Objectives  We studied the changes of iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn) contents in wheat caused by the application of phosphorus fertilizer in calcareous soil and its relationship with crop absorption and soil nutrient availability, so as to provide a scientific basis for the rational application of phosphorus fertilizer and high-yield and high-quality production of dryland wheat.

  Methods  A long-term winter wheat field experiment has been established since 2004 in a calcareous soil of Yangling, Shaanxi Province. Five P₂O₅ application rates (0, 50, 100, 150, 200 kg/hm²) were set upon the basis of N application rate of 160 kg/hm². Samples were taken during three harvest periods from 2013 to 2016. The biomass and Fe, Mn, Cu and Zn contents in the aboveground organs of wheat were measured, and soil samples were collected at the same time to test the available Fe, Mn, Cu and Zn contents.

  Results  Compared with no P (control), application of P increased wheat yields, Fe and Mn contents but decreased Cu and Zn contents in grains. P application also increased the soil available Fe, Mn and Zn content, but did not change soil available Cu. Regression analysis showed that the P rate for the maximum grain yield (6492 t/hm²) was 165 kg/hm², for the maximum grain Fe (41.7 mg/kg) and Mn (37.5 mg/kg) were 100 and 94 kg/hm² respectively, and for the minimum of grain Zn (25.4 mg/kg) was 136 kg/hm². And every 100 kg/hm² of P₂O₅ application would reduce grain Cu content by 0.4 mg/kg. In 0–20 cm soil, the highest available Mn and Zn were recorded when the P rate was 100 kg/hm², the maximum available Fe was obtained under 200 kg/hm² P rate, but the available Cu showed no significant change under different P₂O₅ application rates. When the yield was 95% of the highest yield, the phosphorus application rate was 108 kg/hm². The yield increase decreased when the phosphorus application rate exceeded this level, and the grain Fe and Mn content did not increase, while the grain Cu and Zn concentration continued to decrease.【 Conclusions 】The increase in Fe and Mn contents of wheat grain in the dry land of the Loess Plateau was mainly caused by the enhanced soil available Fe and Mn under long-term P application, Fe and Mn uptake and transport to grain. Decrease in grain Cu content was related to the no change in available soil Cu, and the insufficient Cu transports to grain in the aboveground. The low available soil Zn and Zn uptake at high P application rate was the main reason for the decreased grain Zn contents. Therefore, in order to balance the grain yield and nutrient contents of wheat, the P₂O₅ application rate should be lower than 108 kg/hm² in this area to prevent grain Cu and Zn contents from further decrease and keep the contents of Fe and Mn at suitable levels.