Soil samples collected from the 20-year long-term fertilizer experiment carried out on the manual loessial soil situated
in semiarid farmland ecological system were used to study the soil organic nitrogen
components and
microbial biomass nitrogen. Results showed that the application of straw and manure could significantly change the organic nitrogen components and microbial biomass nitrogen, and the amount and proportions of acidic hydrolyzable nitrogen increased apparently after application of organic materials, but the amount and proportions of unhydrolyzable nitrogen decreased. Among acidic hydrolyzable nitrogen components, the effects of organic materials on amino acid nitrogen (the average increment was148.7 mgN·kg-1) and ammonia nitrogen (theincrement was45.8 mgN·kg-1) were themost significant with no fertilization, the microbial biomass nitrogen at 0-20 cm depth was 102.2 and 110.4 mgN·kg-1 with application of both N and P fertilizers, whereas addition of 9 375, 18 750, 37 500 kg fresh maize straw and 37 500 kg of organicmanure per ha peryear on the basis of applying N and P fertilizers raised the micrbial biomass nitrogen to 147.5, 163.2, 286.4 and 265.3 mgN·kg-1, respectively.The results of correlation analysis show that there existed a very significant positive correlation between microbial biomass nitrogen and acidichydrolyzable nitrogen after fertilizer application (r=0.901, n=7). In acidic hydrolyzable nitrogen, the correlation coefficient between microbial biomass nitrogen with amino acid nitrogen was largest (r=0.963), with ammonia nitrogen was larger (r=0.886), and the correlations between amino sugar nitrogen or unknown nitrogen and microbial biomass nitrogen did not reach to a significant level. No matter what the case was, amino acid nitrogen and ammonia nitrogen are the easiest mineralizable components of organic nitrogen, the relationship between microbial biomass nitrogen and easily mineralizable organic nitrogen components was most closely.