Abstract: 【Objective】The soil organic carbon content is low but the nitrogen fertilizer application rate is high in apple orchards in China. This study was conducted in order to provide a theoretical basis for the appropriate application of straw and biochar in apple production. 【Method】Two-year-old ‘Fuji’ apple trees (Malus domestica Borkh. cv Red Fuji/Malus hupehensis) trees were used to study the effect of straw and biochar on soil bulk density, cation exchange capacity (CEC), tree growth, and 15N transformation (tree uptake, ammonia volatilization, N2O emission, and soil residual) using 15N trace technique. There were four treatments: CK (control), N (only nitrogen fertilizer), N + B (nitrogen fertilizer + biochar) and N + S (nitrogen fertilizer + straw). 【Result】 The variation trend of soil bulk density in 0-5 and 5-10 cm soil layers was consistent in the four different treatments. There was no significant difference between CK and N treatment, but both were significantly higher than those in N + B and N + S treatments. For the two added exogenous carbon treatments, soil bulk density in N + B treatment was significantly lower than that of N + S treatment. Compared with the N treatment, soil bulk density in 0-5 and 5-10 cm soil layers in N + S and N + B treatments decreased by 0.06, 0.09 and 0.07, 0.11 g•cm-3, respectively. Compared with CK (18.32 cmol•kg-1) and N treatment (19.61 cmol•kg-1), the CEC of 0-10 cm soil layer increased significantly in N + S treatment (22.27 cmol•kg-1) and N + B treatment (25.35 cmol•kg-1). The highest total weight of apple trees, 15N uptake amount and 15N utilization efficiency existed in N + B treatment, followed by N + S treatment, and the lowest of those values were found in N treatment. Compared with CK, the amounts of ammonia volatilization significantly increased in the three N application treatments (N, N + S and N + B). Compared with N treatment, N + S and N + B treatments significantly reduced the N loss through ammonia volatilization, especially in N + B treatment. Compared with CK, the amounts of N2O emission were significantly increased in the three N application treatments (N, N + S and N + B), and the highest was found in N + B treatment, followed by N + S treatment, and N treatment was the lowest. So, addition of exogenous carbon could increase N2O emission rate, but no significant difference was found among the three nitrogen application treatments. When the CK background value was removed, total N gaseous losses (ammonia volatilization + N2O emissions) in N, N + S and N + B treatments accounted for the proportion of N application rate were 6.54%, 4.33% and 3.04%, respectively. The highest 15N residual rate in 0-50 cm soil layer was found in N + B treatment, followed by N + S and N treatment; while the highest 15N residual rate was found in N treatment, followed by N + S and N + B treatment in 50-100 cm. The highest N recovery rate was found in N + B treatment (42.26%), followed by N + S treatment (37.22%), and N treatment (31.54%) was the lowest; so the highest N loss rate appeared in N treatment (68.46%), followed by N + S treatment (62.78%) and N + B treatment (57.74%). 【Conclusion】Application of straw and biochar into apple orchard soil could decrease the soil bulk density, increase the soil cation exchange capacity, improve plant growth, promote N uptake by plant, increase the N fixed by soil and decrease N gaseous loss. The result will get better when application of biochar.

Key words: apple , straw , biochar , soil bulk density , CEC , N absorption and loss