Effects of chemical fertilizer and biogas slurry on cotton (Gossypium hirsutum L.) yield and soil properties under different irrigation levels

This study was carried out during the 2024 growing season in Şanlıurfa, Turkey, between May and October. In this study, the effects of different irrigation levels (I₁:%75, I₂:%100, I₃:%125 irrigation) combined with chemical (N) and organic (biogas residue; B) fertilizer applications on cotton yield components, soil chemical properties, and microbial enzyme activities were investigated. The study was conducted using the cotton cultivar Fiona according to a randomized complete block design with split plots, with three replications. The results revealed statistically significant interactions ( p < 0.01; p < 0.05) between irrigation and fertilizer applications. The highest boll number was obtained in the I₃N combination, while the highest boll weight was recorded in I₃B (I 3 :%125 irrigation + B: biogas residue). In terms of fiber yield, the optimum value was measured in the I₃N (I 3 :%125 irrigation + N: chemical fertilizer) treatment (2338.9 kg ha⁻¹), indicating that nitrogen application under high irrigation may be critical for yield. Organic fertilizers significantly enhanced catalase, urease, and dehydrogenase enzyme activities, stimulating microbial life and supporting biological productivity. The highest ammonium and nitrate contents were generally observed in the I₂B combination, suggesting that the synergy between optimal irrigation and organic fertilization improved the soil nitrogen cycle. Overall, the results indicate that organic fertilizers strengthen sustainability in cotton production by serving as an alternative or complementary source to chemical inputs. Overall, these results suggest that organic fertilizers could serve as an alternative or complementary source to chemical inputs in cotton production.