Biological Activity of Humic Substances Is Related to Their Chemical Structure

Abstract

To understand if the biological activity of humic substances may be related to their molecular weight or chemical structure, two humic substances, derived from an Ah horizon of uncultivated couch grass [Elytrigia repens (L.) Desv. ex Nevski] and an Ah horizon of forest soil, were extensively characterized by means of different spectroscopic techniques (diffuse reflectance infrared Fourier transform [DRIFT] and 1H nuclear magnetic resonance [NMR]). The two humic substances, each separated in fractions with low (<3500 Da) and high (>3500 Da) relative molecular mass were compared for their effects on Pinus nigra J.F. Arnold callus. Growth of callus, the soluble sugar content, free amino acid pool, and the activities of the key enzymes involved in C and N metabolism were investigated. Callus was grown for a subculture period (28 d) on basal Murashige and Skoog medium plus humic matters with or without different hormones: indole‐3‐acetic acid, 2,4‐dichlorophenoxyacetic acid, or 6‐benzylaminopurine. The results of 1H‐NMR spectra and the DRIFT spectroscopy showed significant differences in the chemical composition between forest and grass humic substances. A large amount of aliphatic and H‐sugarlike component and an intense chemical shift of the β‐CH3 region in both grass humic fractions were observed, while high contents of betaine, organic acid, and COOH groups in both forest humic fractions were detected. A different biological activity between the grass and forest humic fractions was also observed. Thus, the different activity of the two humic substances used seems related to the diverse chemical composition rather than to different molecular weights.

The effect of humic substances on germination and early growth of Lamb’s Quarters (Chenopodium album agg.)

“…An inhibitive effect on seed germination was connected above all with HA isolated from O h -horizon of podzol from the mountain spruce stand (E) in the initial days. This negative activity is in accordance with testing of HS isolated from forest soil (Halley & Pellisier 1997;Nardi et al 2002;Muscolo et al 2007). HS activity may be related to their physical and chemical properties.…”

“…HS activity may be related to their physical and chemical properties. High contents of betaine, organic acid, and -COOH groups were determined in forest humic fractions (Muscolo et al 2007). Phenolic compounds contained in coniferous forest soils (podzolic horizon) are responsible for allelopathic effect on spruce seedlings (Pellisier 1993;Halley & Pellisier 1997).…”

“…Positive effect of HS on growth and yield of main crops has been widely confirmed (Sanders & Prince 1988;Reynolds et al 1995;Sharif et al 2002;Olk et al 2007). The mechanism by which HA and FA stimulate plants is difficult to clarify due to the chemical heterogeneity of HS (Christl et al 2000;Veselá et al 2005;Muscolo et al 2007) and uncertainty about their characterization (Gonzales-Perez et al 2006). Different biological activities of prepared HS are influenced mainly by soil type or their origin (Lulakis & Petsas 1995;Asenjo et al 2000;Muscolo et al 2002;Nardi et al 2002;von Lutzow et al 2006).…”

Humic Acid on Germination and Vigor of Corn Seeds1

“…The inhibitory effect of humic acid is caused when the applied dose is high, which is detrimental for the germination of some species. Another factor that is not favorable for germination is the source of humic acid used; as some sources of humic acid from the decomposition of conifers are responsible for allelopathic effects on germination of some species (MUSCOLO et al, 2007). Humic acid specifically influences the germination of different plant species depending on its source and dose, making it a subject of further research to better understand the mode of action to develop specific protocols for each culture.…”

“…The activity mechanism of humic acid in promoting plant growth is not fully understood, and the beneficial effects to plants is difficult to comprehend due to its chemical heterogeneity (VESELÁ et al, 2005;MUSCOLO et al, 2007).…”