Abstract: The stability and adaptability of the critical nitrogen concentration in drip-irrigated cotton fields in Southern Xinjiang were investigated by measuring the nitrogen uptake (N_uptake), nitrogen nutrition index (NNI), and accumulated nitrogen deficit (N_and). A two-year field study was conducted using five nitrogen levels (0, 75, 150, 300, and 450 kg·hm^-2), with cotton cultivars ‘Xinluzhong55’ and ‘Xinluzhong78’ as the test materials. We analyzed the effects of nitrogen addition on plant dry matter (PDM) and critical nitrogen concentration dilution curves (PNC_c). The N_uptake, NNI, and N_and models as well as a model of the relationships between NNI, N_and, and the relative seed cotton yield (RY) were established based on the PNC_c for both cultivars. The results showed that the PNC_c, maximum nitrogen concentration (PNC_max), and minimum nitrogen concentration (PNC_min) dilution models in drip-irrigated cotton showed power function relationships with plant dry matter (PDM), with determination coefficients (R²) of 0.905 and 0.960, 0.919 and 0.950, 0.934 and 0.969 for Xinluzhong55 and Xinluzhong78, respectively. Model validation with independent experimental data showed that the critical nitrogen concentrations in cotton grown under drip-irrigated conditions were simulated satisfactorily. The Xinluzhong55 and Xinluzhong78 1:1 line R², root mean square error (RMSE) and relative estimation error (RE) of the simulated and observed critical nitrogen concentrations were 0.987, 0.051 g·kg^-1, 1.95% and 0.984, 0.044 g·kg^-1, 1.91%, respectively, showing good stability and adaptability between the two years. According to the N_uptake, NNI, and N_and model, we concluded that 300 kg·hm^-2 was the optimum level of nitrogen application in drip-irrigated cotton in Southern Xinjiang. Based on the fitting results of nitrogen level and seed cotton yield, the theoretical appropriate nitrogen applications for Xinluzhong55 and Xinluzhong78 were 322.7 and 336.4 kg·hm^-2, respectively. The R² of NNI, N_and and RY in different growth stages were greater than 0.900, the NNI, N_and and RY estimation model of seed cotton yield based on 1:1 lines of R², RMSE, and RE were 0.899-0.989, 0.05-0.13, and 4.1%-9.9% and 0.902-0.981, 0.04-0.12, and 5.1%-9.5%, all of which were highly significant. These results could provide new directions for the evaluation and diagnosis of optimum nitrogen application levels and nitrogen status, as well as the estimation of seed cotton yield in drip-irrigated cotton in Southern Xinjiang.

Key words: critical nitrogen concentration, Southern Xinjiang, drip irrigation, cotton, nitrogen dilution curve, nitrogen nutrition index, model