Effects of different proportions of organic carbon input on nitrogen pool activity of tobacco-growing soil under fertilizer reduction

In order to explore the long-term effects of different proportions of organic carbon input on nitrogen supply capacity of tobacco-planting soil under fertilizer reduction, four treatments were set up based on 11 consecutive years of field location experiments: no fertilizer (CK), local conventional recommended fertilizer (100% fertilizer, CF), 20% chemical fertilizer reduction with 3 000 kg·hm⁻² organic fertilizer (OF-1), and 40% chemical fertilizer reduction with 6 000 kg·hm⁻² organic fertilizer (OF-2).By analyzing and comparing the contents of soluble nitrogen components, soil nitrogen pool activity (NL), soil nitrogen pool management index (NPMI), tobacco yield and economic traits, the effects of different proportion of organic carbon input on soil nitrogen pool activity of tobacco planting soil under continuous fertilizer reduction were investigated. The results showed that continuous years of no fertilizer (CK) significantly reduced the tobacco yield, the proportion of medium and top-grade tobacco, and soil soluble nitrogen fraction content and nitrogen pool management index. Under continuous fertilizer reduction conditions, different proportions of organic carbon inputs (OF-1 and OF-2) significantly increased flue-cured tobacco yield, soil soluble nitrogen content, soil nitrogen pool activity and nitrogen pool management index. Compared to the CF treatment, the OF-1 and OF-2 treatments with different organic carbon input ratios significantly increased the content of total dissolved nitrogen (TDN) in tobacco-planted soil by 52.11% and 57.36%, respectively. The content of dissolved organic nitrogen (DON) increased by 99.25% and 102.64%, respectively. Additionally, the ammonium nitrogen (NH₄⁺-N) content rose by 18.08% and 19.22%, while the nitrate nitrogen (NO₃⁻-N) content increased by 48.09% and 54.61%, respectively. Furthermore, the soil nitrogen pool activity improved by 53.58% and 50.36%, and the soil nitrogen pool management index increased by 48.21% and 50.57%, respectively. Tobacco leaf yield also showed significant improvements, rising by 18.69% and 19.22%. The proportion of high-grade tobacco leaves increased by 94.95% and 67.61%, while the proportion of medium-high-grade tobacco leaves increased by 13.91% and 12.25%, respectively. The effects of OF-1 and OF-2 treatments on soil nitrogen pool activity and nitrogen pool management index were not significantly different from each other. The results of correlation analysis showed that nitrogen composition, soil nitrogen pool activity and nitrogen pool management index of tobacco-planting soil were significantly positively correlated with tobacco yield and economic traits. Random forest analysis showed that soil nitrogen pool management index, soil soluble nitrogen content and soil nitrogen pool activity were important predictors of changes in yield of tobacco. In conclusion, under continuous chemical fertilizer reduction, organic carbon input enhances tobacco leaf yield by increasing soil soluble nitrogen content and improving soil nitrogen pool activity and nitrogen pool management index. This approach serves as a green and ecological fertilization strategy to achieve chemical fertilizer reduction and efficiency improvement. It also enhances soil nitrogen supply capacity and promotes high-quality tobacco production.