Coupling Effects of Organic Fertilizer Substituting Chemical Fertilizer on Potato Yield, Quality and Soil Nitrogen Content in the Erhai Lake Basin of China

Rational fertilization boosts crop yields and enhances nutritional value, but over-fertilization is counterproductive. Furthermore, water eutrophication caused by excessive use of nitrogen fertilizers has become a major agricultural non-point source pollution problem in the Erhai Lake Basin of China. This study took high-fertility soil as the research object and set up six treatments: no fertilization (CK), local recommended fertilization (T1), optimized chemical fertilizer (T2), organic fertilizer replacing 20% (T3), 40% (T4), 60% (T5) of chemical fertilizer with equal nitrogen. The results show that replacement of chemical nitrogen fertilizers with organic nitrogen fertilizers at an appropriate ratio can optimize soil nitrogen supply, enhance the activity of soil nitrogen cycle enzymes, thereby promoting the activity of nitrogen metabolism enzymes and nitrogen assimilation capacity in potato plants, and ultimately achieve a synergistic effect of increased yield, improved quality and higher fertilizer use efficiency. Among the treatments, the nitrate reductase (S-NR) activity in potato leaves was 36.74% and 41.66% higher under T3 than T1 and T4, respectively. For potato quality, Vitamin C (VC) content was 17.41% higher under T3 than T2; soluble protein content was 11.44%, 10.63%, and 9.44% higher under T3 than T1, T2, and T4, respectively. The replacement of chemical fertilizers with organic fertilizers mainly enhances the protein content in potato tubers by increasing soil urease (S-URE) activity and leaf relative chlorophyll content (SPAD) value. Based on the comprehensive differential combination evaluation model, considering potato metabolic absorption, yield, quality, and soil nitrogen content, the T3 treatment is the optimal fertilization method in the Erhai Lake Basin of China.