Abstract: Forty six top soil samples covering 20 soil types were collected from eastern China mainly in the summer of 1998 and early spring of 2000. Water soluble organic matter (WSOM) of the samples were extracted with 2∶1 weight ratio of soil to water. Fluorescence excitation spectroscopy was recorded with Hitachi F 2500 fluorescent photometer and molecular size distribution of WSOM was determined with Sephadex G 25. All WSOM samples were not processed with pH adjusting and concentrating. Correlation analysis was used to study spatial variation of the intensity of fluorescence excitation per unit carbon of WSOM and its relationship with average molecular size. Also, the same analyses were performed on fulvic acids extracted from part of the soil samples. It was found that no significant negative correlation existed between the intensity of fluorescence excitation per unit carbon of WSOM and molecular size of WSOC, but partial correlation became true with pH as the controlling factor, showing pH is important to excitation intensity of WSOM without pH adjustment. No similar relationship was found for FA samples, which was considered to be partly due to the small amount of samples of FA (only 12 samples). Both the intensity of fluorescence excitation per unit carbon of WSOM and FA showed the same trend, increasing from south to north in the study area, which was similar to the trend of pH, average molecular size of both WSOM and FA, but opposite to that of the temperature and precipitation. The pH value of soil may play an important role in regulating spatial variation of fluorescent property of WSOM, which needs further investigation.

Key words: eastern China, soil, water soil organic carbon, molecular weight, fluorescence, spatial variation