1. On the other hand, π-cymene, caryophyllene, α-pinene, thymol, and camphene were observed at higher levels in the unfertilized plots. Carvacrol was the dominant constituent of the essential oil content ranging from 56.46% to 84.88% among organs, treatments and seasons. It was followed by π-cymene (4.19–21.4%) and α-pinene (0.11–1.88%). Thymol was detected at low levels (0.20–1.44%). Carvacrol percentage was higher in the drier and warmer season (70.75–84.88%) in both leaves and inflorescences, whereas a number of compounds (α-thujene, α-pinene, camphene, myrcene, α- and γ-terpinene, π-cymene, cis-hydrosavinene, linalool, α-terpineol, α-caryophyllene, and β-disavolene) tended to accumulate at higher levels during the wetter and colder season.

► N-fertilization significantly increased oregano oil yield at a rate of 80 kg ha−1.
► Oil concentration in both leaves and inflorescences was unaffected by N-levels.
► Nitrogen positively affected oil composition only in the more humid season.
► Carvacrol was the dominant constituent of the essential oil (from 56.46% to 84.88%).
► Carvacrol content was higher in the drier and warmer season.