The varying promotion effects of fulvic acid with different molecular weights on the enhancement of grain yield and quality of winter wheat

This study aims to verify the application effect of fluvic acid (FA) with different molecular weights (MW) on the growth and quality of winter wheat. FA extracted from lignite was divided into 3 MWs (W1 <_ 3 000 D, 3 000 D < W2 <_ 10 000 D, and W3 > 10 000 D) by dialysis, and its structure was analysed. Three application rates were set for each MW FA in the pot experiment, which was 10, 25 and 50 mg/kg in soil, respectively, and water was the control. There were significant interactive effects of MW and application rates of FAs. Compared with the control, all the FA treatments significantly increased grain yields, nitrogen uptake efficiencies, grain iron concentration, and soil available nitrogen concentration. Heatmap analysis revealed that the W1C2 (10 mg/kg W1) treatment had the most significant impacts for all analysed indexes, whereas W3C3 (50 mg/kg W3) showed the weakest impacts. The results showed that at a low application rate (10 mg/kg in soil), the promotion effects of the three MW FAs were similar. W1 showed the most significant promotion effects, which was attributed to the combined effects of its lower MW and functional group characteristics.