Objectives  Fulvic acid has growth-promoting and anti-stress effects. However, the complex structure and wide molecular weight of the fluvic acid lead to an unstable effect. Here, we studied the effect of fulvic acids with different molecular weights on the growth and nutrient uptake of winter wheat.

  Methods  The tested fulvic acid was divided into three grades based on molecular weight through a biological semipermeable membrane: <3000 D (W1), 3000 to 10000 D (W2), and >10000 D (W3). The structure and functional groups of the three fulvic acids used in a winter wheat pot experiment were determined. We applied the three groups of fulvic acid solution to the soil at the wheat jointing stage at 0 (CK), 10 (C1), 25 (C2), and 50 (C3) mg/kg separately. Plant and soil samples were collected at the flowering and maturity stages to measure biomass, yield, and nutrient content.

  Results  Structural analysis showed that low molecular weight fulvic acid had lower aromatic hydrocarbon saturation, more oxygen-containing functional groups, developed side chains, and higher hydrophilicity. Compared with the CK, fulvic acid (P<0.05) increased grain yield by 21.62%−49.06%, grain Fe concentration by 62.81%−100.67%, grain N accumulation by 19.47%−38.49%, N uptake efficiency by 10.67%−19.66%, and nitrogen fertilizer partial productivity by 21.63%−49.07%. The above indicators decreased with an increase in the W1 application rate, and increased with an increase in the fluvic acid application rate in W2. For W3, the indicators increased first and then decreased with the fulvic acid application rate increase. Fulvic acids (P<0.05) promoted the translocation of dry matter and N accumulation to grains before flowering. The quantity and efficiency of dry matter translocation increased by 24.30%−40.27% and 20.21%−33.57%, while the quantity and efficiency of N translocation increased by 20.12%−51.22% and 8.44%−22.90%. The fertilizer partial productivity, yield, and grain N accumulation of winter wheat were significantly and positively correlated with fulvic acid O/C atomic ratio, H/C atomic ratio, oxygen content, nitrogen content, hydrophilicity index, and carbohydrate carbon content. The cluster analysis results showed that the application of fulvic acid (P<0.05) affected the measured indicators of winter wheat, differed from the control, and they had different clusters.

  Conclusions  Applying all the fluvic acids with different molecular weights improved the biomass, grain yield, grain N, Fe uptake, and nitrogen fertilizer utilization efficiency of winter wheat. The molecular weights influenced the suitable dosage of fulvic acid. Fulvic acid with a molecular weight of 3000 D or less had a higher functional group and hydrophilicity. Therefore, a low dosage of fluvic acid potentially improves winter wheat yield and N utilization efficiency. In contrast, fluvic acid with medium and large molecular weights requires a high dosage to receive the similar yield and N efficiency effect.