An investigation was carried out to study the soil organic carbon fractions in paddy and associated non-paddy soils of Assam, India. Three districts viz. Sivasagar, Jorhat and Golaghat in the Brahmaputra valley of Assam were selected for this study. Soil samples were collected from six profiles, three each from mono-cropped paddy and associated non-paddy areas were collected. Horizon -wise, soil samples were analyzed for organic carbon fractions. Organic carbon, Humus C, Humin C, Humic acid C and Fulvic acid C were all analyzed from the samples. Organic carbon content in soil varied from 0.90 -7.90g kg⁻¹. Surface horizons of paddy soils contained lower amounts of humus C (0.255-0.330 per cent) and fulvic acid C (CFA) (0.135 – 0.180 per cent) and higher amounts of humin carbon (0.210 -0.475 per cent) and humic acid C (CHA) (0.120-0.150 per cent) as compared to that of non-paddy soils (0.300-0.435 and 0.195-0.300, 0.180-0.490 and 0.105-0.135 per cent respectively). It was concluded that the ratio of (CHA+CFA)/CTOC indicated less humification on the surface horizons of paddy soils. This is a direct result of prolonged submergence which led to humic acid carbon becoming less oxidized or humified.

Keywords: Organic carbon, humic acid, fulvic acid, humin, humification, paddy soils.