Contents of the main metabolites in maize affected by humic acids with different molecular weight derived from weathered coal

Objectives The molecular weight of humic acid determines its function. The effect difference of humic acids with different molecular weight was studied in this paper, in order to provide theoretical basis for the efficient utilization of humic acid in China.

Methods A hydroponic experiment was carried out using Hoagland nutrition solution as the basic cultural liquid and maize cultivar of ‘Zheng Dan 958’ as material. Three kinds of humic acids with molecular weight of > 50 kDa (HA_H), 10 kDa to 50 kDa (HA_M) and < 10 kDa (HA_L) were separately added into the cultural solution in ratio of 10 mg C/L, and with no humic acid addition as control (CK). Maize seedlings were grown for 20 days, then harvested. The whole plant was divided into root, stem and leaf, dried and weighed. The fourier transform infrared spectroscopy (FTIR) (Bruker VERTEX 70) was applied to detect the contents of the main chemical components.

Results The addition of humic acid could significantly improve the dry weight of maize root, stem and leaf, which was increased by 91.07%, 89.27% and 88.53%, respectively, compared with the control. Among the three humic acids, the greatest promoting effect was obtained by HA_L, in which the dry weight of root, stem and leaf was increased by 143.14%, 123.41% and 150.54%, respectively; followed by HA_M and HA_L. The transmissivity of the FTIR spectrum with the wave lengths of 3420 cm^–1 and 1655 cm^–1 was lower in the humic acid treatments in the roots than that in the control, while those in stems were higher than in the control, indicating that humic acid could increase the content of carbohydrate, protein, polypeptide and amino acid in maize roots, and reduce their accumulation in stems. For the leaves treated with humic acid, the transmissivity of spectrum with wavelength of 3420, 2920, 1735, 1655, 1518, 1380, 1250 and 1050 cm^–1 in the humic acid treated leaves were all lower than those in the control, and those in HA_L were significantly lower than HA_M and HA_H.

Conclusions Exogenous addition of humic acid can increase the dry biomass and change the contents of carbohydrates, lipids, proteins and nucleic acids in maize. Humic acid with molecular weight of < 10 kDa performs better than that having larger molecular weight. So small molecular humic acid should be considered for the effective use of humic acid products in the practical production.