Diffuse reflectance Fourier transform spectroscopy and thermal analysis applied to humic substances

Spectroscopic (DRIFT) and thermal (TG-DTA) analyses were carried out to investigate the structure of humic acids formed in different environments: a soil developed under Norwegian spruce (Picea abies, (L.) Karst) subalpine forests of north Italy and two kinds of coals (Sphagnum peat and leonardite-North Dakota). The samples taken from forest soil were classified in relation to vegetal cover at different old age (grassland, young and old forest). The thermal patterns and DRIFT spectroscopy showed different structural modifications can be ascribed to interaction between soil and vegetal cover. The main structural changes in grassland and both forests appeared in carbohydrates, aliphatics and aromatics content. Particularly different was the humic acid composition from old forest because it did not preserve any memory of molecules derived from plants. Similarly, the thermal and DRIFT analyses of humic acids from peat and leonardite showed considerable structural differences in relation to their formation ways. In leonardite the disappeared of all features of plant residues, contrary to peat, and the carbon skeletons containing aromatic units indicated an increase of humification rank.