Construction Of Critical Nitrogen Concentration Dilution Model And Nitrogen Diagnosis Of Drip Irrigation Cotton Under Water And Nitrogen Operation

【Purpose】Water and nitrogen operation is closely related to cotton yield formation.The critical nitrogen concentration can accurately reflect the nitrogen nutrient status of cotton,but it is affected by water supply.Therefore,in this study,Xinshi K-18,the main cotton variety recommended by Xinjiang Eighth Division,was selected.A dilution model of critical nitrogen concentration of drip irrigation cotton was established through water and nitrogen operation test,and the influence of water condition on critical nitrogen concentration was analyzed to clarify the robustness of the model under different water conditions and the difference of nitrogen nutrient diagnosis results based on critical nitrogen concentration.In order to provide theoretical and technical support for cotton nitrogen nutrition diagnosis and efficient nitrogen application management,the characteristics of cotton fertilizer requirements under different water conditions were studied.【Methods】In this study,four irrigation levels were set up:moderate water deficit W1(3000 m³·hm^-2),mild water deficit W2(3750 m³·hm^-2),normal water W3(4500 m³·hm^-2)and surplus water W4(5250 m³·hm^-2).Five nitrogen application levels were:N0(0 kg·hm^-2),N1(225 kg·hm^-2),N2(262.5 kg·hm^-2),N3(300 kg·hm^-2)and N4(337.5 kg·hm^-2).Dry matter content,nitrogen content,leaf area index,yield and yield component factors were measured during each sampling period.Fiber quality was also measured.The dilution models of critical nitrogen concentration of drip irrigation cotton under different water conditions were established and verified based on above-ground dry matter.The models of nitrogen nutrient index（NNI）,nitrogen uptake(N_upt)and cumulative nitrogen deficit(N_and)under different water conditions were established based on critical nitrogen concentration（N_c）to perform nitrogen nutrient diagnosis.【Results】（1）Water and nitrogen management had significant effects on the content of dry matter and nitrogen in the upper hinterland of drip irrigated cotton at each growth stage,and both increased with the increase of irrigation and nitrogen application.（2）Water and nitrogen preparation had a significant effect on cotton yield.The yields of seed cotton and lint reached the highest values under W3N3 treatment,which were 5892.2 kg·hm^-2 and 2747.0 kg·hm^-2,respectively.The contrast intensity of water and nitrogen preparation had a significant effect,while the uniformity and elongation had no significant difference under different water and nitrogen conditions.（3）The critical nitrogen concentration dilution curves R² under each water condition of W1～W4 were 0.999,0.912,0.952 and 0.974,respectively.Based on the measured and simulated values of two years’data,1:1 Histogram shows that RMSE are 0.284,0.280,0.243 and 0.269,respectively,indicating that the model has good inter-year stability.（4）NNI,N_upt and N_and models based on Nc showed that N3(300 kg·hm^-2)level was the optimal nitrogen concentration for drip irrigation cotton under different water conditions.The optimal nitrogen application rates were W1:262.5 kg·hm^-2 and W2,W3 and W4 were 300 kg·hm^-2,respectively.There was a linear correlation between the NNI and Nand of drip irrigation cotton and the relative seed cotton yield（RY）under different water conditions,and the correlation was the best at the bolling and flopping stages of W2,W3 and W4,which could provide theoretical support for yield estimation.【Conclusions】The critical nitrogen concentration of drip irrigated cotton increased with the increase of irrigation amount.Considering the results of yield and nitrogen nutrition diagnosis,the critical nitrogen dilution curve of drip irrigated cotton based on shoot dry matter,NNI,N_upt and N_and models could accurately diagnose the nitrogen nutrition status of cotton under mild water deficit,normal water and water surplus conditions.However,when water deficit is moderate,the nutritional diagnosis is not suitable for nitrogen nutrition diagnosis due to a certain deviation.The optimal nitrogen application rate was 262.5 kg·hm^-2 under moderate water deficit,and 300kg·hm^-2 under mild water deficit,normal water and water surplus.