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Decrease in Humification of Organic Matter with Intensified Lowland Rice Cropping

A Wet Chemical and Spectroscopic Investigation

^a IRRI, P.O. Box 3127, Makati Central Post Office (MCPO), 1271 Makati City, Philippines
^b Istituto di Chimica Agraria, Università di Bari, Via Amendola, 165/A, 70126 Bari, Italy

View larger version (25K): [in a new window]   Fig. 1 Fourier-transform infrared spectra of the mobile humic acid fraction from the (a) dryland rice, (b) rice–soybean, (c) double-cropped prilled urea, and (d) triple-cropped optimal N fertilizer soils

View larger version (23K): [in a new window]   Fig. 2 Fourier-transform infrared spectra of the calcium humate fraction from the (a) dryland rice, (b) rice–soybean, (c) double-cropped prilled urea, and (d) triple-cropped optimal N fertilizer soils

View larger version (16K): [in a new window]   Fig. 3 Fluorescence excitation spectra showing the relative fluorescence intensity (RFI) of the (left) mobile humic acid and (right) calcium humate fractions from the (a) dryland rice, (b) rice–soybean, (c) double-cropped prilled urea, and (d) triple-cropped optimal N fertilizer soils

View larger version (25K): [in a new window]   Fig. 4 Wide scan range (500 mT) electron spin resonance spectra of the (a) mobile humic acid and (b) calcium humate fractions from the double-cropped prilled urea soil. The intensity of the organic free radical signal (a',b') is reduced by one order of magnitude. The Fe+3 signal was also obtained at higher gain (b'')

View larger version (16K): [in a new window]   Fig. 5 Correlations between the free radical concentration and (a) H concentration, (b) C:H ratio, and (c) light absorption at 465 nm (E465) for all mobile humic acid (MHA) and calcium humate (CaHA) samples. Data for light absorption are from Olk et al. (1996, 1998)(and unpublished). Units are optical density units (g HA–C L-1)-1