Dissecting the relationship between yield and mineral nutriome of wheat grains in double cropping as affected by preceding crops and nitrogen application

Preceding crop and nitrogen (N) fertilization affect N cycling and productivity of subsequent wheat in double cropping. However, the effects of preceding crop and N fertilization on relationships between yield and "mineral nutriome" or nutritional quality (amount and bioavailability) in wheat grains have not been well characterized, particularly not in the case of multiple mineral nutrient elements and when considering cumulative effects of fertilization strategies in long-term field experiments. Here we present effects of preceding crop (monocultures of maize, peanut or soybean, and intercropping of maize with peanut or soybean) at 4 levels of annual fertilizer N input (0-440 kg/ha) on yield traits, major mineral nutrients and their relationships in wheat grains in a 4-years continuous double-cropping field experiment in the North China Plain. Nitrogen fertilizer increased wheat grain yields, mass concentrations of N and copper (Cu), and micronutrient (mainly zinc: Zn and iron: Fe) bioavail-ability, while no N supply was associated with low grain yield, N and Cu concentrations and micronutrient bioavailability, high mass concentrations of Zn, manganese, phosphorus (P), calcium, magnesium and phytate-P, and a high ratio of phytate-P/P. Moderately lowering N input from the in-season 240 to 200 kg/ha (but not 120 kg/ha) would not harm grain yield and most nutritional traits. Compared to significant effects of N application level on almost all grain yield and nutritional traits, and soil properties, preceding crops had less impact and only grain yields and Zn concentrations were significantly affected. Grain yields, N and Zn concentrations varied dramatically from an average of 2.7 to 7.6 t/ha, 16.5-19.3 g/kg and 15.1-19.8 mg/kg with different N appli-cation levels, respectively; while varied relatively little from 5.9 to 6.6 t/ha, 17.8-18.3 g/kg and 16.1-18.1 mg/ kg under different preceding crops, respectively. Compared to the conventional wheat-maize rotation, a legume pre-crop monoculture but not maize/legume intercropping increased grain yields by 5.0-11.9% on average, but reduced grain Zn concentrations from 17.5-17.8 to 16.1 mg/kg significantly. The legume effect was most sig-nificant at zero or low N supply. The relationship between grain yields and Zn concentrations was adequately described by a concave-up parabola (R2 = 0.3001, P < 0.0001, n = 288), suggesting appropriate N fertilization and rotation can achieve high-yield while ameliorating the dilution of grain Zn density. Therefore, our findings provide new insight in relationships among agronomic practices, wheat grain yield and grain nutritional quality (particularly for Zn).