Chemical and biochemical properties of humic substances isolated from forest soils and plant growth

To investigate the relationships among litter composition and 13C–nuclear magnetic resonance spectra (13C–NMR), we studied d13C 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. Pinus sylvestris L. 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 carbox- ylic 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 d13C values of the humic fractions ranged from 224.96 to 225.84‰, 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.