Changes in aromaticity and carbon distribution of soil organic matter due to pedogenesis

Changes in aromaticity and carbon species distribution of eight soil profiles were studied using CPMAS ¹³C NMR spectroscopy. In soils of cooler climates and with C/N ratios of usually more than 20, proportions of alkyl carbon increase and those of aromatic carbon decrease or remain constant with soil depth. In soils of warmer climates and with C/N ratios usually less than 20, alkyl carbon decreases and aromatic carbon increases with soil depth. Concomitantly, signals around 130 ppm appear in the NMR spectra, indicating the formation of stable, C-substituted, probably condensed aromatic substances. The statistical interpretation of the data shows that 73% of the variation of the aromaticity and 75% of the variation of the aliphatic carbon contents of the deepest soil horizons can be explained by the ratio of precipitation/temperature.