The effects of humic substances within the Ah horizon of a Calcic Luvisol on morphological changes related to invertase and peroxidase activities in wheat roots

The chemical and biological characteristics of humus within the Ah horizon (Calcic-Luvisol) have been studied. Attention was paid to variation in the NMR spectra of humic fractions and delta C-13 values and to how these changes are related to different biological humic fraction activities. The chemical changes in particular involve the decrease of the aromatic component and the increase of the non-aromatic component within the horizon and the different delta C-13 value not only within the horizon but also among the humic fractions distinctive of different molecular sizes. An attempt has been made to explain the vertical chemical changes in terms of processes affecting the biological characteristics of the high and low molecular size humic fractions. The main conclusions are that the low molecular size humic fractions, in the upper part of the horizon, are of greater importance with respect to the other humic fractions in influencing the enzyme activities linked to growth metabolism. The biological role of the high molecular size humic fractions characterised by a relevant content of peptidic- and carbohydratic-C is also presented.