Establishment of a critical nitrogen dilution curve for drip-irrigated cotton under reduced nitrogen application rates

The goal of this study was to establish the feasibility of using the critical nitrogen (N) dilution curve of cotton leaves for diagnosing N nutrition, so as to provide a theoretical basis and guidance for accurate fertilization in drip-irrigated cotton fields. We conducted field experiments using Lumianyan 24 under four different reduced N treatments including (506 kg hm−2 (N1), 402.5 kg hm−2 (N2), 299 kg hm−2 (N3), and 195.5 kg hm−2 (N4)) between 2018 and 2019. We subsequently measured N concentration and leaf dry matter (LDM). We also calculated the critical N concentration (Nc) dilution model, allometric growth model, N nutrient index (NNI), and N accumulation. Our results revealed that with the development of growth period, the leaf NNI and N accumulation displayed an increasing trend followed by a decreasing pattern in 2018. On the other hand, the leaf NNI in 2019 showed a similar performance as presented in 2018, while the N accumulation depicted a gradually increasing trend. We also found that NNI and N accumulation of cotton leaves reduced with decreasing N application. The Nc dilution curve model for drip-irrigated cotton fields was Nc = 5.163LDM−0.171 (LDM > 1.65), Nc = 4.7% (LDM ≤ 1.65), R2 = 0.933, with a highly significant fit and RMSE = 0.374, accompanied by excellent model simulation performance. In conclusion, this study established the Nc dilution curve model. Additionally, the calculated NNI can efficiently diagnose the status of N nutrients, thus providing a valuable theoretical basis for accurate fertilization of drip-irrigated cotton fields.