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Plants have evolved with roots in close contact with the solid phase of the soil. Therefore, root exudates may be a better medium for extracting low molecular size (LMS) organic fractions than currently used alkaline solutions. Our objective was to compare the chemical and biological activity of LMS extracts using maize (Zea maysL.), Picea abies, and Pinus sylvestrisroot exudates to humic substances (HS) extracted with alkaline solution. Gas chromatographic/mass spectrometric (GC/MS) spectra revealed that the LMS fractions had a greater variety of fatty acids than the HS. Fourier transform infrared (FT‐IR) spectra of LMS fractions also indicated different amounts of functional groups by comparison with HS. The possible biological role of LMS fractions with respect to HS was assessed by measuring hormone‐like activity and nitrate uptake in P. sylvestrisseedlings. The LMS fractions from agricultural soil stimulated nitrate uptake and nitrate reductase (NR) and glutamine synthetase (GS) activities, whereas those from a forest soil increased ammonium uptake, NR, and glutamate dehydrogenase (GDH) activities. The stimulation of nitrate and ammonium uptake via a NR‐GS or NR‐GDH metabolic pattern was consistent with the different chemical composition of the LMS fractions. This indicates LMS fractions in soil have consequential affects on the plant's capacity to adapt to different environmental conditions.

To investigate the relationships among litter composition and 13C–nuclear magnetic resonance spectra (13C–NMR), we studied δ13C values and biochemical activities of the humic constituents extracted from the Ah horizons of two undisturbed forest soils, located in a unique climatic area under different vegetative covers, Pinus mugoT. and Pinus sylvestrisL. The two litters differed greatly in composition, while the 13C–NMR spectra of the humic substances extracted from the two Ah horizons were similar. The 13C–NMR spectra of the low molecular humic size (LMS) 1–2 revealed high aromatic and carboxylic contents and low aliphatic and carbohydratic ones. The total humic extracts (TQ) 1–2 and the high molecular humic size (HMS) 1–2 exhibited an opposite trend. The δ13C values of the humic fractions ranged from −24.96 to −25.84‰, indicating an advanced stage of humification. The biochemical activities of the humic fractions were studied by evaluating the invertase, peroxidase, and esterase activities in Pinus mugoand Pinus sylvestrisseedling roots grown in the presence of their humic matter. These forest species were differently affected by their humic substances. In particular, the Pinus sylvestrishumic matter, endowed with a higher indoleacetic acid (IAA) content, positively influenced all the enzymes tested in Pinus sylvestrisseedlings, while the Pinus mugohumic matter only increased the peroxidase activity in its seedlings. The plant species differ in their capacity to respond to biological humus activity, which is reflected in their natural distribution.