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 crop ped lowland rice soils of tropical Asia, we have been investigating the eff ects of continuous rice cropping on the chemical nature of soil organic mat ter (SOM). In this study, the labile mobile humic acid (MHA) fraction and t he more recalcitrant calcium humate (CaHA) fraction were extracted from soi ls of four long-term field trials on the International Rice Research Instit ute (IRRI) farm, which varied in the number of annual irrigated rice crops and hence degree of soil submergence. The two humic acid (HA) fractions wer e analyzed by Fourier transform infrared, fluorescence, and electron spin r esonance spectroscopies for elemental composition and acidic functional gro ups. With increasing soil submergence, the HA fractions became less oxidize d or humified, with higher S and H and lower O concentrations, more amide o r amino, hydroxyl, and methoxy groups, and fewer carboxyl groups and organi c free radicals. The HA extracted from submerged soils appeared to have gre ater 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. Th e MHA was less humified than the CaHA, having fewer acidic functional group s, smaller C:N and C:H ratios, richer concentrations of amides and carbohyd rates, lower concentrations of COOH, higher fluorescence intensity, and sho rter wavelength of fluorescence emission maximum, and lower concentration o f organic free radicals. Free radical concentrations for both HA fractions were highly correlated with other indices of humification reported here and elsewhere.