Abstract: Soil samples from two long-term fertilization experiments carried out on the paddy soil in subtropical area of China for 17 years were measured. The soil samples were collected from the plough layers of different fertilizer treatments. Soil organic nitrogen fractions and microbial biomass nitrogen were studied. Results showed that long-term fertilization increased the soil organic nitrogen fractions in topsoil of paddy soil. The application of chemical fertilizers combined manure or straw could significantly increased the content of soil total N、available N、microbial biomass N and acid hydrolysable N, but had no apparent effects on non-hydrolysable N. The content of the different fractions of acid hydrolysable N varied with different treatments. Compared with the check, application of manure or straw with chemical fertilizers greatly increased the contents of amino acid N, ammonium N, amino sugar N while the contents of hydrolysable unidentified nitrogen (HUN) were decreased after long-term fertilization. Of the total N in the soils studied 69%-89% was acid hydrolysable N. The results showed a very significant positive correlation between microbial biomass N and acid hydrolysable N after fertilizer application. The soil microbial biomass N was also closely related with the amino acid N, amino sugar N and ammonium N. There was no significant correlation of microbial biomass N with HUN. It was concluded that application of organic manures along with chemical fertilizers is an effective way of maintaining the quantity and quality of soil organic N and the size of the microbial biomass. The traditional use of manures should therefore be promoted in order to maintain long-term soil productivity even where chemical fertilizers are being used to achieve high yields.

Key words: Long-term fertilization, Paddy soil, Soil organic nitrogen fractions, Soil microbial biomass nitrogen