Objective To investigate the changing patterns of water extractable organic matter (WEOM) in soda saline-alkaline paddy soil in the fields with different planting years of rice, and provide a theoretical basis for increasing organic carbon fixation in such soil.

Method Three-dimensional fluorescent spectroscopy was performed to study the contents and fluorescence characteristics of WEOM in soda saline-alkaline paddy soil with 0, 1, 3, 5, 15 and 20 planting years of rice.

Result With the increase of planting age, the content of WEOM in soil first increased and then decreased, reached the maximum (259.29 mg·kg^-1) after planting rice for three years, and was the lowest (19.29 mg·kg^-1) after planting rice for 20 years. The absorbance of WEOM at UV 254 nm increased with planting age. The shape of the three-dimensional fluorescence spectra of WEOM was different from fluorescence linear density which increased with planting age. There was a blue shift in the emission wavelength of the fluorescence peak in region I and region II of paddy soil, and a red shift in region Ⅲ and region Ⅴ. The fluorescence regional integration values were region Ⅴ> region Ⅲ> region Ⅳ> region Ⅱ> region Ⅰ. The proportion of humic acid-like substance in WEOM was reduced after planting rice for 20 years.

Conclusion The humic substances, fulvic acid-like substance and unsaturated aromatic compounds in soda saline-alkaline paddy soil increase with planting age. Three-dimensional fluorescence technique can be applied to study the structure and composition of WEOM in soil.