DYNAMIC AND BALANCE OF SOIL NUTRIENTS UNDER LONG-TERM FERTILIZATION CONDITIONS Ⅱ. Effect on N availability and composition of humus N

Long term application of organic manure, or inorganic fertilizers, or combination, of the two forms increased total nitrogen (N), alkali-hydrolyzed N and acid-hydrolyzed N in the soil, stimulated soil N availability and plant uptake. Its residual effect could be found in the third years after fertilization. As the soil humus, humic acid-N reduced, while fulvic acid-N increased due to fertilization. The fulvic acid-N can be easily hydrolyzed in 6mol/L HC1 solution, 71.4% ~ 74.2% of the total fulvic acid-N was hydrolyzed. The humc acid-N is more stable than fulvic acid-N, only 40%~45% of humic acid-N can be hydrolyzed in the HC1 solution. As the form of hydrolyzed N, 2/3~3/4 was amino-N while l/4~1/3 was NH4-N.The majority of the unhydrolyzed N in the humic acid was heterocyclic compound- N (37.5% - 52.5%), followed by benzene compound-N (31. 0%~35. 2%) and non-benzene compound-N (10. 6%~26. 7%). Due to long term application of organic manure and inorganic fertilizers, the portion of N in heterocyclic compound was reduced, while benzene and non-benzene compound was increased. About 1/3 of humic acid-N was on the surface, while 2/3 was in core. Application of organic and inorganic fertilizers increased the amount of N on humic acid surface, and reduced that in core of the humic acid.