Diagnosis and comprehensive evaluation of nitrogen nutrition in non-mulched drip irrigation cotton in southern Xinjiang based on critical nitrogen concentration

The critical nitrogen (N) dilution curve (CNDC) describes the process of achieving the highest yield with the lowest nutrient concentration. It is influenced by species and environment, providing a new strategy for diagnosing cotton nitrogen status and its impact on cotton growth and yield. This study investigated the effects of two irrigation amounts (45 and 54 mm) and three N application rates (150, 225, and 300 kg ha−1). Results indicated that compared with low water treatment (W1), increasing irrigation water led to a 21.3–24.7% increase in dry matter, a 22.2–29.6 % increase in N accumulation, a 1.4–26.1 % decrease in soil N content, and a 3.9–9.0 % reduction in N percentage of reproductive organs. The nutrient network index (NNI) decreased by 2.2–6.4 %, while irrigation water productivity (WPI) decreased by 5.4–22.1 %. Compared with low nitrogen (F1), increasing N fertilization application enhanced dry matter by 7.1–20.8 % and N accumulation by 2.7–24.8 %, while also increasing the nitrogen percentage in reproductive organs by 4.0 –9.9 %. Experimental results demonstrated that moderate increases in irrigation and nitrogen application improved seed cotton yield and the partial factor productivity of N (PFPN). Under the W1 treatment, higher nitrogen application brought cotton plant N closer to a optimal condition. However, excessive water and N exacerbate N loss and result in a nutrient network index (NNI) value below 1, indicating prolonged nutrient deficiency. The TOPSIS comprehensive evaluation indicates that an irrigation water amount of 45 mm and a nitrogen application rate of 300 kg ha−1 represent optimal water and nitrogen application strategies under water resource constraints. Conversely, an irrigation water amount of 54 mm and a nitrogen application rate of 225 kg ha−1 are identified as the optimal strategies for achieving high cotton yields.