Hormone-like activity of humic substances in Fagus sylvaticae forests

• Soil A horizons from a large area typical for beech (Fagus sylvatica) in northern Italy were investigated to understand the phyto-hormone-like activity of humic substances (HS) with respect to the vegetation. These soils had developed under thermophilous (Cephalonthero-Fagion with Carici albae-Fagetum), mesophilous (Fagion sylvaticae with Dentario pentaphylli-Fagetum and Galio-odourati-Fagetum) or acidophilous (Luzulo-Fagion with Luzulo niveae-Fagetum) beech forests. • The development of organic and humic matter in the three forest types was assessed by quantifying chemical parameters. Phyto-hormone-like activity of HS was evaluated by measuring auxin-like (IAA_like ) and gibberellin-like (GA_like ) activity as well as IAA concentration, invertase and peroxidase activities. • Soil pH differed between the forest types. Soil C: N distinguished the thermophilous and the mesophilous from the acidophilous types but did not distinguish thermophilous from the mesophilous types. Phyto-hormone-like activity of HS varied significantly across the beech forests. Acid conditions were essential for the release of IAA_like activity whereas neutral conditions promoted GA_like activity. Plant isoenzymatic polymorphism confirmed the different auxin activities of HS. • The phyto-hormone-like activity of HS is highly suitable, and better than soil chemical parameters, for differentiating between these ecosystems.