Comparative effects of Azotobacter vinelandii and urea on soil enzyme activities and fruit quality of papaya (Carica papaya L.)

Azotobacter are potential bio-fertilizers that promote plant growth and improve soil properties. To optimize a sustainable nitrogen fertilization strategy for papaya, a study was carried out to compare the effects of inorganic nitrogen fertilizer (urea) and Azotobacter on the activities of important soil enzymes, mineral acquisition efficiency and their influence on yield and quality attributes in papaya. Different doses of Azotobacter vinelandii (10⁷ CFU g⁻¹) applied: 25 g (T₃), 50 g (T₄), 100 g (T₅), 200 g (T₆) and 400 g (T₇) plant⁻¹ were compared with a control (unfertilized and un-inoculated) (T₁) and a recommended dose of nitrogen, applied through urea (T₂). A. vinelandii 200 g plant⁻¹ (T₆) significantly enhanced soil organic matter (∼65.7 %), available phosphorus (∼65.52 %), potassium (∼27.32 %) and resulted in the highest activities of soil enzymes like acid phosphatase (∼86.19 %), alkaline phosphatase (∼67.76 %), urease (∼68.59 %), dehydrogenase (∼192.32 %) and FDA hydrolytic activity (∼185.20 %). The same treatment also recorded the highest microbial count. However, T₂ recorded the highest nitrogen content (∼122.40 %), while T₆ significantly recorded the highest phosphorus (∼153.85 %) and micronutrient contents (Fe, Mn, Zn, Cu, B). There was no significant difference between T₂ and T₆ in terms of total carbohydrate, plant biomass and fruit yield (∼37.09 % increase). However, T₆ and T₇ significantly enhanced the total phenol (∼40.54 %), total flavonoid (∼58.33 %) and anti-oxidant activity (∼112.29 %) in fruit pulp. The Principal Component Analysis signified the relevance of A. vinelandii in enhancing phosphorus and micronutrient uptake. Redundancy analysis indicated that nitrogen-fixers, phosphorus-solubilizers, fungi, bacteria, and actinomycetes collectively explained 43.90 % of the variance in soil biochemical attributes, with strong positive associations to acid phosphatase, dehydrogenase, and FDA hydrolysis. The factor analysis of yield and nutrient uptake parameters identified two major factors: Factor-1 (represented by T₂), designated as ‘Nitrogen acquisition factor’ and Factor-2 (represented by T₆), designated as ‘Phosphorus and Micronutrient acquisition factor’. The latter reflects the role of A. vinelandii. in enhancing papaya yield primarily by facilitating the uptake of phosphorus and essential micronutrients.