Chemical and Biochemical Properties of Humic Substances Isolated from Forest Soils and Plant Growth

To investigate the relationships among litter composition and [sup.13]C-nuclear magnetic resonance spectra ([sup.13]C-NMR), we studied [[Delta].sup.13]C 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 mugo T. and Pinus sylvestris L. The two litters differed greatly in composition, while the [sup.13]C-NMR spectra of the humic substances extracted from the two Ah horizons were similar. The [sup.13]C-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 [[Delta]sup.13]C 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 mugo and Pinus sylvestris seedling roots grown in the presence of their humic matter. These forest species were differently affected by their humic substances. In particular, the Pinus sylvestris humic matter, endowed with a higher indoleacetic acid (IAA) content, positively influenced all the enzymes tested in Pinus sylvestris seedlings, while the Pinus mugo humic 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.