Co-Application of Sheep Manure and Azotobacter Biofertilizer Enhances Growth, Yield, Essential Oil Profile, and Antioxidant Activity in Summer Savory

Overuse of chemical fertilizers can threaten the agro-ecological balance, including an excessive accumulation of certain elements, such as nitrogen and phosphorus. On the other hand, organic fertilizers and biofertilizers, which are eco-friendly and cost-effective, increase biological nitrogen fixation and enhance the availability of nutrients to plants. The aim of this research was to study the possibility of using a full (22.50 t/ha) and 50% (11.25 t/ha) treatment of sheep manure with azotobacter (100 mL/20 L) instead of inorganic fertilizers for increasing savory (Satureja hortensis L.) growth production and yield value as well as improving chemical and biological properties. The results showed that the treatment with 50% sheep manure recorded the highest total dry herb (3.18 t/ha) yield. The inorganic fertilizer resulted in the highest essential oil content (1.43% v/w) and γ-terpinene (10.38% v/v), cymol (5.90% v/v), and α-bisabolene (5.28% v/v) values. The maximum carvacrol value (42.54% v/v) was recorded in the savory herb after applying no fertilization to the plants, while the highest concentration of thymol (16.09% v/v) was obtained by applying the full sheep manure treatment. The full sheep manure + azotobacter treatment had the highest mean α-terpinene value (7.22% v/v), and the 50% sheep manure + azotobacter treatment had the highest mean α-phellandrene value (6.44% v/v). The highest DPPH activity (60.86%) and FRAP value (69.64 mg TE/g DW) were observed with the azotobacter + full sheep manure treatment, while the highest total phenolic content (96.87 mg GAE/g DW) and total flavonoid content (45.97 mg QE/g DW) in the savory herb were obtained from the combination treatment of 50% sheep manure doses + azotobacter. Principal coordinate analysis (PCA) revealed distinct clustering of treatments, with PC1 and PC2 explaining >60% of the variance, highlighting the dominant role of sheep manure doses in morphological/yield properties. Heatmap analysis grouped the treatments (right) and examined properties (bottom) as two main groups. The full sheep manure + biofertilizer and inorganic fertilizer treatments were found in the first group, depending on the treatments. Moreover, the heatmap analysis revealed that the full and 50% sheep manure (SM) treatments played critical roles in separating the examined properties, and the DPPH and carvacrol properties were grouped together compared to other properties. Thus, the results suggest that treatment with azotobacter could be employed in combination with appropriate rates of sheep manure to obtain the maximum benefits regarding herb yield, biological activity, and essential oil components.