Humic-like substances from agro-industrial residues affect growth and nitrogen assimilation in maize (Zea mays L.) plantlets

Humic-like substances (HL) extracted from the agro-industrial residues of rape (Brassica napus) (B-HL), castor-oil (Ricinus communis) (R-HL) and flax (Linum usitatissimum) (L-HL), and from a digestate (D-HL) were analyzed for their elemental composition and characterized by FT-IR and 13CNMR spectroscopy. The indoleacetic acid (IAA), total phenol (TP) and flavonoid (FL) content, and their auxin-like activity were also determined. HL were then applied to maize plantlets in order to evaluate their effect on plant growth, nitrogen metabolism, and photosynthesis. Results showed that all the HL had a physiologically active IAA, TP and FL content. HL positively affected the growth of maize with a noticeable increase in leaf biomass. The HL from digestate showed the highest values of glutamine synthethase and glutamate synthase enzyme activity, whereas both L and B-HL were more efficient in affecting the chlorophyll a, b and total carotene content. In conclusion, the biostimulant activity of the four HL varied, but the greater bioactivity of HL from digestate was a result of its high acidity and carbon distribution together with its high IAA and TP content, and high auxin-like activity.