Chemical Characteristics and Biological Activity of Organic Substances Extracted from Soils by Root Exudates

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 mays L.), Picea abies, and Pinus sylvestris root 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. sylvestris seedlings. 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.