Optimization of nitrogen rate based on grain yield and nutrient contents in dryland wheat production

Objectives It is of great significance to explore the wheat grain yield, soil nitrate nitrogen (N), available phosphorus (P), available potassium (K) and grain nutrient contents under the condition of a long term N application at different rates, for the purpose of appropriate N application, soil fertility improvement, wheat yield increase and grain nutritional quality improvement in drylands.

Methods Field experiments were conducted to investigate the effects of different N rates on soil nitrate N, available P and K contents, N, P and K absorption and utilization of wheat, grain N, P and K contents, wheat biomass, yield and yield components based on the long-term location-fixed field experiment, which was initiated in 2004 in the Loess Plateau. Soil and plant samples were collected in the consecutive experimental years of 2015–2017.

Results The three-year averaged results showed that long-term application of N increased wheat grain yield by 67.1%, biomass by 52.0%, harvest index by 9.5%, spike number by 32.5% and grains per spike by 40.0%, respectively, while the 1000-grain weight was decreased compared with no N application. The grain wheat yield and biomass were quadratically correlated with the N rate, and the maximum yield was 6587 kg/hm² obtained at a N rate of N 215 kg/hm². The P content of grain decreased with the N rate increase, K content of grain showed no significant change, while the N content showed an opposite trend. There was a significant positive correlation between the soil nitrate N content and the N application rate. The soil nitrate N content was 7.2 mg/kg at sowing and 10.3 mg/kg at harvest when the maximum yield was obtained. Soil available P decreased with the N application rate increase, and soil available K showed no significant change. The N absorption and utilization efficiencies (i.e. N harvest index, N physiological efficiency, N partial factor productivity and agronomic efficiency) decreased with the increase of N rates.

Conclusions For improving the winter wheat grain yield and grain key nutrient contents, the target grain yield should be 6300 kg/hm² in drylands of the experimental area, and the corresponding fertilizer application rate should be kept at N 150 kg/hm² and P₂O₅ 100 kg/hm², and the nitrate N would be sustained at 6.0–8.0 mg/kg in 0–20 cm soil at sowing or harvest, respectively, and available P be kept at 12.0–15.0 mg/kg, and available K at 139–140 mg/kg.