Decrease in humification of organic matter with intensified lowland rice cropping: A wet chemical and spectroscopic investigation

To address recent concerns of impaired nutrient cycling in intensively cropped lowland rice soils of tropical Asia, we have been investigating the effects of continuous rice cropping on the chemical nature of soil organic matter (SOM). In this study, the labile mobile humic acid (MHA) fraction and the more recalcitrant calcium humate (CaHA) fraction were extracted from soils of four long-term field trials on the International Rice Research Institute (IRRI) farm, which varied in the number of annual irrigated rice crops and hence degree of soil submergence. The two humic acid (HA) fractions were analyzed by Fourier-transform infrared, fluorescence, and electron spin resonance spectroscopies for elemental composition and acidic functional groups. With increasing soil submergence, the HA fractions became less oxidized or humified, with higher S and H and lower O concentrations, more amide or amino, hydroxyl, and methoxy groups, and fewer carboxyl groups and organic free radicals. The HA extracted from submerged soils appeared to have greater capacity for complexing Cu+2, Fe+3, and VO+2 than did HA from aerated soils. Fertilizer treatments had little effect on HA chemical structure. The MHA was less humified than the CaHA, having fewer acidic functional groups, smaller C:N and C:H ratios, richer concentrations of amides and carbohydrates, lower concentrations of COOH, higher fluorescence intensity, and shorter wavelength of fluorescence emission maximum, and lower concentration of organic free radicals. Free radical concentrations for both HA fractions were highly correlated with other indices of humification reported here and elsewhere.