The effects of substituting organic fertilizer for nitrogen fertilizer on seed cotton yield and soil enzyme activity under drip irrigation

Introduction The application of organic fertilizers can increase carbon and nitrogen sources in the soil. This could affect the quantity of microorganisms and the metabolism of nutrients in the soil, thereby influencing soil enzyme activity and cotton yield. This study explored the effects of substituting organic fertilizer for different proportions of nitrogen fertilizer on cotton yield, fiber quality, and soil enzyme activity. Methods The experiment involved a field trial in Xinjiang with a total of 11 treatments. T1 was the no-fertilizer treatment (control), T2 was the single fertilizer treatment (chemical fertilizer only), and T3–T11 were the combined fertilizer treatments (three different types of organic fertilizers replacing 10%, 20%, and 30% of nitrogen fertilizer). Results T11 achieved the highest plant height and fruit shoot number, while T8 and T10 respectively contributed to the greatest leaf SPAD and ginning outturn. The combined fertilizer treatments significantly increased cotton yield and fruit shoot number compared with the control and single fertilizer treatment. When different types of organic fertilizers were applied, there were no significant differences in seed cotton yield among T5, T8, and T11 (30% nitrogen replacement). The yield reached 6243.05 (T5), 6661.11 (T8), and 6933.47 (T11) kg ha -1 , being 59%, 70%, and 77% higher than that of the single fertilizer treatment (T2). Fiber length, uniformity, and strength were all highest in T11. Both cotton biomass and nitrogen accumulation were enhanced under the combined fertilizer treatments. In particular, T5, T8, and T11 increased soil microbial biomass carbon and nitrogen contents. T5 stimulated soil β-xylotetraerase activity; T8 elevated β-1,4-N-acetylamino glucosaminidase, cellulose β-glucosidase, and β-1,4-glucosidase activities; T11 improved alkaline phosphatase activity. The comprehensive enzyme activity index of T8 was significantly higher than other treatments. Conclusion Substituting liquid organic fertilizer for 30% of nitrogen fertilizer (T8 and T11) is the most suitable strategy for cotton planting in Xinjiang.