Large rainfall fluctuation and excessive nitrogen application are the main factors that limit the nitrogen fertilizer efficient utilization and stable yield and high quality in winter wheat production in Weibei dryland. In this study, the purpose was to construct different critical nitrogen concentration dilution curves of winter wheat under two precipitation patterns to analyze the feasibility of diagnosing the nitrogen nutrion status of dryland winter wheat by the nitrogen nutrient index. From 2017 to 2021, a 4-year nitrogen application test was conducted in Heyang County, Shaanxi Province, with Jinmai 47 as the test material. Five nitrogen application rate treatments were set at 0, 60, 120, 180 and 240 kg/hm2. According to the classification of precipitation patterns, normal years were 2017-2018 and 2020-2021 , and dry years were 2018-2019 and 2019-2020. The effects of different nitrogen application rate on nitrogen utilization efficiency, grain yield and its components of winter wheat under the two precipitation patterns were srudied. Based on the relationship between the aboveground biomass and plant nitrogen concentration of winter wheat in dryland under the two precipitation patterns, the critical nitrogen concentration dilution curve model and nitrogen nutrient index (NNI) was established. The results show that: 1) nitrogen application, precipitation patterns and their interaction effects have a significant impact on the grain number per spike, 1000-grain weight and grain yield. 2) The aboveground biomass and critical plant nitrogen concentration of winter wheat under normal years and dry years were conform to the power function relationship. However, there are variances in models parameters of normal years and dry years(the model parameter a were 3.33 and 2.79 g/kg in normal and dry years, and the parameter b was 0.40 and 0.31 in normal and dry years, respectively). The model validation showed that the plant nitrogen concentration fitted by the critical nitrogen concentration dilution curve model was linearly correlated with the actual nitrogen concentration. The root mean square error (RMSE) was 0.20 and 0.14 g/kg in normal years and dry years, respectively, and the normalized root mean square error (NRMSE) was 10.30% and 7.69%, respectively, indicating that models had good stability. 3) According to the grain yield and nitrogen nutrition index, the appropriate nitrogen rates in normal and dry years was 160-180 kg/hm2 and 101-120 kg/hm2. In this study, the critical nitrogen concentration dilution curve and nitrogen nutrient index were established based on the aboveground biomass and plant nitrogen concentration of dryland winter wheat during each growth states under the two precipitation patterns. The critical nitrogen concentration dilution curve and nitrogen nutrient index better predicted the nitrogen nutrient status of winter wheat in different growth stages under the two precipitation patterns. The results can provide valuable information for the nitrogen assessment and precise nitrogen application.
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This study investigated the effects of different nitrogen and phosphorus application rates on summer maize aboveground biomass, nitrogen accumulation, and drew a critical nitrogen concentration dilution curve. The nitrogen status of maize plant was diagnosed and evaluated based on a model of nitrogen nutrition index (NNI) under different nitrogen and phosphorus interaction conditions, which provided a theoretical basis for the rational application of nitrogen and phosphorus fertilizers in summer maize.
By using Zhengdan958 (ZD958) and Yuyu22 (YY22) as tested materials, the field experiments in Guanzhong Plain, Shaanxi included four phosphorus application rates and five nitrogen application rates, such as 0 (P0), 60 (P1), 120 (P2), 180 (P3) kg P2O5·hm-2 and 0 (N0), 75 (N1), 150 (N2), 225 (N3), 300 (N4) kg N·hm-2 during 2019-2020. The aboveground samples were taken during the jointing, tasseling, filling, and maturity stages of summer maize to analyze the effects of nitrogen and phosphorus application rate on maize dry matter accumulation, dynamic changes of nitrogen concentration and grain yield. The field test data was used to construct and verify the critical nitrogen dilution curve model of summer maize.
The results showed that nitrogen and phosphorus application rate significantly increased aboveground biomass, plant nitrogen concentrations and grain yield of summer maize. The grain yield and aboveground biomass of summer maize increased as the nitrogen application rate increased within the same phosphorus application condition. The nitrogen concentration of maize plants showed a decreasing trend with the extension of growth period and the increase of aboveground dry matter weight. There was a power exponential relationship between nitrogen concentration and aboveground biomass. In addition, the phosphorus application could promote maize plant nitrogen absorption and aboveground dry matter accumulation. The overall performance of the phosphorus application treatments was P2>P3≈P1>P0 under the same nitrogen application conditions, appropriate phosphorus application could improve the capacity of maize plant for nitrogen absorption and relieved the decline of nitrogen concentration. The critical nitrogen concentration (Nc) curves of maize (P0, Nc=27.98DM-0.249; P1, Nc=29.77DM-0.182; P2, Nc= 30.81DM-0.138; P3, Nc=30.06DM-0.187) were constructed according to the aboveground dry matter (DM) weight and its nitrogen concentration under different phosphorus application conditions; the relatively stable model had a linear correlation between the fitted and actual plant nitrogen concentrations, which showed that the n-RMSE were 10.23%, 6.67%, 6.95% and 7.19%, respectively. The NNI values were calculated based on the critical nitrogen concentration curves. NNI increased with the increase of nitrogen application in each growth stages within the same phosphorus application conditions, which was also positively correlated with relative aboveground biomass (RDW) and relative yield (RY).
Based on the model of nitrogen nutrition(NNI) in this study, N2-N3 and P1-P2 were the best conditions. Based on the fitting curve of comprehensive nitrogen application rate and grain yield, the nitrogen rate of 187.5-205.7 kg·hm-2 and phosphorus rate of 60-120 kg·hm-2 was the optimal fertilization option for summer maize in Guanzhong Plain, Shaanxi.
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