A field experiment was conducted to study the impact of irradiated and non-irradiated sewage sludge applied to sandy soil on the productivity of fennel plants (Foeniculum vulgare L.). Four rates of irradiated and non-irradiated sewage sludge application were used (20, 40, 60, and 80 t ha−1). Samples analysis included the biomass production at the vegetative and flowering stages, seed production, volatile oil content, volatile oil constituents, chlorophyll content, total and reducing sugars, and the heavy metals content of the shoots and seeds.
The biomass production increased as the sludge application rate increased for both irradiated and non-irradiated plots. However, the increase was significantly higher under all irradiated treatments than the corresponding rates of non-irradiated treatments at both the vegetative and flowering stages. At the vegetative stage, the biomass values ranged from 10.2 to 34.1 g plant−1 at 80 t ha−1 for non-irradiated and irradiated sewage sludge, respectively. Whereas at the flowering stage, the values ranged from 23.9 to 65.1 g plant−1 at 80 t ha−1 for non-irradiated and irradiated sewage sludge, respectively. Total sugars, reducing sugar, non-reducing sugar and chlorophyll content increased as the sludge application rate increased. At the 80 t ha−1 of irradiated sludge application rate, the reducing sugar content was 29.39 mg g−1 DW (dry weight) at the vegetative stage and 37.85 mg g−1 DW at the flowering stage. Regarding heavy metals, sewage sludge was a good source to provide fennel plants with essential micronutrients (Zn and Fe) in the meantime, the translocation of (Pb and Cd) to the shoot system was very low. A linear gradual increase in seed yield was observed as the sludge application rate increased. Irradiated sewage sludge treatments showed a higher fennel seed yield than non-irradiated sewage sludge treatments.
Volatile oil percentages exhibited no observable variations due to the use of sewage sludge. A few and limited fluctuations could be observed. However, total oil content (l ha−1) increased due to the increase in seed yield. The magnitude of increase in volatile oil production in response to the sewage sludge application was parallel to the increase in seed yield. The GLC measurements of the fennel volatile oil reveal that the t-anethole is the predominant fraction. However, fenchone was detected in a relatively moderate concentration. The applied sewage sludge treatment induced some variations in fennel volatile oil constituents. The t-anethole is relatively higher in volatile oil obtained from plants grown on sandy soil fertilized with non-irradiated sewage sludge than that fertilized with irradiated sewage sludge. In the meantime, the increase in t-anethole was accompanied by a decline in fenchone content. Under all sludge application rates iron and zinc concentrations of fennel seeds were within the normal plant concentration range, whereas there were only traces of Cd concentration.