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 hor izon (Calcic-Luvisol) have been studied. Attention was paid to variati on in the NMR spectra of humic fractions and delta C-13 values and to how these changes are related to different biological humic fraction a ctivities. 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 differen t molecular sizes. An attempt has been made to explain the vertical ch emical changes in terms of processes affecting the biological characte ristics of the high and low molecular size humic fractions. The main c onclusions are that the low molecular size humic fractions, in the upp er part of the horizon, are of greater importance with respect to the other humic fractions in influencing the enzyme activities linked to g rowth metabolism. The biological role of the high molecular size humic fractions characterised by a relevant content of peptidic- and carboh ydratic-C is also presented.