Abstract:In this paper, the effects of long-term application of organic fertilizer and chemical fertilizer on soil labile nitrogen fractions and some kinds of soil enzyme activities (urease, catalase, alkaline phosphatase and sucrase) were studied based on a 26-year long-term field experiment in Chinese Academy of Agricultural Sciences Research Station in Yucheng City, Shandong province, China. The results show that, compared with CK, long- term fertilization significantly increases soil total nitrogen, particle organic nitrogen (POM-N), dissolved organic nitrogen (DON), microbial biomass nitrogen (MBN) and light fraction organic nitrogen(LFON) contents. Treatments of applying organic fertilizers alone could improve soil total nitrogen and labile nitrogen fractions contents more effectively than the treatments of applying mineral fertilizers alone. The highest soil total nitrogen and labile nitrogen fractions contents appear in treatment of double organic fertilizer input, then is the treatment of common organic fertilizer input. The similar soil total nitrogen and labile nitrogen fraction contents are in treatment of applying 50% organic fertilizer and 50% mineral fertilizer and the treatment of applied double amount of mineral fertilizers. The soil total nitrogen and labile nitrogen fraction contents are improved with the increased input levels of mineral fertilizers. POM-N is the main contributor of the soil total nitrogen, affected by fertilizer types and input levels. There is no difference in the contribution of LFOM-N to the soil total nitrogen among all the treatments, suggesting that fertilizer application types and input levels couldn’t affect the contribution of LFOM-N to the soil total nitrogen. The activities of soil urease, alkaline phosphatase and sucrase are increased significantly by the application of fertilizers. The three enzyme activities are correlated significantly with each other, and also correlated with soil total nitrogen, available phosphorus and organic carbon, and closer correlation exists between the urease activity and soil labile nitrogen fractions contents. But the activity of soil catalase is decreased after long-term application of fertilizers