Organic nitrogen (N) forms in soil were quantified in a 15-year
fertilization trial using the method proposed by Bremner
in 1965. Results showed that soil total nitrogen and organic nitrogen fractions were significantly influenced by long-term application of chemical fertilizer and manure. Compared with application of N and P fertilizers, manure with N and P fertilizers greatly increased the contents of total N, acid
hydrolysable N and nonhydrolysable N. Of the forms of acid hydrolysable N, hydrolysable unidentified N (HUN) was significantly increased but amino sugar N significantly decreased by manure with N and P fertilizers. Similar to manure with chemical fertilizer, N application significantly increased total N, acid hydrolysable N and nonhydrolysable N compared to the plots without N addition. N fertilizer significantly increased ammonium N but decreased amino sugar N, whereas the increase in amino acid N and HUN was not obvious. In contrast, P application did not significantly affect total N and organic N forms.