Long-term impact of organic and inorganic fertilizers on soil enzyme activities in cotton fields

Abstract:

Soil enzymes catalyse every biochemical transformation that converts organic residues into plant-available nutrients, yet their response to decades of contrasting fertiliser regimes in tropical cotton systems has received limited attention. This research evaluated the activity of four key enzymes dehydrogenase, urease, alkaline phosphatase, and β-glucosidase in surface soils (0-20 cm) from a 14-year-old long-term fertiliser experiment on cotton (Gossypium hirsutum L.) at Assiut, Egypt. Five treatments were compared: unfertilised control, farmyard manure alone (FYM, 10 t/ha/yr), recommended NPK (120:60:40 kg/ha), integrated FYM + NPK, and vermicompost (5 t/ha/yr). Soil samples collected after the 2023 cotton harvest showed that FYM + NPK plots had the highest dehydrogenase activity (47.8 µg TPF/g soil/day), 68.3% above the control. Alkaline phosphatase and β-glucosidase followed a similar pattern. Pure NPK treatment raised enzyme levels only marginally above the control, confirming that inorganic fertilisers alone do little to stimulate biological activity. Soil organic carbon and microbial biomass carbon correlated strongly with all four enzyme activities (r > 0.82). The results underscore the importance of integrating organic amendments with mineral fertilisers for sustaining enzyme-mediated nutrient cycling in continuous cotton cultivation.