Abstract:In order to reveal the distribution of soluble organic nitrogen (SON) in soil profile, three types of paddy soils (yellow, grey and yellow podzolic paddy soils) with the same parent material were selected to study the content, composition and main influencing factors of SON in paddy soil profiles. The results showed that the contents of SON, free amino acid nitrogen (FAA-N), amide nitrogen (AN-N) and soluble protein nitrogen (SP-N) had significant differences in different paddy soil profiles, and all were in an order of 0-20 cm > 20-40 cm > 40-60 cm. The contents of SON and its components in different paddy soils were mainly manifested in the 0-20 cm, in which all was in an order of grey paddy soil > yellow podzolic paddy soil > yellow paddy soil. The contents of SON, FAA-N, AN-N and SP-N in grey paddy soil were 50.5%, 41.7%, 44.8% and 2.1% higher than those of yellow podzolic paddy soil, 196.5%, 200.9%, 180.4% and 76.5% higher than those of yellow paddy soil. The contents of FAA-N and AN-N of the three paddy soils in 0-20 cm accounted for 55.4%-58.7% and 45.5%-48.1% of SON, respectively, while the contents of FAA-N and AN-N in 40-60 cm accounted for 33.1%-55.7% and 50.3%-52.8% of SON, respectively. These results indicated that FAA-N and AN-N had a downward accumulation trend, and contribute more to SON than SP-N in soil profile. Neutral amino acids, especially sarcosine, were the main components of FAA-N in 40-60 cm. Grey correlation analysis showed that organic matter, protease, urease and total porosity were closely related to the vertical distribution of SON and components in different paddy soils, and their contribution to SON, FAA-N, AN-N and SP-N were 95.2%, 92.1%, 98.3% and 90.9%, respectively. Therefore, the vertical distribution of SON and its components in soil profile is closely related to soil type and depth, and is affected by soil organic matter, enzyme activities and total porosity.