Abstract: Based on 100 samples from 4 soil depths, the spectrophotometric absorbances of soil solutions at 14 wavelengths (250, 254, 260, 265, 272, 280, 285, 300, 340, 350, 365, 400, 436 and 465 nm) and soil carbons (dissolved organic carbon DOC and soil organic carbon SOC）and nitrogens (dissolved soil nitrogen DON and soil organic nitrogen SON) were concurrently measured. We are aiming to answer the question that absorbance at which wavelength is most correlated with the soil carbon and nitrogen, and whether or not differences exist in these relations between top- and sub-soils. The following conclusions were obtained. Firstly, although absorbance values of surface and deep soil solutions significantly decreased with wavelength, the decrease in top soil was much steeper than that in sub-soils. The lower region of the studied wavelengths was more powerful to distinguish the difference between surface and deep soils, accordingly. Comparing with deep layers, more robust correlations between soil solution absorbance and SOC, DON and SON were found in surface soil layer (0-20 cm). However, the difference in the DOC and absorbance relations between surface and deep soils was much weaker. Secondly, the absorbance values of 8 wavelengths (250-300 nm) were significantly auto-correlated, indicating that each of them could function the same in characterizing soil solution. Using 254 nm as a representative of them, the regression analysis between absorbance (254, 340, 365, 400, 436 and 465 nm) and soil carbon, nitrogen have been carried out by using the entire pooled data. The best regressions (R²=0.53-0.59) for SOC, DON, SON were found in 254 nm, and sharp decreases in R² value with wavelength increases were found. No such large change was found in DOC, and R² from 0.25 to 0.33 were found in wavelengths of 254, 340, 365 and 400 nm. Our findings indicate that the spectrophotometric absorbance of soil solution (particularly from 250 nm to 300 nm) can be used as indicators for monitoring soil carbon and nitrogen changes, and attention should be paid to the soil sampling depths and variation in different soil carbon and nitrogen parameters.

Key words: soil solution absorbance value, soil dissolved organic carbon, soil dissolved organic nitrogen, soil organic carbon, soil organic nitrogen